Document x1KVJJZD9Rqnn3oGYN4RGY5Qg

,788 CHAPTER 36 ' 1953 Guide <r' the thermal valve and is discharged againstthe end-plugin which the indi vidual liquid feeds are closely arranged: - These distributors can be used in either verticalor 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 tubings 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 thecoil 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 AirHeating and Cooling Coils 789 in steam heating coils, the temperature .within the (tubes being substantially uniform ; and the mean temperature difference the same whatever,the direction of -flowj 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 Fia. 5. Types op 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 the1'performance of thesurfaCe. 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 t^ke 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 dpils, however, it is sometimes advantageous to use parallel flow from second tow 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 for Depth Control . 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 multicircuit 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 niay cause freezing of the condensate in 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 inner tube is prevented from freezing, A wide, range of modulation at very low temperatures without danger of freezing, is therefore obtained.. As an added precaution, with both steam and .water coils,, the outside, air inlet