Book - School of Science and Technology

Air-to-air heat exchangers 507Return airTo plantOutside airTo exhaustFigure 17.31 Plate type heat exchangerform narrow passages carrying, alternately, exhaust and supply air. Energy is transferredby conduction through the separating plates and contamination of one air stream by theother thus avoided. The form of the casing is arranged to suit the configuration of thetransfer surface and to provide for convenience of air duct connections; one example isshown in Figure 17.31. Condensation may occur in the return air passageways and drainsare therefore required.Since the separating plates are normally of metal (aluminium or stainless steel) moisturetransfer is not possible and sensible heat only is exchanged. An epoxy or vinyl coating may beapplied to aluminium plates for use in mildly corrosive atmospheres such as swimming pools.Units are available to handle air quantities in the range 60±24 000 litre/s and may bebuilt up in modular fashion to suit individual requirements. Due to the relatively low rateof heat transfer per unit area, the plate surface necessary is large but unit sizes arereasonably compact since air flow passages are kept to minimum width. Temperatureefficiencies in the range of 50±80% are claimed, and resistance to air flow is 140±300 Pa ata face velocity of 3 m/s.This type of heat exchanger offers no method of control and therefore a bypass sectionmay be needed to avoid, for example, heating the outside air above the required supplytemperature only to have to cool it down again.Glass tube heat exchangersThe operation is similar to that of the plate heat exchanger; normally the `clean' supply airwould be passed through the tubes and the `contaminated' exhaust around them to allowfor easier cleaning. These units are particularly suited to handling corrosive fumes fromlaboratories, metal treatment shops, fume cupboards, and for swimming pools.Units may be obtained to handle up to 16 000 litre/s and with a temperature efficiencyof up to 80%. Pressure drop would be of the order of 250 Pa.Thermal wheelsConstructed on the lines illustrated in Figure 17.32, the thermal wheel or regenerative heatexchanger consists of a shallow drum containing appropriate packing which is arrangedto rotate slowly between two axial air streams, transferring energy between the two. Thewheel is mounted in a supporting structure and motor driven at approximately 20 rev/min:the speed may be varied as a means of controlling output.