Document ppdnRJzpjJ01k7VXbG3RON47

784 CHAPTER 36 1953 Guide tures, etc., to perform the same duty with either. In a few cases both sprays arid coils are used. The coils may then be installed within the spray chamber, either in series with the sprays or below them. In making the selection between spray and surface dehumidifiers, certain advantages of each should -be considered. The fact that a spray dehumidifier is usually designed to deliver nearly saturated air, tends to simplify the control prob lem. In this case the dry-bulb temperature is also the dew-point, and hence, a dew-point control can be arranged by Usirig a simple duct thermo stat. Spray dehumidifiers have an advantage over uriwetted boils of ob taining some air cleaning and odor absorption. On the other hand, coils make possible a closed and balanced cooling water circriit, obviating the unbalanced punaping head, the complication of water level control, and danger from possible floods incidental to multiple spray dehumidifiers, especially if located on different levels. The use of coils often makes it possible for the same surface to serve for summer cooling and winter heating by circulating cold water in the one season and hot water in the other, with consequent saving in apparatus and piping. Another advantage, .is that where the surface coil system cari be used with direct, expansion of refrig erant,it is comparatively low in initial and operating, costs. The safety of the occupant must be kept in mind in comfort conditioning applications. Some localities have refrigeration codes which restrict the use of directexpansion coils in the air stream; and hence, local codes should be consulted by the engineer before a system employing direct expansion methods is designed. The choice between spray dehumidifiers and coils depends upon the necessities and the economic .aspects of each case, and no general rule can be given. There are many installations in which either may be used. COIL CONSTRUCTION AND ARRANGEMENT Coils are basically of two types, those consisting of plain tubes or pipe, and those having extended surfaces. The former are little used for the applications covered by this chapter, but are often employed where condi tions cause frost accumulation, and for cooling within spray dehumidifiers. . The heat transmission from air passing over a tube to a fluid flowing within it is impeded by three resistances. The first is that from the air to the surface of the tube and is usually called the outside surface resistance or air-film, resistance. The second is the resistance to the conduction of heat through the metal itself. Finally: there is. another surface or film resistance to the flow of heat between the inside surface of the metal and the fluid in the tube. For the applications under consideration both the resistance of the metal wall to heat conduction, and the inside surface or film resistance are usually low as compared with the air-side surface resist ance. Economy in space, weight and cost makes it advantageous to de crease the external surface resistance, where it is proportionately large, to approach that of the tube wall, and that from the tube to refrigerant. This may be accomplished by increasing the external surface by means of fins Sometimes water spray is applied to the same type surface as would have been used without it. The overall heat transfer is not necessarily increased much, but the water spray may serve other purposes than to increase the flow of heat, such as air and coil cleaning. In fin or extended, surface coils the external surface of the tubes is known as 'primary, and the fin surface is called secondary. The primary surface consists generally of rourid tubes or pipes which may be staggered, or in some cases placed in line with respect to the air-flow. ' The staggered arrangement is usually preferred because it obtains a somewhat higher heat Air Heading and Cooling Coils 785 transfer-value.. Numerous: types of , fin arrangement are,.used,; the most common of which are spiral,, flat-and flat-crinkled or corrugated, all - as shown in Fig. 1. While the spiral fin surrounds each tube individually .in all cases, the flat types may be continuous (including several rows of tubes); or they, may be round or square, with individual fins for .each tube. All of these, as .well as other less common;types, are.in use, the selection for a particular installation being based on economic considerations,, space re quirements and resistances of individual designs of coils. A most impor tant factor in the performance of extended surface coils is the bond between the fin arid the tube. An intimate contact between the tube-and: the firi must, be maintained permanently in order to assure a continuing, rated performance after the heating units have been in service for. a period, of time. In some coils, fins are Wound on the tubes under pressure, in order to upset the metal slightly at the fin root,- and then are given a coating of solder while the fin and tube are still revolving, for the purpose of assuring a uniform coating of solder. In other types, the spiral fin may be knurled into a shallow groove on the exterior of the tube. : The. tube may be ex- Spiral (ins Flat continuous fins o o s: '3 O O Z oo OO Flat corrugated fins Flat square fins .Fio. 1 Types op Fin Coil Arrangement panded after the fins are assembled, or the tube hole flanges of a flat or corrugated fin may be made to override those in the preceding fin and so compress them upon the tube. There are also_types of construction where the fin is formed out of the material of the tube itself. ....... ^ For heating coils, materials most generally used are copper and aluminum. Steel is occasionally used where sodium or calcium chloride brine is circu lated, in the tubes. Aluminum, fins on copper tubes are a common ;construetion. : Generally speaking, brass does not serve as a satisfactory fin material, because of corrosion difficulties. Cooling coils for water. ,or for volatile, refrigerants most frequently have copper fins and tubes; although aluminum fins on copper'tubes;are also used. There are many makes of heating and cooling coils of ;the light weight extended surface type for both heating and cooling with tubes commonly , f, , and 1 in. outside diameter, and with fins spaced three per inch up to eight per inch. .The tube spacing generally varies from about 1 to 2\ in. on centers, depending upon the width of individual fins and on.other considerations of performance. Fin spacing should be chosen for the duty to be performed, with special atten tion .being paid to lint accumulation and, especially in dehumidifying, the consideration of frost accumulation. ' Steam Coils For proper performance of steam heating coils, condensate and' air miist be continuously eliminated arid the 'steam must be evenly distributed to the individual'tubes. This distribution is usually accomplished by iridi-