Document 5kJaMwoKwOwnrQJqLYdGLG04R

ISO . CHAPTER 7 1946 Guide 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 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- Heat Transfer Surface Coils 151 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 andfor.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. 19 are used to collect this water and conduct it to the condensate pan. Sometimes finned surface coils on summer cooling and dehumidifying ' Fig. 9. Typical Arrangement of Cooling: Coils ina 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 aiTangement. 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. 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-eyaporated 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. Coil Arranged with Drip Trough . , Fig. 11. Recirculating Spray System for Cleaning Coils duty are provided with water sprays. These' sprays, are of two types.In the first type a set of spray nozzles is arranged for intermittent cleaning. These sprays are not operative when the system is in use and no recircu lating 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 advantageous 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 23.)