Document 27YaZ9zDbJ3xX7913L5zKnVr

484 CHAPTER 25 - 1948 Guid*? 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. 6 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 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 systems, as shown in Figs. 7 and 8. 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. 7. The thermal expansion valve system depends upon the thermal valve auto- Fig. 7. Direct-Expansion Coil with Flooded System Fig. 8. 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 cqil must be so arranged that the required suction superheat can 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 Radiators, Convectors, Coils 485 Fig. 9. Types of Refrigerant Feed Distributing Heads handles the same`refrigerating load. 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. 9 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 uniformmixture of gas and liquid within the distributor, from which individual