Document O3LyrqLLdnz2RaE35dpJBnjje

472 _________Chapter 25. . - 1945 Guide 'within the tubes being substantially uniform'arid 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. 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, tire 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 Beat Transfer Surface Coils ________ ;______________________.473' exposed tcTfreezing ferhperatures 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 be used beyond the coil to catch any water which may be blown into the air stream. It is customary to include these eliminators when the air velocity exceeds about 450 fpm with the individual fins and about 600 fpm for the continuous flat fin type. Where a number of coil sections are stacked one upon another, and where the velocities are low, so that eliminators need not be used, occasional trouble results when. water splashes down from one coil.to the next and blows out into the air stream. In such cases drip troughs as shown in Fig. 10 are used to collect this water and conduct it to the condensate pan. Sometimes finned surface coils on summer cooling and dehumidifying duty are provided with water sprays. These sprays are of two types. 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 aic temperature is hot 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. 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 Fig. 10. Con. Arranged with Drip Trough Fig. 11. Recirculating Spray System for Cleaning Coils In the first type a set of spray nozzles is arranged for intermittent cleaning. The operator can wash the coils off as frequently as necessary. These sprays are not operative when the system is in use and no recirculating pump is provided. The second arrangement requires a collecting tank and a recirculating pump. The water is in circulation whenever the apparatus' is in operation, and assists in keeping the coil clean and in absorbing odors. Fig. 11 illustrates such an arrangement. Wherever air by-passes are used around a coil on summer duty for control purposes, it is of advantage to direct only return air through the by-pass rather than a mixture of return and outside air. The casing should be arranged accordingly. To maintain the air quantity handled by the fan reasonably constant, and to assure the required design quantity of by-passed air when the by-pass damper is open, cooling coil banks are frequently furnished with both face and by-pass dampers as shown in Fig. 9. Although both heating and cooling coils are made of sufficient strength to take up expansion and contraction arising within themselves, care should be taken to avoid imposing strains from the piping on to the coil connections. (See Chapter 14.)