Objectives
• To appreciate Thermodynamic analysis of refrigeration and
liquefaction processes.
• To describe the Carnot Refrigerator
• To define Coefficient of Performance and perform calculations for
various refrigeration cycles
• To describe practical refrigeration/liquefaction cycles
• Review the factors involved in selecting the right refrigerant for an
application.
• To be able to solve problems involving liquefaction of gases.
• To be able to compute work required for operation of refrigeration
cycles.
Refrigeration and Liquefaction
• A Refrigerator is a device operating in a cycle which maintains a body at a
temperature below that of the surroundings.
• This requires continuous absorption of heat at a low temperature level, usually
accomplished by evaporation of a liquid in a steady-state flow process.
• The vapor formed may be returned to its original liquid state for reevaporating in
either of two ways. Most commonly, it is simply compressed and then
condensed. Alternatively, it may be absorbed by a liquid of low volatility, from
which it is subsequently evaporated at higher pressure.
• Refrigeration finds large-scale industrial application, for example, in the
manufacture of ice and the dehydration of gases.
• Applications in the petroleum industry include lubricating-oil purification, low-
temperature reactions, and separation of volatile hydrocarbons.
• Liquefaction of gases
THE CARNOT REFRIGERATOR
• In a continuous refrigeration process, the heat absorbed at a low
temperature is continuously rejected to the surroundings at a higher
temperature.
• Basically, a refrigeration cycle is a reversed heat-engine cycle.
• Heat is transferred from a low temperature level to a higher one;
according to the second law, this requires an external source of energy.
• The ideal refrigerator, like the ideal heat engine, operates on a Carnot
cycle, consisting in this case of two isothermal steps in which heat IQcI is
absorbed at the lower temperature Tc and heat IQHI is rejected at the
higher temperature TH, and two adiabatic steps.
• The cycle requires the addition of net work
Ideal-Vapour compression refrigeration cycle
• The vapor-compression refrigeration cycle is the most widely used cycle for refrigerators, air-conditioning systems, and
heat pumps. It consists of four processes:
• 1-2 Isentropic compression in a compressor
• 2-3 Constant-pressure heat rejection in a condenser
• 3-4 Throttling in an expansion device
• 4-1 Constant-pressure heat absorption in an evaporator
• In an ideal vapor-compression refrigeration cycle, the refrigerant enters the compressor at state 1 as saturated vapor and is
compressed isentropically to the condenser pressure.
• The temperature of the refrigerant increases during this isentropic compression process to well above the temperature of
the surrounding medium. The refrigerant then enters the condenser as superheated vapor at state 2 and leaves as saturated
liquid at state 3 as a result of heat rejection to the surroundings.
• The temperature of the refrigerant at this state is still above the temperature of the surroundings.
• The saturated liquid refrigerant at state 3 is throttled to the evaporator pressure by passing it through an expansion valve or
capillary tube.
• The temperature of the refrigerant drops below the temperature of the refrigerated
• space during this process.
• The refrigerant enters the evaporator at state 4 as a low-quality saturated mixture, and it completely evaporates by
absorbing heat from the refrigerated space.
• The refrigerant leaves the evaporator as saturated vapor and reenters the compressor, completing the cycle.
• In a household refrigerator, the tubes in the freezer compartment where heat is absorbed by the refrigerant serves as the
evaporator.
• The coils behind the refrigerator, where heat is dissipated to the kitchen air, serve as the condenser
• The steady flow energy equation on a unit–mass basis reduces to
• ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE
• An actual vapor-compression refrigeration cycle differs from the ideal one in several ways, owing mostly to the irreversibilities that
occur in various components.
• Two common sources of irreversibilities are fluid friction (causes pressure drops) and heat transfer to or from the surroundings.
• The T-s diagram of an actual vapor-compression refrigeration cycle is shown
• In the ideal cycle, the refrigerant leaves the evaporator and enters the compressor as saturated vapor. In practice, however, it may
not be possible to control the state of the refrigerant so precisely.
• Instead, it is easier to design the system so that the refrigerant is slightly superheated at the compressor inlet. This slight overdesign
ensures that the refrigerant is completely vaporized when it enters the compressor.
• Also, the line connecting the evaporator to the compressor is usually very long; thus the pressure drop caused by fluid friction and
heat transfer from the surroundings to the refrigerant can be very significant.
• The result of superheating, heat gain in the connecting line, and pressure drops in the evaporator and the connecting line is an
increase in the specific volume, thus an increase in the power input requirements to the compressor since steady-flow work is
proportional to the specific volume.
• The compression process in the ideal cycle is internally reversible and adiabatic, and thus isentropic. The actual compression
process, however, involves frictional effects, which increase the entropy, and heat transfer, which may increase or decrease the
entropy, depending on the direction.
• Therefore, the entropy of the refrigerant may increase (process 1-2) or decrease (process 1-2’)
during an actual compression process, depending on which effects dominate.
The compression process 1-2’ may be even more desirable than the isentropic compression process
since the specific volume of the refrigerant and thus the work input requirement are smaller in this case.
• Therefore, the refrigerant should be cooled during the compression process whenever it is practical and
economical to do so.
• In the ideal case, the refrigerant is assumed to leave the condenser as saturated liquid at the compressor exit
pressure.
• In reality, however, it is unavoidable to have some pressure drop in the condenser as well as in the lines
connecting the condenser to the compressor and to the throttling valve.
• Also, it is not easy to execute the condensation process with such precision
SECOND-LAW ANALYSIS OF VAPOR
COMPRESSION
REFRIGERATION CYCLE
THE CHOICE OF REFRIGERANT
• Although the coefficient of performance of a Carnot refrigerator is independent of the refrigerant. the
irreversibilities inherent in the vapor-compression cycle cause the coefficient of performance of
practical refrigerators to depend to some extent on the refrigerant.
• Nevertheless, such characteristics as its toxicity, flammability, cost, corrosion properties, and vapor
pressure in relation to temperature are of greater importance in the choice of refrigerant. So that air
cannot leak into the refrigeration system, the vapor pressure of the refrigerant at the evaporator
temperature should be greater than atmospheric pressure.
• On the other hand, the vapor pressure at the condenser temperature should not be unduly high,
because of the initial cost and operating expense of high-pressure equipment.
• These two requirements limit the choice of refrigerant to relatively few fluids.
• Ammonia, methyl chloride, carbon dioxide, propane and other hydrocarbons can serve as refrigerants.
• Halogenated hydrocarbons came into common use as refrigerants in the 1930s. Most common were
the fully halogenated chlorofluorocarbons, CC13F (trichlorofluoromethane) or CFC-11) and
CC12F2( dichlorodifluoromethane or CFC-12).
• These stable molecules persist in the atmosphere for hundreds of years, causing severe ozone
depletion. Their production has mostly ended.
• Tables and diagrams for a variety of other refrigerants are readily available.
Further reading
• Cengel, Y. A., & Boles, M. A. (2015). Thermodynamics: An Engineering
Approach 8th Editon (SI Units). The McGraw-Hill Companies, Inc., New
York.
• Smith. J.M., Van Ness, H. C. Abbott, M. M. (2004). Introduction to
Chemical Engineering Thermodynamics. Seventh Edition.
