Document nNaXj0YmQxvZDv2kDa77JY4q2

Heating Ventilating Air Conditioning Guide 1939 are other forms of distributors the above are typical examples. Xh individual liquid connections from the distributor to the coil inlet ar* commonly made of small diameter tubing and are all of the same leneth and diameter in order to impose the same friction between the distribute 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 evaporating surface. Where conditions require a coil which is thin in the d.rection of air flow, with large quantities of air per ton of refrigeration, the temperature difference between leaving air and refrigerant is large and it is, therefore, practical to feed the coil in the direction of the air flow as shown in Fig. 6. If the refrigerant temperature ' : Chapter 24. Heat Transfer Surface Coils of relative flow in common use. Fig. 10A shows parallelflow in which t?air and the medium in the tubes proceed through the coil in the same tion Fig- 10B shows counter-flow in which the media in the tubes oceed in a direction opposite to the flow of air. Fig. 10C shows crossKnain which the air and heating or cooling media pass at right angles to other. Parallel-flow is often used in coils fed with volatile refrigerant eiC-. is characterized by a fall of refrigerant temperature in direction of gow dUe to the pressure drop through the refrigerant circuit. Crossis common in steam heating coils, the temperature within the tubes being substantially uniform, and the mean temperature difference the came whatever the direction of flow, relative to the air. Cross-flow is also lsed in very thin coils with brine or water or volatile refrigerants, it being ! practical to arrange these coils any other way. The counter-flow arrangement or some modifications of it is used almost universally in is high and the air quantity per ton low so that the leaving temperature difference becomes small, i t is expedient to arrange the circuits so that the ; refrigerant is brought back to the air inlet side pf the coil to take ad- * vantage of the large temperature difference at that point for. superheating, : as in Fig. 7. Sometimes a single thermal valve is used per coil. In other I cases multiple valves are used, with the coil divided across the air flow ' or parallel to it as shown in Figs. 8 and 9. The arrangement of Fig- 9 ! has the disadvantage of unequal load on the two parallel circuits. The choice between face or thickness division is dictated by the type of auto- i matic temperature and humidity control used. j Flow Arrangements | The relative direction of flow of the air outside the tubes and the media J within them influences the performance of the surface. There are three | 500 brine or water coils which are deep in the direction of air flow, to take advantage of the highest possible mean temperature difference for given entering"water and air temperatures. Counter-flow is frequently used in coils fed with volatile refrigerant to take advantage of the higher air temperature for superheating the leaving gas. This arrangement permits complete evaporation, of the refrigerant and proper operation of the thermal expansion valve. Applications Heating coils in field assembled banks are used for a number of pur poses as described in Chapter 21. They may be arranged with the air flow 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 neces sity for avoiding 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 501