Document 7O2Q4L66Lrx9BRMRwk0E8KL06

490CHAPTER 25 1948 Guide about 150 to 300 for evaporating dichlorodifluoromethane, about 350 to 1200 for water at 2 and 6 fps and about 1200 for condensing steam. The influence of the medium in the tubes oh the over-all, heat transfer rate is, therefore, apparent. Because of these variables, reliable rating and performance information for any design of coil must be'based on actual tests on that coil under the expected conditions of operation. A comparison between the perform ance of two designs, unless based on such tests on each, may lead to entirely erroneous conclusions. For details on coil calculation and performance see Chapter 7: * 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, dryand 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. 7. Peculiarities of individul designs of coils. 8. Individual installation requirements, such, for example, as type of automatic contral to be used. The duties required may be determined from information in Chapters 6, 8, 14 and 15. 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 an air distribution system for new air conditioning apparatus, or may be dictated by requirements of satisfac tory, air distribution or ventilation. The resistance through the air circuit influences 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 opera ting expense low, or it may be limited by the maximum fan peripheral velocity which requirements of quietness may permit. The friction through the water or brine circuit may be dictated by the head available from a given-size of pump and pump motor. As the fan and pump motor inputs-represent a refrigerating load on cooling installations, it is eco nomical to keep them low. Proper performance of a surface heating or cooling coil depends upon correct choice of the original equipment and upon certain other factors. The usual coil ratings are based on a uniform face velocity of air. If the air is brought in at odd angles or if the fan is located so as to block part of the air flow, the performance as given in the manufacturer's ratings cannot usually be obtained.- To obtain this performance it is necessary also that the air quantity be adjusted on the job to that used in determining the coil selection, and must also be kept at. this value. The most common causes for, a reduction of air quantity are the fouling of the filters and collection of dirt in the coils. These difficulties can be avoided by Radiators, Convectors, Coils 491 proper'design and proper servicing.' There are a number of ways iii-which coils may be. cleaned. A common method is-to wash them off with water. They can sometimes, be brushed and cleaned with a vacuum cleaner.; In bad cases of neglect, especially on restaurant jobs where grease and; dirt have accumulated, it is sometimes necessary to remove the coils and wash off the accumulation with steam, compressed air apd water,- or hot water. The most satisfactory-solution, however, is to keep the filters serviced, and thus make the- cleaning of the coils unnecessary:. - - The proper selection of coils requires an understanding of the necessities of eachcase-and should be based on an economic analysis-of the: plant design as a whole. No general rule can, therefore, be laid down for-the selection of heating or cooling coils. It is possible, however, to point out the limits of usual practice and to -indicate' the influence of the variables involved in the coil selection. Heating Coils,. Steam and'hot water heating coils are usually rated within these limits: Air Face Velocity--200 to 1200 fpm, sometimes up to 1500 fpm. Steam Pressure--2 to 200 lb, sometimes up to 350 lb per square inch. Hot Water Temperature--150to225 F. Water Velocity--2 to 6 fps. Individual cases may deviate widely, but the 'tabulation given' here with will serve as a guide to usual heating installation.practice: ' Air Face Velocity--500 to 800 fpm face, 500 being a common 6gure. Delivered Air Temperature--varies from about 72 F for ventilation only to about 150 F for complete heating. Steam Pressure--2 to 10 lb,' 5 lb being common. Hot Water Temperature--150 to 225 F. Water Velocity---2 to 6 fps. Water Quantity--Based on about 20 F temperature drop through a hot:water coil. Air Resistance--The total resistance through heating coils is usually limited to from % to % in. of water gage for public buildings, to about 1 in. for factories. The selection of heating coils is relatively simple as it involves dry-bulb temperatures-and sensible heat only, without the complicatipn of simul taneous latent heat loads, as in cooling coils. For a given duty, entering air temperature, and steam pressure, it is possible to select several arrange ments of the same design of coil depending upon the relative importance of space, cross-sectional area, and air resistance. Cooling Coils Cooling and dehumidifying coils are usually rated within these limits: Entering Air Dry-Bulb--60 to 100 F. Entering Air Wet-Bulb---50 to'80 F. Air Face Velocities--300 to 800 fpm, (sometimes as low as 200 and as high as 1200). Volatile Refrigerant Temperatures--25 to 55 F, at coil suction outlet. Water Temperatures--40 to 65 F. ' Water Quantities:--2 to 6 gpm per ton, or equivalent to a water temperature rise of from 4 to 12 deg. Water Velocity--2 to 6 fps. The ratio of total to sensible heat removed varies in practice from-1.00 to about .1.65, i.e., sensible heat is from 60 to 100 .per cent of total,