Document ykz7Y1YQEQDZjQ4LYYgzMe5Xn

HEATINC VENTILATING AIR CONDITIONING GUIDE 1944 valve actuated by a dew-point thermostat located in the washer air outlet or by humidity controllers located in the conditioned space. ATMOSPHERIC WATER COOLING EQUIPMENT In the operation of a refrigerating plant or a condensing turbine, one of the main problems is the removal and dissipation of heat from the compressed refrigerant or the discharged steam. This is accomplished ordinarily by first transferring the heat of the gas to water in a heat exchanger, from which water it may then be dissipated5'in a number of ways. If the plant is situated on the banks of a river or lake, an intake may be taken up-stream or at a considerable "distance from the discharge, to prevent mixing of the heated discha'rged water with the inlet water. If the source of cooling water is a city supply or a well, the discharge water may be run into'the nearest sewer or open waterway. Lacking an unlimited water supply, or in cases where city water is too expensive or where the water available contains dissolved salts which would form scale on the heat-exchanging apparatus, it is necessary to recirculate the water, and to cool it after each passage through the heat-exchanger by exposure to air in an atmospheric water cooling apparatus. Air has a capacity for absorbing heat from water when the wet-bulb temperature of the air is lower than the temperature of the water with which it is in contact. The rapidity with which this transfer of heat occurs depends upon (1) the area of water in contact with the air, (2) the relative velocity of the air and water, and (3) the difference between the wet-bulb temperature of the air and the temperature of the water. The rate of heat dissipation is influenced further by many small factors2 prevailing upon these primary ones, making the comprehensive design problem difficult. Selection of equipment for any specified service must ulti mately rest in overall economic considerations established from. reliable performance data. As the heat content of the air increases, its wet-bulb.temperature rises. (See Chapter 1.) Because it is impractical to leave the air in contact with water for a long enough time to permit the wet-bulb temperature of the air and the temperature of the water to reach equilibrium, atmos pheric water cooling equipment aims to circulate only enough air to cool the water to the desired temperature with the least possible expenditure of power. In an air washer, humidifier or dehumidifier, the air is first conditioned by water to change its moisture and temperature, and it is then sent to the place where it is to be used. In water cooling equipment the tem perature of the water is reduced by air, and the cooled water is carried to its. point of usage. In the air washer, an excess-of water is used to con dition a fixed quantity of air, while in water cooling equipment, an excess of air is used to cool a fixed quantity of water. Both types of equipment have a common basis of design, however, in that the size of the equipment is determined by the quantity of air that must be handled. With the air washer, the size of the equipment is fixed by the quantity of air to be conditioned, and the amount of conditioning *Cooling Tower Performance Studies, by L, M. K. Boelter (A.S.H.V.E. Transactions, Vol. 45, 1939. p. 615). 530 CHAPTER 27. SPRAY EQUIPMENT is controlled by the quantity and temperature of the water supplied and its method of application. With water cooling apparatus, its size and the quantity of air required bear no direct relation to the quantity of water being cooled, but vary through a wide range for different services and conditions. Sizes of Equipment Assuming a definite quantity of water to be cooled, the size and design of atmospheric cooling equipment are affected by the following primary conditions: . 1. Temperature range through which the water must be cooled. 2. Number of degrees above the. wet-bulb temperature of the entering air to which the water temperature must be reduced. 3. Temperature of the atmospheric wet-bulb at which the required cooling must be performed. 4. Time of contact of the air with the water. (This involves height or length of the apparatus and velocity of air.) 5. Surface of water exposed to each unit quantity of air. 6. Relative velocity of air and water. Items 1, 2, and 3 are established by the type of service and geographical location, while items 4, 5, and 6 depend upon the design of the equipment. The establishment of a proper cooling range depends upon: 1. Type of service (refrigerating, internal combustion engine and steam condensing). 2. Wet-bulb temperature at which the equipment must operate satisfactorily. 3. Type of condenser or heat-exchanger used. Because the design of an entire plant is usually affected by the quantity . and temperature of the cooling water supply, plants should be designed for cooling water conditions which can be most efficiently attained. The first consideration is usually the limiting temperature, of the plant. For example, if an ammonia compressor refrigerating plant is to be designed for 185 lb head pressure as a normal maximum, the limiting temperature of the ammonia in ,the condenser is 96 F. Should the ammonia tempera ture go above this figure the head pressure will exceed 185 lb and power Table 2. Condenser Design Data . Gas Maximum Pressure Desired in Condenser Gab Temperature or Condenser Deg F Leaving Hot Water Temperature Dbg F Best Condenser Design Average Condenser Design Steam. 28 in. vacuum____ Steam. 27 in. vacuum_____ Steam. _ 26 in. vacuum____ Ammonia!______ 185 lb gage head pressure___ Carbon dioxide.. 1030 lb gage head pressure__ _ Methyl 102 lb gage chloride___ head pressure___ Dichlorodi- 117 lb gage fluoromethane head pressure___ 101.2 115.1 125.9 96.0 86.0 100.0 100.0 97 110 120 92 83 96 96 93 105 114 88 81 92 93 531