Document zzKN3LZeYjy86Q4y0Oje7ZD3

790 CHAPTER 36 1953 Guide dampers are often closed automatically when the fan is stopped to avoid trouble, caused by very cold outside air drifting.in during ojf 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 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 .Heating and Cooling Coils -. 791 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: the coil connec- : tions. (See Chapter .21.) .- r! COIL SELECTION . In the selection of a coil it is necessary to consider'several factors: 1. The duty required--heating, cooling, dehumidifying. 2. Temperature of entering, air- dry-bulb only if there is no dehumidification; dry-and wet-bulb if moisture is to be removed.: -, 3. Available heating and cooling media.. 4. Space and dimensional limitations. . . .. 5. Air quantity limitations. ' -- 6. Allowable resistances in air circuit and through tubes. Fig. 8. Flow of Media in Tubes in Relation to Aik 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. 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. 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 absorb ing 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 accord ingly. To maintain the air quantity handled by the fan reasonably con 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. 9. Fig. 9. Typical Arrangement of Cooling Coils in a Central System 7. Peculiarities of individual designs of coils. ,- 8. Individual installation requirements, such,, for, example, as type of automatic control to be used. The duties required may be determined from information in Chapter 9, 11, 12 and 13. There may, or may not, be a choice of cooling and heating media, as well as temperatures available, depending upon whether the installation is new or is in combination with present sources of heating or cooling. Space limitations are dictated by the requirements of individual cases. The air quantity is influenced by a number of considerations. The air quantity through heating coils is often made the same as that necessary to handle the summer cooling load. The air handled may: be fixed bythe use of old ventilating ducts as the air distribution system for new air condi tioning apparatus, or may be dictated by requirements of satisfactory air distribution or ventilation. The resistance through the'air circuit influ ences the fan horsepower and speed. This resistance may be. limited to allow the use of a given size of fan motor, or to keep the operating expense low, or it may be limited by the maximum fan peripheral velocity which requirement of quietness mav permit. The friction through the water or