Book - School of Science and Technology

92 The building in summerLossesMotorDriveEquipmentInputMotorshaftEquipmentshaftUsefuloutputMotor Efficiencies %Motor outputrating (kW) 0.75 3.75 7.5 15 38 561 Phase3 Phase0.65 0.78 0.81 0.83 0.85 0.860.74 0.85 0.88 0.90 0.91 0.92Drive efficiencies %Plain bearings 95 - 98 Vee Belt 96 - 98Roller bearings 98 Spur Gear 93Ball bearings 99 Bevel Gear 92Figure 3.15 Typical energy balance for an electric motorenergy, as in the case of water pumped to an elevated tank or goods raised in a hoist. Inconsequence, the energy input to machines constitutes a heat gain and may be evaluatedby reference to the power absorbed and, where appropriate, any diversity of use. Whenboth the motor and the driven machine are in the space, then the total power input mustbe considered, but if either are mounted elsewhere, then reference to Figure 3.15 willprovide guidance as to the proportion of the total which must be allowed for.No useful guidance can be given as to the amount of heat which may be liberated withina space by equipment such as steam presses, hot plates, drying ovens and gas or electricfurnaces and reference must be made to manufacturers' literature. Similarly, for laboratories,etc., standard authorities should be consulted, a wide ranging summary beingincluded in the Guide Section A6.DiversityAs has been mentioned in the two previous paragraphs, it may well be important that thematter of diversity in use be considered. In a workroom, for instance, there may be avariety of machines having 60 or so motors which together have a brochure rating of300 kW but, at the same time, the total instantaneous loading may be only two-thirds ofthat total. While it would clearly be wrong to ignore this seeming diversity in use, thedetail will often require careful analysis as to both timing and the physical positioning ofthe machines. At the risk of stating the obvious, a diversity of 66% does not mean thateach motor is running at 66% full load (although this may be the reasoning of an illtrainedcomputer program) but more likely that of each 60 motors fitted, a randommaximum of 40 are in use simultaneously. Those at one end of the space might, perhaps,all be idle at one time or, conversely, but one of each pair of adjacent machines might berunning. Each case needs to be considered in detail.Temperature controlThe case made in Chapter 2 for temperature control room-by-room in winter, appliesequally to buildings in summer. In the case of air-conditioning systems, the controlphilosophy requires very careful consideration since the plant configuration to deal withheating, cooling, humidification and dehumidification involves much more complex