Document 5LbvzV9JaZMgrKann8xBL61nD

880 CHAPTER 35 1957 Guide 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 dewpoint, and hence, a dew point control can be arranged by using a simple duct thermo stat. Spray dehumidifiers have an advantage over unwetted coils of ob taining' some air cleaning and odor absorption. On the other hand, coils make possible a closed and balanced cooling water circuit, obviating the unbalanced pumping 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 can be used with direct expansion of refrig erant, 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. 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 round 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 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 Air Heating and Cooling Coils 881- 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 and the tube. An intimate contact between the tube and the fin 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^ 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 wherfe sodium orj Calcium chloride brine is circu lated in the tubes. Aluminum fins on copper tubes are a common con Spiral 8ns OQ OO Flat continuous 8ns O O f>i tntp o o OO oO Flat corrugated fins Flatsqwrafins Fig. 1. Types of Fin Coil Arrangement struction. 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-g to 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 possibility of lint accumulation and, especially .in debumidifying, the consideration of frost accumulation. Steam Coils j For proper performance of steam heating coils, condensate and air: must be continuously eliminated and the steam must be evenly distributer!' to the individual tubes. This distribution is'usually accomplished by indi vidual orifices in the tubes, by distributing plates and orifices in -the steam fieader, or by perforated internal;steam-distributing pipes: extending into the individual tubes. The latter arrangement has the advantage of dis tributing the steam throughout the length of each tube, and is conducive to uniform temperature of delivered-air. The tendency of condensate to freeze at the bottom of the coil with cold entering air and light- heating