11 AC AND FRIDGE for ship[Autosaved].pptx

Ac and fridge system for ships.

1. Refrigerant – Environmental Issue

2. J&E Hall were one of the first companies to see the potential of refrigeration on ships back in 1880 and now the industry is just at the end of another period of significant change. The changes are due to the large increase in the cruise ship market as well as the LNG carrier market, both of these are some of the biggest users of marine refrigeration and air conditioning systems. It is also now known that chlorofluorocarbons (CFCs) that were developed during the last half of the last century, and used in refrigeration systems, are very damaging to the environment. As a consequence the use of these refrigerant gasses has now been phased out. CFCs are ozone depleting substances (ODS) – see below – and the gas cannot be purchased anymore. Alternatives are being used in new and existing systems. REFRIGERATION AND AIR CONDITIONING

3. Ozone Depleting Substances Ozone is made up of three oxygen atoms joined together. This is slightly diff erent to the normal two oxygen atoms that form the bulk of the free oxygen that exists in the atmosphere. The bulk of atmospheric ozone lies in the stratosphere, which approximately 10–25 miles (15–40 km) above the surface of the earth. It carries out the important function of refl ecting, back into space, some of the ultraviolet (UV) radiation that has come from the sun. Any reduction in this protective layer would mean that more harmful radiation would reach the surface of the earth. Ozone is made and destroyed by the natural processes that are part of nature; however, this destruction had accelerated since the development, and subsequent release into the atmosphere, of CFCs. The natural production of ozone has not been able to keep up and the total amount of ozone has reduced.

4. CFCs are a very stable chemical that are not broken down by water and other chemicals in the lower part of the atmosphere. Therefore, CFCs remain intact until they reach the stratosphere where they are struck by intense radiation from the sun. The CFC finally decomposes under the sunlight and the chlorine released interacts with the ozone leading to its destruction. The chemicals that are most potent at reducing the ozone in the atmosphere are CFCs, carbon tetrachloride, methyl bromide, methyl chloroform and halons. Hydrochlorofluorocarbon (HCFC), which is a compound composed of hydrogen, chlorine, fluorine and carbon atoms, is being used as a temporary replacement for CFCs but even this is not allowed as a permanent replacement. The introduction of hydrogen means that the compound breaks down in the lower atmosphere before it is able to reach the stratosphere where the ozone is situated.

5. Desired properties of refrigerant : Low cost and ease of handling. Easily available. Non toxic & poisonous. Non explosive , corrosive & chemically stable. Non flammable. Leakages easily detectable. Moderate & low condensing pressure – otherwise heavy equipment & stronger piping required. High specific heat of liquid – reducing vaporization during throttling. Low boiling point – to avoid high vacuum. High latent heat of vaporization - reduce mass flow of refrigerant Low specific volume in vapour state – reduce size & increase efficency. Stable under working condition. Oil miscible . (capable of being mixed & dissolved) Environmental friendly

6. Ozone Creation : Chemical formula : 0 3 natural occurring gas found in the stratosphere (10 - 50 km from earth surface). ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- On absorbing the UV (ultraviolet) ray, the 0 2 molecules split to form individual oxygen atoms. The individual atom combines with remaining oxygen molecules to form ozone, 0 3 . OZONE IS A MOLECULE OF THREE OXYGEN ATOMS : Oxygen atom Oxygen molecule , 0 2 UV ray Free oxygen molecules Cluster of ozone

7. Ultraviolet (UV) light / ray / radiation is harmful to life. The ozone layer around the earth : a) Keeps the planet warm. In the lower atmosphere, ozone combines with the other greenhouse gases (water vapour, carbon dioxide, methane, nitrous oxide). b) In the upper atmosphere ,it blocks nearly all the sun’s UV rays from reaching surface of the earth.

8. CFCs (Chlorofluorocarbon) Compound consisting of chlorine, fluorine and carbon. Common CFCs : CFC-11,12, 113, 114 and 115. Found in refrigerants used previously, aerosols with CFCs propellant. Easily broken down by strong UV rays in the stratosphere, releasing chlorine atoms. Chlorine atoms deplete the ozone layer. HOW ?

9. Ozone Depletion UV ray CFCI 3 CFCI 2 Oxygen molecules 2. Chlorine atoms from the CFCs attacks the ozone, by taking one oxygen atom away to form chlorine monoxide. Ozone Oxygen molecules 1. CFCs reacting with the UV rays result in formation of a chlorine atom . chlorine atom Chlorine monoxide Oxygen atom Oxygen atom 3. The chlorine monoxide then combines with another oxygen atom to form a new oxygen molecule and a new chlorine atom. 5 . The chlorine atom goes on to destroy more ozone molecules. 4. The newly formed oxygen molecules do not block the UV rays, allowing it to penetrate to the surface of the earth. Oxygen atom Oxygen atom Oxygen atom chlorine atom Oxygen molecules

10. اثر گل خانه ای : زمانی‌که نور خورشید به سطح زمین می‌رسد، مقداری از آن جذب شده و زمین را گرم می‌کند. چون زمین از خورشید سردتر است، آن انرژی را با طول موج‌های بلندتری نسبت به خورشید از خود می‌تاباند. (نگاه کنید به تابش جسم سیاه و قانون جابجایی ویین) پیش از آن‌که آن‌ها در فضای بیرونی از دست بروند، مقداری از این طول موج‌های بلندتر توسط گازهای گلخانه‌ای در جو زمین جذب می‌شوند. جذب این انرژی تابشی باعث گرم شدن جو می‌شود (جو زمین همچنین در اثر انتقال گرمای محسوس و گرمای نهفته حاصل از سطح نیز گرم می‌شود). گازهای گلخانه‌ای، نور خورشید را هم به سمت سطح زمین و هم به سمت خارج از سطح زمین می‌تابانند. به فرایند بازتابش این نور به سمت سطح زمین که توسط جو انجام می‌شود، اثر گلخانه‌ای می‌گویند. بخار آب، دی‌اکسید کربن، متان و ازن مؤثرترین گازهای گلخانه‌ای هستند. با وجودی که نمی‌توان به‌طور دقیق مشخص کرد که سهم هر کدام از این گازها در اثر گلخانه‌ای زمین چقدر است اما بخار آب بین ۳۶٪ تا ۷۰٪، دی‌اکسید کربن بین ۹٪ تا ۲۶٪، متان بین ۴٪ تا ۹٪ و ازن حدود ۳٪ تا ۷٪ در فرایند اثر گلخانه‌ای زمین نقش بازی می‌کنند. گازهای گلخانه‌ای دیگر، که البته به همین‌ها محدود نمی‌شوند، عبارتند از، نیتروژن اکسید، هگزا فلوراید گوگرد، هیدروفلئور کربن ها، پرفلئور کربن ها و کلروفلئور کربن ها (به فهرست IPCC گازهای گلخانه‌ای نگاه کنید). اجزای اصلی جو یعنی: (نیتروژن و اکسیژن) گازهای گلخانه‌ای نیستند، زیرا مولکول‌های دوتایی با هسته‌های یکسان، تشعشع(انتشار) فروسرخ را نه جذب و نه منعکس می‌کنند در نتیجه هیچ تغییر شبکه‌ای در گشتاور دوقطبی در این مولکول‌ها رخ نمی‌دهد

11. Glossary (part 1) : Ozone Depletion Potential, ODP A number which refers to the amount of ozone depletion caused by a substance. ODP = impact on ozone of a chemical / impact on ozone with CFC-11 of similar mass. ODP CFC-11 = 1.0 ; ODP Halon = 10 ; ODP HFC = 0 (No chlorine present) Global Warming Potential, GWP A number that’s refers to the amount of global warming caused by a substance. GWP = warming caused by a substance / warming caused carbon dioxide of similar mass. GWP CO 2 = 1.0 ; GWP CFC-12 = 8,500 ; GWP CFC-11 = 5,000

12. Glossary (part 2): Chloroflurocarbon (CFC) : Compound consisting of chlorine,fluorine & carbon. Halon : Not a refrigerant but included in the glossary as used extensively as a extinguishing agent. Contains Bromine, which is many time more effective in destroying the ozone than chlorine. ODP halon 1301 = 13 ; ODP halon 1211 = 4 Hydrobromofluorocarbon (HBFC) : Compound of hydrogen, bromine , fluorine and carbon. Hydrocarbon (HC) : Compound of hydrogen & carbon. Methane, ethane, propane…… Although highly flammable, ODP=0;low GWP; low toxicity. Hydrochlorofluorocarbon (HCFC) : Compound of hydrogen, chlorine, fluorine & carbon. HCFCs to replace CFCs. Chlorine content is much lesser than CFCs.

13. Glossary (part 3): Hydroflorocarbons (HFCs) : Compound of hydrogen, fluorine and carbon. HFCs are to replace CFCs. Non existence of chlorine = zero depletion of ozone. Refrigerant 134a and R404 are classed HFCs group. They do not attack the ozone layer BUT are classified as GREENHOUSE gases. Hydrofluoroolefins (HFOs) are currently being developed to replace hydrofluorocarbons (HFCs) with high global warming potential (GWP) in air-conditioning and refrigeration systems. The development of low-GWP HFOs requires detailed knowledge of the key thermophysical properties of these refrigerants with commonly used lubricating oils at varying operational conditions

14. CFCs were banned under the Montreal Protocol (1989) due to Ozone Depletion Potential (ODP). HCFCs are in the process of being canceled due to their Global Warming Potential (GWP). Refer to the graph on the next page for refrigerant ODP and GWP ratings

15. 3.3 Ozone depletion potential The ozone layer in our upper atmosphere provides a filter for ultraviolet radiation, which can be harmful to our health. Research has found that the ozone layer is thinning, due to emissions into the atmosphere of chlorofluorocarbons (CFCs), halons and bromides. The Montreal Protocol in 1987 agreed that the production of these chemicals would be phased out by 1995 and alternative fluids developed. From Table 3.1, R11, R12, R114 and R502 are all CFCs used as refrigerants, while R13B1 is a halon. They have all ceased production within those countries which are signatories to the Montreal Protocol. The situation is not so clear-cut, because there are countries like Russia, India, China etc. who are not signatories and who could still be producing these harmful chemicals. Table 3.2 shows a comparison between old and new refrigerants It should be noted that prior to 1987, total CFC emissions were made up from aerosol sprays, solvents and foam insulation, and that refrigerant emissions were about 10% of the total. However, all the different users have replaced CFCs with alternatives. R22 is an HCFC and now regarded as a transitional refrigerant, in that it will be completely phased out of production by 2030, as agreed under the Montreal Protocol. A separate European Community decision has set the following dates. 1/1/2000 CFCs banned for servicing existing plants 1/1/2000 HCFCs banned for new systems with a shaft input power greater than 150 kW 1/1/2001 HCFCs banned in all new systems except heat pumps and reversible systems 1/1/2004 HCFCs banned for all systems 1/1/2008 Virgin HCFCs banned for plant servicing

16. Iso butane is R600a, when you compare the change of temperature with pressure the change is drastic in the case of iso butane than in R134a. secondly isobutane is a bigger compound. From the ASHRE hand book itself it is seen that for normal refrigeration purpose R600a better than R134a. it is because of the following reasons 1) Latent heat of vaporistion of R600a is much higher than R134a 2) With small change in pressure the temperature of R600a can be easily brought to required cold condition condition. 3) Cp of R600a much higher than R134a . but the density is a bit low for R600a. from the above mentioned reasons R600a better than R134a in the performance.

17. Marine refrigerants The refrigerants currently used in marine refrigeration plants, and some of their properties, are listed in Table 2. The uses of the refrigerants in ships are as follows: RlI is used in marine air conditioning, especially for cruise ships, and for cleaning out marine refrigeration machinery. Rl2 is used for marine air conditioning and food stores in ships, and has universal use in refrigerated containers. R22 is used in marine air conditioning and food stores in newer ships, and in most central cargo refrigeration plant, fishing boat refrigerated storage and freezing plant, and liquid gas tanker re-liquefaction plant. R502 has very occasional use for low temperature refrigeration. Ammonia (R717) is used for large freezing and low temperature storage installations on board fish factory vessels, and has very occasional use in central cargo refrigeration plant.

18. CFC and HCFC refrigerants RII, RI2, R22 and RS02 do not react with steel, copper, aluminium and brass, but attack lead, tin, zinc and magnesium and their alloys. They also attack natural rubber, some elastomers and polytetrafluoroethane (PTFE), so it is important to ensure that the correct materials are used for gaskets, seals, jointing and packing. Ammonia Ammonia reacts with copper, zinc and their alloys, so steel only should be used in ammonia plants. It also attacks natural rubber and some elastomers, so it is important to ensure that the correct materials are used for gaskets, seals, jointing and packing. Ammonia gas is extremely toxic, with a long term threshold limit of 35 parts per million (ppm), and may be lethal at concentrations of 2500ppm and above. It has a pungent odour, detectable at concentrations less than 10ppm, which provides a warning against remaining in harmful concentrations. Ammonia is flammable in air at concentrations of 16% to 27%, and may form an explosive mixture.

19. RI34a has been developed. as an alternative for Rl2. Containing no chlorine, it has an ODP of 0, and a GWP one tenth that of R12. It suffers a drawback in being unsuitable for use with mineral oils, and is expensive. Synthetic oils have been developed. but they too are expensive. At present RI34a is considered to be an acceptable refrigerant for small systems (below 5hp), operating at high evaporating temperatures and low condensing temperatures .

20. مبردهای ترکیبی جهت جایگزینی با مبردهای قدیمی ساخته شده اند تا سیستم ها بدون نیاز به تغییرات عمده ای با مبردهای جدید شارژ شوند.البته مبردهایی از این گروه نیز هستند که دارای خواص مخصوص به خود هستند و قابل جایگزینی نیستند. مخلوط های HCFC جهت جایگزینی کوتاه مدت تا پیدا شدن مبردی مناسبی استفاده می شوند اما مبردهای ترکیبی HFC برای جایگزینی بلندمدت ساخته شده اند. آزئوتروپ (مبردهای سری 500) هنگام تغییر فاز به گازهای جداگانه تجزیه نمی شوند و مانند گازهای خالص تنها یک دمای جوش دارند و افزایش دمایی در هنگام تغییر فاز ندارند. آزئوتروپ ها در صنعت تهویه و تبرید جدید نیستند و دو گروه مبرد R-500 و R-502 سالیان سال است که کاربرد دارند. R-502 یک مبرد ایده آل جهت تبرید در دماهای پایین محسوب می شود که امروزه در حال جایگزینی است این گروه مبرد ها اغلب از ترکیب دو مبرد ساخته می شوند. شبه آزئوتروپ ها (زئوتروپ) مبردهایی هستند که در هنگام جوشش هر جزء مبرد در دمای مخصوصی می جوشد و بنابراین این مبرد ها در هنگام تغییر فاز دما ثابت نیستند و افزایش دمایی را در هنگام جوشش نشان می دهند.همچنین این مبرد ها قابل تجزیه هستند. به علت تجزیه شدن زئوتروپ ها در هنگام جوشش افزایش دما را شاهد هستیم چون مبردهای سری 400 در هنگام تبخیر تجزیه می شوند باید به صورت مایع شارژ شوند

21. R404A R404A is a blend of R125 / R143a / R134a with mass percentages of 44% / 52% / 4% having a molar mass of 97.60 kJ/kg

22. Latent heat Latent heat is the quantity of heat absorbed or released by a substance undergoing a change of state, such as ice changing to water or water to steam, at constant temperature and pressure.

23. Basic Principles of the Refrigeration Cycle Ice-water-steam phase changes Most refrigerant’s vapour phase demonstrates similar characteristics and properties to steam except that the medium has a much lower boiling point. Consider changes that occur when 1 kg of ice at say –23°C is converted into superheated steam at say 1.013 bar, 200°C. These temperature-heat energy changes are best illustrated graphically ( Figure 7.1 ). 1. Heat added to raise the temperature of 1 kg of ice at –23°C to 0°C = 1 × 2.094 × 23 = 48.2 kJ. (Specific enthalpy of solid ice is –48.2 kJ at –23°C, i.e. h i .) 2. Heat added for fusion of 1 kg of ice at 0°C to 1 kg of water at 0°C= 1 × 333 = 333 kJ. (Specific enthalpy of fusion for ice is –333kJ, i.e. h if .)

24. 3. Heat added to raise the temperature of 1 kg of water at 0°C to saturation temperature ( t s ) 419.1 kJ. At 1.013 bar, t s = 100°C and h f =419.1 kJ (see Figure 7.1 ). (This is liquid specific enthalpy for water.) 4. Heat added for vaporisation of 1 kg of water at 100°C to 1 kg of steam at 100°C (i.e. constant t s ) is h fg = 2,256.7 kJ (2.257 MJ) (see Figure 7.1 ). (This is specific enthalpy of vaporisation for steam.) 5. Heat added to superheat 1 kg of steam from 100°C ( t s ) to 200°C ( t ) = 299.2 kJ (see Figure 7.1 ). (This is specific enthalpy to superheat above dry saturated steam or dss.) Degree of superheat = (200–100)= 100°C. 6. Heat added to change water at 0°C to dss at 100°C = 2,675.8 kJ (2.676 MJ). Also see the table in Figure 7.1 . (This is specific enthalpy h g for dry saturated steam vapour.)

25. Saturation The term saturation is used in thermodynamics to describe the condition of a fluid at its boiling point. When the fluid is still fully in the liquid state and at its boiling temperature then it is described as saturated liquid. When the same fluid has been totally vaporised and is still at the same temperature it is described as a dry saturated vapour. In between these two extreme conditions at boiling point the fluid is described as a wet vapour. Any additional heat supplied to a dry saturated vapour will result in superheating of the vapour and the temperature will rise above boiling point. In this condition, for most fluids’ the state is described as a gas.

26. Specific volume Specific volume (v) is a result of dividing the volume per mass unit. This term is the opposite of the density. Common units of specific volume are cubic meters per kilo, (m³/Kg).

27. Density Density is a result of dividing the mass per unit volume. The term is applicable to mixtures and pure substances and to the solid, liquid, gaseous matter or plasma state. Sometimes the density is known as specific mass. Density of all matter depends on temperature; the density of a mixture may depend on its composition, the density of a gas depends on its pressure. Common units of density are kilos per cubic meter, (kg/m³).

28. Vapour quality Vapour quality or, as it is often referred to, dryness fraction (x) gives information about the state of a cooling medium, for example, vapour quality of 0.4 means the state of the medium is 40% vapour and 60% liquid.

29. Thermodynamics - temperature and pressure relationship What prevents water from boiling at room temperature? It has nothing to do with the temperature. Water is prevented from boiling at room temperature because of the pressure. When you heat water to 100˚C at atmospheric pressure and then continue to add heat, what you are doing is supplying sufficient energy to the water molecules to overcome the pressure of the air and allow them to escape from the liquid state. If you put a cup of water with room temperature into a chamber with no air pressure, it would flash into steam. If you put the same cup of water into a chamber with 1Mpa absolute pressure, the boiling point would be 180˚C. As a rule of thumb: Lower the pressure and you will lower the boiling point. Increase the pressure and you will increase the boiling point.

30. Fundamental thermodynamics of vapours Gas-like forms of a liquid are often caused by a sudden change of temperature and are called vapours. Their thermodynamic state is near to both the boiling and the condensing process. Conversion of the state of aggregation from the liquid into the gas and the other way round is the very distinctive feature of vapours. First we shall look at what happens when we boil water. This example is when we boil water at standard atmospheric pressure. Let us start with 1kg of water at 20˚C. When we start to heat the water the temperature will start to rise until the water reaches its boiling point at 100˚C. At this point the water starts to evaporate but to fully evaporate the 1 kg of water we have to add more energy. However the mix of water and vapour will still remain at 100˚C until all the water is evaporated. For water, at atmospheric pressure, 100°C is referred to as the saturation temperature as well as the boiling point, At this temperature the liquid changes into a vapour and the vapour will become dry if sufficient heat is added. Adding further heat will cause the vapour to become superheated steam and the temperature will rise above the saturation temperature. The same will happen with all types of refrigerant but the pressures and temperatures will not be the same as for water. For refrigerants, as with most other liquids, we normally refer to the three states as liquid, vapour and gas.

31. The second law of thermodynamics To be able to evaporate a medium at – 20 ˚C and to condense the same medium at +30 ˚C we have to change the pressures. In its simplest form the Second law of thermodynamics can be expressed as a quote from Clausius: “Heat cannot of itself pass from a colder to a hotter body” In terms of a refrigeration process this can be expressed as: “Heat can only be transported from a body with low temperature to a body with higher temperature by the conversion of mechanical work.“ It is this law that is utilised in any refrigeration plant onboard ships. Heat is transferred from the required cold air space to the relatively much warmer seawater. For this to be possible one must perform work on the gas by a compressor, compressing the gas to a higher pressure and temperature than the seawater and then heat can transfer from the gas to the seawater in a heat exchanger.

32. Thermodynamics of cooling Refrigeration is defined as the branch of science and engineering that deals with the process of reducing and maintaining the temperature of a space or material below that of the surroundings. Since heat flows naturally (reference: 2nd law of thermodynamics) from a region of higher temperature to one of lower temperature, there is always a flow of heat from the surroundings into the refrigerated area. The heat flow in this case can be limited by the use of suitable and effective insulation. To maintain a constant temperature in the refrigerated space, the refrigeration system must remove heat from the space at the same rate as the heat is entering from the surroundings. Most modern refrigeration systems use an evaporation process to achieve cooling of the low temperature space. Heat is required to change a liquid into a vapour once it has reached boiling temperature. This is the latent heat of evaporation. During the evaporation process heat is transferred to the liquid refrigerant from the air in the low temperature space thus cooling the space This heat is then removed from the refrigerant in the condensing process later in the cycle.

33. Operating cycle Where a refrigeration system is critical for the safe operation of the vessel or protection of the cargo or crew such as with the provisions refrigeration system or the LNG cargo refrigeration system, marine practice would require complete duplication of all units. If the system was risk assessed as business critical it would also be singled out for special attention in the maintenance system. A typical refrigeration system is shown in Figure 7.2 and would undertake the following steps during the complete cycle. The refrigerant in its vapour phase is discharged from the compressor at 90°C and is condensed in the condenser at a condensation temperature of 25°C. For good heat energy transference rate a temperature differential of about 8°C between cooling water inlet and condensation temperatures is usual. The condenser gauge registers condensation (saturation) pressure and corresponding saturation temperature ( t s ) on a dual scale. Some undercooling will occur in the condenser under standard conditions, typically this will be 5°C so the liquid will leave at 20°C. The liquid now passes through the expansion valve where it is released to the desired vaporisation pressure determined by the evaporator outlet temperature. Some flash-off of the liquid to vapour will also occur but the greater the undercooling the less will be the flash-off percentage. This flash-off represents a loss, as any vapour formed before the evaporator will not extract heat from the brine, giving a resultant loss of refrigeration effect. Ideally the fluid should be totally .

34. wet entering the evaporator and just dry leaving the evaporator which means full absorption of heat happens in the evaporator and the heat is extracted from the brine and not from the associated pipework leading to the evaporator. In an ideal system superheating is not advantageous, but due to the practicalities of system design the assumption is that if the vapour is superheated at entry to the compressor then it will have no liquid present to damage the compressor and there will be a higher degree of superheat upon leaving the compressor. The vapour now leaves the evaporator under assumed standard conditions having 5°C of superheat. The evaporator gauge registers vaporisation (saturation) pressure and corresponding saturation temperature ( t s ) on a dual scale. The actual superheated vapour temperature as read from the thermometer being –10°C. For good heat transference rate, a temperature differential of about 5°C between brine outlet and vaporisation temperatures is usual The vapour now enters the compressor to start the circuit again. It should be clearly understood that Figure 7.2 refers to one set of definite conditions, that is, a sea temperature of 22°C and a particular expansion valve setting which determines that condition. Variations of sea temperature or vaporisation pressure would indicate completely diff erent readings but the basic temperature diff erentials of 8°C and 5°C should still exist.

35. The compound pressure gauge shown in Figure 7.3 illustrates the dual scale of saturation temperature and pressure. Ammonia is shown for illustrative comparison purposes only, that is, normally only one refrigerant pressure and temperature is on the scale. Commonly all readings are taken in temperatures only. Correct diff erentials are an indication of correct working with suffi cient vapour charge. Under correct running conditions the compressor discharge pipe should be fairly hot to the touch and the suction pipe should be just frosting up near the compressor. The compressor discharge and suction lines are commonly provided with cross-over valves in addition to the stop valves. These valves allow the pumping out of the high pressure side to the low pressure side for overhauls and allow an easy discharge for starting. Refrigerant is added, with the machine running normally, at the charging position. Many of the circuits employ a liquid receiver after the condenser and CO 2 types commonly have intermediate liquid cooling receivers. The capacity of a liquid receiver is usually sufficient to cover the outlet to the liquid line. Methods of control of the flow of refrigerant are (a) low side fl oat, (b) high side fl oat, (c) hand manual control, (d) capillary, (e) direct expansion with constant pressure, (f ) direct expansion with constant superheat. These are discussed later. The system should always be kept clear of water, air and dirt. Appropriate filters are fitted in the systems and these should be checked on a regular basis.

36. The correct operation of the refrigeration plant is an area for the engineering staff to concentrate upon to ensure an energy efficient ship. The plant can so easily come out of adjustment and this would cause additional energy to be consumed. All ships must keep a ‘technical file’ containing a Ship’s Energy Efficiency Management Plan, and the correct operation of the refrigeration plant will be part of that plan. Engineers must keep a close watch on the refrigeration plant and to help there are some simple faults listed below. 1kg co2 specific enthalpy of evaporation …….= 275 kj 1kg NH3 specific enthalpy of evaporation ……= 1310 kj 5kg co2 volume ............................................ …..= 0.085 m3 1kg NH3 volume ……………………………………= 0.51 m3 Result : co2 high mass flow Result : NH3 high volume flow 8.5 kg ccl2f2 specific enthalpy of evaporation. = 1310 kj 8.5 kg ccl2f2 volume ………………………………..=0.79m3

37. Simple closed refrigeration cycle The evaporator is at a low pressure and the refrigerant therefore has a low saturation temperature which is below the desired temperature of the space to be cooled. The liquid refrigerant entering the evaporator boils at this low temperature and in doing so takes heat from the surrounding air and so reduces the temperature in the cold space. At the evaporator outlet the refrigerant state should be that of a slightly superheated gas. The gas then passes through the compressor where its pressure and temperature are raised. At the compressor outlet the gas temperature is well above that of the condenser cooling medium which is below the saturation temperature associated with the existing gas pressure. As the gas passes through the condenser it first gives up its superheat as it changes to a vapour and then gives up its latent heat as it is condensed back to a liquid. Ideally the liquid condensate will be slightly undercooled (below its saturation temperature) as it leaves the condenser. Next the liquid flows to the expansion valve and expands to the evaporating pressure and associated saturation temperature. The process of evaporation takes place entirely in the evaporator. This is the simple refrigeration cycle.

38. The refrigeration cycle Practically all marine refrigeration systems operate on the vapour - compression cycle. Any cycle consists of a repetitive series of thermodynamic processes. The operating fluid starts at a particular state or condition, passes through the series of processes, and returns to its initial condition. The vapour-compression cycle consists of the following processes: Expansion Vaporization Compression Condensation

39. T. Δ S= Δ H Δ S= Δ Q/T S = ENTROPY H= ENTHALPY T= TEMPERATUR

40. Specific enthalpy Enthalpy (h) means the sum of the internal energy and the product of the pressure and volume of a thermodynamic system. It is an energy-like or state function. It has the dimensions of energy and its value is determined entirely by the temperature, pressure and composition of the system. If the only work done is a change of volume at constant pressure, the enthalpy change is exactly equal to the heat transferred into the system. For each substance, the zero-enthalpy state can be some convenient reference state that is worthy of being remembered. The term “specific enthalpy” (h, SI unit: kJ/kg), as a result of dividing the enthalpy per mass unit, is also used. The basic unit in the metric system for enthalpy is: kJ

41. فرق آنتالپی با آنتروپی آنتالپی گرمای واکنش است که با آنتروپی بسیار فرق می کند اگر واکنشی در حین واکنش گرما از خود تولید کند می گوییم که این واکنش گرماده است که آنتالپی آن کوچکتر از صفر میشود و به اصطلاح میگوییم منفی است واگر واکنش گرما بگیرد میگوییم این واکنش گرماگیر است که دلتا اچ آن یا همان آنتالپی از صفر بیشتر یا مثبت میشود. آنتروپی نشان دهنده ی بی نظمی واکنش و خود به خودی آن است . وقتی که ذرات یک ماده در حین واکنش فاصله ی بیشتری ازهم می گیرند یعنی اینکه بی نظمی آن ماده زیاد شده است وآنتروپی آن ماده عددی مثبت می شود مانند ذوب شدن یخ که وقتی به آب تبدیل میشود بی نظمی آن زیاد میشود . وقتی آنتروپی واکنش مثبت میشود می گوییم واکنش مورد نظر خود به خودی است وبرای واکنشهای برعکس این مطلب که گفته شد آنتروپی نیز بر عکس میشود. نماد آنتروپی دلتا اس است

42. T. Δ S= Δ H B-C. Isobaric Heat absorption in the evaporator C-D. Isentropic compression in the compressor D-A Isobaric heat removal in condenser A-B Constant enthalpy expansion in expansion valve

43. The circuit appears on the theoretical charts as shown in Figures 7.4 and 7.5 . Entropy being a theoretical property of a fluid that remains constant during frictionless adiabatic operations Heat energy received from cold condenser = area under BC Heat energy rejected from in the condenser = area under DA Heat energy equivalent of work done =heat energy rejected - heat energy received = area under DA - area under BC = area of figure ABCDA +area under throttle curve AB B-C. Isobaric Heat absorption in the evaporator C-D. Isentropic compression in the compressor D-A Isobaric heat removal in condenser A-B Constant enthalpy expansion in expansion valve The compression is taken to be isentropic (frictionless adiabatic) for calculation work, this means the compression line is vertical (constant entropy). Unit mass of refrigerant is the usual basis. Coefficient of performance for Freon is approximately 4.7. It should be noted how undercooling, in moving point A to the left, increases the heat received from the cold chamber thus increasing the refrigerant effect. Thermodynamic cycles

44. increasing the refrigerant effect. A sub cooled or under cooled liquid does a liquid exist at temperature lower than the saturation temperature for that Pressure whilst a liquid exactly at saturation temperature is a Saturated liquid, e. q., water at atmospheric pressure a Sub cooled liquid at 77°C and a saturated liquid at 100°C. Superheating & Sub-cooling

45. Δ H= Δ S.T Hb=150, hc=304 , hd=365 Cop=(304-150)/(365-304) =2.52

46. Refrigeration cycle p ~ v diagram This diagram is shown ( Figure 7.5 VOL 8) to allow comparison of this refrigeration cycle with other more familiar cycles covered in theoretical work on p ~ v diagrams. In practice the p ~ v diagram is rarely used in refrigeration.

47. Refrigeration cycle p ~ h diagram Once basic theory has been established by using T ~ s charts, the emphasis shifts in practice to the p ~ h (Mollier) chart ( Figure 7.5 VOL 8). This diagram has the big advantage that heat extracted, heat rejected and work done heat equivalent can be read off directly from the h axis in kJ/kg. Coefficient of performance = Refrigerating Effect/heat of compression = heat energy received/ Heating effect equivalent of work done C.O.P = Cooling Effect/Heating Effect CO.P =HC’-hB/hD-hC’ The coefficient of performance for Freon is about 4.7 It should be noted that undercooling increases the heat received by moving point A to the left

48. Heat energy E 1 at low temp. being passed to refrigerant Compression energy E 2 Meat room at -18 o C Energy E 3 thrown out to sea

49. OUR OBJECTIVE Energy E 3 thrown out to sea = Heat energy E 1 at low temp. + Compression energy E 2 WE NEED TO SPEND THIS WASTE ENERGY TO SEA

50. Coefficient of Performance COP is ratio of Energy Extracted E1 over Energy Spent E2

51. Intermediate liquid cooling It has been mentioned previously that there is a loss in refrigeration effect due to flash-off to vapour when the liquid is being throttled through the expansion valve. Undercooling before the expansion valve reduces flash-off after throttling, so lowering quality, and increasing refrigeration effect in the evaporator. (Although this applies to all refrigerants the loss would only be taken as serious with CO 2 because of its very low liquid specific enthalpy to specific enthalpy of vaporisation ratio.) The practical flash-off loss is about 20% in terms of refrigeration effect for Freon . This does not justify the complexity of fitting two expansion valves and an intermediate liquid valve between them.

52. Critical temperature Critical temperature is that temperature beyond which the gas cannot be liquefied by isothermal compression, that is, as a gas, no amount of compression will liquefy if the temperature remains above the critical temperature for that substance. CO 2 has a low value (31°C) and once the sea temperature (coolant) reached 23°C the critical had been reached (8°C differential) and from this point the efficiency of the CO 2 plant steadily decreases. The critical temperature for most refrigerant vapours is however well above the normal condensing temperatures 96°C for R22 and 73°C for the HFC R404A.

53. Compressor Type Reciprocating Centrifugal Screw rotary

54. Compressors Reciprocating: Suction must be dry Low capacity High leakage factor; air entry Low specific volume Long pressure different

55. Reciprocating compressors Reciprocating compressors for systems cooling domestic store rooms are usually of the vertical in-line type. The larger reciprocating compressors (Figure 11.4} have their cylinders arranged in either V or W formation with 4,6, 8, 12 or even 16 cylinders. Compressor speeds have been increased considerably over the years from 500 rev/min to the high speed of 1500 to 2000 rev/min. The stroke/bore ratio has diminished to the point of becoming fractional because of improvements in valve design and manufacture. Provision is made for unloading cylinders during starting and for subsequent load control, by holding the suction valves off their seats (Figure 11.5) by suitable oil-pressure operated mechanisms. With this control the compressors can be run at constant speed which is an advantage with a.c. motors. A bellows device, actuated by suction pressure can serve to cut out one or more cylinders. Thus a falling suction pressure, indicating a reduced load on the Figure 11.5 Cylinder unloading mechanism system, can be used to reduce automatically the number of working cylinders to that required to deal with the existing load. Nearly all compressors of this type are fitted with plate type suction and delivery valves, whose large diameter and very small lifts offer the least resistance to the flow of refrigerant gas.

56. Each crank of the spheroidal graphite cast iron crankshaft for the W configuration compressor shown carries four bottom ends. The aluminum alloy pistons operate in cast iron liners , which are honed internally . Piston rings may be of plain cast iron but special rings having phosphor-bronze inserts are sometimes fitted. These assist when running in. Connecting rods are H section steel forgings with white metal lined steel top end bushes. Liners are of high tensile cast iron and the crankcase and cylinders comprise a one-piece iron casting . The two throw crankshaft is of spheroidal graphite cast iron. Each throw carries four bottom ends as mentioned above but in other machines the number of banks of cylinders may be less . Main bearings are white metal lined 344 Refrigeration Gas from the evaporator passes through a strainer housed in the suction connection of the machine. This is lined with felt to trap scale and impurities scoured from the system by the refrigerant particularly during the running-in period, Freons are searching liquids, being similar to carbon tetrachloride. They tend to clean the circuit but the impurities will cause problems unless removed by strainers. Any oil returning with the refrigerant drains to the crankcase through flaps at the side of the cylinder space. The valve assembly is shown in Figure. 11.5 in more detail. The delivery valve is held in place by a safety spring which is fitted to allow the complete valve to lift in the event of liquid carry over to the compressor. The delivery valve is an annular plate with its inside edge seated on the mushroom section and its outside edge on the suction valve housing. The suction valve passes gas from the suction space around the cylinder. The control system includes a high pressure cut-out but a safety bursting disc is also fitted between the compressor discharge and the suction. This may be of nickel with a thickness of 0.05 mm. A ruptured disc is indicated by suction

57. Veebloc In general there are four, six or eight cylinders radially round the upper half of the cast iron crankcase with two to four connecting rods from each of two crank throws (see Figure 7.7 for four-cyinder V and Figure 7.8 for eight-cylinder W types). The aluminium piston is fitted with two compression rings and one scraper ring. Piston and gudgeon details are given in Figure 7.8 . A differential oil pressure switch and overload electrical switch protect the machine from low oil or high vapour pressure. In addition the discharge valve cage is spring-loaded to lift in case of liquid carry-over and there is an overpressure nickel bursting disc to relieve excess discharge pressure to the suction side of the machine. Connecting rods are aluminium with steel-backed, white metal bearings, the crankshaft is SG iron. Provision is made for reducing the capacity of the machine either manually or automatically. Capacity reduction gear lifts and holds open the alloy steel suction valves of a specified number of cylinders, which is operated by oil pressure on a servo piston in the automatic type. This can also provide total or partial unloading for easier starting. The lubrication should be clear from Figure 7.8 . Oil is supplied by a rotor type of pump in which the inner rotor has one less tooth than the outer rotor and oil is induced to flow between the two rotors. An eight-cylinder machine of 178 mm bore and 140 mm stroke running at about 12.5 rev/s would require a drive of about 90 kW for a refrigerating capacity of about 320 kJ/s. For low suction temperature operation (say –20°C or lower) and high temperature of discharge (say 30°C or higher) excessive temperatures may be reached in the reciprocating compressor. This is even more liable to occur in the unloaded state than in the loaded state. It is most often found in the fast running smaller bore-stroke size of compressor. In certain cases an oil cooler – operation direct expansion, thermostatic – must be used particularly when automatic unloading is required, and the above conditions apply. Compound compression units must be provided or special change-over valves can be fitted so that an eight-cylinder unit will operate in a single-stage down to –20°C and for temperatures below this six cylinders can perform the initial compression and the remaining two cylinders perform the final compression. For suction pressures below atmospheric level, the risk of air leakage is an important consideration. Crankcase oil heaters are usually fitted for use with the machine stopped, this prevents formation of liquid refrigerant and oil frothing on starting. Auto compressors should be fitted with solenoid operated liquid stop valves (see magnetic liquid stop valve, Figure 7.15 later).

58. Why Refrigeration Compressor Takes Suction from Crankcase Unlike air compressors, it is common in reciprocating type refrigeration compressors to take suction from crankcase. Outlet from the evaporator coils is led to the compressor crankcase. There are a few advantages with this design. Since crankcase is pressurized, no air can enter the system. No refrigerant gas is wasted by even small blow pasts from the compressor pistons. Refrigerant gas is miscible with oil. This property helps the gas to bring the oil in the system back to the compressor. In some designs, oil in the refrigerant gas drips inside the crankcase before leading it for compression.

59. By-pass valve : The compressor is equipped with a built-in mechanical by-pass valve, Fig. 4.7 and Fig. 4.8, pos. 24, which safeguards the compressor against unintended over-pressure in case the electric safety equipment should fail. The bypass valve acts as a kind of over-pressure safeguard between the discharge and suction side of the Compressor The by-pass valve is delivered pre-set, sealed and adjusted to the following opening pressures: – Standard for SMC and TSMC (HP stage) compressors: 24 bar [348 psi]. – Special for SMC - and TSMC (HP stage) compressors: 22 bar The by-pass valve is of the high-lifting type, which makes it robust and durable. Moreover, the by-pass valve is independent of the pressure on the suction side of the compressor. Consequently, it opens only when the pressure on the discharge side exceeds the set pressure compared to that of the atmosphere.

60. The solenoid control valve is an electromagnetic three-way valve which, with a dead coil, connects the unloading cylinder, pos. 12, with the crankcase ( the passage of the oil flow from pipe 2 to pipe 3 is open), Fig. 4.40 If the coil is energized, the valve will reverse so that the passage of the oil flow from oil discharge pipe 1 to 2 is open and the connection to pipe 3 is closed . The solenoid valves are mounted in joint blocks, Fig. 4.41, with one, two, three or four solenoid valves in each block.

61. Unloading device

62. loaded Due to high suction pressure in crankcase Balance between Pressure of constant oil pressure And spring and suction pressure

63. Off load

64. Capacity Regulation of Compressor All compressors have a built-in capacity regulating system which continually adjusts the compressor capacity to the cooling requirements of the plant. Even at reduced capacity the compressor works very efficiently. This makes it very well-suited for plants with reduced cooling requirements for lengthy operating periods. Fig. 4.37 Capacity Regulating Mechanism The capacity regulating system including the frame, pos. 13, is activated by the compressor oil pressure and controlled by means of solenoid valves fitted on the compressor. At a capacity reduction two suction valves are forced open at a time. In this case no compression takes place in the relevant cylinders as the sucked in gas in the cylinders is pressed back to the suction chamber through the suction valves. The above forced opening of the suction valves is also used when starting up the compressor. The system works as follows: At compressor standstill all the suction valves are forced into an open position and cannot be closed until the compressor is in operation and the oil pump has built up the oil pressure in the lubricating system. With an open suction valve there is no compression resistance in the compressor and this reduces its starting torque considerably. Thus, a motor dimensioned to suit the operating conditions of the compressor can easily start up the compressor also by using the star/delta starting system. For compressors fitted with extra capacity stages (extended unloading), one cylinder (SMC 104- 106-108) or two cylinders (SMC 112-116) will be in operation all the time, also at start up. See extended unloading. Capacity Regulation and Unloading of Compressor Capacity Regulation As mentioned in the introduction to this section all SMC and TSMC compressors are fitted with a hydraulic capacity regulating system by means of which the compressor capacity can be adjusted to the refrigerating requirements of the plant. When reducing the compressor capacity, two or more suction valves (on compressors with extended unloading: one or more suction valves) are forced open so that compression does not occur in the cylinders in questionThe suction valve is forced open when the unloading ring together with the pins, pos. 19B, are pressed up under the suction valve, thus keeping the valve in open position as shown in Fig. 4.38

65. Start Unloading As already mentioned the compressor cylinders are unloaded when there is no oil pressure on the unloading cylinders. This means that when the compressor is stopped, i.e. without any oil pressure, all the unloading cylinders are unloaded and consequently there is no compression resistance during start-up. This unloading during start-up reduces the starting torque of the compressor considerably Solenoid Valves for Capacity Regulation The unloading cylinders are controlled by solenoid valves, Fig. 4.40, which receive opening and closing signals from a connected regulator. This could e.g. be a programme device or the Sabroe electronic control system, Unisab II, as described later in this section under Instrumentation.

66. The solenoid control valve is an electromagnetic three-way valve which, with a dead coil, connects the unloading cylinder, pos. 12, with the crankcase (the passage of the oil flow from pipe 2 to pipe 3 is open), Fig. 4.40 If the coil is energized, the valve will reverse so that the passage of the oil flow from oil discharge pipe 1 to 2 is open and the connection to pipe 3 is closed. The solenoid valves are mounted in joint blocks, Fig. 4.41, with one, two, three or four solenoid valves in each block.

67. The oil pump , pos. 11A, Fig. 4.14, is built into the compressor and driven by the crankshaft by means of a coupling. The oil pump is a self-priming gear pump which takes the oil from the oil sump through an oil suction strainer in the crankcase, pos 33A, Fig. 4.13 and forces it through the full flow filter into the lubricating system

68. The oil pressure regulating valve , pos. 22, Fig. 4.12 regulates the oil pressure in the compressor lubricating system. It can be adjusted from the outside by means of a screw driver when the pointed screw, which locks the regulating screw, has been loosened. For variable speed driven compressors, the oil pressure has to be adjusted to the minimum oil pressure at minimum speed. Due to rising pressure drop at high oil flow, the pressure will rise when running at maximum

69. High Pressure Cut-out KP15 Adjusted to stop the compressor if the discharge pressure rises to a pressure 2 bar [29 psi] lower than the setting pressure of the by-pass valve.

70. 2. Low Pressure Cut-out KP15 Adjusted to stop the compressor if the suction pressure drops to a pressure corresponding to 5K lower than the lowest evaporating pressure. The pressostat has an automatic reset function and will therefore restart the compressor once the pressure rises again.

71. 4. Oil Differential Cut-out MP55 Adjusted to stop the compressor if the pressure in the lubricating system drops below 3.5 bar [51 psi] compared to the pressure in the crankcase. The pressure cut-out has a built-in time delay of 60 sec . which keeps it idle during the start-up of the compressor until the correct oil pressure has been established. The pressostat has a manual reset function as well as a yellow indicator lamp which, when illuminated, indicates that the electric circuits are working. Normal oil pressure in the compressor is 4.5 bar [65 psi] which is indicated on the manometer 9 on Fig. 4.50.

72. 5. Discharge Pipe Thermostat KP98 Adjusted to stop the compressor if the discharge gas temperature exceeds 120°C [248°F] for HFC/HCFC This adjustment can, however, be set to 20°C [68°F] above the normal discharge gas temperature, once this is known from experience. This makes it possible to safeguard the compressor against excessive temperatures. The thermostat has a manual reset function.

73. 6. Oil Thermostat KP98 Adjusted to stop the compressor whenever the oil temperature in the crankcase exceeds 80°C [176°F]. The thermostat has a manual reset function.

74. 7. Oil Filter Differential Pressostat Indicates when oil filter pos. 9A needs to be replaced. Connections on the pressostat has a transparent housing and will indicate power supply with green LED and filter replacement with red LED.

75. Charging Compressor with Lubricating Oil Since all SABROE reciprocating compressors are supplied with a special oil charging valve on the crankcase, refrigeration oil may be charged while the compressor is in operation . For this purpose, use a hand-operated oil pump as mentioned earlier or follow the procedure outlined below: Oil charge and oil level are shown in Table 11.2 and Table 11.3. Note: When charging for the first time, use the oil pump. The compressor must never be started unless it is charged with oil. • Reduce pressure in crankcase, e.g. by throttling suction stop valve, until suction pressure gauge shows pressure slightly below atmospheric . • Fill pipe connected to oil charging valve with refrigeration oil and place free end of pipe in a receptacle containing fresh refrigeration oil. • Open oil charging valve carefully. Thus external air pressure will force oil into crankcase. • Avoid getting air or other impurities sucked into compressor. Note: In order to achieve pressure below atmospheric, it will sometimes be necessary to set the low-pressure cut-out so that the compressor can operate down to this pressure . Remember to set the pressure cut-out to its normal setting after oil charging. When in operation, the compressor may be recharged with oil using the hand-operated oil pump

76. Note: Since halocarbon refrigerants such as R22 mix with refrigeration oils, there will always be a good portion of oil blended with the refrigerant in the plant . Therefore, it is often necessary to recharge with refrigeration oil after starting up for the first time and after charging with fresh refrigerant. Therefore, the oil level in the compressor must be watched closely after start up .

77. Searching for Leaks Searching for leaks can be carried out in many ways. The most frequently used method is to use a leak detector. After finding a leak, a good way to find its exact location is to apply soapy water. In case of large leaks, divide the system into sections by closing the valve to prevent needless amounts of refrigerant from leaking out. Proceed as follows to search for leaks: • Ascertain whether there is a shortage of refrigerant by checking the level of the refrigerant in the condenser or the receiver. • Also check that the measuring equipment is functioning properly and do not give false readings. • Search for drops of oil at couplings, flanged joints, valve spindles, shaft seals, safety valves, oil filter and the like . • Ventilate in advance to remove any gaseous refrigerant from the searching area. • Check that the required refrigerant pressure is present in the part of the system in question. • Conduct the search systematically to make certain that all possible sources of leaks are checked. Do not forget any of the following: • Threaded joints and flanged joints • Valve spindles • Shaft seals • Relief equipment • Expansion valves • Pressure transducers and pressure gauges • Safety valves

78. Shaft seal Where motor and compressor casings are separate, a mechanical seal is fitted around the crankshaft at the drive end of the crankcase. This prevents leakage of oil and refrigerant from the crankcase. The type shown (Figure. 11,6) consists of a rubbing ring with an oil hardened face against which the seal operates. The seal is pressed on to the face by the tensioning spring and being attached to bellows, it is self-adjusting. The rubbing ring incorporates a neoprene or duprene ring which seals it to the shaft. The mechanical seal is lubricated from the compressor system and can give trouble if there is insufficient or contaminated oil in the machine. Undercharge may be caused by seal leakage (sometimes due to oil loss). When testing the seal for gas leakage, the shaft should be turned to different positions if the leak

79. The reciprocating compressors is fitted with high quality shaft seal. This consists of a rotating and a stationary unit. A routine inspection of the shaft seal is not normally necessary. With regard to increased operational reliability it is, however, recommended to make an inspection in connection with an oil change, faults in the oil supply and also at regular intervals when operating with high discharge gas temperatures and oil temperatures. Special attention should be given to cracks in the O-ring, as well as wear, scoring and material deposits, carbon and copper plating on the sealing ring. An oil leak rate of 0.05 cm3/h is within the tolerance.

80. Possible Causes of Failure of Mechanical Seal Lack of lubrication (insufficient oil supply, high refrigerant concentration in the oil) Heavy wear of driving parts (high proportion of dirt in the oil) Axial play of crankshaft too large Overheating (hardening and cracking of O-rings, oil carbon) Strong vibration (insufficient fixing of coupling or drive pulley, drive not smooth enough, coupling or drive pulley displaced) Belt tension too high The pressure in the compressor must first be released. According to the drive system, the drive pulley, motor, coupling housing, coupling and key should then be removed.

81. Fitting of Mechanical Seal When strong wear to the drive parts is suspected (contaminated oil, strong deposits) a precautionary compressor exchange or overhaul is urgently recommended. The shaft, flange (gasket remains) and the shaft seal chamber should be cleaned very thoroughly. Any deposits on the shaft must be carefully removed. If necessary the surface can be smoothed with fine polishing cloth soaked in oil (not smoothing cloth). Always exchange the complete shaft seal when possible Never re-use old O-rings Do not touch the sealing surfaces Oil the rotating sealing surface, O-ring and shaft with clean refrigeration oil. Do not oil the asbestos-free gasket or the flange surface. Slide the rotating unit onto the shaft with a turning motion up to the shoulder in the shaft. The drive pin must be located in the slot provided. Lightly oil the sealing surface of the stationary unit, then mount the whole unit including the gasket over the shaft. The gap between the crankcase flange and the cover should be approx 5 mm (spring tension). The fixing screws should be evenly tightened in a crosswise order with a torque wrench with torque as mentioned in the instruction manual

82. Hermetic compressors Hermetic and semi-hermetic machines are conventional compressors but the driving motor is enclosed within the same chamber or casing and is cooled by passage of the refrigerant on the suction side. The motor heat is, of course, an addition to the refrigeration load. Total enclosure removes the risk of gas leakage through the vulnerable shaft seal. These compressors are not intended to be overhauled onboard but to be removed and replaced after a predetermined period of operation. This reduces the risk of moisture ingress into the system during overhaul in possibly far from ideal conditions. The hermetic compressors are normally used in small cooling systems like stand alone air condition units and refrigerators.

83. Semi-hemetic compressors have bolted casings, which provide access to the compressor for maintenance and repairs . This type of compressor is used in refrigerated containers and ships' provision stores. An attendant danger with hemetic or semi-hermetic compressors is that the electric motor may develop a fault and burn out, so contaminating the refrigerant system. To guard against this, current or temperature overload protectors are installed in the rotor windings. When a burn-out occurs, the system should be thoroughly purged to remove contaminated refrigerant and oil, and the compressor, liquid line filter drier and expansion valve should be renewed. A special burn-out filter drier should also be fitted in the suction line, immediately before the compressor. The system should then be commissioned, run for 6- 8 hours, and the filter driers checked and renewed until no traces of contaminants are found. After some 24 hours operation an oil sample should be tested for acidity and, if clear (less than 0.5 acid number), the cleaning is acceptable. The bum-out filter drier should then be removed, and the liquid line filter drier renewed. High discharge pressure is one of the most frequent reasons for motor bum-out. This pressure creates very high discharge temperatures which cause oil decomposition and the formation of corrosive acids which break down the motor winding insulation. It is important, therefore, to monitor the discharge pressure and temperature and to maintain them within safe limits. A high discharge pressure may be due to air in the system, a dirty condenser, or too high a suction temperature.

84. 4.9 Hermetic drives The possible slight leakage of refrigerant through a shaft gland may be acceptable with a large system but would lead to early malfunction of a small circuit. The wide use of small refrigeration systems has led to the evolution of methods of avoiding shaft seals, provided that the working fluid is compatible with the materials of electric motors and has a high dielectric strength. The semi-hermetic or accessible-hermetic compressor (Figure 4.11) has the rotor of its drive motor integral with an extended crankshaft, and the stator is fitted within an extension of the crankcase. Suction gas passes through the motor itself to remove motor waste heat. Induction motors only can be used, with any starting switches outside the crankcase, since any sparking would lead to decomposition of the refrigerant. Electrical leads pass through ceramic or glass seals. Small compressors will be fully hermetic, i.e. having the motor and all working parts sealed within a steel shell, and so not accessible for repair or maintenance. The application of the full hermetic compressor is limited by the amount of cooling by the incoming cold gas, heat loss from the shell, and the possible provision of an oil cooler. The failure of an inbuilt motor will lead to products of decomposition and serious contamination of the system, which must then be thoroughly cleaned. Internal and external motor protection devices are fitted with the object of switching off the supply before such damage occurs.

85. Rotary These types are usually of the form shown in Figure 7.9 . At the position shown, the discharge and suction strokes are half completed, 270°C. At 0° discharging at compression stroke, induction at suction stroke. At 90° start of compression and end of suction. At 180° compression taking place and the suction stroke has just started. Thus the leading flank of the rotor acts as the discharger and the lagging flank acts as the inductor. Such compressors mainly find application in household and domestic units but modern practice is extending their use to cargo purposes. A variation on the above is a multiblade type whereby the eccentric rotor contains spring-loaded blades (or relies on centrifugal force). When any rotary compressor is not in use the oil film between eccentric rotor and cylinder is broken which means pressure equalisation and easy starting but requires the fitting of a non-return valve in the suction line. To reduce sizes these machines are direct drive from the motor.

86. Centrifugal These machines work on a similar principle to the centrifugal pump whereby discharge velocity energy is converted to pressure head. For high-pressure differentials, as normally exist, a series of impellers are required on a fast running rotor, each impeller feeding to the next in series to build up pressure. These machines are best suited to low differential pressure, high-volume capacity work such as air conditioning. Capacity reduction is effected by directional blades at the rotor inlet port. Efficiency is increased if interstage fl ash vapour formed during liquid expansion is returned to an appropriate stage of the compressor

87. Screw These compressors can be visualised as a development of the gear pump. A male rotor with say four lobes on the shaft, meshes with a female rotor of say six lobes on a parallel shaft. Clearance between lobe screws and casing is kept to a minimum with sealing strips and oil fi lm. As the space between two adjacent lobes of the female rotor passes the inlet port at one end of the compressor, a volume of gas is drawn in. With rotation, a lobe of the male rotor progressively fi lls this space so compressing the vapour and, due to the helical screws, forces it axially to the outlet port at the other end. To reduce capacity, sliding sleeves around the barrel can be moved axially to bring the outlet port nearer to the inlet port.

88. Screw compressor LOBES DRIVE SHAFT Min BYPASS GAS OUTLET DISCHARGE PORT INLET UNLOADING PISTON Max SLIDE VALVE CYLINDER NORMAL LOADING Page 12

89. Screw compressor LOBES DRIVE SHAFT Min BYPASS GAS OUTLET DISCHARGE PORT INLET UNLOADING PISTON Max SLIDE VALVE CYLINDER REDUCE LOADING

90. Screw compressors The capacity, range and use of screw compressors (Figures 11.9 and 11.10) has increased over the years. Their economy in space and weight and their capacity for long periods of uninterrupted running are inherent advantages. They can be oil-free, when used within their limited pressure range when oil contamination of the gas cannot be tolerated. For higher pressure range work they are oil-injected. In both types, two steel rotors are mounted in a gas-tight casing, usually of a high quality cast iron such as meehanite. Lobes and mating flutes on Figure 11.10 Cut-away section of an oil-injected compressor (Howden/Godfrey Ltd) the male and female rotors are machined helically and so dimensioned that they mesh like helical gears. Oil-free or dry machines are fitted with timing gears to ensure synchronization of the rotors, without contact. In oil-injected or wet machines, the female rotor is driven by the male counterpart, with surfaces being separated by the oil, which serves also for sealing and cooling. Capacity is varied by Incorporation of a slide valve which, moving axially within the casing, varies the effective length of the rotors. Figure 11.9 shows a pair of rotors and Figure 11.10 a cut-away section of an oil-injected compressor, with the female rotor removed to show the slide valve and the actuating piston in its cylinder. As will be apparent, large, effective oil separators are necessary when rotary oil-injected compressors are used.

91. Screw compressors Two types of screw compressor are used in marine plants; double screw compressors and mono screw compressors. A double screw compressor consists of two rotors with matched helical grooves; a male rotor with four lobes which mesh with the corresponding six flutes on a female rotor (Fig 6). In modem designs the motor drive can be directly connected to the male or female rotor, the driving force being transmitted to the mating rotor by the thin layer of oil sealing the ciearance space between the two rotors. The male and female rotors trap and compress the gas as they mesh and turn together. Suction gas is drawn into the compressor as the interlobe spaces of the rotors pass the inlet port. As they sontinue to rotate, a lobe of the male rotor progressively fills up the space which is available for gas between the female lobes, and the gas is forced forwards axially and compressed. The lobes continue to intermesh until the opposite end of the rotor passes the outlet port, and the compressed gas is discharged. A mono screw compressor consists of a single female rotor with six flutes. Two identical star shaped wheels, each with eleven teeth, mesh with the rotor and are symmetrically spaced around it. The driving motor is connected to the rotor. The principle of operation is similar to that of double screw compressors, the star wheels providing the same function as the male rotor, trapping and compressing the gas as they mesh and turn with the rotating female rotor, To obtain efficient compression and pumping, oil is injected into the compressor to lubricate and seal the rotor mesh and seal the clearances between the rotors and casing walls. The oil also acts as a coolant.

92. It removes some of the heat of compression from the compressed gas ensuring moderate discharge tem- a) peratures. The cooling effect reduces the thermal stress in the rotors and casing. The level of noise emitted by the compressor is also reduced by the from evaporator dampening effect of the injected oil. In the majority of screw compressors, capacity control is effected by means of a regulating slide valve mounted underneath the rotors, connecting via a bypass port with the suction inlet. When open, the valve allows some of the gas in the interlobe spaces of the rotors to return to the suction inlet. The regulating slide movement is controlled by a hydraulic piston, giving continuous regulation from 100% down to about 10% of full output. In some designs lift valves are used, giving capacity regulation in five distinct stages; 100%, 75%, 50%, 25% and 0% of full output.

93. 9.11 Capacity reduction injection valves Where a compressor does not have any capacity reduction device and on–off switching will not give the degree of control required by the process, the cooling capacity can be regulated by injecting discharge gas back into the suction (see Figure 9.7). It has the effect of keeping the evaporator pressure constant, regardless of the load, and can have a wide range of capacity reduction, down to 10% of full load. It is a constant pressure valve, balancing the suction pressure against a pre-set spring. However, since the suction gas to the compressor would then be hotter than its normal slightly superheated condition, the compressor may overheat and the discharge gas become too hot for correct and safe working. This form of capacity reduction is usually combined with a liquid injection valve, thermostatically operated, which introduces liquid also into the suction to keep it cool. The fitting of dual interdependent controls of this sort, both of which have inherent fail–unsafe possibilities, should be approached with caution. A safer circuit injects the discharge gas directly after the expansion valve or into the evaporator outlet and before the sensor of the expansion valve. With this arrangement, the expansion valve will admit extra refrigerant, and gas entering the compressor will be normally cool. These control methods are wasteful of energy.

94. Oil separators Oil separators (Figure 11.7) of the impingement type, may be fitted in hot gas discharge lines from the compressor. The type shown, is a closed vessel fitted with a series of baffles or a knitted wire mesh through which the oil-laden vapour passes. The reduction in velocity of the vapour as it enters the larger area of the separator allows the oil particles, which have greater momentum, to impinge on the baffles. The oil then drains by gravity to the bottom of the vessel where a float valve controls flow to the compressor crankcase. The chief disadvantage of separators placed in the hot gas discharge line is the possibility of liquid refrigerant passing from the separator to the compressor crankcase when the compressor is stopped. In order to minimize Figure 11.7 Float controlled oil trap this risk, the separators should be placed in the warmest position available. It is good practice to drain the oil from the separator into a receiver containing heating elements, where the liquid refrigerant boils off to the compressor suction line and allows the oil to drain to the compressor crankcase through a float valve.

95. The TLT valve works in the following way: A thermodynamic liquid trap uses a disc to control the release of liquid and to trap gas. The trap cycles open and close to discharge liquid and closes tightly between discharges. The disc, which is the only moving part, rises and falls in response to dynamic forces produced by the gas flowing through the trap. Liquid and/or gas enters the trap through the central orifice, lifts the disc and is discharged through the outlet orifice. The gas passes along the underside of the disc at high velocity and collects in the control chamber above. The resulting pressure imbalance forces the disc downward onto the seating surfaces and stops the flow. The trap remains tightly closed until the loss of heat through the trap body lowers the control chamber pressure, allowing the inlet pressure to raise the disc and repeat the cycle. One side of the disc (3) is plain with a single scratch towards the outer edge, whereas the other side of the disc has a machined circular groove. The trap is supplied with the single, radial scratchside of the disc (3) towards the seating faces. If there are iregularities in the oil return, check that the scratch-side of the disc is towards the seating surface. Also check that the strainer (49 is free from impurities. Also, if plant condensing temperature is equal to or lower than machine room temperature, the disc must be fitted with the bleed scratch towards the seating faces.

96. Oil rectifier Some refrigerants are miscible with the compressor lubricating oil which means that the two substances form a mutual solution. Because oil carry over does occur, the miscibility is actually beneficial because the oil tends to be taken around the refrigeration circuit and back to the compressor, by the refrigerant. If the oil and refrigerant are not miscible, as is the case with oil and CO2, then there may be a loss of lubricant from the compressor sump and accumulation in the system. With CO2 the oil collected in the evaporator and was drained off regularly by hand, then returned to the compressor after being strained. The oil rectifier (Figure 11.8) can be fitted to drain oil from the evaporator, automatically, and return it to the compressor. The oil is automatically bled from the evaporator to a heat exchanger in which liquid refrigerant mixed with the oil is vaporized. The heat for vaporizing the refrigerant is obtained by passing warm liquid refrigerant from Figure 11.8 Oil rectifier the condenser, through the heat exchanger. Vapour and oil are passed to the compressor where the oil returns to the sump while the freon passes to the compressor suction. The regulator is a thermostatically controlled valve which operates in the same way as the expansion valve in the main system. It automatically bleeds the oil from the evaporator so that the gas leaves the operates in the same way as the expansion valve in the main system. It automatically bleeds the oil from the evaporator so that the gas leaves the rectifier heat exchanger in a superheated condition.

97. Throttling Devices Maintain pressure difference Meter correct amount for varying load Reduce boiling point by reducing pressure

98. 8 Expansion valves 8.1 General The purpose of the expansion valve is to control the flow of refrigerant from the high-pressure condensing side of the system into the low pressure evaporator. In most cases, the pressure reduction is achieved through a variable flow orifice, either modulating or two-position. Expansion valves may be classified according to the method of control.

99. 8.2 Low-pressure float valves Flooded evaporators require a constant liquid level, so that the tubes remain wetted. A simple float valve suffices, but must be located with the float outside the evaporator shell, since the surface of the boiling liquid is agitated and the constant movement would cause excessive wear in the mechanism. The float is therefore contained within a separate chamber, coupled with balance lines to the shell (see Figure 8.1). Such a valve is a metering device and may not provide positive shut-off when the compressor is stopped. Under these circumstances, refrigerant will continue to leak into the evaporator until pressures have equalized, and the liquid level might rise too close to the suction outlet. To provide this shut-off, a solenoid valve is needed in the liquid line.

100. 8.3 Low-pressure float switches Since the low-pressure float needs a solenoid valve for tight closure, this valve can be used as an on–off control in conjunction with a pre-set orifice and controlled by a float switch (Figure 8.2). The commonest form of level detector is a metallic float carrying an iron core which rises and falls within a sealing sleeve. An induction coil surrounds the sleeve and is used to detect the position of the core. The resulting signal is amplified to switch the solenoid valve, and can be adjusted for level and sensitivity. A throttle valve is fitted to provide the pressure-reducing device. Should a float control fail, the level in the shell may rise and liquid pass into the compressor suction. To warn of this, a second float switch is usually fitted at a higher level, to operate an alarm and cut-out. Where a flooded coil is located in a liquid tank, the refrigerant level will be within the tank, making it difficult to position the level control. In such cases, a gas trap or siphon can be formed in the lower balance pipe to give an indirect level in the float chamber. Siphons or traps can also be arranged to contain a non-volatile fluid such as oil, so that the balance pipes remain free from frost.

101. 8.4 High-pressure float valve On a single-evaporator flooded system, a float valve can be fitted which will pass any drained liquid from the condenser direct to the evaporator. The action is the same as that of a steam trap. The float chamber is at condenser pressure and the control is termed a high pressure float (Figure 8.3). The refrigerant charge of such a system is critical, since it must not exceed the working capacity of the evaporator. It is not possible to have a receiver in circuit and this control cannot feed more than one evaporator, since it cannot detect the needs of either. The difficulty of the critical charge can be overcome by allowing any surplus liquid refrigerant leaving the evaporator to spill over into a receiver or accumulator in the suction line, and boiling this off with the warm liquid leaving the condenser. In this system, the low-pressure receiver circuit, liquid is drained from the condenser through the high-pressure float, but the final step of pressure drop takes place in a secondary expansion valve after the warm liquid has passed through coils within the receiver. In this way, heat is available to boil off surplus liquid leaving the evaporator (see Figure 8.4). Two heat exchangers carry the warm liquid from the condenser within this vessel. The first coil is in the upper part of the receiver, and provides enough superheat to ensure that gas enters the compressor in a dry condition. The lower coil boils off surplus liquid leaving the evaporator itself. With this method of refrigerant feed, the evaporator has a better internal wetted surface, with an improvement in heat transfer.

102. The low-pressure receiver system can be adapted to compound compression and can be fitted with hot gas defrost by reverse gas flow. In both circuits the low-pressure receiver provides the safety vessel to prevent liquid entering the compressor. Providing the highpressure float is correctly sized, this system can operate at low condenser pressures, saving compressor energy in cool weather. Where the halocarbon refrigerants are used in this system, an oil distillation device is fitted, working on the same principle as shown in Figure 5.2.

103. 8.5 Thermostatic level control If a small heater element is placed at the required liquid level of a flooded evaporator, together with a heat-sensing element, then the

104. Expansion valves The expansion valve is the regulator through which the refrigerant passes from the high pressure side of the system to the low pressure side. The pressure drop causes the evaporating temperature of the refrigerant to fall below that of the evaporator. Thus, for example, the refrigerant can be boiled off by an evaporator temperature of — 18°C because the pressure drop brings the evaporating temperature of the refrigerant down to say — 24°C The liquid refrigerant leaves the condenser with a temperature just above that of the sea-water inlet, say 15 °C. As it passes through the expansion valve the evaporating temperature decreases to — 24°C and some of the liquid boils off taking its latent heat from the remainder of the liquid and reducing its temperature to below that of the evaporator. There are six basic types of refrigerant controls or expansion devices, which can be summarized as follows. Manually operated expansion valves These were used for CO2 refrigeration installations where the compressor was started and stopped by a watchkeeper. The compressor was started with the expansion valve open. The valve was then closed in to bring up pressure on the condenser side until the saturation or condensing temperature for the pressure (shown on the gauge) was five or six degrees above that of the cooling sea water. After the manual expansion valve had been set in this way, the gauge on the compressor suction (or evaporator side) was checked. Equivalent saturation or boiling temperature shown for the suction or evaporator pressure had to be about five or six degrees lower than the brine temperature. Any discrepancy indicated undercharge due to CO2 leakage or overcharge due to over enthusiastic topping up with gas. Many modern refrigeration systems have an emergency expansion valve which can be set manually in a similar way. Manually operated expansion valves have the disadvantage of being unresponsive to changes in load or sea-water temperature and must be adjusted frequently. The valve itself is a screw down needle valve dimensioned to give fine adjustment

105. Automatic expansion valves These consist of a needle with seat and a pressure bellows or diaphragm with a torsion spring capable of adjustment. Operated by evaporator pressure their chief disadvantage is their relatively poor efficiency compared with other types. Constant pressure in the evaporator also requires a constant rate of vaporization, which in turn calls for severe throttling of the liquid. There is also the danger of liquid being allowed to return to the compressor when the load falls below a certain level This type of valve is used principally in small equipment with fairly constant loads, such as domestic storage cabinets and freezers.

106. EVAPORATOR Refrigerant (liquid) IN Diaphragm Throttling orifice Spring Adjusting screw to alter spring pressure Refrigerant Refrigerant OUT Spring Pressure - Opening Action Refrigerant Pressure - Closing Action AUTOMATIC EXPANSION VALVE

107. Thermostattc expansion valves These valves are similar in general design to automatic valves, but having the space above the bellows or diaphragm filled with the liquid refrigerant used in the main system and connected by capillary tube to a remote bulb. This remote bulb is fixed in close contact with the suction gas line at the outlet from the evaporator and is responsive to changes in refrigerant vapour temperature at this point. These valves are the most commonly employed (as in the automatic freon system Figure 11.3} and are suitable for the control of systems where changes in the loading are frequent. Unlike the automatic valve, based on constant evaporator pressure, the thermostatic valve is based on a constant degree of superheat in the vapour at the evaporator outlet, so enabling the evaporator at any load to be kept correctly supplied with liquid refrigerant without any danger of liquid cany over to the suction line and thence to the compressor. .

108. The aperture in the expansion valve is controlled by pressure variation on the top of a bellows. This is effective through the push pins (Figure 11.12) and tends to open the valve against the spring. Spring pressure is set during manufacture of the valve and should not be adjusted. The pressure on the bellows is from a closed system of heat sensitive fluid in a bulb and capillary connected to the top of the bellows casing. The bulb (Figure 11.13) is fastened to the outside of the evaporator outlet so that temperature changes in the gas leaving the evaporator are sensed by expansion or contraction of the fluid. Ideally the gas should leave with 6° or 7°C of superheat. This ensures that the refrigerant is being used efficiently and that no liquid reaches the compressor. A starved condition in the evaporator will result in a greater superheat which through expansion of the liquid in the bulb and capillary, will cause the valve to open further and increase the flow of refrigerant. A flooded evaporator will result in lower superheat and the valve will decrease the flow of refrigerant by closing in as pressure on the top of the bellows reduces. Saturation temperature is related to pressure but the addition of superheat to a gas or vapour occurs after the latent heat transaction has ended. The actual pressure at the end of an evaporator coil is produced inside the bellows by the equalizing line and this is in effect more than balanced by the pressure in the bulb and capillary acting on the outside of the bellows . The greater pressure on the outside of the bellows is the result of saturation temperature plus superheat. The additional pressure on the outside of the bellows resulting from superheat overcomes the spring loading which tends to close the valve . A hand regulator is fitted for emergency use. It would be adjusted to give a compressor discharge pressure such that the equivalent condensing temperature shown by the gauge at the compressor outlet was about 7°C above that of the sea-water temperature and the suction gauge showed an equivalent evaporating temperature about the same amount

109. EVAPORATOR Refrigerant (liquid) IN Diaphragm Throttling orifice Spring Adjusting screw to alter spring pressure Remote bulb Refrigerant Refrigerant (slightly superheated gas) OUT Refrigerant THERMOSTATIC EXPANSION VALVE (TEV) ( internally equalised)

110. Refrigerant (slightly superheated gas) OUT Refrigerant (liquid) IN Diaphragm Throttling orifice Spring Adjusting screw to alter spring pressure Remote bulb Refrigerant Equalising connection To overcome this problem, an externally equalised thermostatic THERMOSTATIC EXPANSION VALVE (TEV) ( externally equalised)

111. Capillary tube control This is the simplest of all refrigerant controls and consists of a length of small diameter tubing inserted in the liquid line between the condenser and the evaporator. For a given tube bore and length the resistance will be constant, so that the liquid flow through the tube will always be proportional to the pressure difference between the condensing and evaporating pressures of the system. Although self-compensating to some extent, this type of control will only work at maximum efficiency under one set of operating conditions, and for this reason is principally employed on close coupled package systems using hermetic or semi-hermetic compressors.

112. Electronic expansion valve system The system is designed to provide precise, rapid and remote control of the liquid supply to dry expansion evaporators, in response to the temperature differential between the evaporator outlet and inlet. This provides accurate control which allows maximum utilisation of the evaporator surface (high degree of filling), and a rapid response to changes in evaporator load. It is also unaffected by changes in condensing pressure, which allows the use of lower pressures in cooler climates, and hence reduced compressor power consumption. The system shown in Fig 12 comprises three main components: expansion valve with electric valve actuator; electronic controller; and two plutonium 1000 ohm temperature sensors. The expansion valve is opened and closed by the actuator, Fig 13, which replaces the thermostatic element of the TEV. The actuator comprises a pressure reservoir, which holds a given amount of liquid, an electric heating element, and a negative temperature coefficient sensor (measuring resistance with negative coefficient). During normal operation the heating element keeps the liquid in the actuator at such a temperature (pressure) that stable equilibrium between the evaporator pressure under the diaphragm, and pressure in the actuator over the diaphragm is maintained.

113. Operation The measured temperature differential (52 - 51) is compared in the controller with the required temperature differential (set on the controller). If the measured value deviates from the set value, power to the heating element is changed to cool or heat the actuator. The pressure in the actuator changes slightly, causing the valve to move in an opening or closing direction, increasing or restricting the liquid supply, to restore the required temperature differential. The proportional integration (PI) regulation of the controller ensures that the measured value (52 - 51 ) does not deviate from the set value o n variations in load, evaporating pressure, sub-cooling, and pressure drop across the expansion valve. In the even t of a malfunction, the system should be checked in accordance with the operating manual.

114. 10.6 Expansion valve The expansion valve is a passive orifice, through which the liquid refrigerant is forced by the pressure difference between the condensing and evaporating conditions. Capacity ratings are given in the catalogues of manufacturers and suppliers. Types in general use are: 1. Capillary tubes, for small hermetic systems. These are factory selected and cannot be adjusted. 2. Solenoid valves with liquid level sensors or liquid level valves for most flooded evaporators. 3. High-pressure float valves plus handset throttle valves for some flooded and low-pressure receiver circuits. 4. Thermostatic expansion valves or electronic expansion valves for most dry expansion circuits. Troubles arise with the selection of thermostatic expansion valves, since this is the type generally used in custom-built systems and, for these, selected outside a factory.

115. 9.22 Overheat protection Small compressors will have motor overheat protection adjacent to the hermetic shell or built into the winding (see Section 4.8) and larger motors will have contactor–starters with overcurrent devices. Overheat protection is also fitted on many machines, to guard against high motor winding, cylinder head or oil temperatures. These usually take the form of thermistor detectors, connected to stop the motor.

116. Thermostats Thermostats are temperature-controlled electric switches, which can be used for both safety and control functions. When fitted to compressor discharge lines, they are set to stop the compressor if the discharge temperature is too high. Thermostats are also used to control the temperature in a refrigerated space by cycling the compressor 'on and off', or by 'opening and closing' a solenoid valve in the liquid line. Three types of element are used to sense and relay temperature changes to the electrical contacts. 1 . A fluid-filled bulb connected through a capillary to a bellows. 2. A thermistor. 3. A bi-metal element The above controls should be set in accordance with the plant's instruction manual, and should be checked regularly for refrigerant leaks from the bellows and connecting tubes. The electrical contacts should be examined for signs of wear and arcing.

117. Low pressure controller The low pressure control (Figure 11.3) stops the compressor when low suction pressure indicates closure of all cold compartment solenoids. When the pressure in the compressor suction rises again due to one or more solenoids opening, the low pressure control restarts the compressor. The controller shown (Figure 11.16) is of the Danfoss type operated through a bellows which monitors pressure in the compressor suction. A pressure differential between cut out and cut in settings is necessary to avoid hunting. The push pin operates the switch through a contact which is flipped open or closed through a coiled spring plate. With the contacts open the spring is coiled as shown. Outward movement of the pin compresses the spring and this then flips the contact to close the compressor starting circuit.

118. High pressure cut-out In the event of overpressure on the condenser side of the compressor (Figure 11.3) the high pressure cut-out will cause the compressor to shut down. The device is re-set by hand. There are a number of faults which cause high discharge pressure, including loss of condenser cooling, air in the system and overcharge. The bellows in the cut-out (Figure. 11.14) is connected by a small bore pipe between the compressor discharge and the condenser. The bellows tends to be expanded by the pressure and this movement is opposed by the spring. The adjustment screw is used to set the spring pressure. During normal system operation, the switch arm is held up by the switch arm catch and holds the electrical contact in place. Excessive pressure expands the bellows and moves the switch arm catch around its pivot. The upper end slips to the right of the step and releases the switch arm so breaking the electrical contact and causing the compressor to cut-out. The machine cannot be restarted until the trouble has been remedied and the switch re-set by hand.

119. Room temperature control The temperature of the refrigerated spaces with a direct expansion system (Figure 11.3) is controlled between limits through a thermostatic switch and a solenoid valve which is either fully open to permit flow of refrigerant to the room evaporator, or closed to shut off flow. The solenoid valve (Figure 11.15) is opened when the sleeve moving upwards due to the magnetic coil hits the valve spindle tee piece and taps the Figure 11.15 Solenoid valve valve open. It closes when the coil is de-energized and the sleeve drops and taps the valve shut. Loss of power therefore will cause the valve to shut and a thermostatic switch is used to operate it through simple on/off switching. The thermostatic switch contains a bellows which expands and contracts under the influence of fluid in a capillary and sensing bulb attached to it. The bulb is filled with freon or other fluid which expands and contracts with the temperature change in the space in which it is situated. As the temperature is brought down to the required level, contraction of the fluid deflates the bellows. The switch opens and the solenoid is de-energized and closes. A temperature rise operates the switch to energize the solenoid which opens to allow refrigerant through to the evaporator again. The switch is similar in principle to the high pressure cutout and low pressure controller .

120. Pressure Relief Safety Device Refrigeration systems are designed to withstand a maximum working pressure (MWP) which, if exceeded as a result of a fire, extreme temperature conditions, or faulty electrical controls, may cause some part of the system to explode. To avoid an explosion or sudden rise in pressure compressors and pressure vessels are fitted with a pressure relief device. There are three types of relief devices Spring-loaded relief valves remain set to open at the MWP and close when the pressure drops to a safe level. Relief valves must not be interfered while in service and must be locked or sealed to prevent unauthorized adjustment. Bursting discs, which comprise thin metal diaphragms designed to burst at a pressure equal to the MWP. 3.Fusible plugs, which contain a metal alloy, melts when the temperature in the system corresponding to the MWP. Commonly the discharge from relief device vented directly into the atmosphere. In some plants, relief devices are arranged to discharge toward the low-pressure side of the system .

121. Liquid indicators These can be either cylindrical or circular glasses installed in the liquid line, providing a means of ascertaining whether or not the system is fully charged with refrigerant. -If undercharged, vapour bubbles will appear in the sight glass. To be most effective indicators should be installed in the liquid line as close to the liquid receiver as possible. - Some types incorporate a moisture indicator which, by changing colour indicates the relative moisture content of the liquid These are fitted so that the refrigerant flow may be observed. -A full glass indicates that the system is fully charged, a stream of bubbles indicates a partially charged system, and rapid frothing of the liquid indicates a shortage of refrigerant. -Moisture indicating sight glasses have a colour indicator which changes colour when the moisture content of the refrigerant exceeds the critical value. -The colour indication is reversible, changing back to the original colour when the plant has been dried by replacing or recharging the filter drier.

122. Driers Where halogenated hydrocarbon refrigerants are used it is absolutely essential that driers are fitted in the refrigerant piping and most Classification Societies make this mandatory. Water can freeze on the expansion valve so causing excess pressure on the condenser side and starvation of refrigerant to the evaporator. When this occurs, the compressor will cut out due to operation of the high pressure cut-out or low pressure controller. The presence of a small amount of water can have an effect on plant performance and driers are essential. These are usually simple cylindrical vessels, the refrigerant entering at one end and leaving at the other. For modern installations the strainer/drier pack is replaced complete after opening the bypass and isolating the one to be replaced. Older systems are likely to have a strainer/drier partly filled with renewable drying agent. The drier, usually silica gel or activated alumina, is supported on a stiff gauze disc, overlaid with cotton wool with a similar layer above. In most installations the driers have bypasses so that they can be isolated without interfering with the running of the plant and the drying agent renewed or re-activated (by the application of heat). If the drier is located in the liquid line it should be arranged so that the liquid enters at the bottom and leaves at the top. This is to ensure that there is uniform contact between the liquid refrigerant and the drying agent and that any entrained oil globules will be floated out without fouling the particles of the drying agent. If located in the suction line, the gas should enter at the top and leave at the bottom so that any oil can pass straight through and out.

123. Author note Should water enter the system, acids may be formed by the reaction with the refrigerant gas. -This is especially true for Freon systems. -These acids attack the copper in the system- typically the pipe work- and allow it to be transported through the system. -It is not uncommon to find this deposited on the suction valve plate. - More troublesome is when the deposit finds its way to the crankcase seal destroying the running face. -Thus the importance of maintaining filter dryers in good condition can be seen. -These should be changed at least on a schedule determined by the ships planned maintenance system. -In addition to this it is common to have liquid line flow bulls eye which incorporate a water detection element. - Blockage of the filter dryer can be gauged by feeling the filter. If it is cooler than the surrounding pipe work then the gas is being throttled through it.

124. Topping up the refrigerant A filling connection is fitted in way off the filter dryer, either directly onto it or on the inlet line after the inlet shut off valve. This allows additional refrigerant to be introduced into the system via the dryer element. The normal procedure is to shut or partially shut the inlet to the filter. The compressor is now sucking from the system and delivering to the condenser where the gas liquefies. The filter dryer is on the outlet from the condenser therefore with its inlet valve shut the liquid level begins to rise in the reservoir. As the only gas entering the system is now coming from the top up line the compressor will tend to reduce the suction side pressure as it evacuates the system into the condenser. The inlet valve can be briefly opened to allow more refrigerant into the system.

125. Back pressure regulator valve This valve is fitted to the higher temperature rooms, vegetable and flour (+5oC) only and not to the Meat and Fish rooms (-20oC). They serve two main purposes. Firstly when all solenoid valves are opened they act as system balancing diverters, that is they restrict the liquid flow to the rooms which can be kept at the higher temperature and deliver the bulk to the colder rooms. Secondly they serve to limit the pressure drop across the expansion valve by giving a set minimum pressure in the evaporator coil. This in turn limits the temperature of the refrigerant thereby preventing delicate foodstuffs such as vegetables from being damaged by having air at very low temperatures blown over them. Ultimately they may also be set to provide a safety limit to the room temperature by restricting the pressure to give a corresponding minimum saturation temperature of 0oC.

126. Defrost system Moisture freezes onto the evaporator eventually causing a restriction and reducing the efficiency of the plant. This must be periodically removed. For Veg and Flour rooms, were not restricted to 0oC minimum by the back pressure valve, this is carried out once per day. For the Meat and Fish rooms this has to be carried out two or more times. Due to the low temperature in the rooms it is necessary to fit a drain heater. When on defrost the solenoid valve is shut and the fan is off. On some systems at end of defrost the solenoid valve is opened momentarily before the fan is started. This allows moisture to be snap frozen onto the surface of the element, creating a rough increased surface area and thereby increasing the heat transfer rate.

127. Defrosting The air coolers in the meat and fish rooms are fitted with an electrical defrosting system i.e. evaporator and drip trays are provided with electric heating elements. The frequency of defrosting is chosen by means of a defrosting relay built into the starter panel. The defrosting procedure is as follows: a) All solenoid valves in the system close and the compressor stops. b) The fans in the meat and fish rooms stop working but the fans in the vegetable and dairy rooms continue the circulation of the warm air over the coolers, in this way keeping the cooling surfaces free from ice. c) The electric heating elements in the meat and fish rooms switch on. d) As long as the coolers are covered with ice, the melting takes nearly all of the heat supplied and the temperature of the cooler and the refrigerant is constantly kept near zero. When the ice has melted, the refrigerant temperature rises in the meat and fish rooms. When the temperature reaches the set point (approximately.+10°C) of the defrosting thermostat, the heating elements are switched off. e) The compressor starts and the solenoid valves open, f) When the coil surface temperature has gone below freezing point, the fans in the meat and fish room start. The system is now back on the refrigerating cycle again. If the defrosting is not completed at the expiration of the predetermined defrosting period, the defrosting will be restarted by the timer and a new cycle will commence.

128. Hot gas after oil separator to evaporator of all room (after exp valve) and back to cop suc

129. gas directly from the compressor discharge through the meat and fish room evaporators in the opposite direction of flow , The cooling effect of the frosted evaporator changes the gas to liquid form, which is then dispersed through the vegetable and dairy rooms in the normal manner.

130. Hot Gas Defrosting The hot gas defrosting process takes place manually and is carried out by means of circulating hot R-22 gas directly from the compressor discharge through the meat and fish room evaporators in the opposite direction of flow according to the following procedure: With No.1 compressor in use and all valves on No.2 compressor closed. a) Stop the compressor. b) Shut the gas inlet valve and liquid outlet valve on the condenser. c) Open valve A-1. d) Open valve C-1, supply to the meat room. e) Open valve E-1, supply to the fish room. f) Close the inlet valves to the expansion valves on both rooms. g) Open the non-return valve that bypasses the solenoid valve and expansion valve on both rooms. h) Start the compressor. The refrigerant now flows directly from the compressor to the meat and fish room evaporators at the outlet side. The refrigerant then flows through the evaporators in the reverse direction, bypassing the solenoid valve and expansion valve. The cooling effect of the frosted evaporator changes the gas to liquid form, which is then dispersed through the vegetable and dairy rooms in the normal manner. WARNING Care must be taken to observe the compressor pressures and temperature as the compressor is not receiving its usual cooling effect from the gas, so the time of this operation is limited. It should be carried out under continuous supervision.

131. Condensers Marine condensers are generally of the shell and tube type, designed for high pressures though there may be a few coil-in- casing or other types still in use. The coolant passes through the tubes with the refrigerant condensing on the outside. Condenser tubes provide heat transfer surfaces in which, heat from the hot refrigerant on the outsides of the tubes, passes through the walls of the tubes to the cooling water inside. Seawater is the usual cooling medium for shell and tube condensers, but fresh water from central cooling systems is increasingly being used. The slightly superheated refrigerant gas is first cooled to saturation temperature (vapour state) and then cooled to a temperature slightly below the saturation temperature to ensure it is in the liquid state. The design of condensers is largely dictated by the quantity and cost of the circulating water and, where water is plentiful as at sea, a large number of short circuits may be used to keep pressure drop to a minimum. Water velocity is restricted to prevent erosion of the tubes, usually being kept below 2.5 m/s.

132. Marine condensers are very susceptible to corrosion and much research has been done to lengthen their useful life. With halogenated hydrocarbon refrigerants, the use of aluminium, brass or cupronickel tubes and brass tube plates is acceptable and has reduced the rate of corrosion on the seawater side. However, Refrigerant 717 (ammonia) tends to corrode these materials and it has been found necessary to use stainless steel or even bimetallic tubes and clad tube plates, with ferrous metal being in contact with the refrigerant, and non-ferrous with the sea wafer. The use of sacrificial anodes in the water ends of the condensers is common and sometimes a short length of ungalvanized pipe is fitted between the condenser and the galvanized steel pipe system

133. Evaporators In the direct expansion system, evaporation takes place in air coolers consisting of pipe grids, plain or finned, enclosed in a closely fitting casing, through which air from the holds or chambers, is circulated by forced or induced draught fans. This type of evaporator can be operated either; partly flooded, or fully flooded with incorporated accumulators, or dry. In the latter, the refrigerant flow is controlled by the expansion valve in such a way that as it passes through the grids, it is completely vaporized and slightly superheated.

134. Where brine is used as the secondary refrigerant, evaporators may be of the shell and tube type. In a shell and tube evaporator, the area of tube surface in contact with the liquid refrigerant determines its performance. The brine to be cooled may be circulated through the tubes with the refrigerant being on the outside of them. This involves either a high liquid level in the shell, or the placing of the tubes in the lower part of the shell only, the upper part then forming a vapour chamber. Modern flooded evaporators incorporate finned tubes. Shell and tube evaporators may also be of the dry expansion type, in which the refrigerant passes through the tubes, and the brine circulated through the shell. The advantages of this type are a smaller refrigerant charge and a more positive return of lubricating oil to the compressor

135. Heating Rods for Refrigeration Compressor Oil Heating The refrigeration compressors are delivered in a standard execution with built-in heating coils or rod in the crankcase. The purpose of the heating coil or rod is to keep the oil in the crankcase warm even during standstill of the compressor. This ensures a low content of refrigerant in the oil. Too much refrigerant in the oil makes it loose its lubricating properties. This may lead to damage of the movable parts in the compressor. Further, the danger exists that the oil, during start-up of the compressor, foams so vigorously that the lubricating pressure will disappear. Before start-up the heating rod should be switched on for at least 8 hours. The heating coil or rod must not be switched on if the oil level in the vessel is below minimum in the sight glass. While the compressor is operating, it is usually switched off. Further, remember to switch off the heating rod if the compressor crankcase is opened for inspection.

136. Suction Line Accumulators Compressors are extremely susceptible to damage from liquid refrigerant. Excessive liquid refrigerant return may cause not only oil dilution but complete loss of the compressor oil charge which results in equipment damage due to lack of proper lubrication. In addition, refrigerant flashing may occur causing the oil pump to lose pressure. The right selection and installation of a Suction Accumulator in the line, as close as possible to the compressor, is a lifesaver for the equipment, assuring adequate oil return and that refrigerant return only, is returning to the compressor. Suction Accumulators can also function as suction traps that prevent liquid refrigerant fl oodback, one of the most common causes of compressor failure. In most cases liquid floodback can be controlled. However, an accumulator assures control and protection of the compressor. In summary, Suction Line Accumulators: 1. Provide compressor protection, preventing compressor failure due to liquid slugging. 2. Assure adequate oil return and assures that refrigerant vapor only is returning to the compressor. 3. Provide a minimum system pressure drop while maintaining the maximum refrigerant flow required .

137. 9.20 Suction separators Suction line accumulators are sometimes inserted in halocarbon circuits, to serve the same purpose of separating return liquid and prevent it passing over to the compressor. Since this liquid will be carrying oil, and this oil must be returned to the compressor, the outlet pipe within the separator dips to the bottom of this vessel and has a small bleed hole, to suck the oil out (see Figure 9.11). Suction traps are now widely used, particularly on rolling piston and scroll compressors, to prevent liquid passing into the compressor.

138. Suction Filter Between suction stop valve and compressor a fine-meshed filter has been fitted.  The purpose of this filter is to prevent that impurities from the plant are conveyed with the gas flow into the compressor.  Clean the suction filter at regular intervals as stated in the section on servicing the reciprocating compressor.  The filter and gaskets can be removed without the use of any tools. Clean the filter in a suitable solvent and blow clean with pressurized air.

139. Cleaning of oil filter The oil filter should be cleaned at regular intervals. Please note in this connection that often the filter must be cleaned already after a short operating period following the initial start-up.  This is a consequence of the tiny dirt particles that will be coming from the plant during its first operating period. Clean the oil filter in a suitable dissolvent and blow clean with pressurized air before refitting.

140. Stop Valves Suction and discharge stop valves are used to cut off the compressor from the plant. They are closed tightly by manual tightening. Hence, it is not advisable to use any tools in order to close the valve as this would just lead to overloading of the valve parts. The valve spindle is fitted with a maintenance-free gasket which needs no replacement. Further, the valve is fitted with a back sealing, which is brought into operation when the valve is completely open and the valve cone screwed back towards the cylinder head (anticlockwise rotation). Note: In case the compressor is operating, the valve cone should not be screwed completely back against the cylinder head as any safety pressure controls connected to the valve housing will hereby be blocked.

141. Oil systems The primary function of oil in a refrigeration compressor is to lubricate the bearings and other rubbing surfaces. It is also required to: seal the clearance spaces between the discharge and suction sides of the compressor; b) act as a coolant, removing the friction heat from the rubbing surfaces and,; c) actuate capacity control mechanisms, d) dampen the noise generated by the compressor. The oil for all these purposes is supplied from the crankcase or separate reservoir, and circulated under pressure by a pump or, in the case of some screw compressors, by the pressure difference existing across the compressor. Oil strainers and filters are fitted to prevent solid particles damaging the compressor and oil pump, and sludge blocking the system. Small reciprocating compressors below75 kW are generally splash lubricated. In the splash method of lubrication, oil in the crankcase is thrown by the crank throw or eccentric up onto the cylinder walls, bearings and other rubbing surfaces

142. Refrigeration oil Lubricating oils for refrigeration compressors are selected for their suitability with the different refrigerant, compressor type and the plant's operating temperatures. Refrigeration oils should possess the following properties: Good chemical stability. There should be little or no chemical reaction with the refrigerant or materials normally found in the system. 2. Good thermal stability. They should not form hard carbon deposits at hot spots in the compressor(such as valves or discharge ports). 3. Good viscosity index . This is the ability of an oil to maintain good lubrication properties at high temperatures and good fluidity at low temperatures, i.e. to provide a good lubricating film at all times. 4. Low wax cntent . operating at low evaporating temperatures, as separation of wax particles from the refrigerant oil mixture may cause problems by blocking expansion and regulating valves. 5. Low pour point. Ability of the oil to remain in a fluid state at the plant's lowest evaporating temperature. 6. Moisture free. Any moisture added with oil may cause corrosion,

143. When adding oil to a compressor, or doing an oil change, it is therefore important that only the type specified in the manufacturer's operating manual is used. The oil must be clean and have no moisture content. Oil should always be stored in tightly sealed containers, in a warm place, to ensure it does not absorb moisture from the atmosphere. It is important that the procedures given in the compressor operating manual for changing and topping- up the oil are strictly followed. The lubricant is required only in the compressor where its temperature will be in the region of 50°C. It must have the correct viscosity, film strength and stability both chemical and thermal for the operating conditions. Oxidation is not a problem because the system is Freon filled . Water contamination may cause the oil to emulsify. With some refrigerants, water contamination produces acidity and corrosion . Oil is also carried over to the low temperature side of the system where it must not interfere with heat exchange by congealing in the evaporator. It must not form deposits on control valves and in passages , which would also interfere with the operation of the system . Low pour point is required. Some oils have a lower pour point than others due to their nature.

144. These are the naphthenic oils which have generally a lower pour point than the corresponding grades of paraffinic oils. Either may be used because they are refined and dewaxed to the lowest pour point possible. Pour point depressants are also used as additives in the oil. Because the oil is in solution it does not settle out on to surfaces. When the refrigerant boils off in the evaporator the agitation and velocity of the gas carries the oil mist along to the compressor suction . Cooling of oil in solution with Freon causes a wax to precipitate initially to produce a cloudy appearance and finally as crystals of wax . In this state the wax is called a flocculent, hence the term flock point . Tests for wax precipitation are carried out with a 10 per cent solution of oil in freon which is cooled until wax appears. The temperature at which this occurs, is the floc point. Flocculation in a system can cause wax to deposit on regulating valves and interfere with operation . Pour point applies to all oils but floc point to oils in freon systems. Oil viscosity is important because of the variation produced by miscibility with the gas. In general , the oil used must be compatible with the refrigerant . It is a straight mineral with additives to prevent foaming and to inhibit against chemical action with refrigerants and system metals . A naphthenic oil may be used for low temperatures. Oils are de-waxed and refined for low pour point and floc point. Because the system must be moisture free, it is important that oils are supplied with no water content.

145. FLOC POINT – temperature at which waxy materials in a lubricating oil separate from a mixture of oil and Freon (Registered trademark of E.I. Dupont de Nemours, Inc.) R-12 refrigerant, giving a cloudy appearance to the mixture; also called Freon floc point. Generally used to evaluate the tendency of refrigeration oils to plug expansion valves or capillaries in refrigerant systems. Not to be confused with cloud point, the temperature at which wax precipitates from an undiluted oil.

146. Cloud point The cloud point of an oil is the temperature at which crystallization of paraffin wax begins to be observed when the oil is being cooled down .

147. The operating conditions of refrigeration machines dictate to oils the following requirements: at low temperatures, solid paraffin particles should not drop out of the oil, and it should remain sufficiently fluid, at high temperatures coking and formation of asphalts and resins should not occur, the oil should be stable for many years of operation. The viscosity of the oils used at 50 0C should be at least 16 mm2 / s. The pour point of the oils should be below their operating temperatures. For the operation of oils in refrigeration machines, the socalled cloud point, at which heavy hydrocarbons (paraffins) drop out of the oils, is of great importance [15]. Precipitation of paraffins in evaporators, in narrow sections of the throttling organs, disrupts the operation of the chiller. Therefore, the cloud point must in all cases be below the boiling point of the refrigerant in the evaporator. Lubricating oils must be thoroughly drained. The water content of the oil should not exceed 20 parts per million parts of oil. Dehydrated oil is very hygroscopic, it absorbs up to 1% of water, so it should be contained in a sealed container and possibly less contact with outside air. Cloud point

148. Cloud point is the temperature below which wax in diesel forms a cloudy appearance. And that waxes thickens the oil and clogs fuel filters and injectors in engines. And pour point is the minimum temperature at which a lubricants turns into semi solid and almost losses its flow characteristics.

149. SOLUBILITY Oil in a refrigeration system must do more than simply meet the normal requirements of lubrication. It must be able to mix with the refrigerant and travel through the system. Therefore, you must consider the solubility relationships between refrigerants and oils. Refrigerants fall into three groups: • Those that mix in all proportions with oil in the range of temperature operation. These include R-12, R-113, ethane, propane, isobutane , and the other hydrocarbons . • Those that separate into two phases in the operating range, depending on temperature, pressure, and type base stock (chemical category) of the oil. These include R-21, R-22, R-114, and R-115 . • Those that are insoluble in oil in the operating temperature range. These include R-13, R-14, ammonia, and carbon dioxide. Of the refrigerants listed in the last two groups, only ammonia and carbon dioxide are lighter than oil. They float on top when the oil/refrigerant mixture separates into two phases.

150. Direct Expansion System : Provisional Refrigeration System Condenser Cooling water in / out Fan / blower expansion valve Solenoid stop valve Thermostat Temperature sensor MEAT ROOM LP pressure switch HP pressure switch Refrigerant compressor Sight glass Drier Evaporator Capillary tube : Refrigerant flow From FISH ROOM From VEGETABLE ROOM To FISH ROOM To VEGETABLE ROOM Oil separator Oil return to compressor sump Bulb T1 T2 receiver Oil pressure switch Purging line LP pressure gauge Oil pressure gauge HP pressure gauge Back pressure regulating valve

151. Controlled atmosphere Controlled atmosphere is an inert gas system used to extend the storage life of seasonal perishable products and has been used for many fruits and vegetables; primarily apples and pears in the past, and now mainly for bananas. To successfully store fruit for long periods, the natural ripening of the produce has to be delayed without affecting the eating quality. This is achieved by reducing the temperature of the fruit to the lowest level possible without causing damage through freezing or low temperature breakdown. To further delay ripening, the oxygen supply in the space is reduced to levels below that of the natural atmosphere. This level is below the level required to support human life. The precise levels of temperature, oxygen and carbon dioxide required to maximize storage life and to minimize storage disorders are extremely variable, depending on type of produce, growing conditions and maturity. Optimum storage conditions can vary from farm to farm and from season to season. On reefer vessels, oxygen (02) and carbon dioxide (C02) levels and relative humidity (RH) in controlled atmosphere zones (cargo chambers) can be independently controlled within close tolerances, irrespective of type, temperature and volume of cargo carried and the length of the voyage. A typical modem controlled atmosphere marine system would be expected to have flexibility to control gas levels within the following ranges: 02: < 1-8% CO2: O- 15 % RH: 40-90%

152. It should be remembered that during transport fruit and vegetables are still living organisms absorbing oxygen and giving off carbon dioxide, water vapour and heat. If the chambers loaded with such cargo were absolutely air tight, the oxygen level would decrease and carbon dioxide level increase, but complete airtightness can not currently be achieved. Air leakage is to be compensated by injecting the required amount of gas to produce the desired result. The Lloyd's Register Provisional Rules for Controlled Atmosphere are applicable to any gas system, permanent or portable, gas generating or storage type, which would achieve the above goal

153. Refrigeration is used in the carriage of some liquefied gases and bulk chemicals (Chapter 6), in air conditioning systems (Chapter 12) to cool bulk CO2 for fire fighting systems (Chapter 14) and to preserve perishable foodstuffs during transport or storage, as described in this chapter. The principles of refrigeration, briefly stated below, are the same for each of the applications. Details of the different types of plant are described in the relevant chapters. The main consideration with a ship built solely for the carriage of refrigerated cargo, is the high value of the produce which could be lost in the event of serious failure of the refrigerating machinery. A second very important consideration is that the produce should reach the consumer in good condition. The purpose of refrigeration in the carriage of perishable foodstuffs, is to prevent or check spoilage, the more important causes of which are: 1 excessive growth of micro-organisms, bacterial and fungal; 2 changes due to oxidation, giving poor appearance and flavours; 3 enzymatic or fermentive processes, causing rancidity; 4 drying out (dessication); 5 The metabolism and ripening processes of fruit and vegetables. The perishable foodstuffs carried as refrigerated cargo or as stores on ships can be categorized as dead produce such as meat and fish or as live produce such as fruit and vegetables. The dead cargoes tend to be carried frozen, an exception being made for meat on voyages of moderate length which may be carried chilled. A 10% carbon dioxide level has been found beneficial in cargo spaces for chilled meat. Fruit and vegetables are regarded as live cargoes until consumed, because they continue to ripen albeit slowly under refrigerated conditions. Fruit and vegetables continue a separate existence during which oxygen is absorbed and CO2 is given off, with the generation of heat. The rate of respiration varies with the type of fruit and also directly with the temperature. Apples can produce CO2 at the rate of 0.06 mVtonne/day at carrying temperature and evolve heat at a rate of some 12 W/tonne/h in the process. Each commodity has its own specific storage condition for the best result. Table 11.1 gives some carrying temperatures.

154. Capacity control methods Manual start/stop Speed variation Cylinder unloading reciprocating compressor Suction side throttling centrifugal compressor Hot gas bypass Compressor in parallel Slide valve Screw compressor - control effective working length of rotor. To maintain constant temperature, a constant pressure must be present in the EVAPORATOR. Ideally, the compressor should remove from the EVAPORATOR exactly the volume of refrigerant that boils off in it. Change in loading : change in quantity of boiling off the refrigerant.

155. Primary Refrigerants :are those refrigerants which directly absorb the heat from the storage space and undergo a cycle. There is a latent heat transfer for these refrigerants(they undergo phase change).. Secondary Refrigerants :- are those refrigerants which are first cooled by primary refrigerant and then they absorb heat from the storage space. There is a sensible heat transfer for these refrigerants(no phase change). For example: Brine(high concentration Sodium Chloride solution -

156. Primary refrigerants Primary refrigerants are the working fluids used in vapour compression systems

157. Secondary refrigerants A secondary refrigerant is one which is used as a heat transfer medium, with a change of temperature but no change of state. The secondary refrigerants used in marine plants today are brine and trichloroethylene .

158. Secondary Refrigerant Save on: primary refrigerant high pressure piping Intermediate temperature range possible with injection Improve suction as need for high velocities to improve return of oil reduced

159. Brine Mixture of Calcium Chloride & Water To prevent corrosion maintained at Ph 8 to 8.5 by adding say hydrochloric acid

160. Specific Gravity 15 o C 1.24 1.25 1.26 1.265 1.275 1.28 Operating Temperature o C -21 -23 -26 -29 -32 -34 Freezing Temperature o C -30 -32 -35 -37 -41 -43 Specific Heat Kcal./Kg .685 .678 .671 .667 .661 .658

161. Indirect Expansion (Brine System) Condenser / Receiver Cooling water in / out expansion valve Solenoid stop valve Thermostat Temperature sensor LP pressure switch HP pressure switch Refrigerant compressor Sight glass Drier Evaporator Capillary tube : Refrigerant flow Oil separator Oil return to compressor sump Bulb T1 T2 pump Brine header tank Secondary refrigerant to various compartment Oil pressure switch

162. What is meant by direct expansion? ‪ Direct expansion , or DX cooling, uses the principles of thermodynamics to transfer heat from one area to another through the evaporation and condensation of a refrigerant , which serves as the medium through which heat is captured and removed from one area and released in another

163. Brine Circuits Properties of brine -It is an advantage if the coolant coil through the cold chamber contains a fluid which is virtually non-harmful to the contents of the space in the event of leakage. -Small domestic units circulate the refrigerant through the evaporator in direct expansion and have the evaporator arranged in the space that is to be cooled. - Larger systems cooling cargo holds or container units usually employ a secondary refrigerant (brine) which is circulated through the evaporator and then circulated through the pipework to the cold chambers and back. -A big advantage is that the brine pipes have a much larger reserve of cold than refrigerant coils when the plant is stopped, --- -also having the advantage that various different circuits can easily be arranged, for example, cooling +4°C), chilling (–10°C) and defrosting if required.

164. The brine as used is a mixture of distilled water (preferably) and calcium chloride (CaCl 2 ). The colder the brine circuit the more dense the brine in circulation has to be to avoid any freeze up. Table 7.3 gives the densities and corresponding freezing points. -The brine should be maintained in an alkaline state under all conditions, this can easily be checked by the use of litmus paper, phenolphthalein, etc. -Brine density should also be taken regularly by standard hydrometer test at 15.5°C -and a regular check should also be taken for brine leakage at the brine header tank which serves to keep a head on the system

165. There is a possibility that the air content of brine rooms could become explosive or inflammable under conditions of hydrogen gas liberation due to corrosive action, hence it is advisable not to allow naked lights. The brine circuit consists of a brine room, containing distribution headers, mixing tanks, evaporators, pumps, etc., then the various piping systems to cold storage spaces maintained under pressure. The piping is usually tested to 7 bar, or 2.5 × WP , whichever is the greater, pipes commonly of mild steel externally galvanised and painted, about 40 mm bore.

166. It is usual to regulate the flow of brine by the return valves on the distribution and return headers. For chilling chambers about 3m 3 of chamber would require about 1 m 2 of pipe cooling surface, increased to 4.5 m 3 if air circulated. For freezing chambers the ratio is about 1.5 m 3 /m 2 (2.2 m 3 /m 2 air circulated). Normally 1,250 kg/m 3 density would be satisfactory for most brine circulation with a pH value of 8.5. Non-freezing solutions can also be based on organic fluids; ethylene and propylene glycol are in general use .

167. Indirect (brine) system The system in Fig 19 incorporates three brine evaporators (or chillers), three circulating pumps and one brine mixing pump, the steam heater, the brine making and balance tanks, the brine delivery and return manifolds with valves, called the brine regulating stations, and the brine injection valves. The system is entirely filled with brine and solely connected to the atmosphere through the balance tank placed at the highest point of the system. Such a system is known as a closed system. -The pumps circulate the brine from the evaporators through the delivery regulating station to the air cooler and back via the suction station to the evaporator. -The brine temperature can also be regulated automatically by the use of brine injection which has an accuracy of±O.l°C. The evaporators, however, can produce brine at different temperatures. -Defrosting is achieved by circulating the warm brine from the brine heater to the air coolers and back to the heater. Steam is usually preferred as a heating media, but electrical heaters are sometimes used. -The brine regulating stations and pumps are usually located in a separate insulated room above the engine room which may also accommodate the brine evaporators. The brine expansion tank is equipped with a level switch, which allows for a low level alarm.

168. Refrigeration system for cargo Direct expansion grid system Direct expansion battery cooling system Brine grid cooling system Brine battery cooling system

169. Direct expansion grids Direct expansion grids (Figure 11.17) provide a simple means of cooling a small refrigerated chamber. Such a system could be costly in terms of the quantity of refrigerant required and the cooling would rely on convection currents. Leakage of refrigerant into the cargo space could be a problem. A further objection would be that multiple circuits of liquid refrigerant could give control problems.

170. Direct Expansion Grid System compressor Condenser Cargo hold Grid

171. Cold brine grids The pipe grids for this type of system (Figure 11.18) were arranged so that they cover as much as possible of the roof and walls of the chamber. The greatest coverage was needed on those surfaces which formed external boundaries and the least on divisional bulkheads and decks. As the actual cooling of the cargo also depended on movement of air by natural convection, this type of chamber cooling required good, careful and ample dunnaging of the cargo stowage. This appreciably diminished the amount of cargo that could be carried so that the system is no longer favoured. Brine as a cooling medium (or secondary refrigerant) is cheap and easily regulated.

172. Direct expansion batteries and air This is a commonly used system (Figure 11.19) where the refrigerant circulates through batteries enclosed in trunkings or casings. Air from the refrigerated chambers, is circulated through the batteries by fans. Its great advantages are economy in space, weight and cost, and also the use of circulated air as the cooling medium or secondary refrigerant.

173. Direct Expansion Battery Cooling compressor Condenser Cargo hold evaporator fan

174. Brine Grid Cooling compressor Condenser Cargo hold Brine Grid expansion valve Brine circulating pump

175. Brine battery and air This system, in which brine instead of primary refrigerant is circulated through the batteries, continued to be employed for reefer ships carrying such cargoes as chilled meat or bananas where extremely close control of temperature was required when direct systems were gaining favour elsewhere. Brine is relatively easy to regulate. The system shown (Figure 11.20) is arranged with two separate refrigeration and brine circuits with connections from both brine systems to the air cooler batteries (or grids). (A more detailed diagram of a brine distribution system is shown in Figure 11.24.) Brine is inexpensive, being made with calcium chloride and fresh water to a gravity of about 1.25. Sodium dichromate or lime may be added to maintain the brine in an alkaline condition. Systems have been designed in which brine is replaced by one of the Glycols, for example ethylene glycol. The glycols have the advantage of being non-corrosive, and may be used at much lower working temperatures than brine. Trichlorethylene has also been used as a secondary refrigerant, but has the disadvantages of being toxic and a solvent of many of the synthetic rubbers and other materials normally used as jointing.

176. Brine Battery Cooling compressor Condenser cargo hold brine evaporator expansion valve brine circulating pump

177. Battery system This system is to blow air across a brine or direct expansion grid and circulate the storage space. It is well suited to higher temperature storage, for example, shellac, as there is no dripping from overhead grids on to the cargo. Also this system gives some control over the humidity as moisture will be deposited on the cooling coil. The supply of air circulation to any storage room will reduce the brine cooling surface required by as much as 50%. Direct expansion grids employ only about 40% of brine cooled grid pipe surface but do not have the same large reserve of cold.A

178. What is the principle of operation of vapor absorption system? There are many combinations of refrigerant and absorbents. The most important are : -ammonia as refrigerant and water as absorbent, -water as refrigerant and lithium bromide as absorbent. These have widely different boiling points and also have very high affinity (attraction) for each other. -A small amount of absorbent can absorb large quantity of the refrigerant. On absorption, the heats released are latent heat of vaporization and the heat of mixing. Because of their widely different boiling points, refrigerant is easily released from the mixture.

179. Absorption Type Refrigerator Unit This device has no moving parts and is continuous in operation when provided with a heat source, such as a town gas burner or electric element. The total pressure, which is the sum of the partial pressures, is constant through the system. Consider Figure 7.16 : Hydrogen vapour, which is insoluble in water, leaves the absorber and rises until it meets ammonia liquid falling into the entry to the evaporator. The hydrogen pressure causes a lowering of the ammonia pressure (two media exerting the same pressure as previously exerted by only one) and this assists in vaporization of the ammonia. Ammonia and hydrogen vapour are carried down to the absorber where water absorbs and dissolves the ammonia and the hydrogen vapour re-cycles. .

180. 2. Ammonia vapour, which is highly soluble in water, rises with the water vapour from the generator to the separator where the water vapour and some ammonia vapour condenses. Ammonia vapour then rises, is liquefied in the condenser, reduced in pressure and vaporised in the evaporator, and falls to be absorbed in the absorber. Ammonia, dissolved in water, falls down into the lower pipe to the generator.

181. 3. Water vapour leaves the generator, is condensed in the separator, falls through the absorber dissolving the ammonia vapour and returns to the generator. -The unit requires no compressors, or pumps, and is silent and vibration less. It is fairly often used in domestic units on shore, but rarely on-board ship, as a correct and steady

182. level is critical for correct working. Condenser, evaporator and vapour–liquid separator are air-cooled, with fins welded or brazed on to the piping to give extended surface heat transfer. In many designs the hydrogen vapour rises up through the absorber to the underside of the evaporator. It then passes up through the evaporator and carries ammonia vapour down to the reservoir where ammonia and hydrogen separate out, ammonia condensing and hydrogen rising through the absorber. The precise method adopted depends on the dimensions of the equipment and the thermodynamics and flow pattern of the media in the unit

183. Hydrogen vapour, which is insoluble in water, leaves the absorber and rises until it meets ammonia liquid falling into the entry to the evaporator -The hydrogen pressure causes a lowering of the ammonia pressure and this assists in vaporization of the ammonia. -Ammonia and hydrogen vapour are carried down to the absorber where water absorbs and dissolves the ammonia and the hydrogen vapour re-cycles. -Ammonia vapour, which is highly soluble in water, rises with the water vapour from the generator to the separator where the water vapour and some ammonia vapour condenses. Ammonia vapour then rises, is liquefied in the condenser, reduced in pressure and vaporised in the evaporator, and falls to be absorbed in the absorber. Ammonia, dissolved in water, falls down into the lower pipe to the generator

184. Maintenance of reciprocating refrigeration compressors to be carried out as per the time intervals or running hours specified in the product manual. The main causes of operating malfunctions of the plant are: Incorrect control of liquid supply to the evaporator Moisture in the plant Air in the plant Anti-freezing liquid is missing Congestion due to metal shavings and dirt Congestion due to iron oxides Congestion due to copper oxides Inadequate refrigerant charge

185. Explain about one method of refrigerant charging ? Normally charging is made through the liquid charging valve at the high pressure side. Firstly, weighting the gas bottle. Connect the gas bottle and charging valve with the connection pipe. Before tightening the cap on charging pipe, open bottle valve to remove air in the pipe. Then tighten the cap and open bottle valve fully, charging valve is still closed. Change compressor to manual running and start it. Close the condenser outlet valve. Pumping down the entire charge to the condenser. Open the charging valve slowly when suction pressure just above zero. Control the valve opening slowly that no frost formed on the compressor suction pipe. Check the level in the condenser sight glass. Close the charging valve and pumping down the entire charge until suction pressure just above zero. Stop the compressor and close the discharge valve. Cooling water kept running for some hour. Then air is purged out through purging valve on condenser until the refrigerant gas appear at the valve. Calculate the amount of refrigerant (charging) and enter the engine log book.

186. How to fill fridge plant compressor oil ? Change the compressor to manual running. Pumping down the entire charge to condenser. Connect the L.O hand pump to L. O filling valve after air is purged out. When compressor suction pressure just above zero, open the oil filling valve, inject the L.O into crank case. Then stop the compressor and close compressor discharge valve. Then cool down the refrigerant. Then purged out the air through the purging valve until refrigerant appears at purging valve.

187. To remove the air from the system: Change the compressor to manual position Closed the liquid stop valve after the condenser Pumping down the entire charge into the condenser Until the suction pressure just above zero. Stop the compressor and closed the consider inlet valve Then allowed to cool the consider contents. Air is expelled through the purging valve until the refrigerant gas appears at the valve.

188. The Compressor is Operational Run the compressor at minimum capacity at normal operating temperature. Adjust the low-pressure control so that the compressor stops at a suction pressure of approx. 0.1 bar. Throttle the suction stop valve very slowly. Keep an eye on the suction pressure gauge. The suction pressure must be lowered slowly enough to give the refrigerant dissolved in the oil time to escape without the oil foaming. This is of great importance in compressors running on HFC/HCFC. Once the pressure is down to approx. 0.1 bar, stop the compressor and perform the following steps in the order specified. Close suction stop valve. Cut off power to compressor motor. Close discharge stop valve. Drain off last remains of refrigerant gas. Having ensured that power to compressor motor cannot be inadvertently connected, the compressor is ready for opening. For this purpose, remove all fuses to the electric motor.

189. What is secondary refrigerant? State the purposes of secondary refrigerant? Define: 1) Short cycling 2) Dew point 3) Specific humidity