A basic introduction to thermodynamic cycles in refrigeration systems and refrigerents

1. REFRIGERATION SYSTEMS
VAPOR REFRIGERATION SYSTEMS
The purpose of a refrigeration system is to maintain a system at a temperature
below the temperature of its surroundings.
CARNOT REFRIGERATION CYCLE
Carnot vapor refrigeration cycle
 The refrigerant enters the evaporator as a two­phase liquid–vapor mixture
at state 4. In the evaporator some of the refrigerant changes phase from
liquid to vapor as a result of heat transfer from the region at temperature
TC to the refrigerant. The temperature and pressure of the refrigerant
remain constant during the process from state 4 to state 1.
 The refrigerant is then compressed adiabatically from state 1, where it is a
two­phase liquid–vapor mixture, to state 2, where it is a saturated vapor.
During this process, the temperature of the refrigerant increases from TC to
TH, and the pressure also increases.

2.  The refrigerant passes from the compressor into the condenser, where it
changes phase from saturated vapor to saturated liquid as a result of heat
transfer to the region at temperature TH. The temperature and pressure
remain constant in the process from state 2 to state 3
 The refrigerant returns to the state at the inlet of the evaporator by
expanding adiabatically through a turbine. In this process, from state 3 to
state 4, the temperature decreases from TH to TC, and there is a decrease in
pressure.
Area 1–a–b–4–1 is the heat added to the refrigerant from the cold region per unit
mass of refrigerant flowing. Area 2–a–b–3–2 is the heat rejected from the
refrigerant to the warm region per unit mass of refrigerant flowing. The enclosed
area 1–2–3–4–1 is the net heat transfer from the refrigerant. The net heat transfer
from the refrigerant equals the net work done on the refrigerant. The net work is
the difference between the compressor work input and the turbine work output
COP­ The coefficient of performance β of any refrigeration cycle is the ratio of the
refrigeration effect to the net work input required to achieve that effect.
DEPARTURES FROM THE CARNOT CYCLE
Actual vapor refrigeration systems depart significantly from the Carnot cycle and
have coefficients of performance lower than Carnot.
The ways actual systems depart from the Carnot cycle are­
In actual systems, these heat transfers are not accomplished reversibly as
presumed above. To achieve a rate of heat transfer sufficient to maintain the
temperature of the cold region at TC requires the temperature of the refrigerant in
the evaporator T ’C, to be several degrees below TC. Similarly, to obtain a
sufficient heat transfer rate from the refrigerant to the warm region requires that
the refrigerant temperature in the condenser, T ‘H, be several degrees above TH.

3. Maintaining the refrigerant temperatures in the heat exchangers at T ‘C and T ‘H
rather than at TC and TH, respectively, has the effect of reducing the COP.
This conclusion about the effect of refrigerant temperature on the COP also
applies to other refrigeration cycles.
Compression process from state 1 to state 2 occurs with the refrigerant as a two­
phase liquid–vapor mixture, this is commonly referred to as wet compression. Wet
compression is normally avoided because the presence of liquid droplets in the
flowing liquid–vapor mixture can damage the compressor and this makes carnot
cycle impractical.
The expansion process from the saturated liquid state 3’ to the low­quality, two
phase liquid–vapor mixture state 4, produces a relatively small amount of work
compared to the work input in the compression process.
Ton of refrigeration is equal to 200 Btu/min or about 211 kJ/min.

4. VAPOR COMPRESSION REFRIGERATION SYSTEMS
Vapor­compression refrigeration systems are the most common refrigeration
systems in use today.
vapor compression refrigeration system
T­S CURVE FOR IDEAL VCRS

5. WORK AND HEAT TRANSFERS
 As the refrigerant passes through the evaporator, heat transfer from the
refrigerated space results in the vaporization of the refrigerant.
For a control volume, rate of heat transfer per unit mass of refrigerant
flowing­
is the mass flow rate of the refrigerant
is referred to as the refrigeration capacity.
 The refrigerant leaving the evaporator is compressed to a relatively high
pressure and temperature by the compressor.
Rate of power input per unit mass of refrigerant flowing­
 The refrigerant passes through the condenser, where the refrigerant
condenses and there is heat transfer from the refrigerant to the cooler
surroundings
The rate of heat transfer from the refrigerant per unit mass of refrigerant
flowing is­
 Refrigerant at state 3 enters the expansion valve and expands to the
evaporator pressure. This process is usually modeled as a throttling
process for which­

6. The refrigerant pressure decreases in the irreversible adiabatic expansion,
and there is an accompanying increase in specific entropy. The refrigerant
exits the valve at state 4 as a two­phase liquid–vapor mixture.
In the vapor­compression system, the net power input is equal to the compressor
power, since the expansion valve involves no power input or output
Therefore COP of VCRS is­
Dry compression is presumed
Process 1–2s Isentropic compression of the refrigerant from state 1 to the
condenser pressure at state 2s.
Process 2s–3: Heat transfer from the refrigerant as it flows at constant pressure
through the condenser
Process 3–4: Throttling process from state 3 to a two­phase liquid–vapor mixture
at 4.
Process 4–1: Heat transfer to the refrigerant as it flows at constant pressure
through the evaporator to complete the cycle
PERFORMANCE OF ACTUAL VAPOR­COMPRESSION SYSTEMS
t­s curve for actual vcrs

7. p­h curve for vcrs
Heat transfers between the refrigerant and the warm and cold regions are not
accomplished reversibly.
The refrigerant temperature in the evaporator is less than the cold region
temperature, TC, and the refrigerant temperature in the condenser is greater than
the warm region temperature, TH.
Coefficient of performance decreases as the average temperature of the refrigerant
in the evaporator decreases and as the average temperature of the refrigerant in
the condenser increases.
The effect of irreversible compression can be accounted for by using the isentropic
compressor efficiency­
Additional departures from ideality results from frictional effects that result in
pressure drops as the refrigerant flows through the evaporator, condenser, and
piping connecting the various components.

8. ABSORPTION REFRIGERATION
These cycles have some features in common with the vapor­compression cycles but
differs in following important respects­
 Instead of compressing a vapor between the evaporator and the condenser,
the refrigerant of an absorption system is absorbed by a secondary
substance, called an absorbent, to form a liquid solution. The liquid
solution is then pumped to the higher pressure. Because the average
specific volume of the liquid solution is much less than that of the
refrigerant vapor, significantly less work is required so, absorption
refrigeration systems have the advantage of relatively small work input
compared to vapor­compression systems.
 The other main difference between absorption and vapor­compression
systems is that some means must be introduced in absorption systems to
retrieve the refrigerant vapor from the liquid solution before the refrigerant
enters the condenser. This involves heat transfer from a relatively high­
temperature source. Natural gas or some other fuel can be burned to
provide the heat source, and there have been practical applications of
absorption refrigeration using alternative energy sources such as solar and
geothermal energy.
SIMPLE AMMONIA­ WATER ARS

9.  In this case, ammonia is the refrigerant and water is the absorbent.
Ammonia circulates through the condenser, expansion valve, and
evaporator as in a vapor­compression system. However, the compressor is
replaced by the absorber, pump, generator, and valve shown on the right
side of the diagram.
 In the absorber, ammonia vapor coming from the evaporator at state 1 is
absorbed by liquid water. The formation of this liquid solution is
exothermic. Since the amount of ammonia that can be dissolved in water
increases as the solution temperature decreases, cooling water is circulated
around the absorber to remove the energy released as ammonia goes into
solution and maintain the temperature in the absorber as low as possible.
The strong ammonia–water solution leaves the absorber at point “a” and
enters the pump, where its pressure is increased to that of the generator.
 In the generator, heat transfer from a high­temperature source drives
ammonia vapor out of the solution (an endothermic process), leaving a
weak ammonia–water solution in the generator. The vapor liberated passes
to the condenser at state 2, and the remaining weak solution at c flows
back to the absorber through a valve. The only work input is the power
required to operate the pump, and this is small in comparison to the work
that would be required to compress refrigerant vapor between the same
pressure levels.
MODIFIED AMMONIA–WATER ABSORPTION SYSTEM
In this cycle, a heat exchanger is included between the generator and the absorber
that allows the strong water–ammonia solution entering the generator to be
preheated by the weak solution returning from the generator to the absorber,
thereby reducing the heat transfer to the generator .
The other modification shown in the figure is the rectifier placed between the
generator and the condenser. The function of the rectifier is to remove any traces
of water from the refrigerant before it enters the condenser. This eliminates the
possibility of ice formation in the expansion valve and the evaporator.

10. modified ammonia water ars
Another type of absorption system uses lithium bromide as the absorbent and
water as the refrigerant To achieve refrigeration at lower temperatures than are
possible with water as the refrigerant, a lithium bromide–water absorption
system may be combined with another cycle using a refrigerant with good
low­temperature characteristics, such as ammonia, to form a cascade
refrigeration system.

11. GAS REFRIGERATION SYSTEMS
In gas refrigeration systems working fluid remains a gas throughout. They are
used to achieve very low temperatures for the liquefaction of air and other gases
and for other specialized applications such as aircraft cabin cooling.
BRAYTON REFRIGERATION CYCLE
 The refrigerant gas, which may be air, enters the compressor at state 1,
where the temperature is somewhat below the temperature of the cold
region, TC, and is compressed to state 2.
 The gas is then cooled to state 3, where the gas temperature approaches the
temperature of the warm region, TH.
 Next, the gas is expanded to state 4, where the temperature, T4, is well
below that of the cold region.
 Refrigeration is achieved through heat transfer from the cold region to the
gas as it passes from state 4 to state 1, completing the cycle.

12. COP is­
The magnitude of the work developed by the turbine of a Brayton refrigeration
cycle is typically significant relative to the compressor work input.
To obtain even moderate refrigeration capacities with the Brayton refrigeration
cycle, equipment capable of achieving relatively high pressures and volumetric
flow rates is needed.
For most applications involving air conditioning and for ordinary refrigeration
processes, vapor­compression systems can be built more cheaply and can operate
with higher coefficients of performance than gas refrigeration systems
Gas refrigeration systems can be used to achieve temperatures of about 21508C
(22408F), which are well below the temperatures normally obtained with vapor
systems.
BRAYTON REFRIGERATION CYCLE WITH HEAT
EXCHANGER

13. The heat exchanger allows the air exiting the compressor at state 2 to cool below
the warm region temperature TH giving a low turbine inlet temperature, T3.
Without the heat exchanger, air could be cooled only close to TH, as represented on
the figure by state a.
In the subsequent expansion through the turbine, the air achieves a much lower
temperature at state 4 than would have been possible without the heat exchanger
Accordingly, the refrigeration effect, achieved from state 4 to state b, occurs at a
correspondingly lower average temperature.
SELECTING REFRIGERANTS
Refrigerant selection for a wide range of refrigeration and air­conditioning
applications is generally based on three factors­
1) Performance­It refers to providing the required cooling or heating capacity
reliably and cost effectively
2) Safety­ It refers to avoiding hazards such as toxicity and flammability
3) Environmental impact­ It primarily refers to using refrigerants that do not
harm the stratospheric ozone layer or contribute significantly to global
climate change.
The selection of a refrigerant is based partly on the suitability of its pressure–
temperature relationship in the range of the particular application. It is generally
desirable to avoid excessively low pressures in the evaporator and excessively high
pressures in the condenser.
Other considerations in refrigerant selection include chemical stability,
corrosiveness, and cost.
The type of compressor also affects the choice of refrigerant. Centrifugal
compressors are best suited for low evaporator pressures and refrigerants with
large specific volumes at low pressure. Reciprocating compressors perform better
over large pressure ranges and are better able to handle low specific volume
refrigerants.

14. Refrigerant Types and Characteristics