Document VG88vJNXg3YQGmd4aqzqO4ddN

HEATINC VENTILATING AIR CONDITIONING GUIDE 1944 then passes through an expansion valve to an evaporator, where heat is absorbed from a cooling load. From the evaporator the vapor and resi dual absorbent passes to an absorber where it meets absorbent which is initially 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 the heat of absorption and maintain the absorptive power of the absorber at a maximum. Like the steam jet system, the absorption system compares most favorably when a cheap source of cooling water and steam or other heat is available. Unlike the steam jet system, the comparative performance is usually best with a wide range of temperature between the evaporator and absorber, since with a good refrigerant-absorbent combination, the amount of heat and water required for a given refrigerating effect in creases slowly with an increase of evaporator-condenser temperature range. At the present time the most used refrigerant-absorbent combinations are: (1) water and ammonia, and (2) dichloromonofluoromethane arid 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. EXPANSION VALVES The thermostatic expansion valve is a device to regulate the flow of liquid refrigerant so that the evaporator will always be used to best advantage. The evaporator coil must be kept as full as possible without any chance of liquid refrigerant entering the suction line. The expansion valve accomplishes this'by regulating the supply of refrigerant, so that' the temperature of the gas leaving the evaporator is always slightly higher than the temperature of the boiling refrigerant inside of it. This difference in temperature between the outgoing (suction) gas and the liquid refrigerant in the evaporator is called the superheat of the gas. The operation of the thermostatic expansion valve can best be ex plained by means of a diagram, Fig. 7. A small refrigerant charge.in the control bulb exerts a pressure through the tube to the upper side of the diaphragm, which tends to open the valve. The magnitude of this pressure is determined by the temperature of the suction gas leaving the evaporator, as- the control bulb is attached to the suction line at this point and is at approximately the same temperature. The suction pressure in the evaporator is transmitted through the equal izer tap and exerts an opposirig. force on the other side of the diaphragm in the. direction to close the valve. This pressure corresponds to the tem perature of the boiling refrigerant. The resulting force on the diaphragm is determined by the differential between the temperature of the suction gas and the boiling point of the refrigerant, which is the amount of super heat in the gas. If this temperature differential becomes greater (super heat increases), the resultant force on the diaphragm opens the valve and admits more refrigerant. The. reverse is true if the superheat decreases, 478 CHAPTER 25. REFRICERATION and the valve partly closes, thus admitting less refrigerant. The spring keeps the valve closed until the resultant force on the diaphragm cor responds to the desired superheat. The adjustment of the spring will change the amount of superheat to be maintained in the suction gas. The selection of the expansion valve is, of course, determined by the capacity of the valve. The capacity of a valve with a given orifice is determined by the refrigerant used, the differential of pressure across the valve and the amount the liquid is sub-cooled as it enters the valve. _ The expansion valves are usually rated at zero sub-cooling of the liquid, or 100 per cent liquid. Oftentimes special devices are used to properly distribute the refrigerant among the parallel paths of the evaporator. These' distributing devices usually have considerable pressure drop. Where they are used, the pressure drop across the expansion valve is not the difference between suction and discharge pressures, as allowarice must be made for the pressure drop across the distributing device. An equal izer connection from the evaporator suction line must be made to the underside of the diaphragm (see Fig. 7) whenever the valve outlet is not at the evaporator pressure so as to insure suction pressure at this point. When distributing devices tire used, this equalizer connection is essential for proper operation of the valve. Another pressure drop allowance must be made for the liquid line, particularly when the liquid line has an appreciable vertical rise. CONDENSERS Condensers used for liquifying the refrigerant are of three general designs: (1) air, (2) water, and (3) evaporative (combination air and . water). Air Cooled Air cooled condensers are seldom used for capacities above 3 tons of refrigeration, unless an adequate water supply is extremely difficult to .479