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888 CHAPTER 35 1957 Guide 11, 12, 13 and 29. -There may, or may not, be a choice of cooling and heat ing 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 limited by the use of old ventilating ducts as the air distribution system or may be dictated by requirements for satisfactory 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 operating expense low, or it may be limited be cause of sound level requirements. The allowable friction through the water or brine circuit may be dictated by the head available from a given size of pump and pump motor. The performance of a surface heating or cooling coil depends upon correct choice of the original equipment, and upon proper application and maintenance. Coil ratings are based on a uniform face velocity. Interference with uniform air flow through the coil will affect performance. Such air flow interference may be caused by air being brought in at odd angles or by inadvertent blocking of a portion of the coil face. To obtain rated per formance it is necessary that the air quantity be adjusted on the job to that used in determining the coil selection, and that it be kept at this value. The most common causes of 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 regular servicing. There are a number of ways in which coils may be cleaned. A common method is to wash them 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 best practice, however, is to keep the filters serviced, and to inspect and wash the coils at regular intervals. The proper selection of coils requires an understanding of the require ments of each case, and should be based on an economic analysis of the plant design as a whole. While no general rule can be established 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 psig, sometimes up to 350 psig. 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 psig, 5 psig being common. . . Hot Water Temperature--150 to225 F. Water Velocity--2 to 6 fps. Air Heating and Cooling Coils 889 Water Quantity--Based on about 20 deg temperature drop through a hot-water coil. Air Resistance--The total resistance through heating coils is usually limited to from in. of water 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 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 percent of total, depend ing on the application. (See Chapter 29.) 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 most com fort installations air face velocities between 400 and 600 fpm are frequent, 500 being a common value. Refrigerant temperatures ordinarily vary between 35 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. Dehumidifying Coils The performance of coils accomplishing both cooling and dehumidifi cation is determined by a series of tests carried on under laboratory con ditions. Coil ratings can be prepared from such laboratory test data to facilitate the selection of coils for cooling and dehumidifying duty. The coil capacity should be in balance with the capacity of related equipment, such as compressors, and the temperature of the circulating medium. The selection of cooling coils for factory-assembled self-contained air conditioners is generally accomplished in conjunction with laboratory test ing. The current industry standards call for ratings at 33.4 cfm per thousand Btu of cooling capacity and this is approximately 400 cfm per ton of refrigeration. The use of an entering air condition of 80 F dry bulb and 67 F wet bulb is representative of the entering air conditions actually encountered in many comfort operations because, while the indoor con ditions are usually lower than 67 wet bulb, the introduction of outside air will usually bring the mixture of air to the cooling coil up to an approxi mation of the 67 wet bulb entering air condition at design conditions. The selection of cooling coils for field assembled projects is usually ac-