Air refrigeration system is one of the earliest methods of cooling developed. It became obsolete for several years because of it’s low coefficient of performance (C.O.P). Air cycle refrigeration systems belong to the general class of gas cycle refrigeration systems, in which a gas is used as the working fluid.
The gas does not undergo any phase change during the cycle, consequently, all the internal heat transfer processes are sensible heat transfer processes. Gas cycle refrigeration systems find applications in air craft cabin cooling and also in the liquefaction of various gases.
The main characteristic feature of air-refrigeration system is that throughout the cycle the refrigerant remains in gaseous state.
Types of Air- Refrigeration System :
- Closed System
- Open System
1.Closed System (or Dense Air Machine) :
In this system the air refrigerant is contained within the piping or components/parts of the system at all times and with refrigerator usually pressure is above the atmospheric pressure.
A closed system has the following thermodynamic advantages :
(a.) It can work at suction pressure higher than atmospheric pressure. This reduces the volume in compressor and expander .
(b.) The operating pressure ratio can be reduced, resulting in higher C.O.P.
2. Open System :
In this system the refrigerator is replaced by the actual space to be cooled with the air expanded to atmospheric pressure, circulated through the cold room and then compressed to the cooler pressure.
The pressure of operation in this system is inherently limited to operation at atmospheric pressure in the refrigerator.
In this system, the refrigeration is obtained by three basic steps viz, compression, cooling and expansion, accompanied by expraction of work.
The air after expansion is directly led to the conditioned space. It is therefore necessary that air be expanded to one atmosphere pressure. This requires larger volumes to be handled. So the size of the compressor and expander, now a days are not affected significantly since both of them are turbo-type.
This system, therefore has an advantages over closed system , in respect that it does not require a heat exchange for refrigeration process. This saves weight and cost of the equipment .
Advantages of Closed System over Open System :
- Compressor may be at high pressure, the sizes of expander and the compressor can be kept within reasonable limits by using dense air.
- In open system the air picks up moisture from products kept in refrigearated chamber,the moisture may freeze during the expansion and is likely to choke the valves. Thus the drier in the open system is required where as it does not happen in closed system.
- Coefficient of performance is high.
- In open system the expansion of the refrigerant can be varied only upto atmospheric pressure prevailing in the cold chamber but for closed system there is no such restriction.