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from the cassettes. It also allows the use of water straight from the tap with no need for water treatment (i.e. demineralisation plants). Minerals and pollutants stay behind in the cassette material to be washed away with the discharge water, keeping the total humidification process pure.
The theory of evaporative cooling Figure 2[1]
shows air passing through a water
spray chamber, which could also be the corrugated packing medium saturated and sprayed from the micro jet nozzles in our application. Text books refer to air passing over or through a wetted surface. This process results in heat and mass transfer and can be represented by the ‘straight line law’. This law states that when air is transferring heat and mass (water) to or from a wetted surface, the condition of the air at point A, shown on the psychrometric chart, drives towards the saturation line at the temperature of the wetted surface. The condition of the air leaving the spray chamber or wetted surface at point B has dropped in dry bulb temperature and increased in humidity or moisture content. The straight line law states that point B lies on a straight line drawn between point A and the saturation curve at the wetted surface temperature at point C. The warm air at point A dry bulb temperature drops when in contact with the water at temperature tC. This application of heat and mass transfer is one of the most complex encountered in heat exchange theory, but this article will consider the underlying application of the basic principles. Further reading on this is given at the end of the article.
The process that is taking place is known
as adiabatic saturation and as such occurs without any external exchange of heat into or out of the system. Air at the start of the process is at a dry bulb temperature tA and moisture content gA and as it passes through the saturated cassette, water is evaporated from the surface of the cassette material so that the air leaving the cassette and entering the condenser coil, in our case, has a dry bulb temperature tB and moisture content gB. For water to evaporate, heat must be supplied and, in an adiabatic process, this heat can only come from the air itself. The latent heat of evaporation gained by the air must equal the sensible heat loss by the air, which means there will be a drop in air dry bulb temperature to compensate for the increase in moisture content: i.e. (gB – gA)hfg = Cpair(tA – tB)
The theoretical process line follows the 56 CIBSE Journal July 2010 A gA
Figure 1: Typical air cooled chiller arrangement
A
B tA
wet-bulb temperature
pump C B
gC gB
dry-bulb temperature
tC
tB Figure 2: Psychrometric chart for evaporative cooling process
adiabatic saturation temperature (which may be assumed to follow the wet bulb temperature lines that are printed on the psychrometric chart). But a close approximation, which usually assists psychrometric calculations, is to consider the process as a constant enthalpy one. The main factors that affect the performance of the cassette in reducing the
ambient dry bulb temperature entering the condenser coil are: • Ambient air wet bulb temperature: At summer ambient conditions of say 30C dry bulb, 20C wet bulb, air leaves the cassette and enters the condenser coil at approximately 23C. The lower the actual ambient wet bulb, the more the potential for the dry bulb to be reduced, but of course, the opposite is also
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tA
moisture content
sat. line
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