Document K6GgYaj9Xj7DDRZLo0Y5GJbdK

146 CHAPTER7 1946 Guide * 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 Water outlet | Heat- Transfer Surface Coils 1*7 provision for cleaning of individual tubes is of advantage. It is important to arrange water coils for complete drainage if located where they will be exposed to freezing. Fig. 2 shows such construction. The drains may be provided in the water piping although they are often arranged in the coil headers. Direct-Expansion Coils Coils for volatile refrigerants present more complex problems of fluid distribution than do water, brine or steam. It is desirable that the coil be effectively arid uniformly cooled throughout, and necessary that the compressor be protected from entrained, unevaporated refrigerant. There are two types; namely, flooded systems, and thermal expansion valve systems, as shown in Figs. 3 and 4. In a flooded coil, the circulation is similar to that in a water tube boiler. The liquid is maintained at the proper level by the action of a float regulator as. shown in Fig. 3. The thermal expansion valve system depends upon the thermal valve auto- Fig. 2. Various Water Circuit Arrangements heating loads is also minimized.- This is'especially valuable for outside air preheaters. 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 qf 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,' Fig. 3. Direct-Expansion Coil with Flooded System. Fig. 4. Direct-Expansion Coil with. Thermal Valve System matically feeding just as much liquid to the coils as is required to main tain the superheat at the coil suction outlet within predetermined limits which vary from about 6 to 10 deg. The thermal valve arrangement is in common use for the type of coils covered by this chapter, while the flooded system is comparatively rare. With the flooded system the refrigerant distribution through the tubes depends on. properly selecting: the length of the feeds and the head of liquid imposed upon the liquid inlets. No auxiliary distributing devices are required. With the thermal valve system there are two factors to consider. There must be, generally, more than one refrigerant feed through the coil per thermal valve to keep the pressure drop through the refrigerant circuit within practical limits' and to reduce the corresponding penalty in increased evaporating temperature. At the same time the coiF must be so arranged that the required suction superheat nan be attained with a minimum sacrifice in the performance of the coil as a whole. It is general practice to attain this superheat within the coil itself and not . by the use of external heat exchangers or other auxiliary devices. With thermal expansion valves it is advantageous to keep the pressure drop through the refrigerant feeds as low as possible. The feeds are'laid out to expose each to the same mean temperature difference so that it