Book - School of Science and Technology

Traditional systems 3770.50.60.70.80.91.00.90.80.70.60.20.30.40.10Sensible/total heatratio for wateradded at 30°C0.75–10 –5 0 5 10 1520 25 30 35 40Specific enthalpy kJ/kg45 50 55 60 650.80Specific volume m /kg0.853258 09 0Percentagesaturation0.5200.10.40.30.20.2 0.1 0.300.61.00.30.1152.11.00.12.90.61.93.7 5.30.2100.85.22.76.50.76.41.156.62.50.25.71.96.40.85.20.10 4.7 5.5 1.3 0.43.6 3.6 1.0 0.1–5 1.6 2.7–10 0.20.60.3 1.3 0.10.3Wet-bulb temperature °C (sling)7 06 05 04 03 02 01 00.0200.0180.0160.0140.0120.0100.0080.0060.0040.002Moisture content kg/kg (dry air)65605550450.000–10 –5 0 5 10 15 20 25 30 35Dry-bulb temperature °C–10 –5 0 5 10 1520 25 30 35 40Specific enthalpy kJ/kgFigure 14.2 Annual percentage frequencies of coincident dry-bulb and wet-bulb temperatures for theLondon areasatisfactorily through the range of annual external conditions. To this end it is necessaryto have an understanding of the range and frequency of occurrence of coincident wet-bulband dry-bulb external conditions. Figure 14.2 shows, on a psychrometric chart, theannual percentage occurrence of external conditions lying within specified limits. yTraditional systemsCentral plantThe basic elements of air-conditioning systems of whatever form are:Fans for moving air.Filters for cleaning air, either fresh or recirculated, or both.Cooling plant connected to heat exchange surface, such as finned coils for cooling anddehumidifying air.Heater batteries for warming the air, such as hot water or steam heated coils or electricalresistance elements.Humidifiers, such as by steam injection, water sprays/washers or heated pan type.A control system to regulate automatically the amount of cooling, warming, humidificationor dehumidification.y Holmes, M. J. and Adams, F. S., Coincidence of Dry and Wet Bulb Temperatures. BSRIA Technical NoteTN2/77.