Document 15DjZ9VrdQXe89Rez3vrg5qKa

HEATING VENTILATING AIR CONDITIONING GUIDE 1944 adsorption heat is approximately 10 F for each grain of moisture removed per cubic foot of air at atmospheric pressure. This temperature rise occurs progressively through the adsorbent bed and is an important con sideration in predetermining the performance of a given design of ap paratus. Data such as these together with information covering other characteristics, such as specific heat, resistance to air flow, etc., are of value in the basic design of adsorption dehydration apparatus. In the solution of air conditioning problems, however, reference must be made to performance data on established apparatus designs such as are pre sented later in this chapter. Absorbents Any absorbent substance 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. Solid Absorbents. The substances used are in general the solid forms of the liquid absorbents, more commonly calcium chloride due to its low cost. At present they are used principally in small dessicating chambers, and in small dryers of the cartridge type, through which air is forced under pressure. Liquid Absorbents. These are characteristically water solutions of materials in which the vapor pressure is reduced to a suitable level by governing the concentration and temperature of the dehydrating solution. In addition to having suitable vapor pressure characteristics a practical absorbent must also be widely available at economical cost, be non-corrosive, odorless, non-toxic, non-inflammable, chemically inert against any impurities in the air stream, stable over the range of use and especially it must not precipitate out at the lowest temperature to which the apparatus is exposed. It must have low viscosity and be capable of being economically regenerated or concentrated after having been diluted by absorbing moisture. . Water solutions, or brines, of the chlorides or bromides of various inorganic elements such as lithium chloride and calcium chloride are the absorbents most frequently used in connection with air conditioning applications and detailed attention is confined to these two in this chapter. The application consists of bringing the air-vapor stream into intimate contact with the absorbent, permissibly by passing the air stream through a finely divided spray of the brine but more generally by passing the air over a contacting pack where the liquid absorbent presents a large surface to the air stream. The difference in vapor pressure causes some of the vapor in the air-vapor mixture to migrate into the brine. Here it condenses into liquid water and decreases the concentration of the absorbent. . As the water vapor is added to the absorbent and condenses, it gives up its; latent heat of condensation. For, every pound of water absorbed and condensed the heat released is obtainable from steam tables. For in stance, at a dew-point temperature of 60 F the amount of this , heat is about 1057 Btu. In addition to this heat there is involved also the socalled heat of mixing which is frequently considerable. The heat thus produced in the contacting packs of- the average air conditioning instal lation is to a large extent transferred to the chemical solution itself and ..the balance to the air being dried. A more complete cycle involves heat removal from the contacting medium, either within or' external to the interchanger. Thus the tem perature of the medium may be higher than, equal to, or lower than that of the air, depending on the agent used and the function to be performed. 450 COOLING, DEHUMIDIFICATION AND DEHYDRATION CHAPTER 24. In such a cycle, the dehydration process may be accompanied by cooling or heating, or neither, and such effect, if present, may be either a neces sary by-product of the process, or for the specific purpose of obtaining both latent and sensible heat removal simultaneously. In many in dustrial applications it is not necessary to aftercool the air. In applica- C0NCENTRAT10N. PER CENT 5n 40 50 IS . 20 25 CONCENTRATION, M0LAL Fig. 2. Temperature--Pressure--Concentrations for Lithium Chloride tions requiring low dry-bulb temperatures excess sensible heat must be remSionvceedtbhey aanbsaoftreprtcioonoleprr.ocess can continue only as long as there is a difference in vapor pressure between the absorbent and the air-vapor mixture and since at a given temperature of the absorbent the vapor pressure depends on the concentration of the solution, evidently there must be a relation between these quantities which if known would state the limits of the process. The relationship would also depend on. the absorbent being used, and would have to be determined for each sub stance used as an absorbent. This relationship is shown graphically in Tr:~ O'*- lithium chloride, and Fig. 3 presents similar data for calcium 451