Document 8VRZG5kVp3Dyrya2pMMDzwR3y

470 ~ Chapter 25 ~ ___________ '' ~-1945 Guide . which vary from about 6 to 10 F. The thermal valve arrangement is in common use for the type of coils covered by this chapter, while the flooded system-is1 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 practicaf limits and to reduce the corresponding Beat Transfer Surface Coils' _____________ - - :________________ ' ,,' . 471 ' velocity , through a central jet against the end plate, forming a uniform mixture of gas and liquid within the distributor, from which individual. connections are led as shown.. In type C the refrigerant enters at high velocity from the thermal valve and is discharged against the end plug in which the individual liquid feeds are closely arranged. These distributors can be used in either vertical or horizontal position. Although-there are other forms of distributors those mentioned are typical examples.' . The individual liquid connections 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 \yith the least pos sible sacrifice of active evaporating surface. Sometimes a single thermal penalty in increased evaporating temperature. 'At the same time the coil 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 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. 5 shows three typical types of distributors. In distributor A the liquid and gasmixture 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 S 'the refrigerant is discharged at a high 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 should be avoided since it offers the disad vantage of unequal load on the two parallel circuits. Flow Arrangement The relative direction 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 parallelflow 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 angle to each other. Parallel flow is seldom used with watercoils for the reason that a lesser mean temperature difference results than with counter-flow. The counter-flow arrangement is almost universally used in brinfe or water coils to.take advantage of. the highest possible mean temperature difference for given entering water and air temperatures. It is also invariably used in coils fed with volatile refrigerant to take advantage of the higher air temperature for superheating the leaving gas. This arrangement assists complete evaporation and superheating of the refrigerant which is essential to proper operation of the thermal'expansion valve. Cross-flow is common in steam heating coils, the temperature.