Building Design and Construction Handbook - Merritt - Ventech!

13.94 SECTION THIRTEEN
13.38 CHEMICAL COOLING
When the dew point of the conditions to be maintained is 45�F 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 45�F, 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 70�F 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
53�F; i.e., the conditions leaving may be 75�F and 19% RH or 80�F 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.
13.39 YEAR-ROUND AIR CONDITIONING
Electronic-computer rooms and certain manufacturing processes require temperature
and humidity to be closely controlled throughout the year. Air-conditioning
systems for this purpose often need additional controls for the condensing equipment.
When city water is used for condensing and the condensers are equipped with
a water regulating valve, automatic throttling of the water by the valve automatically
compensates for low water temperatures in winter. When cooling towers or evap-
FIGURE 13.46 Controls for cooling with a chemical moisture absorber.