Document 0qVMZYRjX3NOLVNaQMyrBkJKJ

754 CHAPTER 35 1951 Guide vidual orifices in the tubes, by distributing plates and orifices in the steam header, or by perforated internal steam-distributing pipes extending into the individual tubes. The latter arrangement has the advantage of dis tributing the steam throughout the length of each tube, and is conducive to uniform'temperature of delivered air. The tendency of condensate to freeze at the bottom of the coil with cold entering air and light heating loads, is also minimized. This is especially valuable for outside air pre heaters. 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 fWater outlet Air Heating and Cooling Coils 755 be effectively and 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 sys-' terns, 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 expan sion valve system depends .upon the thermal valve automatically feeding just as much liquid to the coils as is required to maintain the superheat at! the coil suction outlet within predetermined limits, which vary from about1 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 rarely used. With the flooded system the refrigerant distribution through the tubes depends on properly selecting the lengthof the feeds, and the head of liquid imposed upon the liquid inlets. No auxiliary distributing devices are re quired. With the thermal valve system, there are two factors to consider. Fig. 2. Various Water Circuit Arrangements elimination is taken care of in the system piping as described in Chapter 21. To assure a pressure drop sufficient for adequate distribution, but at the same time to provide against axcessive 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, 24, or -36 parallel water circuits, as conditions may require. Orifices in individual tubes are occasionally employed, but are usually unnecessaiy as the .resistance of individual water circuits is generally sufficient to effect a satisfactory distri bution. In precooling coils using well water, where there may be consider able sand and other foreign matter in the water, provision for cleaning of .individual tubes is of advantage. It is important to arrange water coils for complete drainage (see Fig. 2). The drains are usually provided'in the water piping at the coil header. Direct-Expansion Coils Coils for..volatile refrigerants present more complex problems of fluid' distribution.than do water, brine of steam.. It is desirable that the coil Fiq. 3. Direct-Expansion Coil with Flooded System Fiq. 4. Direct-Expansion Coil with Thermal Valve System 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 coil must be so arranged that the required suction superheat can be attained with a minimum sacri fice 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' handles the same refrigerating load. Here, a distributing means is imposed between valve and coil liquid inlets to divide the refrigerant equally among the feeds. Such a distributor must be effective for distributing both liquid and vapor, because the entering refrigerant is a mixture of the two. Fig.; 5 shows three typical types of distributors. In distributor A the liquid and gas mixture from the thermal valve is led tangentially into a chamber.' The coil feed connections extend outward radially at the top of this' chamber. In distributor B the refrigerant is discharged at a high velocity! through a central jet against the end plate, forming a uniform mixture of gas and liquid within the distributor, from which individual connections are led as shown. In type C the refrigerant enters at high velocity from