Document wDeQGVvj5xEnvx6ezjKrDbNz4

488 Chapter 25 1945 Guide off the-accumulation"with-steam, compressed-air-and waterTor 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 oh 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--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 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 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 complication 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 The usual range of ratings for cooling and dehumidifying coils is enumerated herewith: 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 range of from 4 to 12 F. 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, depending on the application. (See Chapter 20.) . Required ratios may demand wide variations in air velocities,..refrigerant temperatures, and coil depth, so that general rules as to these values may be misleading. On usual comfort installations air face velocities between 400 and 600 fpm Heat Transfer. Surface Coils 489 are frequent, 500 being a common value. Refrigerant temperatures will ordinarily vary between 40 and 50 F where cooling is accompanied by dehumidification. Water velocities will range from 2 to about 6 fps. When no dehumidification is desired, for which condition the dew-point of the entering air will be equal to or lower than the cooling coil surface temperature, the coil selection is made on the basis of dry-bulb tempera tures and sensible heat transfers 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 sideration of both dry- and wet-bulb, air temperatures. It is further complicated by the fact that the proportional amount of dehumidification Table 1. Various Cooling Coil Arrangements Selection 1` Total cooling capacity, tons____ 100: Sensible cpoling 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, in. water.___________ 0.11 Coil face area, sq ft- __________ 147 Coil rows deep.__ 1___________ __ 4 Coil evaporator temp, deg F........ 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 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 iii 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 will give low air velocity and resistance but high air quantities per ton. The coil of small face area and great depth will require 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 correspondingly small compressor operating expense can be attained but at the expense of coil surface, air quantity or both. The choice will be determined by the necessities of individual installations. For a given quantity and condition of entering air the evaporating temperature of a volatile refrigerant coil will be determined by a balance