Refrigeration and Air Conditioning - Notes.pdf

Refrigeration

1. Refrigeration and Air Con
Heatis a form of energy. It is not a solid, liquid or gas and it cannot be measured by
weight or volume. Heat is considered to be the lowest form of energy since all
other energies finally degenerate in to heat.
Latent Heat-Heat that is absorbed/rejected by a material resulting in a change of
physical state (occurring at constant temperature). The heat that cannot be
measured on thermometeris called latent heat and is associated with change of
state of a substance.uses the SI unitjoule per kilogram[J/kg].or Kj/Kg
Latent Heat of Fusion-The quantity of heat (Kj/Kg) required to change Kg. of material
from the solid phase into the liquid phase.
Latent Heat of Vaporization-The quantity of heat (Kj/Kg) required to change Kg. of
material from the liquid phase into the vapor phase.
Sensible Heat-Heat that is absorbed/rejected by a material, resulting in a change of
temperature. The heat that can be sensed/measured on thermometer is called
sensible heat and during sensible heating the substance does not change its state.
Specific heat: = energy required to change a unit mass of a material by 1°C. Units:
energy per unit mass per degree.
Latent heat = energy required to change the state (gas, liquid, solid) of a unit mass of
material. Units: energy per unit mass.
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2. Refrigeration and Air Con
Heat content is also known as enthalpy. It is a measure of how much heat a gas or
liquid can hold. Enthalpy is the total amount of heat in one Kg of a substance. The
metric counterpart is kJ/kg. (kilo joules/kilogram)
Enthalpy = Sensible Heat + Latent Heat
When enthalpy is constant, the condition of the gas is called adiabatic.
Entropyis thermodynamic property of a working substance, which increases with
addition of heat, and decreases with its removal. Entropy (Specific) is expressed as
kJ/kgK. Entropy is property where as enthalpy is energy.
It is the ratio of heat content of a gas to its absolute temperature. It remains the same
when a gas is compressed, if no heat is added or removed. When entropy is
constant, the condition of the gas is called isentropic.
Condensing pressure:The pressure at which the refrigerant is phase changing from a
vapor to a liquid.
Evaporating pressure:The pressure at which the refrigerant is phase changing from a
liquid to a vapor.
Vapor pressure:Pressure exerted on a saturated liquid.
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3. Refrigeration and Air Con
Saturation Temperature-That temperature at which a liquid starts to boil (or vapor
starts to condense). The saturation temperature (boiling temperature) is constant
at a given pressure, and increases as the pressure increases. A liquid cannot be
raised above its saturation temperature. Whenever the refrigerant is present in two
states (liquid and vapor) the refrigerant mixture will be at the saturation
temperature.
Superheated vapor:Any vapor above its saturation temperature for a given pressure.
Degree of Superheat -At a given pressure, the difference between a vapor’s
temperature and its saturation temperature is called degree of superheat.
Superheating is the sensible heating of refrigerant vapor at invariable pressure in the
evaporator to a temperature more than the temperature of saturation
corresponding to the evaporator pressure. superheating ensures total evaporation
of the liquid refrigerant before it goes into the compressor.
Degree of Subcooling-At a given pressure, the difference between a liquid’s
temperature and its saturation temperature.
Subcoolingin refrigeration implies cooling the refrigerant in liquid state, at uniform
pressure, to a temperature that is less than the saturation temperature, which
corresponds to condenser pressure.A liquid at a temperature below its saturation
temperature for a given pressure.
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4. Refrigeration and Air Con
Gases can be converted to liquids by compressing the gas at a suitable temperature.
However, they become more difficult to liquefy as the temperature increases
because the kinetic energies of the particles that make up the gas also increase. At
the critical temperature they cannot longer be liquified.
Critical Temperature:The temperature which above, a substance can not exist as a
liquid,no matter how much pressure is applied. Every substance has a critical
temperature.
Critical Pressure:The pressure required to liquifya substance vapor at its critical
temperature
Critical point:The end point of the pressure-temperature curve that designates
conditions under which aliquidand itsvaporcan coexist. At higher temperatures,
the gas cannot be liquefied by pressure alone. At the critical point, defined by the
critical temperatureT
cand the critical pressurep
c,phaseboundaries vanish.
Triple point:The temperature and pressure at which the three phases (gas, liquid,
and solid) of a substance coexist in thermodynamic equilibrium.
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5. Refrigeration and Air Con
Triple Point and Critical point
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6. Refrigeration and Air Con
Unit of Refrigeration is expressed in terms of ‘tone of refrigeration’ (briefly written as
TR). A tone of refrigeration is defined as the amount of refrigeration effect (cooling
effect) produced by the uniform melting of one tone (1000 kg) of ice from and at
0
0
C in 24 hours. In actual practice, one tone of refrigeration is taken as equivalent
to 210 kJ/min or 3.5 kW (i.e. 3.5 kJs).
Basic vapourcompression cycle
The basic refrigeration cycle makes use of the boiling and condensing of a working
fluid at different temperatures and, therefore, at different pressures. Heat is put
into the fluid at the lower temperature and pressure and provides the latent heat
to make it boil and change to a vapour.
This vapouris then mechanically compressed to a higher pressure and a
corresponding saturation temperature at which its latent heat can be rejected so
that it changes back to a liquid. The total cooling effect will be the heat transferred
to the working fluid in the boiling or evaporating vessel, i.e. the change in
enthalpies between the fluid entering and the vapourleaving the evaporator.
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7. Refrigeration and Air Con
The Vapor-Compression Refrigeration Cycle
The vapor-compression refrigeration cycle has four components: evaporator,
compressor, condenser, and expansion (or throttle) valve.
The most widely used refrigeration cycle is the vapor-compression refrigeration cycle.
In an ideal vapor-compression refrigeration cycle, the refrigerant enters the
compressor as a saturated vapor and is cooled to the saturated liquid state in the
condenser. It is then throttled to the evaporator pressure and vaporizes as it
absorbs heat from the refrigerated space
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8. Refrigeration and Air Con
The ideal vapor-compression cycle consists of four processes.
Process Description
1-2 Isentropic compression
2-3 Constant pressure heat rejection in the condenser
3-4 Throttling in an expansion valve
4-1 Constant pressure heat addition in the evaporator
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9. Refrigeration and Air Con
Actual Vapor-Compression Refrigeration Cycle
Actual vapour-compression cycle
Two main differences with ideal cycle:
Fluid frictions, causing pressure drop
Heat transfer to or from surroundings.
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10. Refrigeration and Air Con
The cycle processes can be described as follows:
7-1 Evaporation of the liquefied refrigerant at constant temperature T
1= T
7.
1-2 Superheating of the vapor from temperature T
1to T
2at constant pressure P
L.
2-3 Compression (not necessarily adiabatic) from temperature T
2and pressure P
Lto
temperature T
3and pressure P
H.
3-4 Cooling of the super-heated vapor to the saturation temperature T
4.
4-5 Condensation of the vapor at temperature T
4= T
5and pressure P
H.
5-6 Subcoolingof the liquid from T
5to T
6at pressure P
H.
6-7 Expansion from pressure P
Hto pressure P
Lat constant enthalpy.
A further difference between the real cycle and the ideal is that temperature T
1at
which evaporation takes place is lower than the temperature T
Lof the cold region
so heat transfer can take place. Similarly the temperature T
4of the heat rejection
must be higher than the hot region temperature T
Hto bring about heat transfer in
the condenser.
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11. Refrigeration and Air Con
Advantages Of Subcooling
Refrigeration is improved when a liquid refrigerant is subcooledby a circulation of cold
water in the heat exchanger or by some other methods. As a general rule, a 1% increase
in refrigeration can be achieved for every 2 degrees of liquid subcoolingobtained. Due
to this characteristic, designs of condensers have been changed to achieve obtain liquid
subcooling.
Production of flash gas is reduced during the process of expansion.
Greater latitude (distance) is achieved in management of piping and location of
evaporator.
Methods Of Subcooling
Refrigerant can be subcooledby following methods:
By improving and carrying out modifications in condensers so that subcooling
arrangement is included.
By Installing internal and external heat exchangers to provide subcooling
Coefficient of performance
Since the vapourcompression cycle uses energy to move energy, the ratio of these two
quantities can be used directly as a measure of the performance of the system. This
ratio,isthe coefficient of performance. For practical purposes in working systems, it is
the ratio of the cooling effect to the input compressor power.
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12. Refrigeration and Air Con
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Refrigeration System Line Diagram

13. Refrigeration and Air Con
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14. Refrigeration and Air Con
Compressor
The function of a compressoris to remove the vapor produced by the evaporator and
to deliver it at a required higher pressure. The compressor pumps gas from the
evaporator and compresses it, i.e. increases its pressure. In addition to their
different working principles, compressors can also be distinguished according to
their basic type of construction
In an open compressor, the motor and the compressor housing are mounted
separately. Because the open compressor lacks a seal around it, there is risk of
refrigerant leakage. The advantages are that the compressor components are easily
accessible for maintenance and the costs of a shell can be avoided.
A hermetic compressor also houses both the motor and the compressor housing
inside a shell. However, the steel shell is welded, which provides a true hermetic
seal against the surroundings. It is impossible to open the welded shell of a
hermetic compressor, and the compressor must therefore be scrapped in the event
of damage to the motor or compressor.
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15. Refrigeration and Air Con
Reciprocating Compressor
The advantage of reciprocating compressors is the relatively simple working principle
and construction.
A disadvantage of reciprocating compressors is that they have many moving parts,
which makes it almost impossible to avoid vibrations. Another disadvantage is the
"dead space". When the piston is at its top position, some of the compressed gas
will be trapped in the space between the top of the piston and the cylinder roof.
The gas in the dead space results in lower volumetric efficiency, because less fresh
gas is compressed on each piston stroke than the total volume of the cylinder could
actually admit.
The valves controlling the flow of gas to and from the compressor are sensitive to
droplets in the gas. If a considerable amount of liquid enters the compressor
housing, a very high pressure can be built up when the piston reaches its top
position, which may cause severe damage to the valves or crankshaft. This
phenomenon is called Liquid Hammer.
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16. Refrigeration and Air Con
Scroll compressors
Scroll compressors capture the gas in the volume formed between one fixed and one
orbiting scroll. The orbiting scroll is driven by an electric motor, which rotates a
shaft. Note that the scrolls perform an orbiting motion. They do not rotate.
Figure above explains the scroll compressor function. Superheated gas (blue) enters
at the outer ends of the spirals and is compressed on its way through the scrolls
due to the orbiting motion of one of the spirals. The discharge hole, where high-
pressure gas (red) leaves, is located in the center of the scrolls.
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17. Refrigeration and Air Con
Scroll compressors are available in both open and hermetic design. They have several
advantages over reciprocating compressors:
The absence of suction and discharge valves eliminates pressure drops and
consequential noise and vibrations.
The scrolls have no dead space, which results in volumetric efficiencies close to
100%.
Fewer moving components, leading to a lower failure rate.
They are relatively insensitive to liquid droplets in the suction gas from the
evaporator
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18. Refrigeration and Air Con
Compressors are lubricated for three main purposes:
To reduce frictional wear on bearings and other moving parts of the compressor
To cool the refrigerant gas during compression
To seal against refrigerant gas leakage
If the viscosity of the oil-refrigerant mixture is too low, it leads to incomplete or
inefficient separation of the metal surfaces, which increases friction and wear.
Various anti-wear additives can counteract this to some extent, but this solution
cannot be used to its full extent in refrigeration systems due to the risk of reaction
between the additives and the refrigerant.
With high-viscosity oil-refrigerant mixtures, there may be problems, such as
obstructed flow, that may lead to poor pumping efficiency. In order to work
properly, oil-refrigerant mixtures should have a dynamic viscosity that is sufficiently
high to give satisfactory sealing and lubrication in the compressor. In addition, the
mixture must be thermally and chemically stable, so as not to react with
components and materials in the refrigeration system.
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19. Refrigeration and Air Con
Lubricating oil may have negative impacts on other parts of the refrigeration system.
An oil separator is therefore often mounted directly after the compressor outlet to
reduce the flow of lubricant into the condenser and evaporator. Heat transfer will
be impaired if oil droplets become trapped in these components. The refrigerant is
protected from most of the oil, because the oil separator continuously returns
lubricant to the crankcase of the compressor.
Partly miscible oils and refrigerants may separate in the condenser. If so, a refrigerant-
rich phase is carried over to the expansion valve, while the oil accumulates in an
oil-rich phase in the refrigerant reservoir. This may restrict the return of oil to the
compressor, leading to insufficient lubrication.
In the evaporator, the lubricant is subject to low temperatures, which may lead to
problems with wax formation and phase separation. If the solubility of the
refrigerant in oil at low temperatures is low, there may be problems with returning
the oil to the compressor.​​​
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20. Refrigeration and Air Con
Condenser
The normal function of a condenser is to transform hot discharge gas from the
compressor to a slightly sub-cooled liquid flow,by transferring heat from the
refrigerant gas to the secondary cooling liquid.
The basic operation of condensers is divided into three parts: de superheating,
condensation and sub-cooling. All three operations can be carried out inside the
condenser.
The total heat transfer is called the Total Heat of Rejection (THR). Alternatively, the
desuperheatingor sub-cooling operation can be carried out in a separate heat
exchanger.
Evaporator
The evaporator is one of the five essential components in the refrigeration system,
together with the condenser, compressor, expansion valve and refrigerant.
In the evaporator, the refrigerant boils off by absorbing energy from the warmer
secondary fluid, thus reducing the temperature. The secondary fluid may be a gas
or a liquid, depending on the system.
A refrigerant in liquid form will absorb heat when it evaporates and it is this
conditional change that produces cooling in a refrigerating process.
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21. Refrigeration and Air Con
Refrigerant selection criteria:
Selection of refrigerant for a particular application is based on the following
requirements:
Thermodynamic and thermo-physical properties
Environmental and safety properties, and
Economics -Economic properties: The refrigerant used should preferably be
inexpensive and easily available
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22. Refrigeration and Air Con
Thermodynamic and thermo-physical properties: The requirements are:
a) Suction pressure: At a given evaporator temperature, the saturation pressure
should be above atmospheric for prevention of air or moisture ingress into the
system and ease of leak detection. Higher suction pressure is better as it leads to
smaller compressor displacement
b) Discharge pressure: At a given condenser temperature, the discharge pressure
should be as small as possible to allow light-weight construction of compressor,
condenser etc.
c) Pressure ratio: Should be as small as possible for high volumetric efficiency and low
power consumption
d) Latent heat of vaporization: Should be as large as possible so that the required
mass flow rate per unit cooling capacity will be small.
In addition to the above properties; the following properties are also important:
e) Isentropic index of compression: Should be as small as possible so that the
temperature rise during compression will be small
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23. Refrigeration and Air Con
f) Liquid specific heat: Should be small so that degree of subcoolingwill be large
leading to smaller amount of flash gas at evaporator inlet
g) Vapourspecific heat: Should be large so that the degree of superheating will be
small
h) Thermal conductivity: Thermal conductivity in both liquid as well as vapourphase
should be high for higher heat transfer coefficients
i) Viscosity: Viscosity should be small in both liquid and vapourphases for smaller
frictional pressure drops
The thermodynamic properties are interrelated and mainly depend on normal boiling
point, critical temperature, molecular weight and structure.
The freezing point of the refrigerant should be lower than the lowest operating
temperature of the cycle to prevent blockage of refrigerant pipelines.
Critical temperature is the temperature above which it is impossible to liquefy the
refrigerant regardless ofpressure. It should be as high as possible and certainly
above the maximum expected condensertemperature.
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24. Refrigeration and Air Con
Environmental and safety properties:
Next to thermodynamic and thermophysicalproperties, the environmental and safety properties
are very important. In fact, at present the environment friendliness of the refrigerant is a
major factor in deciding the usefulness of a particular refrigerant. The important
environmental and safety properties are:
a) Ozone Depletion Potential (ODP): According to the Montreal protocol, the ODP of refrigerants
should be zero, i.e., they should be non-ozone depleting substances. Refrigerants having non-
zero ODP have either already been phased-out (e.g. R 11, R 12) or will be phased-out in near-
future(e.g. R22). Since ODP depends mainly on the presence of chlorine or bromine in the
molecules, refrigerants having either chlorine (i.e., CFCs and HCFCs) or bromine cannot be
used under the new regulations
b) Global Warming Potential (GWP): Refrigerants should have as low a GWP value as possible to
minimize the problem of global warming. Refrigerants with zero ODP but a high value of GWP
(e.g. R134a) are likely to be regulated in future.
c) Total Equivalent Warming Index (TEWI): The factor TEWI considers both direct (due to release
into atmosphere) and indirect (through energy consumption) contributions of refrigerants to
global warming. Naturally, refrigerants with as a low a value of TEWI are preferable from
global warming point of view
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25. Refrigeration and Air Con
d) Toxicity: Ideally, refrigerants used in a refrigeration system should be nontoxic.
e) Flammability: The refrigerants should preferably be non-flammable and nonexplosive.
For flammable refrigerants special precautions should be taken to avoid accidents
Other important properties are:
f) Chemical stability: The refrigerants should be chemically stable as long as they are inside
the refrigeration system.
g) Compatibility with common materials of construction (both metals and nonmetals)
h) Miscibility with lubricating oils: Oil separators have to be used if the refrigerant is not
miscible with lubricating oil (e.g. ammonia). Refrigerants that are completely miscible
with oils are easier to handle (e.g. R12).
i) Dilelectricstrength: In hermetic arrangements, the motor windings are cooled by
refrigerants vapor on its way to the suction valve of the compressor. Therefore, the
dielectric strength of refrigerant is an important property in hermetically sealed
compressor units.High dielectric strength (for compressors withintegral motors)
Refrigerant with a high dielectric strengthavoids short circuitwhen refrigerant comes
directly in contact with motor windings in a hermetically sealed compressor.
The dielectric strength of any material is a measure of its insulation property.
j) Ease of leak detection: In the event of leakage of refrigerant from the system, it should
be easy to detect the leaks.
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26. Refrigeration and Air Con
Refrigerants Other Properties
The refrigerant ought not to be poisonous. Where this is impossible, the refrigerant must have a
characteristic smell or must contain a tracer so that leakage can quickly be observed.
The refrigerant ought not to be flammable nor explosive.
The refrigerant ought to have reasonable pressure, preferably a little higher than atmospheric
pressure at the temperatures required to be held in the evaporator
To avoid heavy refrigerator design the pressure, which corresponds to normal condensing
pressure, must not be too high.
Relatively high evaporating temperature is required so that heat transmission can occur with
least possible circulating refrigerant.
Refrigerant vapourought not to have too high a specific volume because this is a determinant for
compressor stroke at a particular cold yield.
The refrigerant must be chemically stable at the temperatures and pressures normal in a
refrigeration plant.
The refrigerant ought not to be corrosive and must not, either in liquid or vapourform, attack
normal design materials.
The refrigerant must not break down lubricating oil, must be easy to obtain and handle, must not
cost too much.
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27. Refrigeration and Air Con
Thermo Static Expansion Valve
The expansion valve is situated in the liquid line between the condenser and the inlet
of the evaporator. Whereas the compressor operates to increase the pressure and
pump the refrigerant through the system, the expansion device releases the
pressure be​tween the high-pressure condensation side and the low-pressure
evaporation side.
A liquid-charged bulb has a large charge of refrigerant and will never "run dry". It will
always contain both liquid and gaseous refrigerant. The pressure inside the bulb
increases as the superheating increases, due to additional evaporation. Historically,
the refrigerant in the bulb was the same as the working refrigerant in the system
(parallel-charged). However, better characteristics have been achieved by using
different refrigerants (cross-charged), which is now the most common
arrangement.
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28. Refrigeration and Air Con
Expansion valves serve two purposes:
Controlling the amount of refrigerant entering the evaporator: As much of the
evaporator surface as possible should be covered with​ liquid refrigerant without
liquid being carried over to the compressor. If the capacity of the evaporator
increases, the expansion valve should allow a larger flow of refrigerant, and vice
versa. A smaller refrigerant mass flow results in a higher level of superheating,
because less surface area is required for evaporation.
Maintaining the pressure difference between the condenser (high pressure) and
the evaporator (low pressure): The pressure difference created by the work of the
compressor is maintained by the expansion device.
Expansion valves do not directly control the evaporationtemperature. Instead, they
regulate the superheating by adjusting the mass flow of refrigerant into the
evaporator, and maintain the pressure difference between the high pressure and
low-pressure sides.
The evaporation temperature depends on the capacity of the compressor and the
characteristics and efficiency of the evaporator.
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29. Refrigeration and Air Con
Thermal expansion valves, or thermostatic expansion valves -TEVs are popular
expansion devices due to their simplicity and availability, and their relatively good
sensitivity an​d accuracy in regulation.
The disadvantage of TEVs is the necessity for relatively high superheatin​g, which
reduce heat transfer area from the evaporation process.
The bulb, which transmits the corresponding pressure of the superheated gas, consists
of a hollow metal container filled with a refrigerant fluid.
A capillary tube connects the bulb to the valve housing. The bulb is fitted in direct
contact with the suction pipe ( evaporator outlet), close to the compressor inlet.
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30. Refrigeration and Air Con
Working of Expansion Valve -During normal operation, evaporation will stop some
distance up in the evaporator. Then, saturated gas appears which becomes
superheated on its way through the last part of the evaporator. The bulb
temperature will thus be evaporating temperature plus superheat, e.g. at –10 °C
evaporating temperature the bulb temperature could be 0 °C.
If the evaporator receives too little refrigerant ,the vapourwill be further superheated
and the temperature at the outlet pipe will rise. The bulb temperature will then
also rise and with it the vapourpressure in the bulb element, since more of the
charge will evaporate. Because of the rise in pressure the diaphragm becomes
forced down, the valve opens and more liquid is supplied to the evaporator.
Correspondingly, the valve will close more if the bulb temperature becomes lower.
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31. Refrigeration and Air Con
Working Principle -If the superheating increases, the pressure inside the bulb will
increase, because more refrigerant inside the bulb evaporates. The increased
pressure is transmitted through the capillary tube, and depresses the membrane
inside the head of the TEV. This moves the needle, opening the valve orifice and
thus increasing the refrigerant mass flow.
An increased mass flow of refrigerant requires more heat surface area to evaporate,
and therefore results in less superheating. The outlet gas temperature will
therefore decrease. This in turn will cool the bulb, resulting in the condensation of
some bulb refrigerant and thus decreasing the pressure on the membrane. The
force of the spring will close the valve slightly, and less refrigerant will be allowed
into the evaporator, again increasing the superheating. The system will soon find a
balance.
The balance across the membrane is adjusted with a spring that may be adjustable
manually or set at the factory. The stiffer the spring, the higher the level of
superheating required to open the valve.
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32. Refrigeration and Air Con
32
Working Principle
If the saturation pressure increases instead, the bulb will still detect an increased
temperature. More refrigerant will boil off, but the increased pressure below the
membrane will balance the higher pressure above the membrane. Thus, there will
be no change in the needle position.
Movement of the membrane controls the position of a needle and hence the mass
flow of refrigerant entering the evaporator.
The degree of opening of the valve is determined by:
The pressure produced by the bulb temperature acting on the top surface of the
diaphragm.
The pressure under the diaphragm, which is equal to the evaporating pressure.
The pressure of the spring acting on the underside of the diaphragm.

33. Refrigeration and Air Con
Internal Pressure Equalization
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34. Refrigeration and Air Con
Thermostatic expansion valve -Internal Pressure Equalization
There are three pressures acting inside the thermostatic expansion valve.
P1 is the pressure at the top of the thermostatic expansion valve acting inside the
power element above the diaphragm. Due to this pressure the diaphragm tends to
move down due which the needle also moves down and the valve tends to open.
When the evaporator temperature becomes higher the gas in the feeler bulb
expands due to which the gas pressure inside the power element increases. This
causes the downward movement of the needle to open the valve.
The pressure P2 is the pressure acting on the lower side of the diaphragm due to the
refrigerant pressure inside the evaporator. This pressure tends to move the
diaphragm upwards and close the opening of the valve.
The pressure P3 is the spring pressure that tends to close the opening of the valve.
This pressure remains constant.
The pressures P2 and P3 act against the pressure P1. The pressure P1 tends to open
the valve while the pressure P2 and P3 tend to close the thermostatic expansion
valve. Thus if the valve has to open P1 should be greater than combined forces of
P2 and P3.
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35. Refrigeration and Air Con
A thermostatic expansion valve-Bulb Charge Fluid
There are two common approaches to what makes up bulb charge fluid.
The first approach is to use the same refrigerant that is used in the system, i.e., using
R-410A in the bulb for an R-410A system.
The other common approach—and the one that Danfossrecommends—is called a
cross charge. Cross charged bulbs mix a combination of different refrigerants with
gases to flatten the pressure-temperature (P-T) curve. Cross charges enable the TXV
to perform similarly regarding the change in opening degree for a given change in
superheat across a range of evaporator temperatures.
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36. Refrigeration and Air Con
Defrosting
Evaporators working below 0°C will accumulate frost which must be removed
periodically, since it will obstruct heat transfer. Evaporators of suitable and robust
construction can be defrosted by brushing, scraping or chipping, but these methods
are labourintensive and may lead to damage of the plant.
Where the surrounding air is always at + 4°C or higher, it will be sufficient to stop the
refrigerant for a period and allow the frost to melt off (as in the auto-defrost
domestic refrigerator). This method can be used for cold rooms, packaged air-
conditioners etc., where the service period can be interrupted.
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37. Refrigeration and Air Con
For lower temperatures, heat must be applied to melt the frost within a reasonable
time and ensure that it drains away.
Methods of defrosting used are as follows:
Electric resistance heaters. Elements are within the coil or directly under it.This
method of defrosting is employed for fined coil evaporator. A bank of electric
heaters is located near the coil. During defrosting, the system remains closed and
heater starts to melt the frost accumulated on the evaporator. The time required
for defrosting varies from few minutes to 30 minutes depending on the size of the
evaporated and level of frost deposited on the evaporator.
Hot gas -A branch pipe from the compressor discharge feeds superheated gas to
the coil. The compressor must still be working on another evaporator to make hot
gas available. The process of defrosting is performed at regular interval (6-10 hours)
by the action of solenoid valve which supplies hot refrigerant gas after compression
to the evaporator. Hot gas supply for few minutes melts the ice accumulated on the
evaporator. The condensed refrigerant is re-evaporated in the re-evaporator before
it goes to compressor.
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38. Refrigeration and Air Con
Hot Gas Defrosting
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39. Refrigeration and Air Con
Lubrication
Oil is supplied to the bearings and crankshaft seal by means of a gear pump driven
from the crankshaft. The oil is filtered through an Auto-Kleanstrainer and/or an
externally mounted filter with isolating valves. A pressure gauge and sight glass are
fitted and protection against oil failure is provided by a differential oil pressure
switch. Oil loss from the compressor is sometimes the result of it being carried into
the system by the refrigerant.
Cylinder walls are splash lubricated and some of the oil is carried around with the
refrigerant.If oil and refrigerant are miscible (i.e. if they form a mutual solution)
the oil tends to return via the circuit to the crankcase. When the refrigerant is not
miscible with the oil, the latter may be precipitated in the system, usually in the
evaporator.
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40. Refrigeration and Air Con
Oil Separators
Oil separators of the impingement type, may be fitted in hot gas discharge lines from
the compressor. The reduction in velocity of the vapouras 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.
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41. Refrigeration and Air Con
Driers
The presence of a small amount of water can have an effect on plant performance.
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 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 .
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.
41

42. Refrigeration and Air Con
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 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 duprenering 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.
42

43. Refrigeration and Air Con
Back Pressure Valve
Back pressure valve is a spring loaded non-return valve. It is fitted just at the
exit of the refrigerant from the evaporator coil in a multi temperature
rooms system. This being fitted at the exit of the compartments whose
temperature is set higher (usually at about 4-5 degrees centigrade).
Onboard ship, it is fitted outside vegetable room compartment.
Function of back pressure valve
The function of the valve is to maintain equilibrium of the system as the
pressure of the gas at the exit of each compartment differs. The vegetable
room is maintained at a temperature of+4°C to -+6°C while fish room and
meat room is at a temperature of-12°C to -14°C.
Hence, flow of refrigerant at fish room and meat room is more as compared
to veg room.
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.
43

44. Refrigeration and Air Con
Back Pressure Valve -Its purpose is to maintain a constant pressure and,
consequently, the constant of saturation temperature in the evaporator, regardless
of the pressure changes elsewhere in the system. 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.
44

45. Refrigeration and Air Con
Low Pressure controller or LP cut off: This is a compressor safety which cut off
thecompressorin the event of pressure drop in the suction line. The pressure of the suction
line is continuously sensed by the control unit and when it goes below the set value, which
means the room is properly cooled, the LP cut out will auto trip the compressor. When the
pressure rises, indicating there is flow of refrigerant in the line due to increase in room
temperature, the LP switch will start the compressor.
The reasons of excessive low pressure are as under.
Very low load at the evaporator. Starving of evaporator.
Defective expansion valve/ blockage of expansion valve.
45

46. Refrigeration and Air Con
High pressure cut-out
For protecting the compressor from high pressure and subsequent failure, a high
pressure cut out is provided that take a pressure tapping from the discharge line
and when it detects an over pressure, it stops the compressor. The HP cut out is not
resettable automatically but has to be reset manually by the operator. This is
because the high pressure is a serious fault and the cause must be investigated and
corrected before the plant is started again.
High pressure can be caused in a refrigeration plant due to various causes like
Over charge,
Loss of cooling water,
Air, or other incompressible gases in the system
Obstruction in the discharge line of the compressor
Poor rate of heat transfer at the condenser. The refrigerant can not liquefy rapidly
The discharge pressure will abnormally raise and highpressure cut out will take in
action.
46

47. Refrigeration and Air Con
Construction of High Pressure Cut Out
47

48. Refrigeration and Air Con
Oil differential cut out: This safety is again for compressor as it is the only machinery
in the circuit having rotational parts which requires continuous lubrication. In the
event of low supply or no supply of lube oil to the bearing, the differential pressure
will increase and activates a trip signal to safeguard thebearing and crankshaft.
Relief valves: Relief valves are fitted in discharge side of compressor and will lift and
safeguard the compressor in the event of over pressure. One relief valve is also
fitted in thecondenserrefrigerant line to avoid damage to the condenser if there is
high pressure in the discharge line.
Solenoid valves: Master solenoid valve is fitted in the common or main line after the
condenser discharge. It closes when compressor stops or trips to avoid over flow of
refrigerant in to evaporator. All holds or rooms are fitted with individual solenoid
valve which control the flow of refrigerant to that room.
Oil heater: Oil heater is provided for the compressor crank case oil and prevents
compressor from getting excessively cold which may effect the lubrication of the
parts.
48

49. Refrigeration and Air Con
The safety devices inrefrigerationsystem -Summary
L. P cut-out on compressor suction side: Set at a pressure corresponding to 5 °C below the
lowestexpected evaporating gauge reading
H.P cut-out on compressor discharge side:Set at a pressure corresponding to 5 °C above
thehighest expected evaporating gauge reading
Lube oil low pressure cut-out:Oil pressure usually set at 2 bar above crankcase pressure
Cooling water L .P cut-out in condenser side
Safetyspring loaded liquid shock valveon compressor cylinder head
Bursting discon cylinder head, between inlet and discharge manifold
Bursting discon Condenser, [if fitted]
Relief valve onCondenser; air purging valve on condenser
Master solenoid valve: to prevent liquid being entered into Compressor, when the plant
isstandstill, especially in Large Plant
49

50. Refrigeration and Air Con
50
Refrigeration System Line Diagram

51. Refrigeration and Air Con
CAUSE EFFECTS AND REMEDIES
51

52. Refrigeration and Air Con
The Effects And Remedies OfUnder Charge In Refrigeration -Symptoms
Low discharge pressure
Low liquid level in the condenser sight glass
The machine runs for longer periods between cut out.
Remedies
To charge the refrigerant until the bubbles disappears in the liquid sight glass andcharging will
correct the pressure gauge reading.
To find out the leaky points by halide torch or soap solution or electronic leak detector.
The Effects And Remedies OfOver Charge In Refrigeration -Symptoms
It is indicated by high compressor discharge pressure.
Excess frost on compressor suction line after evaporator.
Full of sight glass.
Remedies
The excess refrigerant is to be recovered through the purging valve fitted on thecondenser
until discharge pressure return to normal.
52

53. Refrigeration and Air Con
The Effects OfMoisture In Refrigeration System-Symptoms
Starving of evaporator and rapid condenser pressure rise cause compressor short cycling.
Frost at expansion valve inlet.
Remedy
Clean expansion valve filter.
Renew drying agents of dehydrator or drier.
Cause of expansion valve chocking
When a expansion valve is functioning correctly, frost forms on the outlet side of the valve, and if
the inletside does show frosting then this is a sign of blockage.
It is caused by dirt or freeze up by water present in the system.
Starving of evaporator and rapid condenser pressure rise cause compressorshort cycling.
Remedy
Clean the expansion valve filter
Renew the drying agent of dehydrator or drier. (activated alumina or silica gel )
53

54. Refrigeration and Air Con
The Effects Of Oil In Refrigeration System On Ships -Symptoms
The System Running Longer Than Normal.
Difficult To Cool Down Room Temperature due to reduced heat transfer rate
Remedy
Pumping down the entire charge to condenser.
Along the system must be blown out with compress air by stripping individual unit
component.
Repair the oil separator.
Renew the compressor piston rings.
The Effects OfAir In Refrigeration System -Symptoms
High condenser and compressor temperature.
Abnormally high discharge pressure
Very small bubbles in liquid sight glass.
Remedies
To remove the air from the system:
Air is expelled through the purging valve until the refrigerant gas appears at the valve.
54

55. Refrigeration and Air Con
Lube oil foaming in crankcase
When a cold compressor is started, pressure drops rapidly and the temperature of the oil rises.
As a result, the solubility of the refrigerant is sharply decreased. There will be a rush of gas out
of the oil over a very short period of time. The drop in crankcase pressure causes oil foaming.
When this occurs, an excessive amount of oil may be carried with the refrigerant through and
beyond the compressor.
Causes
Liquid in suction line
Too much refrigerant dissolvedin oil
Remedy
See liquid in suction line
Before starting compressor,heating element must havebeen on for at least 8 hours inorder
to boil refrigerant out ofoil. During start-up phase,capacity should be connectedat a slow
rate to prevent asudden drop of pressure onsuction side with resultantfoaming.Under
normal operatingconditions, compressor shouldoperate under as stablepressure conditions
as possible
55

56. Refrigeration and Air Con
Crankcase “sweating” or frosting up -Causes
Liquid in suction line
Expansion valve sensormisplaced
Expansion valve supplying too much liquid
Remedy
See liquid in suction line
Check positioning of expansionvalve sensor –see instructionsfor expansion valve
Increase superheating onthermostatic expansion valve
Too low condenser pressure -Causes
Excessive condenser cooling
Defective piston rings or worncylinders
Discharge valves are defectiveor leaky
By-pass between high pressureside and suction sideof compressor
Compressor lacks capacity
Remedy
Regulate condenser cooling
56

57. Refrigeration and Air Con
The indications of leaky suction and discharge valves in refrigeration system
Defective Suction valve
Continuous running of compressor
Insufficient cooling effects
Noisy operation
High suction pressure
Defective Discharge valve
Continuous running of compressor
Insufficient cooling effects
Noisy operation
High suction pressure during running
Low discharge pressure during running
Suction pressure increase faster after compressor is shut down
Warm cylinder head
57

58. Refrigeration and Air Con
Frosting On Suction Line
The control system is faulty and the inside temperature is way down so the refrigerant isn’t
fully evaporating before it exits the evaporator and is therefore still expanding in the suction
line.
The system is overfilled so as above, the refrigerant is still expanding in the suction line
When the refrigerant is low, the temperature of the refrigerant at the beginning of the
evaporator coils will be colder than the freezing point of water (less than 32°F). Because the
coils are so cold, the condensation that forms on the coils will freeze. As ice builds up on the
coils, it restricts the air flow through the coils. Because of the restriction, the refrigerant can't
absorb as much heat from the indoor air moving over the coils. This causes the refrigerant to
boil later in the evaporator, which causes ice to form further along the coils. This situation
continues to progress, until the whole evaporator is a block of ice. Once that happens, the
refrigerant will start to boil in the suction line. This cause the temperature of the suction line
to drop, and just like in the evaporator, cause the condensation to freeze.
58

59. Refrigeration and Air Con
Common Faults and Simple Detection –Summery
1. Undercharge: low discharge gauge reading, lengthy running times.
2. Air in system: high discharge gauge reading (assuming sufficient vapour), jumping of gauge
pointers, inefficient working.
3. Dirty condenser or insufficient cooling water: high discharge gauge reading and incorrect
condenser temperature differentials.
4. Overcharge: unlikely, but gives high discharge gauge reading and very sensitive expansion valve
working.
5. Oil on cooling coils: incorrect condenser and evaporator temperature differentials (oil is an
insulator), excess frost on suction pipe.
6. Choked expansion valve: caused by dirt or freeze up by water, gives starving of evaporator and
rapid condenser pressure rise.
7. HP cutout : condenser coolant restriction causing hp
59

60. Refrigeration and Air Con
TROUBLESHOOTING
60

61. Refrigeration and Air Con
Compressor Starts But Stops immediately
61
Reasons Solutions
Low pressure cut out gets activated
Ensure that all the suction line valves are in open condition,
the refrigeration is properly charged and the low pressure cut out
is not defective
Defective oil pressure cut out
Check for proper functioning of oil pressure cutout
and replace the defective cutout
Defrosting timer is getting activated
frequently
If the defrost timer is getting activated frequently,
leading to cutout of compressor, check and repair defrost timer
The lube oil level is below required
level
This can be because of leakage of lube oil from seal
or carry over of oil. Rectify the leakage and refill the oil level
Foaming of oil leading to reduced
oil pressure
Ensure no foaming takes place, renew the oil if required
Motor overload cutouts are
activating
Ensure that electrical motor trips are working properly

62. Refrigeration and Air Con
Compressor Start and Stops Frequently
62
Reasons Solutions
Wrong Setting of Cutouts: It may be because the high
pressure (HP) cutout is set too high or LP cutout is set too
low
Check and change the setting to advisable
limit
Differential Setting Span is Small: The low pressure (LP)
cut out is provided with starting and stopping pressure
setting. If the setting span is too small, it will lead to
frequent cut-in and cut-out of the compressor
Change the setting and increase the span
between starting and stopping compressor
pressures.
Defective Valves: If the compressor discharge valve is
leaky or the line solenoid valve is not closing properly,
this will lead to variation in sensor pressure and result in
frequent cut-in and cut-out of compressor
Replace all the defective valves
Clogged Suction Filters: Compressor is provided with a
filter in the suction line. If that is clogged, it will lead to
frequent LP cut out
Clean the filter

63. Refrigeration and Air Con
Compressor is Running Continuously
63
Reasons Solutions
Refrigerant not sufficient for cooling evaporator
Ensure thermostatic expansion valve is working
properly and clean the filters inside TEV
Thermostat low pressure cut-out not activated
at low temperature/ pressure
Correctly set the LP cut-out to correct setting
Refrigerant charge is low in the circuit
Check for leakage of refrigerant and charge with
required refrigerant

64. Refrigeration and Air Con
Unusual Sound from Compressor
64
Reasons Solutions
The oil pressure is low
Ensure that the oil level is maintained and no foaming
of oil is developed. Replenish or refill the lube oil if
required
Incorrect alignment of compressor and motor
Check the alignment and set motor and compressor in
one line
Loose foundation
Ensure that the compressor foundation is secured and
all foundation bolts are checked for tightness
Loosening of driving belt
Ensure to check the elasticity of the belt and renew if
the belt is slack

65. Refrigeration and Air Con
High Compressor Discharge Temperature
65
Reasons Solutions
Excessive suction temperature due to less
refrigerant in the circuit
Recharge the circuit to maintain refrigerant quantity in the
circuit. Ensure TEV is set properly and supplying enough to the
evaporator, else degree of superheat will result in increase of
suction and discharge temperatures of the compressor
Leak in the discharge valve leads to
generation of heat
Renew the leaky valve
Open bypass between suction and
discharge
Control the bypass to avoid this

66. Refrigeration and Air Con
Evaporator Coil Icing
66
Reasons Solutions
Too low temperature setting
Increase the coil temperature by adjusting TEV or it’s
sensor
The coil capacity is less Install large capacity evaporator coils
Defrost is not operational
Check if the defrost system is functioning at regular
intervals

67. Refrigeration and Air Con
Reduced Cooling Capacity
67
Reasons Solutions
Inadequate refrigeration Charge refrigerant in the circuit
Insufficient or damaged insulation in the roomCheck and renew the insulation
Room or hold is over packed Ensure that the room is not filled above its capacity
Malfunctioning solenoid or TEV
Check the functioning of these valves and renew if not
functioning properly
Poor thermostat location that senses cold
temperatures
Place the sensor of the TEV in proper location i.e
discharge of the evaporator
Room door is kept open
Ensure to close the door while exiting the provision
room

68. Refrigeration and Air Con
Reducing oil level in compressor
68
Reasons Solutions
Foaming of oil due to liquid in suction
line
Foaming of oil may arise due to liquid refrigerant entering the
crankcase. Replenish oil and troubleshoot cause of liquid in
suction of compressor
Drive side seal leaking
The compressor is provided with oil seal at the drive side. Ensure
it is not leaking and renew it if required
Worn out piston rings/ liner leading to
oil carryover in the system
Renew the compressor piston rings or liner

69. Refrigeration and Air Con
Heating, Ventilation and Air Conditioning (HVAC)
69

70. Refrigeration and Air Con
The basic principles of air conditioning:
Air conditioning is the process of treating air so as to control simultaneously its
temperature, humidity, cleanliness and distribution to meet the requirements of
the conditioned space.
Action involved:
Temperature control and Humidity control
Air filtering, cleaning and purification
Air movement and circulation
Winter conditioning relates to increasing temperature and humidity of air whilst
summer conditioning relates to Air movement and circulation
70

71. Refrigeration and Air Con
The objectives of air conditioning on ships
To extract excess heat
To raise air temperature when required
To add moisture as required
To reduce moisture content as required
To maintain sufficient air flow
To remove dust
71

72. Refrigeration and Air Con
Important definitions in air-conditioning:
Dry bulb (d.b.) temperatureis the temperature as measured by an ordinary
thermometer which is not affected by radiated heat.
Wet bulb (w.b.) temperatureis the temperature registered by a thermometer with
wetted fabric around the bulb.
(When moisture evaporates from a surface, i.e. the skin, the latent heat required, is
drawn from the surface causing it to be cooled. If a thermometer bulb is covered by
a wetted fabric and exposed to the air, the rate of evaporation will depend upon
the humidity of the surrounding air. As the heat required must come from the bulb,
this results in a lower temperature reading than if the bulb was dry.)
72

73. Refrigeration and Air Con
Relative humidity (r.h.): The relative humidity is a measure of the amount of water
vapor in the air (at a specific temperature) compared to the maximum amount of
water vapor air could hold at that temperature, and is given as a percentage value.
Relative humidity depends on the temperature of the air, as warm air can hold more
moisture than cold air. A relative humidity of 100 percent indicates that the air is
holding all the water it can at the current temperature and any additional moisture
at that point will result in condensation. A relative humidity of 50 percent means
the air is holding half the amount of moisture that it could.
As the temperature decreases, the amount of moisture in the air doesn't change, but
the relative humidity goes up (since the maximum amount of moisture that cooler
air can hold is smaller).
73

74. Refrigeration and Air Con
Hygrometer or Psychrometer:
Hygrometer is an instrument to measure the humidity of air. This consists of an
ordinary thermometer which gives the dry bulb temperature and a wet bulb
thermometer (wetted with gauze cover).The wet bulb reading will be less than the
dry bulb reading, the difference is quoted as the wet bulb depression.
The drier the air, the more rapid the moisture evaporation from the gauze giving a
cooling effect. Thus greater the difference between the dry and wet bulb readings,
drier the air and lesser the relative humidity.
74

75. Refrigeration and Air Con
Dew Point
The atmospheric temperature (varying according to pressure and humidity) below
which water droplets begin to condense and dew can form. The dew point is the
temperature to which the air must be cooled at constant pressure in order for it
become saturated, i.e., the relative humidity becomes 100%. A higher dew point
indicates more moisture present in the air.
Specific volumeis defined as the number of cubic meters occupied by one kilogram
ofmatter.It is the ratio of a material's volume to itsmass, which is the same as the
reciprocal of itsdensity. In other words, specific volume is inversely proportional to
density. Specific volume may be calculated or measured for any state of matter, but
it is most often used in calculations involvinggases.
The standard unit for specific volume is cubic meters per kilogram (m
3
/kg), although it
may be expressed in terms of milliliters per gram (mL/g) or cubic feet per pound
(ft
3
/lb).
Specific volumes are measured for different materials atstandard temperature and
pressure, which is defined as 0 degrees Celsius and 1 atm(or atmosphere).
75

76. Refrigeration and Air Con
Physcometry:It is the study of properties of mixture of air and water vapour. This
subject is important to air conditioning because the systems handle air-water vapor
mixtures, not dry air.
Some air-conditioning processes involve the removal of water from the air-water
vapor mixture (dehumidication) while some involve the addition of water
(humidication).
A convenient way to represent the properties of air-water vapor mixtures is the
psychrometricchart.
On the chart, such properties as dry bulb temperature, wet bulb temperature, dew
point, relative humidity, humidity ratio, specific volume, and enthalpy are
presented in graphical form.
Psychometric chart or tableis used to find relative humidity from dry bulb and
wet bulb readings taken at the same location in a space. (The thermometers
may be in a fixed position or in a football rattle type device.)
76

77. Refrigeration and Air Con
PsychrometricChart and comfort zone
77

78. Refrigeration and Air Con
Comfort Zone:The condition of the air in a space depends on its temperature,
humidity and movement. The effect of the air on people in a space varies
considerably between one person and another, so it is only possible to stipulate a
fairly wide zone. Under summer conditions relative humidity between 30% and
70%,-average about 50% and , thermometer readings 19 deg to 25deg, average
22deg gives the best degree of summer comfort.
78

79. Refrigeration and Air Con
Air handling unit
Compressor protection:Compressors are under similar protection systems as of
refrigeration plant, low pressure cooling zone, high pressure part (manual reset)
and this cuts the differential oil pressure. In addition, it has a lock for the
compressor installed, can not be started when the air handling unit fan is not
started. When the fan stops, the compressor is turned off. The compressor is
triggered with a low suction pressure. The purpose is to prevent liquid back to the
compressor.
79

80. Refrigeration and Air Con
Air Conditioning Circuit
80

81. Refrigeration and Air Con
In the diagram above a single unit contains an evaporator fed through a gas
compressor. A belt driven fan supplies air to the evaporators through an air filter
with fine mesh. This filter is removed and washed regularly in a soapy solution
containing a disinfectant. The air flows over the evaporator, where it is cooled. The
water condenses and is transported in a collecting basin and pipelines. A perforated
tubeis installed after the evaporator allows the low steam quality to be introduced
into the air to improve the moisture when it is too dry.
The fresh air is taken from the outside atmosphere and the recirculatedair is the
return air housing. The air is distributed to the ships during their stay in the port or
during navigation .
Air is recirculatednormally on tankers during port stay or during sailing when any
cargo or IG related operations are on to prevent cargo vapoursfrom entering the
accommodation spaces.
81

82. Refrigeration and Air Con
Now as the accommodation temperature starts going up above the desired set point
the thermostat energizes the liquid line magnet valve, the suction pressure now
goes up as the gas starts coming to the compressor, the compressor then
immediately starts on L.P cut in thereby the cooling now again commences, this
cycle is repeated to maintain the desired temperature in the accommodation
spaces.
Thermostats are normally located in the air handling unit; they sense the temperature
of Return air. Alternately they are also located in one of the cabins on the top deck.
Normally in warm weather area the AC plant is always running at full load, ideally
the air conditioning plant will start unloading when the ship goes into colder
weather. The thermostat control will come into action only when further drop in
temperature takes place, and this will stop the air-con compressor
82

83. Refrigeration and Air Con
Athermostatis the component of an Air-con system which regulates the temperature
of the space to be cooled so the temperature is maintained near a desired set point
temperature. The thermostat does this by indirectly switching the compressor on
or off, to maintain the correct temperature. As long as the desired temperature in
the accommodation space is not reached the Air con compressor keeps working
and thereby cooling the accommodation, when the desired temperature is reached
the thermostat actuates and closes the liquid line magnet valve(solenoid valve)
located on liquid line after the condenser, the compressor then eventually stops on
low suction pressure cut off. However the air handling unit blower keeps running all
the time.
83

84. Refrigeration and Air Con
Compressor Starting Unloader
Solenoid valves are used in conjunction with servo valve to operate the opening and
closing of the suction valve. It is fitted on top of the cylinder (near suction valve).
During energized position, the solenoid closes the access between the two
cylinders or stages in the compressor, by keep the suction valve open and bypassing
the hot discharge gas directly to the suction line making the pressure of the unit as
zero bar, and reducing the capacity of the compressor by half.
84

85. Refrigeration and Air Con
Capacity control valve-Solenoid Operated
Capacity control or regulation in refrigeration compressors enables the refrigeration
plant to run at part loads according to the demand of the refrigerant in the system.
Capacity control is obtained when the solenoid valve, fitted in the top cover, closesthe
access to the two cylinders, positioned under the same top cover. This makes
theinlet pressure to the cylinder drop to zero bar. At the same time the compressor
capacityis reduced to 50%. However, a little gas will be flowing through the closed
solenoidvalve, hereby ensuring the necessary cooling and lubrication of the
cylinders.This capacity control controlpermits a certain reduction in power
consumption.
With the solenoid valve de-energized, the gas ports in the valve plate and cylinder
head are open.
The only disadvantage of this type is that the spring in the solenoid valve may
malfunction and its operation is affected by high variation in temperatures.
85

86. Refrigeration and Air Con
Capacity control valve-Oil Operated
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.
Capacity control of an air-conditioning plant can be defined as a system which controls
the output of the plant as per the load in demand. Refrigerating capacity control
with reciprocating compressors running at constant speed consists of controlling
the quantity of gas delivered to match the fluctuating load .
Holding the valves open: This is the most common method used in unloading in multi
cylinder V & W type compressors. It is accomplished by lifting of suction valves,
usually of 2 cylinders together by means of push pins. When the suction valve is
lifted the gas drawn during suction stroke is pushed back into the suction line
during the upward stroke of the piston. No work is done except frictional work
during such idling.
The push pins are operated by oil pressure. More and more cylinders are unloaded as
the suction pressure or evaporator temperature continues to drop.
86

87. Refrigeration and Air Con
Capacity Control oil operated
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.
87

88. Refrigeration and Air Con
Capacity Control oil operated
The compressor lube oil pump supplies oil to all the bearings, and one connection is
provided to the capacity control valve.
The capacity control valve is provided with high-pressure oil from the lube oil supply
pump of the compressor. This valve had several grooves bored into its periphery
and connected to the unloadermechanism of different units.
A spring piston is provided which controls the spreading of high-pressure oil supply
into the bore chamber. The spring piston is pressed by the oil supplied through an
orifice which pushes the piston and aligns the un-loader holes, providing high-
pressure oil to the unloaderunit.
The un-loader assembly comprises a un-loader piston held by a spring. The un-loader
piston is connected to a rotating cam ring having lifting pins attached to the suction
valve. The lifting pins always act on the suction valve i.e. un-loading the unit at stop
condition.
When the bores on control valve align with the unloaderbores, oil will pass and press
the un-loader piston rotating the cam and releasing the un loader pins from the
suction valve.
88

89. Refrigeration and Air Con
Capacity Control oil operated
89

90. Refrigeration and Air Con
Capacity Control oil operated
The capacity control regulating valve is responsible for controlling the pressure
(opening and closing of capacity control valve ports with un-loader ports). Its one
end is connected to the crankcase and another end to the capacity control valve.
As the pressure in the crankcase drops due to a reduction in load, oil in the capacity
control valve is drained into the crank case leading to the closing of un-loader
ports, lifting of the suction valve, and cutting of the cylinder unit.
This likewise means that all such cylinders are unloaded at start up releasing the
unnecessary load on the motor during the starting period.
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91. Refrigeration and Air Con
Capacity Control oil operated
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92. Refrigeration and Air Con
Recharging Of Refrigerant -First of all we should know that why the requirement of recharging
of refrigerant came. The main reason can be leakage of refrigerant. hence before recharging
leakage test is a must and once the leakage is found and rectified then only the recharging
should be initiated:
Pumping Down: Close the outlet of the recieverand keep the compressor running. let it run till
the L.P cut out is not initiated. This way gas get collected in reciever. Once the cut out occur
wait for a while and let the compressor start again. keep on collecting the gas in recieveruntill
the suction pressure gauge shows all most zero bar. Now close the suction and discharge valve
and check the sight glass of recieverto check the amount of refrigerant in the reciever. if the
gauge is one-third full then no need of charging but if it is less the charging is required.
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93. Refrigeration and Air Con
Gas Charging
While in operation, the refrigerant used in the refer plant gets consumed or is reduced in quantity
because of leakage in the system. Reduction in quantity of refrigerant may lead to troubles in
the plant such as-
Short Cycling of Compressor
Too low suction pressure
Difficult to maintain temperature of rooms and holds
Reduction in the efficiency of the plant
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94. Refrigeration and Air Con
Gas Charging of Refrigeration Plant:
For gas charging, a special T piece valve block with mounted pressure gauge is provided to
combine three connectors inter-connecting:
-Vacuum pump
-Charging Cylinder
-Charging Point
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95. Refrigeration and Air Con
Following steps are to be taken for charging gas into the reefer plant:
1. Connect gas bottle or charging cylinder, vacuum pump and charging point in the reefer system
to the valve block.
2. The discharge of the vacuum pump is to be connected in the empty recovery bottle
3. First open the valve between vacuum pump and charging bottle located in the valve block
without opening the main valve of the charging cylinder. This will remove all the air inside the
pipe. Once vacuum is reached, close the valve of charge cylinder in the valve block
4. Now open the valve of the charging point pipe in the valve block and run the vacuum pump
until the vacuum is reached. This will remove the trapped air from this pipe. Then shut the
valve in the valve block
5. Now keep the system idle for 5 minutes to check there is no pressure drop. This will ensure
there are no leakages in the system
6. Now open charging bottle pipe valve and the charging point pipe valve located in the valve
block. This will set the line for charging. Ensure that the vacuum pump valve is shut
7. Now open the main valves in the charging cylinder and charging point of the reefer system
8. Do not overfill the system. Make sure the receiver has 5 % space for expansion
Ensure that no refrigerant is leaked out in the environment as these effects the ozone layer in the
atmosphere.
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96. Refrigeration and Air Con
Replenishing Luboil in Fridge Compressor.
Oil may be added to the crankcase of larger compressors simply by pumping down and reducing
the crankcase pressure to enable oil to be poured when the filler cap has been removed.
However, filler holes tend to be small on some compressors and an oil pump may be used.
The oil charging pump is similar to a cycle pump and needs no explanation.
Change the compressor to manual running.
Pumping downthe 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.Ointo crank case.
Then stop the compressor and close compressor discharge valve. Then cool down
therefrigerant.
Too much oil in a crankcase can cause damage to a compressor by creating a dynamic pressure
during operation
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