Book - School of Science and Technology

510 Fans and air treatment equipmentFrom buildingControl asrequiredInlet to plantFinnedcoilExpansionFinnedcoilPumpTo exhaustOutside airFigure 17.34 Run-around coils with water circulationTable 17.5 Properties of water/ethylene glycol solutionsGlycolconcentration(%)Freezingpoint( C)Specific heatcapacity (at 10 C)(kJ/kg K)0 0 4.210 5 4.120 10 3.925 13 3.830 16 3.740 25 3.5system is a pair of conventional finned tube heating/cooling coils, one fitted in each airstream, connected by a pipeworksystem for pumped circulation of the working fluid,often a 25% solution of glycol anti-freeze in water. Table 17.5 gives the freezing point andspecific heat capacity of water and ethylene glycol solutions in concentrations 0±40%glycol by mass. The specific heat capacity of the solution affects the efficiency of heattransfer; a reduction in specific heat capacity giving lower efficiency.The flexibility of the system derives from the obvious ease by which the coils may beconnected together; there is no need to disturb the routes of what may be large air ducts tobring them, inlet and outlet for both supply and exhaust, to the heat exchanger. Furthermore,coils may be fitted to any number of exhaust ducts and the heat therefrom collectedand distributed to any number of similar coils fitted to supply air ducts. Diversity ofenergy availability and energy demand between air handling plants may thus be used tobest advantage.There are, of course, compensating disadvantages, those of most consequence being theneed for double heat exchange (exhaust air to fluid and fluid to supply air), the relativelysmall temperature differentials available for such energy transfer, the need for waterpumping power and the matter of heat loss and gain from and to the pipeworksystem.Direct transfer of latent heat is not possible with this system, but in winter the coil in theexhaust air stream would run wet as would that in the supply air stream during somesummer conditions: energy transfer would thus be assisted and efficiency improved as inthe case of plate type heat exchangers. In the context of what has been said before,however, heat transfer would be sensible only.