Document RJXx427Mr0kQo2jqE8pBbgypv

sssg 312 CHAPTER 22 1959 Guide attic ventilating fans (ASHVE Transactions, Vol. 40. 1934, p. 155). A. P. Krats and 8. Konzo: ASHVE RefigABr Report No. 979--Study of summer cooling in the research residence for the summer of 1933 (ASHVE Transactions, Vol. 40, 1934, p. 167). W. A. Hinton and A. F. Poor: ASHVE Research Report No. 1198---The effect of attic fan operation on the cooling of a structure (ASHVE Transactions, Vol. 48, 1942, p. 145). W. H. Badgett: The Installation and Use oj AUic Fans (Agricultural and Mechanical College of Texas Bulletin No. 52, 1940). W. A. Hinton and W. G. Wanamaker: Some, effects of attic fan operation on comfort: (ASHVE Transactions, Vol. 50, 1944, p. 371). BIBLIOGRAPHY A. H. Church: The Centrifugal Pumps and Blowers (John Wiley A Sons, New -York).' Pan Engineering (Buffalo Forge Co.). Theodore Baumeister, Jr.: Fans (McGraw-Hill Co,, New York). Curt Keller: The Theory and Performance of Axial Plow Fans, adapted by L. S. Marks and J. R. Weske (McGrawHill Co, New York). `3 t 3 Uses for Coifsf Coif Construction and Arrangement; Steam Coils, Wofer Coils, Direct-Expansion Coils, Flow Arrangement, Applications; Coil Selection; Heating, Cooling, and Dehumidifying Coils/ Heat Transfer and Air-How Resistance; Performance of Heeding and Dry Cooling Coils; Overall Coefficient of Heat Transfer; Performance of Dehumtdrfying CotJ^,* Determining Refrigeration Load COILS described in this chapter are used for heating or below them. In making the selection between spray and or cooling an air stream under -forced convection. surface dehumidifiers, certain advantages of each should Surface-coil equipment may be made up of a number ofbe considered. The fact that a spray dehumidifier is usu banks assembled in the field, or the entire assembly may ally designed to deliver nearly saturated air, tends to be factory constructed. The applications of each type of simplify the control problem. In this case the dry-bulb coil are limited to the field within which it is rated. Other temperature is also the dew point, and hence, a dew-point limitations are imposed by code regulations, by proper control can be arranged by using-a ample duct thermo choice of materials for the fluids used and the condition of stat. Spray dehumidifiers have an advantage over unwetted the air handled, or by an economic analysis of the possible coils of obtaining more air cleaning and odor absorption. alternates for each installation. On the other hand, coils make possible a closed and balanced USES FOR COILS cooling-water circuit, obviating the unbalanced pumping head, the complication of water-level control, and danger For heating service, coils are used as tempering coils, preheaters, reheaters, or booster beaters. The function of the coils is air heating only, but the apparatus assembly may include means for humidification and air cleaning. Steam or hot water are the usual hating media, although others are used in special cases, such as reheating by means of discharge gas from a refrigerating system. Coils are used for air coding with or without accompany ing dehumidification. Examples of cooling applications with out dehumidification are precooling coils using well water or other relatively high temperature water to reduce the from possible floods incidental to multiple-spray dehumidi fiers, especially if located on different levels. The use of coils often makes it possible for the same surface to serve for summer cooling end winter heating by circulating cold water in one season and hot water in the other, with conse quent saving in apparatus and piping. Another advantage is that where the surface-coil system can be used with direct expansion of refrigerant, it is comparatively low in initial and operating costs. For comfort-conditioning applications, the regulations of the applicable national and local safety codes should be consulted by the design engineer. load on the refrigerating machinery, or water-cooled coils removing sensible heat in connection with chemical mois COIL CONSTRUCTION AND ARRANGEMENT ture-absorption apparatus. A major portion of coil equip Coils are basically of two types, those consisting of plain ment is designed to handle both sensible cooling and de- tubes or pipe, and three having extended surfaces. The humidification. The assembly usually includes air cleaning former are little used for the applications covered by this means to protect the coil from accumulation of dirt, and to chapter, but are often employed where conditions cause frost keep dust and foreign matter out of the conditioned space. accumulation, and for cooling within spray dehumidifiers. Although cooling and dehumidification are the usual func The heat transmission from air passing over a tube to tions, cooling coils are additionally and purposely wetted a fluid flowing within it is impeded by three resistances. to aid in air cleaning and odor absorption. The first is that from the air to the surface of the tube and The usual cooling media used in surface coils are cold is usually called the outside surface- resistance or air-film water or Group I (ASA Classification) refrigerants, but resistance. The second is the resistance to the conduction others are used in special cases. Brines are seldom required of heat through the metal itself. Finally there is another for the range of applications covered by this chapter, al surface or film resistance to the flow of heat between the ^ though there are cases, where low entering-air temperatures inside surface of the metal and the fluid in the tube. For with large latent heat loads require a .refrigerant tempera the applications under consideration both the resistance ture so low that use of water becomes impracticable. Some of the metal wall to heat conduction, and the inside surface times, also, brine from an industrial system already installed or film resistance are usually low as compared with the is the only convenient source of refrigeration. air-side surface resistance. Economy in space, weight, and For combined cooling and dehumidifying, surface coils cost' makes it advantageous to decrease the external sur present an alternate to spray dehumidifiers. For many face resistance, where it is proportionately large, to ap applications it is possible, by proper selection of apparatus, proach that of the tube wall, and that from the tube to choice of air velocities, refrigerant temperatures, etc., to refrigerant. This may be accomplished by increasing the perform the same duty- with either. In a few cases both external surface by means of fins. Sometimes water spray sprays and coils are uged. The coils may then be installed is applied to the same type surface as would have been within the spray chamber, either in series with the sprays used without it. The overall beat transfer is not necessarily ' 313