Document X4Yw25Kerz97qxgO2w6BB94J

Heating Ventilating Air Conditioning Guide 1939 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 nof provided with internal distributing pipes, or an equivalent arrangement If these are used, there is little danger of freezing the condensate as lon as the leaving air temperature is not allowed to fall below about 40 pS 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 water cooling coils is shown in Fig. 11. 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 Chapter 24. Heat Transfer Surface Coils i 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 1 ravs are not operative when the system is in use and no recirculating sPp is provided. The second arrangement requires a collecting tank P d a recirculating pump. The water is in circulation whenever the naratus is in operation, and assists in keeping the coil clean and in absorbing odors. Fig. 13 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 --and to assure the required design quantity of by-passed air Drain Insulation Drip pan Fig. 11. Typical Arrangement of Cooling Coils in a Central System 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 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. 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. 12, 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. 502 Fig. 12. Coil Arranged with Drip Trough Fig. 13. Recirculating Spray System for Cleaning Coils when the by-pass damper is open, cooling coil banks are frequently furnished with both face and by-pass dampers as shown in Fig. 11. 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 Chapters 16 and 17). 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 magnitude of the driving force, i.e., the temperature difference. 2. The design and surface arrangement of the coil. 3. The velocity and character of the air stream. 4. The velocity and character of the medium in the tubes. The driving force is usually taken as the logarithmic mean temperature difference for heating or cooling without dehumidification. For combined