Desiccant dehumidification and evaporative cooling

In some environments a combination of high temperature and high humidity defies a remedy by conventional air conditioning which is biased towards temperature rather than humidity. Dehumidification is merely a by-product of bringing the temperature down to below the dew point of the air, causing condensation.

A desiccant is a hygroscopic material, liquid or solid, which can extract moisture from humid air, gas or liquids. Liquid desiccants work by absorption where moisture is taken up by chemical action. Solid desiccants have a large internal area capable of absorbing significant quantities of water by capillary action. Examples of efficient desiccants are

• activated alumina

• lithium salts

• triethylene glycol.

This method of dehumidification requires a heating stage in the process. This is to dry or regenerate the desiccant material and requires a temperature range of 60-90°C. One option is to supply the heat by means of evacuated tube solar collectors, backed up by natural gas

Evaporative

Evaporative

Figure 3.6 Desiccant wheel and thermal wheel dehumidifying and cooling

when insolation is inadequate. Alternatively waste heat, for example from a Stirling CHP unit, may be exploited.

As a full alternative to air conditioning, desiccant dehumidification can be used in conjunction with evaporative cooling. After being dried by the revolving desiccant wheel the air passes through a heat exchanger such as a thermal wheel for cooling. If necessary further cooling may be achieved by an evaporative cooler before the air is supplied to the building.

The exhaust air at room temperature also passes through an evaporative cooler and then through the thermal wheel, chilling it in the process. This enables the thermal wheel to cool the supply air. After passing through the thermal wheel the air is heated and directed through the desiccant wheel to remove moisture and then expelled to the atmosphere.

There are problems with the system. It is not amenable to precise temperature and humidity control and it is not so efficient in dry climates. On the positive side it is providing a full fresh air system (see Fig. 3.6).

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