Book - School of Science and Technology

Application 523Difference in enthalpy, entering to leaving:(44:46 32:80) ˆ 11:66 kJ/kgThus, cooling capacity(11:66 15:52) ˆ 181 kWPlant dutiesFrom the results calculated, it is now possible to arrive at the capacities of the variousitems of equipment which, together, will go to make up the plant for summer use. Theseare, excluding any margin which might be added in practice:Supply fan, handling air at 14 C, and specific volume at 0:81 m 3 /kg:(15:52 0:81) ˆ 12:6m 3 /s ˆ 12 600 litre/sExhaust fan, handling air at 21 C, and specific volume 0:83 m 3 /kg (4.82 kg/s exhaust tooutside and 10.7 kg/s recirculation):(15:52 0:83) ˆ 12:9m 3 /s ˆ 12 900 litre/sCooling coil, 12 600 litre/s with:air onˆ 23:4 C DB and 16:1 CWBair offˆ 14:0 C DB and 11:5 CWBWater chiller, excluding any calculated allowancefor heat gains in pumps, and water circulatingpipes, etc:ˆ 181 kWFigure 18.4 shows, graphically on a small section of the chart, the psychrometricchanges taking place during the processes outlined in the example. Figure 18.5 illustratesthe various conditions on a diagram of the plant and system.10Wet uºCb lb temperature15Heat & moisturegain in roomCoolingcoilRecirc airMixturepointSupply to roomOutside air4.82 kg/s50%9.51 kg/s0.0120.0110.0100.0090.0080.007Moisture content kg/kg (dry air)Duct & fan gain0.00610 15 20 25 30Dry bulb temperature ºCFigure 18.4 Diagram of psychrometric changes (see example calculation)