Document G6oqvG8Kjw3JqYXbqg6J1QdNx

742 CHAPTER 45 1965 Guide And Data Book liquid subcooling is read on the horizontal scale at the bottom of Fig.-41 as 12 F deg. REFERENCES 1 M. M. Bolstad and R. C. Jordan: Theory and use of the capiUary tube expansion device (Refrigerating Engineering^ December 1948, p. 519). * N. E. Hopkins: Rating the restrictor tube (Refrigerating Engineering, November 1950, p. 1087). 1 H. Al Whitesel: Capillary two-phase flow (Refrigerating Engineering, April 1957, p. 42)." 4 H. Ai' Whitesel: Capillary two-phase flow--Part II (Re frigerating Engineering, September 1957, p. 35). _ ; BIBLIOGRAPHY M. M. Bolstad and R. C. Jordan: Theory sad use of the capil- laryexpansion device--Part II, Nonadiabatic flow (Refrigerat ing Engineering, June 1949, p. 577): L. Cooper, W. R. Brisken, and C. K. Chu: Single selection method for capillaries derived from physical flow conditions (Refrigerating Engineering, July 1957, p. 37). H. F. Lathrop: Application and characteristics of capillary tubes (Refrigerating Engineering, August 1948, p. 129). G. P. Marcy: Pressure drop with change of phase in a capillary tube (Refrigerating Engineering, January 1949, p. 53). J. R. Prosek: A practical method of selecting capillary-tubes (Refrigerating Engineering, June 1953, p. 044). F. G. Smith: Turbulent flow of air through capillary tubes (Refrigerating Engineering, October 1956, p. 48). L. A. Staebler: Theory and use of a capillary tube for liquid refrigerant control (Refrigerating Engineering, January 1948, p. 54). L. A. Staebler: The CapiUary Tube and its Applications to Small Refrigerating Systems {Refrigeration Service Engineers Society Service Manual, Section 18, 1950). R. H. Swart: Capillary tube beat exchangers (Refrigerating Engineering, September 1946, p. 221). . Safety Code for Mechanical Refrigeration ASA B9.1 (American Standards Association, Inc., New York, N. Y.). CHAPTER 46 COOLING TOWERS AND SPRAY PONDS Types of Cooling Equipment, Comparing Types of Equipment, Coo/mg Tower Theory, Tower Coefficients, Tower Choree* femtics, Design Conditions, Performance Curves, Selection and Evaluation, Installation and Operation MOST industrial processes generate waste heat that about a 10 deg approach of cold water temperature to the wetmust be removed and dissipated. Small quantities are bulb temperature. readily rejected directly to the atmosphere, but large beat The water leaving a spray nozzle will rise to a height of loads usually rely on a flow of cooling water. The water, if approximately 1 ft per psi of nozzle pressure. It will stiite cheap or plentiful, may be wasted, or it may be cooled and recirculated through the system. the surface of the pond in a circular pattern having a radius of about 10 ft. The cooling occurs within an activefilled volume, TYPES OF COOLING EQUIPMENT having a height equal to the elevation of the nozzles above the surface plus 1 ft per psi nozzle pressure, and a plan area ex A cooling tower is a steady-flow device that utilizes a com bination of mass and energy transfer to cool water by exposing it as an extended surface to the atmosphere. The water surface is extended by spraying, which produces drops, or by Ailing, which presents a Aim surface or creates drops due to splashing. The air flow may be caused by natural wind currents, con vection currents due to variations in density, or by nWhAmryl means. The air flow may be cross-flow or counter-flow to the foiling water. The types of cooling equipment are: 1. Spray ponds 2. Atmospheric towers' a. Packed or spray-filled b. Air flow induced by wind or stack effect 3. Mechanical draft towers a. Packed or spray-filled b. Forced or induced draft c. Cress-flow or counter-flow -- tending 10 ft beyond the outer nozzles. The heat is trans ferred to the quantity of air passing through the air area, which is the projected area of a vertical plane through the active filled volume and broadside to the direction of the air movement. The length of air travel is the horizontal distance the air moves through the filled volume. A reasonably accurate estimate of spray pond performance can be based on the outgoing wet-bulb temperature of the air passing through the filled volume.-This temperature, obvi ously, cannot exceed the hot water, temperature.. It can ap proach the hot water temperature if it passes lengthwise through a long spray pond, but its' temperature will rise slightly when it passes broadside through a narrow pond. The values in Table 2 can be used to obtain a reasonably accurate estimate of the cold water temperature with respect to the calculated outgoing wet-bulb temperature. ' Spray Ponds Heat is dissipated from the surface of a body of water by evaporation, radiation, and convection. A spray pond divides the water into small drops, greatly extending the water sur face, and brings it into intimate contact with the air.1 The heat transfer is largely due to .evaporative cooling, as ex plained in the section. Cooling Tower Theory. The driving force is the difference in enthalpy rather than the temperature difference. The water temperature tends to approach the wetbulb rather than the dry-bulb temperature of the air. This offers an inherent advantage in making it possible to cool the water to a temperature lower than the dry-bulb. Cooling occurs in a spray pond as die water is propelled Example l: Consider a spray,pond consisting of 3 laterals on 25 ft centers. Each lateral contains 8 lengths of 12 ft pipe,-with 6 spray nozzles per length, or a total of 144 nozzles. The spray sys tem is 6 ft above the surface of the pond, operates at 6 psi, and circulates 6000 gpm. Determine performance with a 5 mphwind, both broadside and from the end, with 10 F cooling range and 73 F wet-bulb temperature. Solution: The heat load is 6000 X 10 X 8.33 = 500,000 Btu per min. The spray system covers an area 96 ft long by 50 ft wide and the spray extends an average of 5 ft beyond on all aides, so the overall dimensions are 106 ft X 60 ft. The spray rises 6 ft (1 ft per psi), so the total spray height is 12 ft. The broadside air area is 106 ft X 12 ft - 1270 sq ft. The end area is 60 X 12 ft = 720 sq ft. Wind at 5 mph corresponds to 440 fpm. The specific volume at 93 F wet-bulb (assuming 92 F dry-bulb) is 14.2 cu ft per lb diy air. " upward and then falls to the surface.of the pond. The pond itself acts largely as a collecting Haain A typical nozzle ar rangement is shown in Fig. 1. The dwrign criteria in Table 1 provide a guide that can be used in laying out a pond. The wind varies constantly in both direction And velocity. The wet-bulb temperature is usually also changing These variations result in a continually changing temperature of the water as it strikes the surface of the pond. The pond acts as a reservoir so that the water temperature at the suction of the pump represents an average of past operating conditions. These characteristics make it difficult to test a spray pond or to develop ratings. As a result, spray ponds are not used when water temperatures must meet narrow limitations. They are usually designed for a 10 to 15 F deg cooling range *nd for Tcn^ far thia chapter k awcaed to TC Coofing 743