Book - School of Science and Technology

200 Heat emitting equipmentSwivelnozzlesChassisCentrifugalfansPipeconnectionsHeating coilFiltersectionFigure 7.24 Projector type unit heater, recirculatingunits have been used, the latter being floor mounted as shown in Figure 7.24. It isimportant, when applying the latter, to ensure that the discharge air pattern is unimpededby partitions or other large objects.Since the capacity range of unit heaters of the various types is from 10 to 300 kW, itwould serve no purpose to include a list here, particularly since dimensions are small andmost are suspended rather than floor mounted. Selection must be from makers' lists,taking account of air volume, discharge temperature, mounting height and the throwwhich is produced. The principal criteria determining choice are that a relatively lower airdischarge temperature (40 50 C) and a larger air volume are to be preferred over theconverse since the buoyancy of air at higher temperatures will affect the throw availablefrom a given mounting height as well as encouraging an adverse temperature gradient.Unit heaters may be used to provide an outside air supply to industrial premises usinga ducting arrangement such as those shown in Figure 7.25, item (a) being a purpose madeunit, again protection against wind pressure effects and heating element freezing aredesirable ± the filter is omitted for clarity. In instances where the design incorporatessome units having an outside air supply and others which recirculate, it is desirable thatoutput ratings for the two different duties are so selected that all will provide the samedischarge temperature. The recirculation opening and damper on the duct in example (b)are provided to allow for a boost discharge after a period of shut-down and also toprovide a measure of frost protection. It should be noted that if the fan motor to such aunit were to fail, or be shut off by thermostatic or timer control, a reversed air flow will beinduced by the heating element and, unless the damper is in the recirculating position, willthen be discharged to outside. For this and other reasons, it is as well to provide formotorised operation of the damper, interlinked with the supply to the unit motor.Convectors generallyIn retrospect, and in the context of supply of medium and high temperature hot water andsteam, it has not perhaps been emphasised adequately that it is only the odd isolatednatural convector which would, in normal current practice, be connected to such systems.Offices and other subsidiary areas related to an industrial building so serviced would most