The virtues of air refrigeration, including cheapness, availability and safety of the refrigerant used made it quite appealing to the early inventors of refrigeration machines. Air refrigeration dates back to the middle of nineteenth century, but it was not until 1877 that a reliable unit was developed. It was used mostly for cold-storage of meat on ships.

However, the equipment was bulky and inefficient. The bulkiness was due to the reciprocating compressor and expanders and low speed of rotation. Rapid developments of the more efficient and less bulky vapour compres­sion systems made air-refrigeration rather short-lived.

Recent developments in the rotary compressors and gas turbines have renewed interest in air refrigeration systems. Rotary machines caused great reduction in weight per unit of refrigeration.

Refrigeration Load:

As the temperature of the body or space is less than that of the surroundings, it is but natural that heat will flow into the space by the combined effect of conduction, convection and radiation. In addition to this heat flow, there is always some air leakage and ventilation and hence additional heat removal is required.


Products when placed fresh, then the temperature of the products or bodies is more than the surroundings. And, therefore, to maintain the temperature constant in the refrigerator, all heat coming in should be removed. This total heat to be removed is called the refrigeration load. This is the actual refrigerating effect.

Hence the heat sources in a refrigerator are as follows:

1. Heat transmission/transfer

2. Product heat


3. Air leakage and ventilation

4. Miscellaneous sources

Heat Pump:

A heat pump is a thermodynamic system operating in a cycle which removes heat from a low temperature body and delivers heat to a high temperature body. In accomplishing this, the heat pump receives external energy in the form of work. The system or working substance undergoes a series of processes which constitute the heat pump cycle.

The heat pump may be used as a refrigerator where the primary function is the transfer of heat from a low temperature system. In a heating system, utilising a heat pump, the primary function is the transfer of heat to a high temperature system.

An index of performance of a heat pump is the coefficient of performance, COP.

Referring to Fig. 36.10, let

Q1 = Heat rejected by system

Q2 = Heat transferred to system


WNet = Net work required by system

Applying the first law of thermodynamics to the cycle gives

Carnot Cycle for Refrigeration:

A refrigeration system removes heat from a low-temperature region and transfers heat to a high-temperature region.


The Carnot cycle can serve as the model of the ideal refrigeration cycle. Considering all refrigerators operating between the same two thermal reservoirs, the maximum coefficient of performance is attained by the Carnot refrig­eration cycle.

Figure 36.11 shows the Carnot refrigeration cycle on T-S and P-V diagram. The working substance is a con­densable vapour or it may be any gas or air (non-condensable).

Starting at state 1 and following the arrows, the four processes of the cycle are:

1- 2 reversible adiabatic compression

2- 3 reversible isothermal heat rejection-

3- 4 reversible adiabatic expansion

4- 1 reversible isothermal heat absorption

During the heat rejection process (2-3), condensation takes place at a constant temperature Th and the area a-3- 2-b-a in the T-S diagram represents the heat removed Qh. Similarly, during the heat absorption process 4—1, evaporation takes place as heat is absorbed from the surrounding space at a temperature Tc, and the area a-4-l-b- a in the T-S diagram represents the refrigeration effect Qc. The coefficient of performance of the Carnot refrigera­tion cycle is given by-

It is not practical, however, to operate a refrigeration unit according to the Carnot cycle requirements. The main difficulty lies in the isothermal heat interaction processes. These processes proceed at a very low rate so that very large heat transfer surfaces are needed.

Furthermore, the presence of the liquid phase of the working fluid in the compressor creates problems, for it causes severe erosion of the compressor. This difficulty is avoided in the Rankine refrigeration cycle. When air or gas are used for Carnot cycle, Coefficient of Performance is given by-

The ideal gas refrigeration cycle operates on a reversed Brayton or Joule cycle. This reversed Joule cycle is called Bell-Coleman Cycle.

Practical Cycle for Air Refrigeration-Bell-Coleman Cycle:

Modification of the ideal reversed Carnot cycle so as to make it practicable has resulted in Bell-Coleman Cycle.

The distinguishing features of this system as compared to vapour system is that the refrigerant air remains in the gaseous phase throughout the cycle and does not undergo liquefaction and evaporation as a vapour refrigeration system.

Air refrigeration cycle may be further classified as- (i) Open cycle and (ii) Closed or dense air cycle.

The isothermal operations are replaced by constant pressure operations:

(i) Open Cycle:

Figure 36.12 shows a flow diagram of the open air system. The name is applied because air is discharged into cold chamber at atmospheric pressure and allowed to come in direct contact with the cold body. The compressor draws the air from the cold chamber, compresses it, and delivers it to the cooler.

As air flows through the cooler, heat is removed from it causing a great decrease in its volume. In the ideal case, the pressure remains constant during cooling; in the actual case there is slight pressure drop. The high pressure air is then allowed to expand and made to do work; this work will be done at the expense of its enthalpy (total heat) and there will be a big drop in its temperature.

This expansion of air is carried out in the expansion cylinder (air motor). The low temperature air leaving the expansion cylinder then enters the cold chamber and abstracts heat from the bodies kept in the chamber and in turn lowers temperature in the cold chambers.

Figure 36.13 shows theoretical combined P-V diagram for both the compressor and expander (air motor). Ideally, compression and expansion are reversible adiabatic or isentropic, the area of P-V diagram (1-2-3-4-1) represents the network input to the system.

1- 2 Isentropic compression

2- 3 Constant pressure heat-rejection

3- 4 Isentropic expansion in expander

4- 1 Constant pressure heat-addition

Let T1, T2, T3, T4 be the absolute temperatures at the points 1, 2, 3 and 4 respectively. From the P-V diagram, we get

but heat during compression and expansion are to be taken into consideration.

(ii) Closed or Dense Air Cycle:

The basic air standard cycle is shown in Fig. 36.15. Air is contained in the system and is compressed from B by a reciprocating or rotory compressor. The compressed air at C is passed on to the air cooler where it cools down at constant pressure, to stare D.

The high pressure gas at D is then passed on to an expansion cylinder. Here the air expands upto A, delivers work and gets cooled. The work delivered may be fed back to the compressor by mounting the compressor and the expander on the same shaft. The cooler air at A is passed through the refrigerator. The heat picked up by the air in the refrigerator causes its state to change to point B and the cycle repeats.

The cycle discussed above is known as closed or dense air system. If the cool air is used as such for air condi­tioning or other purposes and is not taken back to the compressor, then the fresh charge of air is drawn by the compressor and the system is called Open Air System.

The open air system is inherently limited to operation at atmospheric pressure only whereas in the dense air system the pressure after expander may be even lower than atmospheric, hence lower temperatures may be attained.

The P-V and T-S diagrams will be same as for open cycle and the numerical analysis of the cycle or system is also same.

Advantages and Disadvantages of Air Refrigeration:

The main advantage of air refrigeration is that the working substance or air refrigerant is available always in the atmosphere. Further it is free or no cost is involved. The open air cycle system is simple.

Thus we can say that:

1. Availability of air is there

2. Air is free

3. System is very simple

The disadvantages are:

1. Freezing of the moisture in the air used in the system. This is particularly for open system. A deposit of snow is liable to choke up the valves and ducts carrying air.

2. Coefficient of performance (COP) of the system is very low.

3. Refrigerating effect per kg of air is comparatively small or to produce the required refrigerating effect mass of air required is large. Secondly, the volume of air dealt by compressor and expander is very large. Therefore, the size of the compressor and expander is very large and the machine is heavy and bulky.

Limitations of Air Refrigeration System:

After noting down the working of Bell-Coleman air refrigeration systems, we observe certain limitations in using air-refrigeration systems.

These limitations may be enlisted as follows:

1. The power required to get unit of refrigeration is comparatively large and hence the performance is very poor. It is observed that approximately 4 kW are required to produce 1 TR. For vapour compression refrigeration system, power requirement is approximately 1 kW only for every TR capacity. Therefore, the air refrigeration system is to be used when other systems cannot be used.

2. When the air refrigeration plant is to be stationary, then atmosphere should not be dusty. In spite of the dusty atmosphere, if air refrigeration system is to be used, then dust-separators must be provided, otherwise wear and tear of the components will result.

3. Moisture contained in the air is a big problem as moisture condenses and even freezes choking the valve and creating problems for the running of this air system.

4. To have the required capacity of the air refrigerating system, the quantity of air required is very large and the volume of air to be handled is very large as compared to vapour compression system. In this case the size and weight of the compressor is very large and the compressor is heavy and bulky.

Thus when the air refrigerating system is absolutely necessary, then and then only this system is used.