Chapter 3 Vapour Compression Refrigeration Systems.pdf

By: Ali S. MEng5212 2021/22
PART
Refrigeration

By: Ali S. Refrigeration and Air Conditioning 31-Dec-21 2

Vapour Compression Refrigeration Systems
• Vapour cycles can be subdivided into vapour compression systems, vapour absorption systems,
vapour jet systems etc.
• VCRS are the most commonly used among all refrigeration systems
• In VCRS working fluid (refrigerant) undergoes phase change at least during one process.
• The refrigeration effect is produced when the refrigerant evaporates at low temperature
• It is also called mechanical refrigeration systems, b/c the input energy required to run the
compressor is in the form of mechanical energy
• Suit to almost all applications with the refrigeration capacities ranging from few Watts to few
megawatts with wide variety of refrigerants
• The actual vapour compression cycle is based on Evans-Perkins cycle, which is also called as
reverse Rankine cycle.

Comparison between gas cycles and vapor cycles
• In gas cycle, the operating cycle will be away from the vapour dome (saturation liquid and vapour curves).
• Heat rejection and refrigeration take place as the gas undergoes sensible cooling and heating
• In a vapour cycle, phase change takes place and refrigeration effect is due to the vaporization of refrigerant
liquid.
• In VCRS temperature remains constant, If the refrigerant is a pure substance during vaporization and
condensation
• But temperature glide during vaporization and condensation if the refrigerant is a zeotropic mixture (having
d/t vapor and liquid phase composition)
• The required mass flow rates for a given refrigeration capacity is much smaller compared to a gas cycle,
because large amount of heat (latent heat) can be transferred per kg of refrigerant at a near constant
temperature during phase change.

Advantage and dis-advantage of VCRS over Air refrigeration system
Advantages:
• Smaller size for a given capacity of refrigeration
• Has less running cost
• Can be employed over a large range of temperature
• Quite high COP
Dis advantages:
• High initial cost (components eg. evaporator)
• Prevention of leakage of refrigerant, w/h is a major problem in VCRS

Standard Vapour Compression Refrigeration System (VCRS)
• Is Based on a theoretical cycle called Evans-Perkins or reverse Rankine cycle w/h has two
isobaric and two isentropic processes.
• Process 3-4: Isenthalpic expansion of saturated liquid in expansion device.

Analysis of standard VCRS
• Assumption for analysis of VCRS
• Steady flow;
• Negligible kinetic and potential energy changes across each component, and
• No heat transfer in connecting pipe lines
1. Evaporator: Heat transfer rate at evaporator or refrigeration capacity
ሶ
𝑄 = 𝑚𝑟 (h1-h4)
2. Compressor: Power input to the compression
𝑊𝑐= 𝑚𝑟(h2-h1)
3. Condenser : Heat transfer rate at condenser Condenser pressure Pc
Refrigeration effect

Cont…
4. Expansion device: h3=h4, and the exit condition is two phase region, hence from the definition of quality (dryness
fraction), ℎ4=(1-𝑋4)ℎ𝑓,𝑒+𝑋4ℎ𝑔,𝑒=ℎ4 + 𝑋4ℎ𝑓𝑔 where;
• X4 is the quality at point 4,
• ℎ𝑓𝑒, ℎ𝑔,𝑒, ℎ𝑓𝑔, are the saturated liquid enthalpy, saturated vapour enthalpy and latent heat of vaporization at evaporator
pressure, respectively
• At compressor inlet, 𝑚r, 𝑖𝑛𝑡𝑒𝑟𝑚𝑠 𝑜𝑓 volume flow rate(w/h indicates com. size) and specific volume.
• Once all the state points are known, then from the required refrigeration capacity and various enthalpies one can
obtain the required refrigerant mass flow rate, volumetric flow rate at compressor inlet, COP, cycle efficiency etc.

Irreversibilities in VCRC
Standard vapour compression refrigeration cycle introduces two irreversibilities
1. Irreversibility due to isenthalpic throttling (process 3- 4) or Throttling loss
2. Irreversibility due to non-isothermal heat rejection (process 2-3) or superheat horn
• Due to these irreversibilities, the cooling effect reduces and work input increases
• As a result the theoretical COP of standard VCR cycle is smaller than that of a Carnot
system for the same heat source and sink temperatures.

Comparison between Carnot (dry compression) and standard VCRS
• The reduction in refrigeration effect (replacing Isentropic expansion by Iso-enthalpic
throttling) is equal to the area d-4-4’-c-d (area A2) and is known as throttling loss.

Cont…
• The increases in heat rejection of VCRS cycle, compared to Carnot cycle by area 2’2’’2=A1
is called superheat horn (due to replacement of isothermal by isobaric)
•
,Which is increased by A1 Or superheat horn
,Which is increased by A1 and A2

Cont…
• Superheat losses and throttling losses depend very much on the shape of the vapor dome on T-s diagram.
• And also the shape of the saturation curves depends on the nature of refrigerant
• Type 1: symmetrical saturation curves or both losses are significant, Ammonia, CO2 and water
• Type 2: CFC11, CFC12, HFC134a, small superheat losses, but large throttling losses
• Type 3: CFC113, CFC114, CFC115, iso-butane, do not have any superheat losses(danger of wet
compression)

Cont…
➢ Generally,
• The superheat loss increases only the work input to the compressor, it does
not affect the refrigeration effect.
• In heat pumps superheat is not a loss, but a part of the useful heating effect.
• However, the process of throttling is inherently irreversible, and it
increases the work input and also reduces the refrigeration effect.

Sub-cooling and superheating
• Reducing sink temperature lower than condensing temp by adding extra area for heat transfer, is called
sub-cooling (w/h is in the sub-cooled liquid region)
• Increasing temp. of heat source a few degrees higher than the evaporator temp., so that the vapour at the
exit of the evaporator can be superheated is called superheating
• Usefull superheating:- super-heating of refrigerant due to heat transfer within the refrigerated space
• Useless superheating:- super-heating of refrigerant vapor by exchanging heat with the surroundings as it
flows through the connecting pipelines.
• Sub-cooling is beneficial w/h increases refrigeration effect by reducing throttling loss with no additional
specific work input
• And only liquid enters into the throttling device leading to its efficient operation by reducing pressure drop in
the evaporator.

Cont…
• Throttling loss with sub-cooling a-4”-4’-b
• Throttling loss without sub-cooling the b-4’-4-c
• Refrigeration effect increases by an amount equal to (h4-h4’) = (h3-h3’)

Cont…
• With useful superheating, the refrigeration effect, specific volume at the inlet to the
compressor and work of compression increase.
• COP may or may not increase with superheat, depends on the relative increase in
refrigeration effect and work of compression.
• Superheat is desirable to prevents the entry of liquid droplets into the compressor.

liquid-suction heat exchanger (LSHX)
• Sub-cooling and superheating can be achieved by simply increasing heat transfer area if the temperature
difference is large enough b/n condenser and heat sink and b/n evaporator and heat source.
• If change in temp is not sufficient, the required amount of subcooling and superheating is attained by LSHX.
• LSHX is a counter-flow heat exchanger in which the warm refrigerant liquid from the condenser
exchanges heat with the cool refrigerant vapour from the evaporator.
LSHX effect

Cont…
• Heat transferred between the refrigerant liquid and vapour in the LSHX
• If we assume that there is no heat exchange between the surroundings and the LSHX and negligible kinetic and potential energy changes
across the LSHX, then, the heat transferred between the refrigerant liquid and vapour in the LSHX, QLSHX is given by:
• If we take average values of specific heats for the vapour and liquid, then we can write the above equation as;
• This means that, the degree of subcooling (T3-T4) will always be less than the degree of superheating, (T1-T6).

Cont…
• If we define the effectiveness of the LSHX, εLSHX as the ratio of actual heat transfer rate in the LSHX to maximum
possible heat transfer rate, then:
• The maximum possible heat transfer rate is equal to ,
• Because the vapour has a lower thermal capacity, hence only it can attain the maximum possible temperature
difference, which is equal to T3-T6.
• If we have a perfect LSHX with 100 percent effectiveness (εLSHX = 1.0), then from the above discussion it is clear
that the temperature of the refrigerant vapour at the exit of LSHX will be equal to the condensing temperature, Tc,
i.e., T1= T3= Tc.
• This gives rise to the possibility of an interesting cycle called as Grindley cycle, wherein the isentropic
compression process can be replaced by an isothermal compression leading to improved COP.

Actual VCRS
Ideal cycles are internally reversible and no change of refrigerant state takes place in the
connecting pipelines
Irreversibilities in actual VCRS
1. Pressure drops in evaporator, condenser and LSHX
2. Pressure drop across suction and discharge valves of the compressor
3. Heat transfer in compressor
4. Pressure drop and heat transfer in connecting pipe lines

𝐶𝑂𝑃𝑎𝑐𝑡 = 𝜂𝑐𝑦𝑐𝜂𝑖𝑠𝜂𝑚𝑜𝑡𝐶𝑂𝑃𝑐𝑎𝑟
An approximate expression for cycle efficiency (ηcyc) in the evaporator
temperature range of –50oC to +40oC and condensing temperature range of
+10oC to +60oC for refrigerants such as ammonia, R 12 and R 22 is suggested
by Linge in 1966.

Modification of the SSS (Standard Single Stage) cycle
• The single stage cycle can be sufficient for small temperature difference between condenser and
evaporator (temperature lift).
• But in some applications the temperature difference becomes too large either due to very low temperature requirement of
the refrigerated space or availability of relatively high temperature heat sink.
• In case of fluorocarbon and ammonia based refrigeration systems
• Single stage system is used up to Te of –30 ⁰C
• A two-stage system is used up to –60 ⁰C and
• Three-stage system is used for temperatures below –60 ⁰C
• Not only at high temperature lift applications, but also multi-stage systems are used in applications requiring refrigeration
at different temperatures
• Example, in a dairy plant refrigeration may be required at –30 ⁰C for making ice cream and at 2 ⁰C for chilling milk.

Cont…
➢ The viable modified cycles are multi – stage cycles operating with two or more low side pressures.
1. Multi – compression
2. Multi – evaporator
3. Cascade system
Effect of evaporator temperature on cycle performance (T-s and P-h diagram)
• When evaporator temperature decreases significantly,
the SSS cycle performance significantly reduces due
to:
• Increase in throttling loss
• Increase in superheat horn loss
• Increase in compressor discharge temperature
• Quality of the vapour at the inlet to the
evaporator increase
• Specific volume at the inlet of the compressor
increases, which increases the volumetric work
of compression requiring larger compressor.

Multi – compression system
• Two concepts which are normally integral to multi-pressure systems are,
i. Flash gas removal, and
ii. Intercooling .
Flash gas removal using flash tank
Flash gas: is a high quality vapour at the inlet to the evaporator which is developed during the throttling of high
temperature lift applications,
✓ Flash gas doesn’t contribute to the refrigeration effect.
✓ It increases the pressure drop in the evaporator.
✓ Since it has to be compressed to condenser pressure flash gas increases Wcomp
✓ Hence it is possible to increase COP by removing the flash gas w/h can be done by using the flash tank.
✓ A flash tank is a pressure vessel, wherein the refrigerant liquid and vapour are separated at an intermediate
pressure Pi.

Flash Gas Removal
• Working principle of a flash tank • Expansion process using a flash tank on P-h diagram

Intercooling in multi-stage compression
Intercooling using liquid refrigerant in flash tank Intercooling using external water cooled heat exchanger
• It is the reduction of compressor work by reducing the exit temperature, 𝒘 = − ‫׬‬ 𝒗𝒅𝑷 and form 𝑷𝒗 = 𝑹𝑻 implies as T
decreases, 𝑣 (specific volume) and hence work w will decrease.

Multi-stage System with Flash Gas Removal and Intercooling
• This system incorporates both flash tank and water intercooler to ensure complete de–superheating of the refrigerant as
bubbling of the superheated refrigerant in the flash tank may not give sufficient intercooling.
Two-stage vapour compression refrigeration system
with flash gas removal using a flash tank and
intercooling
Two-stage vapour compression refrigeration system
with flash gas removal using a flash tank and
intercooling– P-h diagram

Analysis of the performance of the system
By applying mass and energy balance equations to the individual components the performance of the system can be
determined.
Assumptions
✓ Flash tank is perfectly insulated
✓ PE and KE changes of refrigerant across each component are negligible.
1. Mass and energy balance of the flash tank and expansion valve
2. Mass and energy balance of evaporator & low stage compressor

Cont…
3. Mass and energy balance across water-cooled intercooler
Where 𝑄1 is heat transferred by the refrigerant to the cooling water in the
intercooler and 𝑚1 is mass flow rate of the refrigerant in low stage compressor.
4. Mass and energy balance across high-stage compressor
Where 𝑚𝐼𝐼 is mass flow rate of the refrigerant in High stage compressor.
5. Mass and energy balance across condenser & float valve

Cont…
• From the above equations in the flash tank we have:
• The amount of additional vapour generated due to de superheating of the refrigerant vapour from the water-cooled
intercooler is given by:
Thus the vapour generated will be zero, if the refrigerant vapour is
completely de-superheated (ℎ3=ℎ4), which may not be possible
• COP of the system

Multi-Evaporator
• Applies where refrigeration is required at different temperatures.
Multi-evaporator system with single compressor and individual
expansion valves
h1

Cont…
Multi-evaporator system with single compressor and multiple expansion valves
h1

Cont…
• Multi-evaporator system with individual compressors and multiple expansion valves

Cont…
• Multi-evaporator system with multiple compressors and a flash tank for flash gas removal and intercooling
(h1-h8)
Mass -
Energy
Balance

Cascade Systems
• Some industrial applications require moderately low temperatures, and the temperature range they involve may be too large for a single
vapor-compression refrigeration cycle to be practical. The solution is cascading.
• Cascading improves the COP of a refrigeration system.
Applied in:
• Liquefaction of petroleum vapours
• Liquefaction of industrial gases
• Manufacturing of dry ice
• Deep freezing etc
✓ Advantages of cascade system
• Since each cascade uses a different refrigerant, it is possible to select a refrigerant that is best suited for that particular temperature range.
• Very high or very low pressures can be avoided
• Migration of lubricating oil from one compressor to the other is prevented
•

Cont…

Examples
1. In a R22 based refrigeration system, a liquid-to-suction heat exchanger (LSHX) with an effectiveness of 0.65 is used. The
evaporating and condensing temperatures are 7.2 ⁰C and 54.4 ⁰C respectively. Assuming the compression process to be
isentropic, find:
a) Specific refrigeration effect
b) Volumic refrigeration effect
c) Specific work of compression
d) COP of the system
e) Temperature of vapour at the exit of the compressor
Comment on the use of LSHX by comparing the performance of the system with a SSS cycle operating between the same
evaporator and condensing temperatures.
Solution
Given: Refrigerant : R 22
Te = 7.2 ⁰C
Tc = 54.4 ⁰C
Effectiveness of LSHX, ƐX = 0.65

Cont…

3

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