Book - School of Science and Technology

Chapter 19Refrigeration: water chillers and heat pumpsFor full air-conditioning, some means of cooling and dehumidification is necessary andthis, in the majority of cases, is provided by use of a mechanical refrigeration machine ormachines. This equipment may be similar to the usual run of plant used for ice-makingand cold storage work, etc., except that the temperature to be produced is likely to behigher than that required for such applications.The energy balance of a refrigeration cycle is such that it may be thought of as a`thermal transformer', taking in energy at a (relatively) low temperature and discarding itat some (relatively) higher temperature. Where cooling is required, it is the energy at lowtemperature which is used and that at a higher temperature discarded: in the conversesense, where heating is required, the energy at the higher temperature is used and that atthe lower temperature discarded. The latter application is that of a heat pump which isoften applied in modern practice to make use of a low grade energy source which wouldotherwise go to waste.Mechanical refrigerationThis depends upon the principle that a liquid may be made to boil at a chosen lowtemperature if it is held at a pressure which is reduced to an appropriate level.* Toproduce boiling, the liquid must be supplied with heat from an external source and thissource will thus lose energy and be cooled. Given a suitable liquid, the temperature ofboiling may be chosen to suit the required conditions, without resort to unduly lowpressures, and although many different substances have been used, complex hydrocarbonshave been found to offer the most suitable characteristics as refrigerants. The vapourgiven off in boiling is compressed, which process adds heat, and the hot vapour is thenliquified by removal of that heat, the pressure still being maintained. A sudden release ofthe pressure is then arranged and, in consequence, the fluid returns to the state in which itbegan, ready once again to boil at a low temperature. This sequence of events is known asthe vapour compression cycle.A refrigeration plant, working on the vapour compression principle, as shown in Figure19.1, thus comprises these principal components:* Water at atmospheric pressure boils at 100 C: if that pressure were to be reduced to 1 kPa then theboiling point would be 7 C.