Document 669e3q28ydJwypy2Gv2NdDGd

466 Chapter 25 1945 Guide -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 multiplespray 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. Of course 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, 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 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, 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 tube to refrigerant. This is 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 trans fer 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. 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. 1. While the spiral fin surrounds each tube individually in all cases, the flat types may be con- IIcat Transfer Surface Coils 467 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 be 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 may be expanded 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 con struction where the fin is formed out- of the material of the tube itself. In any case the successful performance of a fin surface depends upon the bond between fin and tube being secure and remaining so in service. For heating coils the materials most generally used are copper, steel and aluminum. Sometimes aluminum or brass fins are used on copper tubes. Spiral fins Flat continuous fins OO "9 O O OO z L IT O O flat corrugated fins Ftat square fur Fig. 1; Types of Fin Coil Arrangement Steel is uncommon except in special cases. Some .types of heating coils are made of cast-iron. There are sufficient practical installations of each of these to demonstrate that they can all give good service. However for equal performances brass and aluminum fins must be of greater thickness than copper fins on account of their lower coefficients of conduction. The copper coils are frequently tin-dipped and steel coils galvanized to protect them from corrosion and to assure a bond between fin and tube. Cooling coils for water or for volatile refrigerants are most frequently of copper, both fin and tube. Aluminum fins on copper tubes are also used; For brines such as sodium or calcium chloride and for ammonia, steel fins and tubes are common. ;>miuugu uiere are many variations tor special cases, tube and fin sizes and spacings for air conditioning coils, both heating and cooling, fall within fairly narrow limits. The tubes are usually %, Yi, or % in. OD, and the fins spaced from 4 to 8 per inch, 7 pier inch being a common design. The tube spacing generally varies from about iy% to 2 in. on centers. Small tube size and close fin spacing give large capacity with small space demand, but the resistance, both over the surface and through the tubes, is higher than with larger tubes and more widely spaced fins. Moreover, too close a fin spacing may result in trouble from dirt accumu- lation, especially on dehumidifying coils, and may also cause trouble from water hold-up between the fins, particularly with air flow vertically upward. This condition increases the air resistance and de-