Book - School of Science and Technology

112 Survey of heating methodsTubular heatersAs the name implies, these are steel or aluminium tubes commonly round or oval in crosssection,as shown in Figure 4.16, with no heat transfer filling other than air. The heatingelement extends from end to end to provide an even surface temperature of about 80 C. Asingle tube at 50 mm diameter has an output of about 180 W per metre length and tubesmay be mounted in banks, one above the other, for a greater output per unit length. Thediagram includes a convective type having a substantial casing with an output of about400 W per metre run. This is not strictly a true tubular heater although it has a similarrange of applications. It is smaller in cross-section than the skirting heaters of Figure 4.15and rather more substantial.Tubulars have been used in churches, placed under the pews, in which position theykeep the lower air warm with no attempt to heat the whole enclosure. Fitted belowclerestory windows in any tall building, they prevent down-draughts.Forced convectorsThis general heading covers a variety of items from the compact portable domestic fanheater rated at up to 3 kW at one end of the scale to large industrial units rated at 30 kWor more at the other. Between these extremes are commercial type heaters, fitted withaxial-flow fans, having ratings of about 3±6 kW and a range of cased tangential-fan unitsrated at up to 18 kW to meet the particular requirement of a source for warm air curtainsat building exits and entrances. Figure 4.17 illustrates two such items.Control of direct systemsThe methods used to control direct systems and individual appliances are legion andrange from simple on/off switches through to quite sophisticated systems for largeinstallations. The following is a brief summary of methods available.Individual domestic size units. Simple switches providing `on/off ' action.. Dial switches providing staged control, i.e. 1, 2 and 3 kW in the case of a gas or electricnatural convector.Round Oval ConvectiveFigure 4.16 Electrical tubular heaters