Document B58807Zwynv3BbzjMy3gO4aYE

480 CHAPTER 25 1948 Guide - such pans, when a radiator is steam heated and the relative-humidity in the room is between 25 and 40 per cent. This source of supply of moisture alone is not adequate to maintain a relative humidity above 25 per cent on a zero day. ' COILS Coils described in this chapter are- used for heating or cooling an air stream under forced convection. Surface coil equipment may be made up of a number of banks assembled in the field, or the entire assembly may be factory constructed.. The applications of each type of coil are limited to the field within which it is rated. Other.limitations are imposed by code regulations, by proper choice of materials for the fluids used and the condition of the air handled, or by an economic analysis of the possible alternates on each installation. For heating service, coils are used as preheaters, reheaters or booster heaters. 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 heating media, although others are used in special cases, such as reheating by means of discharge gas from a refriger ating system. Coils are used for air cooling with or without accompanying dehumidi fication. Examples of cooling applications without dehumidification are precooling coils using well water or other relatively high temperature water to reduce the load on the refrigerating machinery, or water cooled coils to remove sensible heat in connection with chemical moistureabsorption apparatus. By proper coil selection it is possible to handle both sensible cooling and dehumidification together as explained later. The assembly usually includes air cleaning means to protect the coil from accumulation of dirt and to keep dust and foreign matter out of the con ditioned space. Although cooling and dehumidification are the usual . functions, there are cases of cooling coils purposely wetted to aid in air . cleaning and odor absorption. The usual cooling media used in surface coils are cold water or Group I (ASA Classification) refrigerants, but others are used in special cases. Brines are seldom required for the range of applications,covered by this chapter, although there are cases where low entering air temperatures with large latent heat loads require a refrigerant temperature so low that water becomes impractical. Sometimes, 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 tem peratures, 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 dehumi difier is usually designed to deliver nearly saturated air tends to simplify the control problem. In this case the dry-bulb temperature is also the dew-point, and hence a dew-point control can be arranged by using a simple duct thermostat. Spray dehumidifiers have an advantage over - unwetted coils of obtaining some air cleaning and odor absorption. On * Radiators, Convectors, Coils 481 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 refrigerant, 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 direct-expansion coils in the air stream, arid hence local codes should be consulted by the engineer before a system employirig 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 cari 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 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, and the inside surface or film resistance are usually low as compared with the air-side surface resistance. Economy in space, weight and cost makes it advantageous to decrease the external surface resistance, where it is proportionately large, to approach that of the tube wall, and that from the tube.to refrigerant. This riiay 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 over-all heat transfer is not-necessarily increased much by such an arrangement, but the water spray may serve other purposes than to increase the flow of heat, such as air and coil cleaning. (See Chapter 7). 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 ar.e 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. 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. 5. While the spiral fin surrounds each tube individually in all cases, the flat types'may be corn 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