Document 65doJVm97Q4QBkGzQ3Emqzmao

160 CHAPTER 7 1946 Guide '. 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 proper design and proper servicing1. There are a number of ways in 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 and 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 each case and should be based on ah 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--150 to 225 F. Water Velocity--2 to 6 fps. Individual cases may deviate widely, but the tabulation given-herewith will serve as a guide to usual heating installation practice: Air Face Velocity--500 to 800 fpm face, 500 being a common figure. .. 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 54s to 54s 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 complication of simul taneous latent heat loads, as in cooling coils. For a given ditty, 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, dr-equivalent to a water temperature rise of from 4 to 12 deg. Water Velocity--2 to 6 fps. Heat Transfer Surface Coils 161 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, depending on the application. (See Chapter .43.) Since required-ratios may demand wide variations in air velocities, refrigerant temperatures, and coil depth, general rules as to their values may be misleading. On usual comfort installations, air face velocities between 400 and 600 fpm are frequent, 500 being, a common value. Refrigerant temperatures ordinarily vary between 40 and 50 F where cooling is accompanied by dehumidification. Water velocities range from 2 to about 6 fps. When no dehumidification is desired, for which condition the dew-point of the entering air is equal to or lower than the cooling coil surface tem perature, the coil selection is made on the basis of dry-bulb temperatures and sensible heat transfer only, the same as with heating coils. It is possible also to choose various arrangements of face area, depth, air velocity, etc., for the same duty. Dehumidifying Coils The selection of coils for combined cooling and dehumidifying duty is more involved than for heating or sensible cooling and requires con- Table 1. Various Cooling Coil Arrangements Selection 1 Total cooling capacity, tons...'.-- 100 Sensible cooling capacity, tons... 69 Latent cooling capacity, tons.. 31 Ratio total to sensible heat.____ 1.45 Air quantity, cfm - .......... 47,800 Cfm per total ton............. ^______ 478 Face velocity, fpm._____________ 325 Resistance, m. water 0.11, Coil face area, sq ft.._ ................. 147 Coil rows'deep.__l........................ 4 Coil evaporator temp. F deg...... 45 2 100 69 31 1.45 41,700 417 423 0.27 99.0 6 45 3 100 69 31 1.45 37,100 371 500 0.51 74.2 8 45 4 100 69 31 1.45 46,800 468 600 0.37 78.1 '4 38 sideration of both dry- and wet-bulb air temperatures. It is further complicated by the fact that the proportional amount of dehumidification required is also highly variable. The methods outlined previously under Heat Transfer and Resistance may be used to determine whether it is possible for a coil to perform the duty required. If entering and leaving air conditions are arbitrarily specified* the corresponding duty'sometimes . cannot be obtained at all without the use of reheat. As with heating and sensible cooling coils, there are combinations of face areas,^ depth, air velocity and refrigerant temperatures which will give the required per formance. This is illustrated in Table 1. It is possible as shown in Table 1 to perform approximately the same duty at a given refrigerant temperature with small face area and large ~ thickness or vice versa. The large face area coil gives low air velocity and resistance but high air quantities per ton. The coil of small face area and 1 great depth requires small air quantities per ton of refrigeration, high resistance and high air velocities: As shown also in Table 1 the same sensible, latent and total cooling capacity may be obtained with.various refrigerant temperatures by proper choice of coil. This makes it possible . to keep the evaporating, temperature high enough to carry the load with a chosen size of condensing unit. High evaporating temperatures with