Book - School of Science and Technology

Application of refrigerationRefrigerating media 533For application to air-conditioning, using a cooling coil, water is pump-circulated througha closed system returning to the evaporator of the refrigeration plant at a temperaturewhich is generally between 7 and 12 C, depending upon the dew point to be maintained:in passing through the evaporator, this water temperature will be lowered by about 4±6 K.In order that the necessary heat transfer may take place, the refrigerant must be at sometemperature below that of the leaving water but, at the same time, it must generally beslightly above freezing point. Thus, in a typical case, the following conditions might obtain:Apparatus dew point 12 CCooling coil outlet 10 CCooling coil inlet6 CWater at evaporator outlet 5.5 CThe refrigerant in the evaporator would in this case be maintained at about 1 C, givinga differential for 4.5 K for heat transfer. As will be appreciated, this small temperaturepotential means that the cooling surface of a simple tubular type would need to be veryextensive: a variety of devices has been developed to augment the transfer rate.An ethylene glycol solution may be used in cooling coils in order to allow lower airtemperatures to be obtained (for example to achieve a low dew point condition): thetemperatures of the fluid circulating may be 7 C from the evaporator and 3 Creturning to it, or lower as required. To achieve such conditions, it is necessary to considerthe strength of the ethylene glycol solution, data for which are available from standardtables (see also Table 17.5 for information on ethylene glycol solutions).In instances where cooling for an air-conditioning system is provided from a refrigerationmachine by direct expansion, the refrigerant is piped directly to cooling coils in theair stream which thus become the evaporator. The surface temperature of the coils is afunction of the leaving air temperature required, the form of the coil surface and thevelocity of the air flow. Refrigerant temperatures much below freezing point are inadmissibleowing to the riskof build-up of ice on the coil surface when dehumidification istaking place. An apparatus dew point of 3 C is normally considered as the practicalminimum for such coils if frosting is to be avoided.Refrigerating mediaThe factors affecting the choice of a refrigerant from a thermal point of view will nowbe clear. A substance is required which can be liquefied at a moderate pressure and whichhas a high latent heat of evaporation. By these means the size of the compressor will bekept to a minimum and the mass of the refrigerant circulated kept relatively small for agiven amount of cooling. In addition, compatibility with the type of compressor andthe refrigerant system, cost, environmental issues and safety have to be taken intoaccount. Environmentally, it is a legal requirement to comply with EC Regulation3093/94 throughout the European Union. In the UK Statutory Instrument 1996/506.The Environmental Protection (Controls on Substances that Deplete the Ozone Layer)Regulations 1996, places a legal duty on owners and operators to comply. CIBSEGuidance Note GN:1 2000 provides more detailed information on the selection andperformance of CFCs, HCFCs, and Halons.The refrigerating media available include ammonia, carbon dioxide, sulphur dioxideand numerous manufactured gases.