Document vVw94r5BOgqxe5wDmw8XmV0rm

Heating Ventilating Air Conditioning Guide 1939 may be expanded after the fins are assembled, or the tube hole flanges of flat or corrugated fin may be made to override those in the precedin* fin and sc compress them upon the tube. There are also types of C0l5 struction where the fin is formed out of the material of the tube itself. I any case the successful performance of a fin surface depends upon th bond between fin and tube being" secure and remaining so in service6 For heating coils the materials most generally used are copper, steel and aluminum. Sometimes aluminum or brass fins are used on copper tubes. Steel is uncommon except in special cases. Some types of heatine coils are made of cast-iron. There are sufficient practical installations of each of these to demonstrate that they can all give good service. The fWater outlet . Chapter 24. Heat Transfer Surface Coils all space demand, but the resistance, both over the surface and through tubeSi is iiigher than with larger tubes and more widely spaced fins. Moreover, too close a fin spacing may result in trouble from dirt accumul ion especially on dehumidifying coils, and may also cause trouble f m water hold-up between the fins, particularly with air flow rtically upward. This condition increases the air resistance and deV eases the capacity of the coil. Water hold-up sometimes causes flooding trouble in vertical air flow units by accumulating too much water for the drain to handle all at once when the fan is stopped. Steam Coils For proper performance of steam heating coils, condensate and air must be continually 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 orifice in the steam header, or by perforated internal steam-distributing pipes extending Fig. 2. Various Water Circuit Arrangements copper coils are frequently tin-dipped and steel coils galvanized to protect them from corrosion and to assure a bond between fin and tube. For heating applications copper tubes and fins are the most commonly used. Cooling coils for water or for volatile refrigerants are most frequently of copper, both fin and tube. The' coil may or may not be tin-dipped, depending on the type of bond between tube and fin. 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 %, Yi< Y, or % in. OD, and the fins spaced from 4 to 8 per inch, 6 per inch being a. common design. The tube spacing generally varies from about lj-g to 2 in. on centers. Small tube size and close fin spacing give large capacity with 496 Fig. 3. Direct-Expansion Coil with Flooded System Fig. 4. Direct-Expansion Coil with Thermal Valve System 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 delivered air temperatures. The tendency for freezing of condensate at the bottom of the coil with cold entering air and light heating loads is also minimized. This is especially valuable for outside, air preheaters. Methods of air and condensate elimination are discussed in detail in Chapters 15, 16 and 22. 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 17. 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 blit are usually unnecessary as the