Document BaOD8YdRYwbOo2x9J62952Kk

262 CHAPTER 17 1965 Guide And Data Book as activated alumina, silica gel, activated bauxites, and aeti' charcoaL have this property. The action of adsorbents, most of which adsorb some gases and condensable vapors besides i vapor, is selective. Thus, in the case of a mixture containing5 b- oth water and organic vapors, activated charcoal would remove__ organic vapors in preference to the water vapor, while the reversewould be true for the other adsorbents mentioned. The selective' property of adsorbents is utilised in some instances for the re moval of objectionable and contaminating vapors from an air or gas mixture. (See Chapter 11.) In other cases, the component adsorbed from a mixture may be the desirable one. St. wir s:'Sr sHs," '* I niHM-------H LIQUID ABSORBENTS A liquid absorbent has the property of absorbing'moisture from, or adding moisture to, the air, depending upon the vapor-pressure difference between the air and the solution. The equilibrium vapor pressure of the solution is dependent upon tiie temperature and concentration of the solution. Absorbents for a liquid dehumidifying system should have the following characteristics: 1. Suitable vapor-pressure characteristics. 2. Ability to remain in solution without formation of crystals at temperatures 10 to 15 deg below the temperature range used in the operating cycle. 3. Should be noncorrosive, odorless, nontoxic, and nonflam mable. 4. Stability. It should not break down within the range of uV, 5. Low viscosity and good heat-transfer characteristics. 6. Availability and low cost. 7. Capability of being regenerated at temperatures obtainable with low-pressure steam. The equilibrium dew point, plotted against contact tem perature for various concentrations of a typical organic liquid absorbent is shown in Fig. 3. The aum data for a typical inorganic absorbent are shown in Fig. 4. The concentration of the liquid absorbents used in commercial equipment is de pendent on the temperature of the cooling medium, and on the: latent and sensible heat-removal requirements. The equilibrium contact dewpoints in Fig3. 3 and 4 show the dew point of air in equilibrium with the liquid absorbent.' Equilibrium data are never quite achieved in. commercial* dynamic systems. Reference should be made to the published: performance data for commercial apparatus for the solution of air-conditioning and air-dehumidification problems. Typical performance data for commercial liquid absorbent dehumidi fiers are presented in Chapter 85 of the 1964 Guide And. Data Book. LIQUID ABSORPTION SYSTEMS Hie flow diagram for a typical liquid absorption system with extended-surface contactor coils is shown by Fig. 5. For dehumidifying operation the strong absorbent solution ;is pumped from the sump of the unit and sprayed over the, contactor coils. The hygroscopic solution at the required temperature and. concentration is in intimate contact with the air which is flowing over the coil surface in the same direction as the liquid . absorbent. Equipment is also available for flow countercurrent to the flow of liquid absorbent. Fig- 1 Moisture Content of Air at One Atmosphere Pressure Moisture is absorbed from the air by the solution due to the vapor-pressure difference between the air and the liquid-^ perature by cooling with city, well, refrigerated, or cooling- absorbent The moisture content of the outlet air can be" tower Water, or refrigerant flowing inside the tubes of the maintained at a constant condition by. automatically regii-' lating the flow of water through the cooler by ^ 0f a . modulating water valve. . . - .. contactor coil. The quantity of water required is a function of tiie water temperature and the total heat, either sensible, latent, or both, removed from the air by the hygroscopic The heat generated in absorbing moisture from the air con-"' solution. - (. sists of the latent heat of condensation of water:vapor. and\i ` The dry-bulb temperature of the air leaving the liquid ab the heat of solution, or the heat of mixing, of the wateriand ~ sorbent contactor at constant flow rate is a function of the the absorbent. The heat of mixing varies with the liquid ab-" temperature of the liquid absorbent and the amount of con sorbent used and with the concentration and temperature of"' tact surface between the air and the solution. In most' comthe absorbent. The solution is maintained at the required tern- . mercial equipment the dry-bulb temperature of the air leav- Dehumidification By Sorbent Materials 263 ITToi mg the dehumidifier will be within 1 to 5 F deg of the liquid absorbent temperature. `- Cooling and heating coils are installed in the ductwork lead ing from the dehumidifier if the,air is to be cooled or heated. The liquid absorbent ^maintained at the proper concen tration for moisture removal by automatically removing from tiie liquid the water vapor, absorbed from the air. A mudl quantity of the solution, usually 10 to 20 percent of the fiow` to the contactor coils, is passed over the regenerator coil where the liquid is heated' with steam or other hating medium. The liquid absorbents commonly used_ may be re generated with steam at 2 to'25 psig. The-vapor pressure of the liquid absorbent at temperatures corresponding to 2 psig / steam-is considerably higher than that of the-outdoor air. The hot solution at the relatively high vapor pressure is' in contact with outdoor air in the regenerator where water b ab sorbed from the-solution by the scavenger air due to the vapor-pressure-difference between the outdoor air and the hot solution. The hot-moist air from the regenerator b dis charged to the outdoors, and the concentrated solution flows to the sump. The solution b then ready for another cycle.' - The steam flow to the regenerator coil b regulated by a' control responsive to the concentration of-the1 solution circu lated overthe contactor coils' such as a level control,'specific gravity control, or boiling" point control. > . - .i- >' < ' For humidifying operations'the-liquid absorbent-b main-