Refrigerants and Components of refrigerants.ppt

Refrigerants and Components of refrigerants

1. Introduction to P-h Charts
Refrigerants & Components

2. Introduction to P-h Charts

3. Introduction to P-h Charts

4. Entropy Lines

5. Introduction to P-h Charts
 Entropy lines are lines with a fairly steep, upward
slope
 These lines will have a more upward slope than
volume
 The lower the enthalpy, the more vertical the entropy
lines become
 The entropy lines don't change much when they enter
the liquid-vapour dome

7. Introduction to P-h Charts
 Volume lines are upward sloping lines
 The volume lines are usually flatter in slope than
entropy lines
 At lower enthalpies, the volume lines become
steeper
 The volume values will not change that much from
line to line at lower enthalpies
 This is because liquids and solids are relatively
incompressible when compared to vapour
 In the liquid-vapour dome, the volume lines are
usually not listed

8. Temperature

9. Ideal VC cycle on P-h
Diagram
Introduction to P-h Charts
Skeleton P-h Diagram
for water

10. Introduction to P-h Charts
P-h Diagram for R-12

11. Refrigerants
Refrigerants & Components

12. Selecting the Right Refrigerant
Refrigerants are substances that are used to absorb and transport heat for
the purpose of cooling. Requirements for a refrigerant are as follows:
a.High latent heat of vaporization
b.Non-corrosive, non-toxic and non-flammable (e.g. Ammonia is
toxic & flammable)
c.Critical temperature should be outside working range
d.Reasonable working pressure
e.Compatibility with component materials (e.g. Ammonia & copper
reaction
f.Stability

13. Selecting the Right Refrigerant
g.Low Cost
h.Availability
i.Environmental impact
j.Ease of leak detection
k.Mixes well with oil
Examples:
R-12, R-134a, R-22
Propane, Ethane
Ammonia R-717, Water R-718, Carbon dioxide R-744

14. Montreal Protocol (1987): CFC/halon/bromide production phase-out by
1995
Selecting the Right Refrigerant
Refrigerant emissions were about 10% of total.
Kyoto Protocol (1992): At UNCED, established legally binding
commitments for the reduction of six greenhouse gases (carbon dioxide,
methane, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons, and
perfluorocarbons)
Q: Each kWh of electricity used in Pakistan produces how much kg of CO
2?

15. Selecting the Right Refrigerant

16. Selecting the Right Refrigerant

17. Selecting the Right Refrigerant
Overall GWP =
Total equivalent
warming impact
(TEWI)
CO
2 emission
largest
contributor
Focus on
efficiency of Ref.
system and, thus,
refrigerant

18. The R-# numbering system was developed by DuPont and
systematically identifies the molecular structure of refrigerants
made with a single halogenated hydrocarbon. The meaning of
the codes is as follows:
Numbering
•Rightmost digit: Number of fluorine atoms per molecule.
•Tens digit: One plus the number of hydrogen atoms per molecule.
•Hundreds digit: The number of carbon atoms minus one. Omitted
for methyl halides, which have only one carbon atom.
•Thousands digit: Number of double bonds in the molecule. This is
omitted when zero, and in practice is rarely used, since most
candidate compounds are unstable.
•A suffix with a capital B and a number indicates the number of
bromine atoms, when present. This is rarely used.
•Remaining bonds not accounted for are occupied by chlorine
atoms.

19. Numbering
•A suffix of a lower-case letter a, b, or c indicates increasingly
unbalanced isomers.
•Special cases:
 R-400 & R-500 series made up of zeotropic & azeotropic
blends.
R-700 series is made up of natural fluids (e.g. CO
2 (R744),
Ammonia (R717) etc.)
e.g., R-134a has 4 fluorine atoms, 2 hydrogen atoms, 2 carbon atoms,
with an empirical formula of tetrafluoroethane. The "a" suffix indicates
that the isomer is unbalanced by one atom, giving 1,1,1,2-
Tetrafluoroethane. R-134 without the "a" suffix would have a molecular
structure of 1,1,2,2-Tetrafluoroethane — a compound not especially
effective as a refrigerant.

20. Compressors
Refrigerants & Components

21. Air CompressorsAir Compressors Gas CompressorsGas Compressors
(gases other than air)(gases other than air)
Widely used to supplyWidely used to supply
compressed air as ancompressed air as an
energy sourceenergy source
Used in petroleum,Used in petroleum,
plastics and plastics and
chemical industrieschemical industries
Used in refrigerationUsed in refrigeration
and air conditioningand air conditioning
systemssystems
RefrigerantRefrigerant
CompressorsCompressors
General: Types and uses

22. General: Types
Compressors may be of the positive displacement or dynamic type. Positive Positive
displacement compressorsdisplacement compressors trap a volume of gas in an enclosed space and increase
the pressure by reducing the volume of the space while Dynamic compressorsDynamic compressors
accelerate a gas and increase its kinetic energy & this kinetic energy is
converted into pressure.
•Positive displacement compressorsPositive displacement compressors can be classified into two main
groups:
–reciprocating compressorsreciprocating compressors - including trunk, crosshead, labyrinth
and diaphragm types
–rotary compressorsrotary compressors - including vane, screw, liquid ring, and lobe
rotor types
•Dynamic compressorsDynamic compressors
–include centrifugal flow and axial flow machines

23. Positive Displacement Compressor: General
The general form of positive displacement compressor is the piston type, being
adaptable in size, number of cylinders, speed and method of drive. It works on the
two-stroke cycle (see Figure).

24. In multicylinder compressors, four, six and eight cylinders are common for large
systems and, for small systems, it will be of two, three or four cylinders.
• Piston compressors may be generally
classified by the type of valve.
• Lower loads require reduction in
capacity and can be achieved in the form
of bypass ports or by speed control (when
steam driven).
• Cold suction gas provides sufficient
cooling for small machines. Water-cooled
cylinder heads used where high discharge
temperatures are involved.
• Lubricant supply is the minimum
consistent with efficient lubrication for
cylinder lubrication whereas maximum
viscosity is consistent with efficient
lubrication for bearing lubrication.
Positive Displacement Compressors

25. Positive Displacement Compressors

26. LABYRINTH PISTON
LABYRINTH
PISTON

27. Dynamic Compressor: General
• The most common type is the centrifugal compressor.
• Suction gas enters axially into the eye of a rotor which has curved blades, and is
thrown out tangentially from the blade circumference.
• Gas may be compressed in two or more stages.
• General method for capacity reduction is to throttle the flow into the impeller.

28. Refrigerator Oils
•Lubrication
–of the compressor to minimize friction and wear
•Sealing
–between working parts of the compressor
•Cooling
–of the compressor bearings and casing
•Noise reduction
–of noise generated by the moving parts of the
compressor
•Electrical insulation
–of the motor in hermetically-sealed compressors
Functions of a refrigerator oil can be as follows:

29. Refrigerator Oils
What properties should refrigerator oils have?
•Viscosity
–must be sufficiently viscous to lubricate the compressor effectively and
provide adequate sealing where required
–common refrigerant gases are extremely soluble in oil and may produce a
significant decrease in oil viscosity
•Miscibility
–in sealed systems, the oil must be completely miscible with the refrigerant
–if the oil/refrigerant separates into immiscible layers the efficiency of
refrigeration and lubrication will be impaired
–there is a risk that the system will become blocked, and the compressor
may become starved of oil
•Low temperature properties
–the oil must not form waxy deposits in the cold parts of the circuit
–this would lead to a reduction of heat transfer and could lead to oil
starvation of the compressor

30. Refrigerator Oils
•Thermal stabilityThermal stability
–although the temperatures in refrigerator systems are not
normally as high as in air compression systems, the oil must not
break down to form coke-like deposits, e.g. on compressor
chambers and valves
•Chemical stabilityChemical stability
–the oil must not react with the refrigerant, or act as a medium for
reaction between the refrigerant and other system components
–in the presence of small amounts of air, moisture and other
impurities, unsuitable oils may react with refrigerants to form
sludges
•CompatibilityCompatibility
–the oil must be compatible with the materials used in the
refrigeration system, e.g. electrical insulation, varnishes,
elastomers, polymers etc.

31. Condensers
Refrigerants & Components

32. Condenser: Introduction
The heat-rejection ratio (HRR) is defined as the ratio of the rate of heat rejected
at the condenser to that absorbed at the evaporator.
rate of heat rejected at condenser
rate of heat absorbed at evaporator
HRR =
The designer and operator of the refrigeration system will usually characterize plant
size by the refrigeration capacity.
Ignoring small heat gains and losses,
Condenser load = Evaporator load + Compressor power
Condenser rating is stated as the rate of heat rejection. Some manufacturers give
ratings in terms of the evaporator load,
Condenser load = Evaporator load × factor

33. Condenser: Types
The three main types of condensers used in general refrigeration systems are:
(air-cooled)
(water-cooled)
(evaporative)

34. Condenser Types: Air-cooled
To provide a comparison, some of the characteristics of each are enumerated:
Air-cooled condenser:
• Usually lowest first cost of the three, and least maintenance cost as well, because no
water circulates or evaporates.
• Example: condenser of the domestic refrigerator. Above this size, the flow of air
over the condenser surface will be by forced convection, i.e. fans.
• Extended surfaces are almost always used (A
o/A
i = 5 – 10).
• Inlet at the top and outlet at the bottom (gravity-assisted flow).
• The flow of air may be vertically upwards or horizontal.
• Where a single fan would be too big, multiple smaller fans give the advantages of
lower tip speed and noise, and flexibility of operation in winter.
• The low specific heat capacity and high specific volume of air implies a large
volume to remove the condenser heat.
• In practice, the temperature rise of the air is kept between 9 and 12 K.
• Materials of construction are aluminium fins on stainless steel tube for ammonia, or
aluminium or copper fins on aluminium or copper tube for the halocarbons.
• Used in desert areas where the supply of cooling water is unreliable.
Q: On average, approx. what’s the air flow rate if 430 kW is rejected by a condenser?

35. Condenser Types: Water-cooled
Water-cooled condenser with cooling tower:
• Lower condensing temperature than with an air-cooled condenser, because the wet-
bulb rather than the dry-bulb temperature of the air is the sink toward which the
condensing temperature drives.
• The higher heat capacity and lower specific volume of water make it an ideal
medium for condenser cooling.
• General form is shell-and-tube having the water in the tubes.
• Materials can be selected for the application and refrigerant, but all mild steel is
common for fresh water, with cupronickel or aluminium brass tubes for salt water.
• Some condensers have two separate water circuits (double bundle), using the
warmed water from one circuit as reclaimed heat in another part of the system.
• The supply of water is usually limited and requires the use of a cooling tower.
Optional sources may include ground water or industrial water.
Q: By comparison, what’s the flow rate now for the condenser?

36. Condenser Types: Evaporative
Evaporative condenser:
• Compact and provides lower condensing temperatures than the air-cooled
condenser as well as the water-cooled condenser/cooling tower combination.
• The mass flow of water over the condenser tubes must be enough to ensure wetting
of the tube surface, and will be of the order of 80–160 times the quantity evaporated.
• Evaporative condensers have a higher resistance to air flow than cooling towers.
• Most types use forced draught fans.
• Evaporative condensers may freeze in winter. A common arrangement is to switch
off the fan(s) with a thermostat, to prevent the formation of ice. The water-collection
tank will have an immersion heater or the tank may be located inside the building
under the tower structure.
• Materials of construction must be corrosion resistant.
• The atmospheric condenser is a simplified form of evaporative condenser, having
plain tubes over a collecting tank and relying only on natural air draught. This will be
located on an open roof or large open space to ensure a good flow of air. Space
required is of the order of 0.2 m
2
/kW.

37. Evaporators
Refrigerants & Components

38. Classified according to refrigerant flow pattern and function. The refrigerant
flow pattern is usually dependent on the method of ensuring oil removal from
evaporator
Flooded evaporators (see Figure) have a body of fluid boiling in a random
manner, the vapour leaving at the top. In the case of ammonia, any oil present
will fall to the bottom and be drawn off from the drain pot or oil drain
connection. With halocarbons, a proportion of the fluid is bled off and rectified
Dry expansion Evaporators which keep the oil moving by means of continuous
fluid velocity, until it gets back to the compressor suction, are termed dry
expansion . In these, the refrigerant is totally evaporated.
Evaporators: Classification
The function of the evaporator will be to cool gas, liquid or
other product load. In most cases, air or a liquid is first
cooled, and this is then used to cool the load. For example,
in a cold-room, air is cooled and this air cools the stored
produce and carries away heat leaking through the
structure; in a water chiller system, the water is circulated
to cool the load, etc.

39. • Air cooling evaporators for cold-rooms, air-conditioning, etc., will have finned pipe
coils.
• In all but very small coolers, there will be fans to blow the air over the coil.
• Construction materials will be the same as for air-cooled condensers. Aluminium
fins on copper tube are the most common for the halocarbons, with stainless steel or
aluminium tube for ammonia.
• Frost or condensed water will form on the fin surface and must be drained away. To
permit this, fins will be vertical and the air flow horizontal, with a drain tray provided
under.
• The size of the tube will be such that the velocity of the boiling fluid within it will
cause turbulence to promote heat transfer.
• Tube diameters will vary from 9 mm to 32 mm, according to the size of coil.
• Fin spacing will depend on factors like compactness and cost and will vary from 2-
12 mm.
• The number of air-cooling coils in operation in industrial refrigeration plants far
exceeds the number of liquid-chilling evaporators installed.
Air Cooling Evaporators

40. Mostly in shell-and-tube or shell-and-coil evaporators.
Liquid Cooling Evaporators
In the shell-and-tube type, the liquid is usually in the pipes and the shell is some
three-quarters full of the liquid, boiling refrigerant. A number of tubes is omitted at
the top of the shell to give space for the suction gas to escape clear of the surface
without entraining liquid. Further features such as multiple outlet headers, suction
trap domes and baffles will help to avoid liquid droplets entering the main suction
pipe. Gas velocities should not exceed 3 m/s and lower figures are used by some
designers. The speed of the liquid within the tubes should be about 1 m/s or more, to
promote internal turbulence for good heat transfer.

41. Evaporators of this general type with dry expansion circuits will have the refrigerant
within the tubes, in order to maintain a suitable continuous velocity for oil transport,
and the liquid in the shell. These can be made as shell-and-tube, with the refrigerant
constrained to a number of passes, or may be shell-and-coil (see Figure). In both
these configurations, baffles are needed on the water side to improve the turbulence,
and the tubes may be finned on the outside.
Liquid Cooling Evaporators

42. Liquid cooling evaporators may comprise a pipe coil in an open tank, and can have
flooded or dry expansion circuitry. Flooded coils will be connected to a combined
liquid accumulator and suction separator (usually termed the surge drum), in the form
of a horizontal or vertical drum.
Liquid Cooling Evaporators

43. Some liquids, such as vegetable fats and ice-cream mixes, increase considerably in
viscosity as they are cooled, sticking to the heat exchanger surface. Evaporators for
this duty are arranged in the form of a hollow drum surrounded by the refrigerant
and having internal rotating blades which scrape the product off as it thickens,
presenting a clean surface to the flow of product and impelling the cold paste towards
the outlet.
Liquid Cooling Evaporators
Note: There are other methodologies used as well.

44. • Plate evaporators are formed by cladding a tubular coil with sheet metal, welding
together two embossed plates, or from aluminium extrusions.
• The extended flat face may be used for air cooling, for liquid cooling if immersed in
a tank.
• The major use for flat plate evaporators is to cool a solid product by conduction, the
product being in packages and held close between a pair of adjacent plates.
• Has horizontal and vertical types.
• Some of its popularity is attributable to its compactness.
Plate Evaporators

45. Air cooling 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 labour-intensive 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.
For lower temperatures (< 4°C), heat must be applied to melt the frost within a
reasonable time and ensure that it drains away. Methods include electric resistance
heaters, hot gas and reverse cycling.
Defrosting

46. Expansion Valves & Thermostatic Control Devices
Refrigerants & Components

47. Metering Device
Throttling Device
Restrictor Device
Functions
 Reduces high pressure liquid refrigerant to low pressure liquid
refrigerant
 Maintains desired pressure difference between high & low
pressure sides
 Allows the liquid refrigerant to vaporize at desired pressure in
evaporator
 Controls the flow of refrigerant as per the load on the
evaporator
Expansion Devices

48. Fixed opening type or Variable opening type
Hand (manual) expansion valves
Capillary Tubes
Constant pressure or Automatic Expansion Valve (AEV)
Thermostatic Expansion Valve (TEV)
Electronic Expansion Valve
Float type Expansion Valve
High Side Float Valve
Low Side Float Valve
Types - Expansion Devices

49. Used in small capacity hermetic sealed refrigerating units
Domestic refrigerator
Water coolers
Room air conditioners
Freezers
Mostly a copper tube
Small internal diameter (0.5 mm – 2.25 mm)
Varying length (0.5 m – 5 m)
Capillary Tube

50. Capillary Tube

51.  Advantages
Inexpensive
No moving parts therefore no maintenance
Pressure equalizes during off-cycle between condenser & evaporator,
which reduces starting torque requirement of the motor. Hence, a
low cost motor
Ideal for hermetic compressor based systems, which are critically
charged and factory assembled.
 No receiver as the refrigerant charge is critical
 Disadvantages
Cannot adjust itself to changing flow conditions
Susceptible to clogging because of narrow bore of the tube
Capillary Tube

52. Constant pressure or Automatic Expansion Valve (AEV)

53. Thermostatic Expansion Valves (TXVs)
TXVs most widely used expansion devices
Regulates refrigerant flow rate to the evaporator according to the
degree of superheat of vapor refrigerant leaving the evaporator
Consists of
 A valve body
 A valve spring
 A diaphragm
 A sensing bulb at the outlet of the evaporator
When liquid refrigerant passes through the small opening around
valve pin, its pressure is reduced to the evaporating pressure
Liquid refrigerant vaporizes gradually as it flows inside copper tubes
At position x, as shown in the Figure (next slide), all the liquid has
been vaporized
At the outlet o of the evaporator, it is superheated to a few degrees
higher than its saturated temperature.

54. If load increases, more refrigerant vaporizes and degree of superheat increases which
increases temperature of sensing bulb. This exerts higher pressure on diaphram which
lowers the valve pin. More liquid refrigerant enters to match the increase in load. The
degree of superheat of the vapor refrigerant at the outlet can be adjusted by varying the
tension of the spring in the thermostatic expansion valve.
Thermostatic Expansion Valve

55. Used for dry-expansion circuits as there is no
liquid level that can be detected.
Sensing bulb usually contains the same
refrigerant.
Hunting refers to a condition that occurs
when a device continuously undershoots or
overshoots the control point, with resulting
fluctuation and loss of control of the
condition to be maintained. Sometimes called
cycling. May reduce refrigeration capacity.
Superheat setting of the TXV at full-load
operation is between 10 and 20 °F.
Thermostatic Expansion Valve

56. Electric, or more truly electronic or microprocessor-controlled, expansion valves can
provide more sophisticated, effective, and energy-efficient refrigerant flow controls
than thermostatic expansion valves. Currently, three types of electric expansion valves
are widely available: step motor valves (a), pulse-width-modulated valves (b), and
analog valves (c).
Electric Expansion Valve
(a) (b) (c)

57. Electric Expansion Valve
Advantages of Electric Expansion Valves
Compared to the thermostatic expansion valves, the advantages of electric expansion
valves are that they
• Provide a more precise temperature control
• Provide consistent superheat control under fluctuating head pressure
• Are able to be operated at low head pressure during lower ambient air temperature
• Are more energy-efficient
Of the three types of electric expansion valves (step motor, pulse-width-modulated,
and analog valves), the performance of step motor valves has proved superior to the
two others.

58. Thermal Electric Expansion Valve
The signal from a suitable thermistor placed at the evaporator outlet will vary,
depending on whether it senses dry refrigerant gas or traces of liquid. This can be
used directly to control the current through a thermal element to modulate the
expansion valve. This device usually has no separate adjustable controller and so
cannot be incorrectly set.

59. The (low-side) float valve controls the liquid refrigerant feed to maintain a constant
liquid level in the evaporator. As the liquid level in the evaporator drops, the float
ball moves downward and opens the float valve wider so that more liquid refrigerant is
fed to the evaporator. In small refrigeration systems, the float chamber is often placed
directly inside the evaporator or in an accumulator, instead of a separate low-side float
chamber. Operation of the low-side float valve may be continuous or intermittent.
Float Valve