Document G6gaLqpbLy1zO1DJaQ646w4R7

Heating Ventilating Air Conditioning Guide 1939 solid adsorbents include lamisilate and charcoal but details of their H physical properties are not available. j r* Nature oi Adsorption Process Adsorption is accomplished without chemical change between the air and the adsorbent substance. The adsorbent does not go into solution but water vapor is extracted from the air-vapor stream passing through the bed of adsorbent material and is caught and retained in the capillary pores. The exact nature of the process which goes on during adsorption is not known but it is stated that the action is brought about by surface condensation, and also by a difference between the vapor pressure of the water condensing inside the pores and the partial pressure of the water vapor in the air-vapor mixture. The adsorbing process in the bed can continue until the vapor pressures come into equilibrium. The amount of vapor adsorbed will depend on the adsorbent substances being used but for any single substance the amount depends on the temperature of the bed as well as on the partial pressure of the air-vapor mixture being passed over it. As the process of adsorption goes on heat is liberated in the bed. The heat so liberated is the latent heat of the water vapor condensed together ; J' ; j E |i j4, f= : 5 | | with the so-called heat of wetting. For a pound of water vapor at 60 F\ . the latent heat released by condensation is approximately 1057 Btu. \ The heat of wetting for silica gel, for example, is about 200 Btu, making a total heat of adsorption of approximately 1257 Btu per pound of water adsorbed from the air-vapor mixture passing through the silica gel bed. The heat of wetting varies with the substance being used as the adsorbent while the latent heat of condensation depends only on the temperature and pressure of the water vapor. Temperature-Pressure-Concentration Relations Since the adsorptive ability of an adsorbent depends on the temperature M Iof the bed and on the partial pressure difference between the pores and the air-vapor mixture it is important to know the pressures and temperatures at which pressure equilibrium is reached. Evidently the equilibrium conditions represent the limits beyond which adsorption of vapor cannot continue. The relationship can be shown graphically and Fig. 1 is such a chart for silica, gel. Charts of like nature can be plotted for other adsorbent materials. Fig. 1 shows the equilibrium conditions for a gel bed maintained at constant temperature while the water vapor adsorption is allowed to continue until pressure equilibrium is reached. Each curve on the chart shows a certain dew-point temperature, and therefore a certain pressure, of the saturated water vapor. As an example in the interpretation of the chart consider the case when moist air at a temperature of 80 F and a partial vapor pressure of 0.5 in. of mercury flows through a bed of silica gel which is at a temperature of 80 F. The chart indicates that the equilibrium of pressure between the air-vapor mixture and the bed is reached when the dry bed has adsorbed moisture to the extent of 31 per cent of the weight when dry. When this happens the bed can adsorb no more moisture unless its temperature is changed. Chapter 2. Refrigerants and Air Drying Agents In practice however the adsorbent bed is seldom held at a steady tem perature in air conditioning applications and neither is the adsorption process permitted to continue until moisture equilibrium is reached. Instead, the bed temperature varies and the bed is re-activated before equilibrium is approached. While charts of this kind can show the limiting properties of the substances they are seldom directly applicable to the solution of air conditioning problems unless considerable additional information is available. This takes the form of performance data cover ing the characteristics of the equipment in which the adsorbent bed is placed. Such performance data are beyond the scope of this chapter. Liquid Absorbents .vuy ausoment suostance may be used as an air drying agent if it.has a vapor pressure lower than the vapor pressure in the air-vapor mixture from which the moisture is to be removed. Absorbents are character istically water solutions of materials in which the vapor pressure is 43