Document 2O9o6B5DZbGYOZd6jpyOe2oR

714 CHAPTER 39 1948 'Guide compressor with increase in evaporator temperature: - Similarly, the capacities of the steam ejector and centrifugal compressors increase more rapidly than those of the reciprocating compressor with increase inevaporator temperature. Thus, both the steam jet and centrifugal machines tend to be more self-regulating than the reciprocating. It is also evident from Fig. 7 that the steam jet equipment is best suited for operation at high evaporator temperatures. The effect of condenser temperature upon the power and capacity of the different types of compressors is shown in Fig. 8. It may be noted that the power required by the reciprocating compressor increases rapidly with increase in condenser temperature, while the power curve for the Fig. 8. Performance Characteristics of Compression Refrigeration Machines at Constant Speed centrifugal compressor is relatively flat. It is also evident that the capacity of the steam jet compressor is independent of condenser tem perature until a certain point is reached where it drops to zero. As previously stated, isteam jet equipment requires more condensing water than other types of compression systems. ' Consequently, steam jet systems are well suited to those applications where condensing water is cheap, or where condensing water is rather high in temperature. Condensers Condensers used for liquefying the refrigerant are of three general de signs: (1) air cooled, (2) water cooled, and (3) evaporative (combination air and water). 1. 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 Refrigera tion 715 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. 2. 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 refrigerant circulates through the annular space between the pipes. Where possible, there should be counter-flow of the refrigerant and the condensing water to obtain maximum temperature differences. This type is usually 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 compression. 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 of operation throughout the ye&r 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 over 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 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 considerably. It has been established that in these localities. during 50 per cent of the time, the outdoor wet-bulb temperature varies from 60 to 70 F and the cooling tower water, for the same periods, varies from 65 to 75 F. When the outdoor wet-bulb temperature drops below 60 F, ' which occurs approximately 30 per cent of the time, the condensing water temperature is still lower. The cost of water used for condensing is small as the only water required is that used to make up the loss by evaporation