Document DGeJn9p98OE1azve7YQXgyBRN

486 CHAPTER 25 . 1948.Guide ' 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 distribu tors 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 with the least pos sible sacrifice of active evaporating 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. Radiatorsi Convectors, Coils __________ - 487 coils, the temperature within the tubes being substantially uniform and the-mean temperature difference the same whatever the direction of flow, relative to the air. Cross-flow is to be avoided in. coils-with volatile refrigerants on account 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 pur poses- as,described in Chapter 43. They maybe-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- Fig. 12. Flow of Media in Tubes in Relation to Air Flow 10. The arrangement of Fig. 11 should be avoided since it offers the disadvantage 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. 12A shows parallelflow in which the air and the medium in the tubes proceed through the coil in the same direction. Fig. 12B shows counter-flow in which the medium in the tubes proceeds in a direction opposite to the flow of air. Fig. 12C 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 temperature 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 super heating the leaving gas. In deep coils, however, it is sometimes advan tageous 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 cqmmon in steam heating Fig.13. Typical Arrangement of Cooling Coils in a Central System 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 freezing temperature, throttling the steam.supply may result in freezing 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. If. these are used, there is little danger of freezing the condensate as long as the leaving air temperature is not allowed, to fall, below about 40 F. As an added precaution with both steam and water, coils the outside air inlet-dampers are often closed automatically when the fan is stopped to avoid trouble caused by very cold outside air drifting in during off periods. A typical arrangement of cooling coils is shown in.Fig. 13. Some means should be provided to filter all the entering air to keep dirt and foreign matter from accumulating on the coils. The assembly is provided with a drip-pan to catch the condensate during summer dehumidifying duty and to collect the' lion-evaporated water from the humidifying sprays in winter. The drip connection should be made ample in size and-liberally provided with plugged tees and crosses for cleaning. It- should not be