Book - School of Science and Technology

Mechanically ventilated buildings 673A mechanical ventilation system operating in a heating mode will benefit from heatgain to the air as the electrical power driving the fan is down-graded to heat. The quantityof heat dissipated to the air stream will depend upon whether the fan motor is within oroutside of the air stream. Table 23.10 gives approximations adequate for the purpose.The application of heat recovery devices to air systems, together with the thermalefficiencies of available equipment, is described in Chapter 17. The benefit in thermalenergy terms is in effect to reduce the additional load imposed upon the supply plant dueto the introduction of outside air. Any increase in system pressures arising from theimposition of heat exchange equipment in the supply and extract systems must beincluded in the calculations together with any power requirements for additional pumpsor drive motors associated with the heat recovery devices.Since the ventilation plant may run year-round, the effective period over which heatingis introduced may be extended to offset the effect of cold days outside of the normalheating season. In consequence, the degree-day totals may have to be increased, perhapsto the annual totals (see Table 23.4).Power requirementsThe system components driven by electric motors would normally include the supply andextract fans and any drives associated with the heat recovery equipment. Taking thesupply fan as an example the power requirement is given by:W ˆ VP t /(1000)whereW ˆ absorbed power (kW)V ˆ volume flow rate (litre/s)P t ˆ fan total pressure (kPa) ˆ fan, motor and drive efficiencyThe annual power consumption for the supply fan is given by W number of hours inoperation which will, of course, include for the preheat periods where appropriate.ConclusionIt must be appreciated that manual calculations of the type outlined earlier in this chapterfor heating and ventilating plants, relying upon application of degree-day data, etc.,cannot be expected to provide an accuracy in absolute terms of better than plus or minusTable 23.10 Approximation of fan heat gain to an air streamLocation ofmotorMotor size(kW)Heat gain toair stream (%)In air stream All 100Out of air stream Up to 4 75Out of air stream Above 4 85