Document np5Z3ZwNyBBXpD2bYgObpNdYR

828 CHAPTER 36 1955 Guide I' the thermal valve and is discharged against the end plug in which the indi vidual liquid feeds are closely arranged. These distributors can be'used in either vertical or horizontal position. There are also other types of headers such as the centrifugal and weir type. -The individual liquid con nections from the distributor to the coil inlet are commonly made of small diameter tubing, and are all of the same length and diameter in order to impose the same friction between the distributor and the coil. Since the -^thermal valves act in response to the superheat at the coil outlet, this super heat should be produced with the least possible sacrifice of active evaporat ing surface. Sometimes a single thermal valve is used per coil. In other cases, multiple valves are used, with the coil divided across the air flow or parallel to the air flow as shown in Fig. 6. The arrangement of Fig. 7 Air Heating and Cooling Coils 829 in steam heating coils, the temperature within the tubes being substantially uniform, and the mean temperature difference the same whatever the direc tion of flow, relative to the air. Cross-flow is to be avoided in coils with volatile refrigerants, because of unequal loading of parallel circuits, and the danger of short circuiting of liquid refrigerant which disturbs proper functioning of the thermal expansion valve. Applications Heating coils in field assembled banks are used for a number of purposes as described in Chapter 30. They may be arranged with the air flow Fig. 5. Types of Refrigerant Feed Distributing Heads should be avoided, since it offers the disadvantage of unequal load on the two parallel circuits. Flow Arrangement The relative directions of flow of the air outside the tubes and the medium within them, influence the performance of the surface. There are three types of relative flow in common use. Fig. 8A shows parallel-flow in which the air and the medium in the tubes proceed through the coil in the same direction. Fig. 8B shows counter-flow in which the medium in the tubes proceeds in a direction opposite to the flow of air. Fig. 8C shows cross-flow in which the air and the medium in the tubes pass at right angles to each other. The counter-flow arrangement is almost universally used in brine or water coils to take advantage of the highest possible mean tem perature difference for given entering water and air temperatures. It is also commonly used, in coils fed with volatile refrigerant, to take advantage of the higher air temperature for superheating the leaving gas. In deep coils, however, it is sometimes advantageous to use parallel flow from second row to last row, and then to complete the circuit by passing through the first row to take advantage of the higher air temperature for superheating. Complete evaporation and superheating of the refrigerant are essential to proper operation of the thermal expansion valve. Cross-flow is common Fig. 6. Arrangement for Face Control Fig. 7. Arrangement fob Depth Control vertical or horizontal, although the. latter is more common. For steam neatingj the coils may be set with the tubes vertical or horizontal. In the atter case the coil should be sloped to provide for condensate, drainage, because of the multi-circuit feed arrangement and the necessity for avoid- i? and water pockets, water heating coils are generally arranged with tubes horizontal. Certain precautions must be taken.against freezing, tfi e,r^.s*'eam coil are used with entering air below- freezing temperature, nrottling the steam supply may cause freezing of the condensate.in the Hist -C0^ > ihthe tubes are of the variety not provided with internal "^nbuting pipes or an equivalent arrangement, ; V; i-. . There are coils available having inner distributing tubes,; and having the crwl an<^ retem headers cast in one piece. In this type-of coil the ndensate that forms in the outer tube has resulted from steam fed from inn orifices. This condensation flowing back along the warm lonft tUe's Prevented from freezing. A wide range of modulation at very add t^mPerateres without danger of freezing, is therefore;obtained.-.: As an ed precaution, with both steam and water coils, the outside air mlet