When the dew point of the conditions to be maintained is 45F or less, the method of controlling temperature and humidity described in Art. 13.37 cannot be used because the coil suction temperature must be below freezing and the coil will freeze up, preventing flow of air. For room conditions with dew point below 45F, chemical methods of moisture removal, such as silica gel or lithium chloride, may be used.
This equipment is arranged to have the air pass through part of the chemical and thus give up its moisture, while another part of the system regenerates the chemical by driving off the moisture previously absorbed. The psychrometric process involved in this type of moisture removal is adiabatic or, for practical purposes, may be considered as being at constant wet-bulb temperature. For example, if we take room air at 70F and 30% relative humidity and pass the air through a chemical moisture absorber, the air leaving the absorber will be at a wet-bulb temperature of 53F; i.e., the conditions leaving may be 75F and 19% RH or 80F and 11% RH, etc.
It may be noticed that the latent load here is converted to sensible load, and a refrigeration system will be necessary to do sensible cooling. See Fig. 13.46 for the control of such a system.
