Book - School of Science and Technology

Direct systems ± electrical (primarily radiant) 107Equipment of this type passes the air for circulation through or directly over an opencombustion chamber with no separation or flue, the products of combustion passing outwith the air into the heated space. The practicability of this arrangement rests upon twofeatures:. The fuels in question, with the consequent lack of any serious concentration ofpollutants in the products of combustion (with the exception of water vapour).. The method of application of the heater whereby the air drawn in for circulation isalways from outside the building and not recirculated from the heated space.In these circumstances, the purity of the discharge will be within the accepted thresholdvalues laid down by the Health and Safety Executive. Unit ratings range up to about650 kW and, since all the energy in the fuel passes to the air circulated, efficiency may bewell over 90%.Direct systems ± electrical (primarily radiant)Luminous firesThe earlier type of luminous fire, which consisted of an exposed coiled-wire element,mounted in some manner to a refractory block, is seldom seen today. Elements are nowusually silica sheathed and are commonly mounted in front of a polished aluminiumreflector having parabolic form. A great variety of types and designs is available withcapacities ranging from 500 W to 3 kW. The effect of such heaters is localised and their useis generally confined to domestic premises, hotel lounges and the like.Infra-red heatersWall or ceiling models of these are suitable for kitchens and bathrooms in a domesticcontext and more robust patterns may be used in commercial or industrial premises.The elements used are similar to those fitted to luminous fires but, for a given rating, arecommonly longer, as Figure 4.11, and arranged to operate at about 900 C. They aresometimes misapplied to churches where, at that temperature, the usual mounting positionin the eaves is too high to provide effective radiant cover. Ratings are up to 3 kW per unit.Quartz lamp heatersFor application to large spaces either where the requirement is intermittent or where onlylocalised areas require spot heating, quartz lamp heaters operate at a temperature of over2000 C. The elements, each of which is rated at about 1.5 kW, consist of a tungsten wirecoil sealed within a quartz tube containing gas and a suitable halide. As illustrated inFigure 4.12, a rather unlovely casing contains a number of elements (normally a maximumof six), each of which is mounted in front of a polished parabolic reflector.High temperature panelsConsisting of either a vitreous enamelled metal plate or a ceramic tile behind which aresistance element is mounted within a casing, panels of this type operate at a temperatureof about 250 C and have ratings in the range 750 W to 2 kW. Although rather