Book - School of Science and Technology

542 Refrigeration: water chillers and heat pumpsof the extraordinarily high vacuum for long periods. This method is not widely used, buthas been found economical in industrial plants where exhaust steam is available.Choice of refrigeration plantThe selection of refrigeration plant will depend upon a number of factors, among whichare: the location of suitable space; the availability of waste heat; the importance laid uponplant noise and whether condenser heat must be rejected at a distance from the compressor.Where a refrigeration plant serves a single air-conditioning system of the all-air variety,and the two can be located close together, a direct expansion coil or coils may be usedsubject to the practicality of matching the control characteristics of this type of heatexchange surface to the system load. In other circumstances, for single systems of any air/water variety and for all systems which consist of a number of distributed air handling units,a chilled water system is used to transport cooling energy from the evaporator. In this lattercase the refrigeration plant as a whole is often referred to as a water chiller. A typicalarrangement of the principal components of a chilled water system is shown in Figure 19.10.The riskof an outbreakof Legionnaires' disease have led designers to favour the useof air cooled condensers, in preference to any water cooling methods, wherever it ispossible to so site the refrigeration plant that the necessary large quantities of outsideair are freely available for circulation. However, there is an energy and space penaltyfor using air cooling, due to the resulting higher condensing temperature, which may beFeed cisternCoolingtowerWater make-upControl valveCondenser water pump(s)CompressorCondenserEvaporatorWater chiller(s)Chilled water pump(s)LoadFigure 19.10 Chilled water systemChilled watercircuit