Document EvqkV5DZpMe8yd2e8bxRoGxax

HEATINC VENTILATING AIR CONDITIONING GUIDE 1941 practical deterioration. Re-activation may be accomplished at tem peratures of air up to 600 F although it is frequently accomplished with air or other gases at temperatures not over 300 F. Volume of the capil lary pores is reported to be from 50 to 70 per cent of the total solid volume. For most estimating purposes the volume-weight relation can be assumed as from 38 to 40 lb per cubic foot on a dry basis. It also has the property of absorbing certain gas and vapors other than water vapor. Other substances having properties which make them available as solid adsorbents include lamisilate and charcoal but details of their physical properties are not available. Nature of Adsorption Process 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 bed of material adsorbs moisture, its vapor pressure approaches that of the contacting air and the rate of adsorption gradually slows down so that equilibrium may not be reached for 24 to 48 hours. Because of this diminishing rate of adsorption, commercially designed systems do not permit the state of equilibrium to be reached but generally operate on a 10 to 30 min contact time--the period of most rapid adsorption. 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 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 tempera ture of 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. 428 CHAPTER 23. COOLINC, DEHUMIDIFICATION AND DEHYDRATION The equilibrium conditions for a gel bed maintained at constant temoerature while the water vapor adsorption is allowed to continue until pressure equilibrium is reached is shown in Fig. 1. 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 .Fig. 1 Temperature--Vapor Pressure--Concentration Relation for a Silica Gel Bed at Constant Temperature air-vapor mixture and the bed is reached when the dry bed has adsorbed moisture to the extent of 30 per cent of the weight when dry. When this happens the bed can adsorb no more moisture unless its temperature is changed. -While charts of this kind can show the limiting properties of the sub stances they are seldom directly applicable to the solution of air con ditioning problems unless considerable additional information is avail able. This takes the form of performance data covering the character istics of the equipment- in which the adsorbent bed is placed. Such - performance data are presented later in this discussion.