Document 6RqEX0YwqZ991kjDvb5pRb9o6

452 Chapter 24 1-1945. Guide ---- The magnitude of this pressure is determined by the^temperature of the^ auction 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 opposing 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 poi#t 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, 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 Fig. 7. Typical Thermostatic Expansion Valve 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. Frequently 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 allowance 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 are 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 de signs: (1) air, (2) water, and (3) evaporative (combination air and \yater). Refrigeration 453 Air Cooled Air cooled , condensers are seldom used for capacities above 3 tons of refrigeration, unless an adequate water supply is extremely difficult to obtain, as, for instance, in railway air conditioning. Even on fractional tonnage installations, air is used as the condensing medium only where water is expensive or where simplicity of installation warrants the higher condensing pressure, and consequent higher power costs than would be obtained using water as the condensing medium.. The conventional air cooled condenser consists of an extended surface coil across which air is blown by a fan. The hot discharge gas enters the coil at the top and, as it is condensed, flows to a receiver located below the condenser. Air cooled condensers should always be located in a well ventilated space so that the heated air may escape and be replaced by cooled air. The principal disadvantages of air cooled condensers are the power required to move the air and the reduction of capacity on hot days. This loss of capacity due to high condensing pressures on hot days requires that equipment of increased capacity be selected to meet the peak load. Thus at normal loads the equipment is oversized. Water Cooled . Water cooled condensers are of the double pipe type, the shell and tube type, or the shell and coil type. Double pipe condensers are arranged so that water passes through the inner of two concentric pipes and refrig erant circulates through the annular space between the pipes. Where possible, there should be counter-flow of the refrigerant and the con densing" water to obtain maximum temperature differences. This type s is usualjy used only with small condensing units. The amount and temperature of the condensing water determine the condensing temperature and pressure, and indirectly the power required for comg^ssion. It is therefore necessary to determine a balance so that the quantity of water insures economical compressor operation. Because there is a decided tendency to conserve the water in city mains and because most large cities are restricting the use of water for air con ditioning and refrigeration equipment, it is often necessary to install cooling towers.or evaporative condensers. Cooling towers, unfortunately, produce the warmest condensing water at the time when the load on the system is greatest, so that the refrigeration equipment must be designed to meet the maximum load at abnormal condensing water temperatures. If properly designed, this makes little difference in the efficiency ofoperation throughout the year except at those times when the condensing water temperature is highest. As this occurs only for 5 per cent of the entire cooling period it can be disregarded as a factor in establishing yearly operating costs. The cooling tower has a certain advantage oyer the use of water from the city mains. Economies are possible when a cooling tower is used, which cannot be achieved by the use of condensing water from city mains. In certain localities, the lowest city water temperature met during the summer months is from 65 to 70 F. This, temperature range takes place for the entire cooling period, regardless of the outdoor temperature. With a cooling tower, the temperature of the condensing water may rise to 80 or 85 F under maximum conditions, but under less than maximum conditions the temperature of the water leaving the cooling tower drops