Document vVr0eObZ58YoN4r4NegQNa5BZ

758 CHAPTER 35 1951 Guide dampers are often closed automatically when thfi 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 Afr flow f Air flow Air flow Air Heating, and Cooling Coils 759 Although both heating and cooling coil8 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 20.) 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. Fio. 8. Flow of Mbdia in Tubbs in Rblation to Aib Flow sections are'stacked one upon another, and where the velocities are low, so j; jj} that eliminators need not bo used, occasional trouble results when water splashes down from one coil to the next and blows out into the air stream. :1 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. j 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, 10,11 and 12. 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 by. the: 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 may, permit. The friction through the water or