DEPARTMENT OF ELECTROMECHANICAL ENGINEERING
INSTITUTE OF TECHNOLOGY
HAWASSA UNIVERSITY
Refrigeration and Air Conditioning - MEng 5202
By: Million M.
Mr. Million M., Thermal Engineer, Hawassa
1/17/2024 1
University
PART I – REFRIGERATION
CHAPTER TWO
REFRIGERATION CYCLES
Gas Refrigeration System
Steam Jet Refrigeration System
• In this chapter we will discuss refrigeration cycles and their energy and exergy analyses.
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Gas refrigeration cycle (reversed brayton cycle, Bell-Colemann)
Air-standard refrigeration systems
• In these systems, refrigeration is accomplished by means of a noncondensing gas (e.g., air)
• The refrigeration load is only the product of the temperature rise of the gas in the low-side heat
exchanger and the specific heat of the gas.
A large refrigeration load requires a large mass rate of circulation.
The throttling valve used for the expansion process in a VCRS is usually replaced by an expansion engine
(e.g., expander).
The work required for the refrigeration effect is provided by the gas refrigerant.
• The Bell-Colemann air refrigeration cycle is the modification of the reversed Carnot cycle with air
as a working medium.
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• A basic air-standard refrigeration cycle (Closed cycle ) has four main elements:
Isentropic compression: 1–2 that raises the pressure of the refrigerant from its lowest to its
highest value in Compressor
Isobaric heat rejection: 2–3 where the high temperature of the refrigerant is lowered in heat
exchanger
Isentropic expansion: 3–4 where the pressure and temperature of the refrigerant are reduced in
turbines, and
Isobaric heat input: 4−1 that raises the temperature of the refrigerant at a constant pressure in
heat exchanger. This input is known as refrigeration load.
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• The gas refrigeration cycle deviates from the reversed Carnot cycle because the heat transfer
processes are not isothermal.
• Consequently, the gas refrigeration cycles have lower COPs relative to the VCRS or the reversed
Carnot cycle.
• Despite their relatively low COPs, the gas refrigeration cycles have two desirable
characteristics:
They involve simple, lighter components, which make them suitable for aircraft cooling,
They can incorporate regeneration, which makes them suitable for liquefaction of gases
and cryogenic applications.
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Energy and Exergy Analyses of a Basic Air-Standard Refrigeration Cycle
• Basic Air cycle refrigeration system analysis is done by making the following
assumptions:
The working fluid is a fixed mass of air that behaves as an ideal gas
The cycle is assumed to be a closed loop cycle
All the processes within the cycle are reversible
The specific heat of air remains constant throughout the cycle
• An analysis with the above assumptions is called as cold Air Standard Cycle (ASC)
analysis.
For an ideal gas
Isentropic Processes of Ideal Gases
Tv k -1 = constant
TP(1-k)/ k = constant
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Pv k = constant
Energy Analyses
Compressor
For heat exchanger II (i.e., condenser)
For expander (turbine)
For heat exchanger I (i.e., evaporator)
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The energy balance for the entire refrigeration system
The net work for the system
The COP
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Exergy Analyses
Compressor
For heat exchanger II (i.e., condenser)
For expander (turbine)
For heat exchanger I (i.e., evaporator)
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The total exergy destruction in the cycle
where the exergy of the heat transferred from the low-temperature medium ExQL given by
The minimum power input to accomplish the required refrigeration load QL
The second-law efficiency (or exergy efficiency) of the cycle is defined as
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The regenerative gas cycle
• Regenerative cooling is achieved by inserting a
counterflow heat exchanger into the cycle.
• 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
• Lowering the turbine inlet temperature automatically
lowers the turbine exit temperature, which is the
minimum temperature in the cycle.
• Without regeneration, the lowest turbine inlet
temperature is T0, the temperature of the
surroundings or any other cooling medium.
• With regeneration, the high- pressure gas is further
cooled to T4 before expanding in the turbine.
Accordingly, the refrigeration effect, achieved from
state 4 to state 6, occurs at a correspondingly lower
average temperature.
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Cabin cooling in an aircraft
• A small amount of high-pressure air is extracted from the main jet engine compressor and cooled by heat
transfer to the ambient.
• The high-pressure air is then expanded through an auxiliary turbine to the pressure maintained in the cabin.
• The air temperature is reduced in the expansion and thus is able to fulfill its cabin cooling function.
• As an additional benefit, the turbine expansion can provide some of the auxiliary power needs of the aircraft.
• Such system satisfy important considerations in the selection of
equipment for use in aircraft. i.e. Size and weight are
In an open-cycle aircraft cooling system - Atmospheric air
is compressed by a compressor, cooled by the
surrounding air, and expanded in a turbine. The cool air
leaving the turbine is then directly routed to the cabin.
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The various methods of air refrigeration systems used for aircraft are as follows:
i. Simple air-cooling system
ii. Simple air-evaporative cooling system
iii. Boot-strap air cooling system
iv. Boot-strap air evaporative cooling system
v. Reduced ambient air cooling system
vi. Regenerative air cooling system
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Ejector refrigeration cycle
• A boiler, an ejector, and a pump are used to replace the mechanical compressor of a conventional
system.
• High-pressure and high-temperature refrigerant vapor is evolved in a boiler to produce the
primary fluid for the ejector.
• The ejector draws vapor refrigerant from the evaporator as its secondary fluid. This causes the
refrigerant to evaporate at low pressure and to produce useful refrigeration.
• The ejector exhausts the refrigerant vapor to the condenser where it is liquefied.
• The liquid refrigerant accumulated in the condenser is returned to the boiler via a pump while the
remainder is expanded through a throttling valve to the evaporator, thus completing the cycle.
• The working input required to circulate the fluid is typically less than 1% of the heat supplied to
the boiler,
• The COP may be defined as the ratio of evaporator
refrigeration load to heat input to the boiler as follows
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A simple jet ejector refrigeration system
Steam jet refrigeration systems
• In steam jet refrigeration systems, water can be used as the refrigerant.
• It is not used when temperatures below 5◦C are required. but are in the range which may satisfy
air-conditioning, cooling, or chilling requirements.
• The main advantages of this system are the utilization of mostly low-grade energy and relatively
small amounts of shaft work.
• Steam jet refrigeration systems use steam ejectors to reduce the pressure in a tank containing the
return water from a chilled water system.
• The steam jet ejector utilizes the energy of a fast-moving jet of steam to capture the flash tank
vapor and to compress it.
• Flashing a portion of the water in the tank reduces the liquid temperature.
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steam jet refrigeration system for water cooling.
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• In the system shown, high-pressure steam expands while flowing through the nozzle 1. The
expansion causes a drop in pressure and an enormous increase in velocity.
• Owing to the high velocity, flash vapor from the tank 2 is drawn into the swiftly moving steam and
the mixture enters the diffuser 3.
• The velocity is gradually reduced in the diffuser but the pressure of the steam at the condenser 4 is
increased 5–10 times more than that at the entrance of the diffuser
• This pressure value corresponds to the condensing temperature of 40◦C. This means that the
mixture of high-pressure steam and the flash vapor may be liquefied in the condenser.
• The latent heat of condensation is transferred to the condenser water, which may be at 25◦C.
• The condensate 5 is pumped back to the boiler, from which it may again be vaporized at a high
pressure.
• The evaporation of a relatively small amount of water in the flash tank (or flash cooler) reduces
the temperature of the main body of water.
• The cooled water is then pumped as the refrigeration carrier to the cooling-load heat exchanger.
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1. Air enters the compressor of a gas refrigeration system with a regenerator at−20◦C at a
flowrate of 0.45 kg/s. The cycle has a pressure ratio of 4. The temperature of the air
decreases from 16 to−30◦C in the regenerator. The isentropic efficiency of the compressor is
82% and that of the turbine is 84%. Determine
a) The rate of refrigeration and the COP of the cycle and
b) The minimum power input, the second-law efficiency of the cycle, and the total exergy
destruction in the cycle. The temperature of the cooled space is −40◦C and heat is released to
the ambient at 7◦C.
c) The minimum power input, the second-law efficiency of the cycle, and the total exergy
destruction in the cycle if the temperature of the cooled space is−15◦c.
d) The refrigeration load and the cop if this system operated on the simple gas refrigeration
cycle.
In this cycle, take the compressor and turbine inlet temperatures to be−20 and 16◦C,
respectively, and use the same compressor and turbine efficiencies. Use constant specific
heat for air at room temperature with Cp=1.005 kJ/kg·K and k=1.4.
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1. Consider the ejector refrigeration cycle shown. The pressure and temperature of the
boiler are 8 bar and 100∘C. The condenser pressure and temperature are 1.5 bar and
35∘C. The evaporator pressure and temperature are 0.2 bar and−15∘C. The pump
thermal efficiency is 45%, and for all of the other components thermal efficiency is 100%.
If the heat rate to the boiler is 4326 W and the mass flow rate in the evaporator is 1.248
kg/min, calculate
(a) the heat rate out of the condenser,
(b) The pump work,
(c) The exergy destruction rate for the system and its components,
(d) The component exergy efficiencies, and
(e) The COPR of the cycle.
(Assume that the kinetic energy change in the boiler is neglected, and that the velocity of the flow coming out of the
condenser is relatively small and can be neglected.)
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