Document K4jgJe2NMmazo9gZD7MoZg7o

540 CHAPTER 25 1949 Guide 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 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 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 economical 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 proper design and proper servicing. 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 accu mulated, 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 cods unnecessary. The proper selection of coils requires an understanding of the necessities of each case 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--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 healing 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--rThe total resistance through heating coils, is usually, limited to from } to f in. of water gage for public buildings, to about 1 in. for factories. Radiators, Convectors, Coils 541 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 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. Table. 4. Vabious Cooling Coil Abbanoements Selection 1 Total cooling capacity,.tons____ 100 Sensible cooling capacity, tons.-- 69 Latent cooling capacity, tons.--. 31 Ratio total to sensible beat........ 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.__________ ____ 4 Coil evaporator temp. F deg...... 45 2 100 69 31 1.45 41,700 417 423 0.27 99.0 6 . 45 34 100 69 31 1.45 37,100 371 500 0.51 74.2 8 45 100 69 : 31 1.45 46,800 :468 600 0.37 78.1 4 38 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, depends ing on the application. (See Chapter 43.) Since required ratios may de mand wide variations in air velocities, refrigerant temperatures, and coil depth, general rules as to their values may be misleading. On usual com fort 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 temper ature, 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. . . r .- Dehumidifying Coils . ' The selection, of coils for combined cooling and dehumidifying duty is more involved than for heating or sensible cooling and requires considera tion of both dry- and wet-bulb air temperatures. (See Chapter7.) It is further complicated by- the fact that the proportional amount of dehumidi-