Book - School of Science and Technology

Convective heating 201SuspensionWeathercowlDampersRe-circulationWeatherlouvresSuspensionFiltersStandardunit heaterAccess panelto motor and filterHeater(a)Fan sectionDiffuser(b)Figure 7.25 Unit heaters with outside air inletsprobably be heated by water at a lower temperature, produced as was shown in Figure 6.5(p. 152).When control of natural convectors and similar equipment is to be by adjustment of thetemperature of the heating medium, it should be noted that their output falls away morequickly than that from radiant equipment. This situation is reflected in the data which areincluded in Tables 7.6 and 7.9. It has been found in practice that the fundamentalequations quoted at the beginning of this chapter must be adapted slightly to reflect theperformance of actual equipment, output being proportional to the temperature difference,heating medium to air, as follows:Radiant equipmentProportional to (t m t a ) 1:3Natural convectors, etc.Proportional to (t m t a ) 1:5It is, for this reason, inadvisable to supply equipment of the two categories from thesame piping circuit but to provide for them separately using different rates of temperatureadjustment. As to forced convectors and unit heaters, response to change in temperaturedifference is, in practical terms, directly proportional thereto. It would, however, benormal current practice to supply equipment of this type from a constant temperaturecircuit, control then being by either `on/off ' switching of the fan motor or speed change asmentioned previously thereby avoiding the forced circulation of `cooler' air creatingpossible discomfort from draughts.In earlier paragraphs covering convective heating, no mention has been made ofbuilders work enclosures for natural or forced convectors, skirting heaters, etc. These,of course, may be used in any situation where appearance is of importance and maltreatedsheet metal is not acceptable. It is nevertheless important that the form and the dimensionsof the enclosure, and the dimensions of inlet and outlet grilles be discussed with theheater manufacturer. Enclosures having grilles set into a flat top perform badly when