Document MJxNwVeYymOYLvOeQ0vMng0G7

772 CHAPTER 35 1952 Guide -the thermal valve and is discharged against the end plug in which the.indi vidual liquid feeds are:.elosely 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 com flections 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-superheat-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 V.. .: : " 773 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. yaJve. ,i- Applications |' Heating coils in field assembled banks are used for a number of purposes as described in Chapter 29. They may be, arranged with the air flow should be avoided, since, it offers the disadvantage of unequal load on the two parallel circuits.- !.' , Fiow 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;7"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 airand the medium in the tubes passat 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 temperature difference forigiven 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 cofls, 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 yalve. Cross-flow is common vertical or.Horizontal, although the.latter is more common. For steam heating, the coils may be set with the tubes vertical or horizontal. In the latter case the coil should be sloped to provide for condensate drainage. Because of the multi-circuit feed arrangement and the necessity`for avoid ing air and water pockets, water heating coils are generally arranged with the tubes horizontal. . Certain precautions must be taken against freezing. Where steam coils are used with entering'air,'below freezing temperature, throttling.the'Steam supply may cause freezing of the condensate ini the bottom of the coil, if the tubes are of the variety not provided with internal distributing pipes or an, equivalent arrangement. .: There.are coils available having inner distributing tubes, and having the supply and .return headers: cast in one piece..; In this type of . coil, the condensate that forms in. ,the outer tube has resulted from steam fed from the inner tube orifices-., This condensation flowing back along the warm jnner.tube is prevented from freezing. .. A wide range of modulation at very low temperatures without danger qf freezing, is therefore obtained. As an added precaution, with both steam,and.water.coils,.the outside air inlet