Document 7M7GaMMovzLnzpxe667XG1opa

Heating Ventilating Air Conditioning Guide 1939 times, also, brine from an industrial system already installed, is the only convenient source of refrigeration. For combined cooling and dehumidifying, surface coils present an alter nate to spray dehumidifiers. For many applications it is possible, by proper selection of apparatus, choice of air velocities, refrigerant temperatures, etc., to perform the same duty with either. In a few cases both sprays and 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 saturated or nearly saturated air, tends to simplify the control problem. In this case the dry-bulb temperature is also the the dew-point, and hence a dew-point control can be arranged by using a simple duct thermostat. Spray dehumidifiers have the advantage over unwetted coils of a certain degree of 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. Surface-coil dehumidifiers seldom deliver saturated, air, and wet bulb depression of 0.5 to 4 F (or more) is usual. 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. Of course the safety of the occupant must be kept in mind in comfort con ditioning applications. Some localities have refrigeration codes which restrict the use of direct-expansion 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 dehu midifiers and coils depends upon the necessities and the economic aspects of each case and no general rule can be given. There are many instal lations in which either can be used. COIL CONSTRUCTION AND ARRANGEMENT Coils are basically of two types,-those consisting of bare tubes or pipe and those of extended surface construction. The former are little used for the applications covered by this chapter, but are often employed where conditions cause frost accumulation, and for cooling surface within spray dehumidifiers. The heat transmission from air passing over a tube to a refrigerant flowing within it, is impeded by three resistances. The same is true when the air is being heated by steam or hot water in the tube. The first resistance is from the air to the surface of the tube, usually called the outside surface resistance or air-film resistance. Second is the resistance to the flow of heat by conduction 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, 494 Chapter 24. Heat Transfer Surface Coils and the inside surface or film resistance are usually low as compared with the air-side surface resistance. This is especially the case where sensible heating or cooling only is accomplished. Where dehumidification accomanies sensible cooling, or where the external surface of the tube is sprayed with large quantities of water, the resistance to heat flow between the tube and the air flowing over it is much decreased. In the case of the water spray, the surface resistance depends on the amount and the method of application of the water. Economy in space, weight and cost make it advantageous to decrease the external surface resistance, where it is proportionately large, to approach that of the tube wall, and that from tube to refrigerant. This is accomplished in heating coils by increasing the external surface by means of fins. For cooling or dehumidification without external water spray, fin surface is also commonly used. With water spray the external resistance is already low, and the fins are less useful for increasing the overall heat transfer. Sometimes water spray is Spiral fins O O stt: 3 0 0 Z OO 1 1 I 1m.1 ^o Vo Flat corrugated fins Flat square fins Fig. 1. Types of Fin Coil Arrangement applied to the same type surface as would have been used without it. The overall heat transfer is not necessarily increased much by such an arrangement, but the water spray may serve other purposes than to in crease 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. In some cases these are staggered and in others in line with respect to the air .flow. The staggered arrangement gives a somewhat higher heat transfer value but also a higher resistance to air flow and in some cases makes the header and return bend arrangement more complicated. A number of types of fin arrangement are used, the most common of which are spiral, flat and flat-crinkled or corrugated, all as shown in Fig. I. While the spiral fin surrounds each tube individually in all cases, the flat types may be con tinuous (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 requirements and resistances of individual designs of coils. A most important factor in the performance of extended surface coils is the bond between the fin and the tube. An intimate contact is assured in a number of ways. The assembled coil may ue coated with tin, zinc, etc., after fabrication. The spiral type fin may be knurled into a shallow groove on the exterior of the tube. The tube 495