Document DLxJrnwq4pN0MXY6NN8r9m3O

636 CHAPTER 25 1949 Guide low temperatures without dangerof freezing is therefore obtained. 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'avoidtrouble 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 dehumidlfying-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 cleanout fittings. 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. When.coils are 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. Where a number of coil Air flow Radiators, 'Convectors* Coils 537 stant, 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. 13. 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 connec tions. (See Chapter 23.) HEAT TRANSFER AND AIR FLOW RESISTANCE The-transfer of heat between the heating or cooling medium and the air stream is influenced by several variables: 1. The temperature difference. 2. The design and surface arrangement of the coil. 3. The velocity and character of the air stream. Cross flow c Fig. 12. Flow of Media in Tubes in Relation to Air Flow - 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. 14 are used to collect this waterand 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. 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- collectingtank and a recirculating pump. The water is in circulation whenever the: apparatus is in operation, and assists in keeping the cOil clean and in absorb ing odors: Fig. 15 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 thanamixture of return and outside air. The casing should be arranged accord- jngly. - To maintain the air-quantity handled by -the fan reasonably con-: Fig. 13. Typical Arrangement of Cooling Coils in a Central System - The driving-force is usually taken as the logarithmic mean temperature difference'for heating or cooling without-dehumidification. For combined cooling and dehumidification, the logarithmic: difference does not- apply strictly and such problems should be handled as described in Chapter -7. With volatile refrigerants there is often an appreciable pressure drop and: corresponding change in evaporating temperature through the.refrigerant; circuit. The problem-is further complicated .by the fact that the refriger-' ant is evaporating in part of the circuit and superheating in the remainder.. In spite of this, heat transfer and ratings for coils using.volatile refrigerants are usually based on a refrigerant temperature corresponding to the averagepressure in the coil. The design and surface arrangement of the coil include such items as1 materials, type, thickness, height and spacing of the fins, and the ratio of this surface to that of the tube, the use of the staggered or in-line tube arrangement, and provisions to increase the air turbulence such as the use: of corrugated as against flat fins. Staggered tubes increase the total heat, transfer as against the in-line arrangement and corrugated fins may. be more