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482 CHAPTER 25 1948 Guide '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 maybe 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. : For heating coils the materials most generally used are copper, steel and aluminum.; Sometimes aluminum or brass fins are Used on copper tubes. Some types of heating coils are made of cast-iron. For equal performance brass and aluminum, firis- must be of greater thickness than copper fins because of their lower thermal conductivities. Copper coils are frequently Spiral fins oo h 1 oo ` Z o o z +- J- o o Flat corrugated fins Flat square fins Fig. 5. Types of Fin Coil Arrangement . 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 most frequently have copper fins and tubes, although 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. Although there are many variations for 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 or % in. OD, and the fins spaced from 4 to 8 per inch, 7 per inch being a common design. The tube spacing generally varies from about 1% 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 held between the fins, particularly with air flow vertically upward. This condition increases the air resistance and decreases the capacity of the coil.- Water hold-up sometimes causes flooding trouble in vertical air flow units by Accumulating too much-water for die drain to handle all at once when the fan is stopped. Radiators, Convectors. Coils 483 Steam Coils For proper performance of steam heating coils, condensate-and air must be continuously eliminated and the steam must be evenly distributed to the individual tubes. This distribution is usually accomplished by individual orifices in the tubes, by distributing plates and orifices in the steam header, or by perforated internal steam-distributing pipes extend ing into, the individual tubes. The latter arrangement has the advantage of distributing the steam throughout the length of each tube, and is Con ducive to uniform temperature of delivered air. The tendency of-conden sate to freeze at the bottom of the coil with cold entering air and light Fig. 6. Various Water Circuit Arrangements heating loads is also minimized. This is especially valuable.for outside airpreheaters. Methods of air and condensate elimination Are discussed in Chapters.23 and 26. Water Coils The performance of water coils, for heating or cooling, depends on the elimination of air from the system and proper distribution of water. Air elimination is taken care of in the system piping as described in Chapter 24. To assure a pressure drop sufficient for adequate distribution but at the same time to provide against excessive pumping head where lArge water quantities are handled, water coils are provided, with various water circuit arrangements. For instance, a typical coil 18 tubes high and 6 tubes deep in the direction of air flow can be arranged for .6, 9, 18 or 36 parallel water, circuits as conditions may require. Orifices in individual tubes are occasionally employed but are usually unnecessary as the resistance of individual water circuits is generally sufficient to effect a satisfactory distribution. In precooling coils using well water where there may be considerable sand and other foreign matter in the water,