Air cycle

Air cycle refrigeration works on the reverse Brayton or Joule cycle. Air is com­pressed and then heat removed; this air is then expanded to a lower temperature than before it was compressed. Heat can then be extracted to provide useful cooling, returning the air to its original state (see Figure 2.14). Work is taken out of the air during the expansion by an expansion turbine, which removes energy as the blades are driven round by the expanding air. This work can be usefully employed to run other devices, such as generators or fans. Often, it is used to help power the compressor, as shown. Sometimes a separate compres­sor, called a ‘bootstrap’ compressor, is powered by the expander, giving two stages of compression. The increase in pressure on the hot side further elevates the temperature and makes the air cycle system produce more useable heat (at a higher temperature). The cold air after the turbine can be used as a refriger­ant either directly in an open system as shown or indirectly by means of a heat exchanger in a closed system. The efficiency of such systems is limited to a great extent by the efficiencies of compression and expansion, as well as those of the heat exchangers employed.

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Air cycle

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Air cooler

 

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Expander Compressor

 

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Cold chamber

 

Air cycle

Figure 2.14 The air cycle — the work from the expander provides a portion of the work input to the compressor

Originally, slow-speed reciprocating compressors and expanders were used. The poor efficiency and reliability of such machinery were major factors in the replacement of such systems with vapour compression equipment. However, the development of rotary compressors and expanders (such as in car turbo­chargers) greatly improved the isentropic efficiency and reliability of the air cycle. Advances in turbine technology together with the development of air bearings and ceramic components offer further efficiency improvements.

The main application for this cycle is the air conditioning and pressuriza — tion of aircraft. The turbines used for compression and expansion turn at very high speeds to obtain the necessary pressure ratios and, consequently, are noisy. The COP is lower than with other systems.

Posted in Refrigeration and Air Conditioning