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872 CHAPTER 37 1956 Guide designed to operate with reasonable efficiency at capacities as low as 20 per cent of normal load. 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 water is expensive, or where simplicity of instal lation warrants the higher condensing pressure and consequent power costs higher 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 con densers 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. The principal advantages are low installation costs and simplicity, and for these reasons they are frequently used in small self-contained units. 2. Water cooled condensers are commonly used with compressors of one horsepower or larger in size, and they are found almost exclusively on large installations. They usually prove to be the most economical choice if an adequate water supply and means for its disposal are available. Although water cooled condensers may be of many designs, the shell and coil and the shell and tube are most commonly found in present day practice. 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 conditioning 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 equip ment 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 year, except at those times when the condensing water temperature is highest. As this occurs only for 5 percent of the entire cooling period, it can be disregarded as a factor in establishing yearly operating costs. For further information on cooling towers, reference may be made to Chapter 35. 3. Evaporative condensers were developed to alleviate the over-burdened water supply and drainage facilities of communities where many small air conditioning systems using water cooled condensers were applied. The adaptation of cooling towerB to small installations is not practicable. The evaporative condenser combines the functions of the two by using a minimum amount of water on a finned surface, cooling it to approximately the wet-bulb temperature of the surrounding atmos phere. i The end view of a typical evaporative condenser is shown in Fig. 12. The fan draws the air over a finned tube condenser which is kept wet by a water spray. The discharge refrigerant gas from the compressor enters the top of the condenser coil, and the liquid refrigerant is drained from the bottom of the coil into a liquid receiver, and then circulates through the remaining portion of the system in the usual way.! The water is circulated through the spray nozzles, and the level is maintained in the sump by means of a float valve. The eliminator plates are placed in the path oft Refrigeration 873 the water-air mixture so as to remove the entrained water. The air leaving the unit is almost completely saturated, so that care must be taken in locating discharge ducts to prevent condensation. Evaporative condensers arc available in sizes up to 100 tons or more. These units use only a small portion of the water required for a water cooled condenser. The water is vaporized by the heat of the refrigerant so that each pound of water used extracts approximately 1000 Btu from the refrigerant, whereas under standard rating conditions where the water temperature rise is 20 F, each pound of water extracts only 20 Btu from the refrigerant. Including the water lost by entrainment Fig. 12. Schematic View op an Evaporative Condenser .T in the discharge air, by overflow and stand-by evaporation, the water used is about 3 to 5 percent of the amount that would be required for a water cooled condenser. The evaporative condenser requires more maintenance, occupies greater space (must be located where air is available), and has a higher first cost than the water cooled condenser, but where the use of water is restricted or expensive, the evapora tive condenser has become widely accepted. Compared with a water cooled con denser and cooling tower, which combination uses about the same quantity of water, the evaporative condenser has the advantage of lower cost and smaller space re quirements: Evaporators and Coolers Refrigeration evaporators must be designed for efficient removal of heat from the medium being cooled, as well as effective boiling of the refrigerant and a minimum drop of pressure through the coil. There are two general types of evaporators, dry and flooded. In the dry evaporator the re frigerant enters in the liquid state, and the design provides for complete