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HEATING VENTIIATINC AIR CONDITIONING GUIDE 1941 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. 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 influences 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 CHAPTER 25. HEAT TRANSFER SURFACE COILS Applications Heating coils in field assembled banks are used for a number of pur poses as described in Chapter 20. 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 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. Fig. 8. Flow of Media in Tubes in Relation to Air Flow 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. Parallel flow is seldom used for the reason that a lesser mean temperature difference results than with counter flow. 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 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 re frigerant which is essential to proper operation of the thermal expansion valve. Cross-flow is common in steam heating 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 danger of short circuiting of liquid refrigerant which will disturb proper functioning of the thermal expansion valve. 480 Fig. 9. Typical Arrangement of Cooling Coils in a Central System 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 water cooling coils is shown in Fig. 9. 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 non-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 exposed to freezing temperatures in winter if the apparatus is used on winter humidifying duty. Access doors should-be provided for servicing filters, humidifying nozzles, and fan bearings and for cleaning the coils. With certain designs of coils when used for dehumidifying, eliminators must 481 '. I