Document MJVJ65DZn2N58YNZMO5DrwngM

Heating Ventilating Air Conditioning Guide 1939 present in the solution are equal. Actually, no such solutions exist that deviations from Raoult's Law are always found in practice, 'j-v0 deviation is called positive when the observed pressure is greater than tht calculated from Raoult's Law, while the term negative deviation refe to the opposite case. Negative deviations are found wherever chemical attraction exists between the solvent and the solute. Positive deviatio occurs when there is a difference in the internal pressure of the com1 ponents, chemical attraction between them being absent. In order to make an effective absorption machine, large negative deviations from Raoult's Law must be shown by solutions of the refrio erant in the liquid absorbent, because, the larger the negative devia tion, the greater is the amount of refrigerant that can be cycled, usinv a given weight of absorbent. Cycling a large amount of refrigerant for a given weight of absorbent is important because of. the heat required to raise the temperature of the mixture and disassociate the refrigerant Fig. 11. Closed Absorption System and the absorbent. Only the latent heat of the refrigerant can be recovered for useful work. Many refrigerant-absorbent combinations have been -proposed and quite a number have been tested. Fig. 11 is a diagrammatic representation of a typical closed absorption system. In this system a mixture of refrigerant and absorbent is evaporated in the generator, passes to an analyzer and rectifier where it is purified, and then to a condenser where the refrigerant and remaining absorbent is condensed. It then passes through an expansion valve to an evaporator, where heat is absorbed from a cooling load. From the evaporator the vapor and residual ab sorbent passes to an absorber" where it meets absorbent which is ini tially low (weak) in refrigerant concentration. The absorbent absorbs the vapor, and the strong absorbent liquor is transferred to the generator through, an interchanger with the weak liquor returning from the generator. A cooling medium, ordinarily water, is used in the absorber to remove .. 448888 Chapter 23. Cooling and Dehumidification Methods the heat of absorption and maintain the absorptive power of the absorber at a maximum. , i -Lp the steam ejector system, the absorption system compares most rablv when a cheap source of cooling water and steam or other Lt source is available. Unlike the ejector system, the comparative rformance is usually best with a wide range of temperature between ^evaporator and absorber, since with a good refrigerant-absorbent mbination, the amount of heat and water required for a given refrig erating effect increases slowly with an increase of evaporator-condenser temperature range. At the present time the most used refrigerant-absorbent combinations e. (1) water and ammonia and (2) monofluorodichloromethane and dimethyl ether of tetraethylene glycol. With the latter combination the boiling points of the refrigerant and absorbent are sufficiently wide apart that almost pure refrigerant is obtained without the use of a rectifier. EVAPORATIVE COOLING Evaporative cooling is accomplished by passing air through a water spray in which the water is being continually recirculated. The air, entering in an unsaturated condition, evaporates a part of the water at the expense of the sensible heat. As this is an adiabatic transfer, the total heat content of the air remains constant, while the dew-point rises and the dry-bulb falls until the air is saturated. The reduction in dry-bulb temperature is a direct function of the wet-bulb depression of the air entering the spray chamber and the re sulting air temperature is governed entirely by the entering wet-bulb temperature of the outside air and the efficiency of the spray. Evaporative cooling is being used advantageously in many parts of the country. By using all outside air in sufficient volume to increase the air motion in an occupied space and to limit the temperature rise of the air to approximately 8 F an entering wet-bulb temperature as high as 70 F will result in effective evaporative cooling. THE REVERSE CYCLE The idea of heating by the reverse refrigeration cycle has captured the imagination of many people and has been much discussed. In principle, heat is absorbed in an evaporator from some available source of heat, pumped to a higher fefnperature and delivered to a condenser. The heat from the condenser is used for heating purposes. The compressor acts as a heat pump whose fundamental function is to raise the potential of the heat. The theoretical work of compression in relation to the heat delivered is: where Ti = absolute temperature of evaporator. Ti = absolute temperature of condenser. Thus, with a small spread of temperature between the evaporator and the condenser, 6 or 8 times as much heat may be obtained theoretically 489