Document MMxOmakbaGR5enVveJQyRQoDk

808 CHAPTER 36 ; 1954 Guide d 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 represents' 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 rated performance 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 Fig. 10. Coil Arranged with Drip Trough Fig. 11. Recirculating Spray System for Cleaning Coils 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 coils unnecessary. 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. 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 variable?, involved in the coil selection. : Heating Coils Steam and hot water heating coils are usually rated within these limits: ,i Air Face Velocity--200 to 1200 fpm, sometimes up to 1500 fpm. Steam Pressure--2 to 200 psig, sometimes up to 350 psig. ' Air Heating and Cooling Coils 809. 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 instciiaiion pr^tice:, ;, Air Face Velocity-^-500 to 800 fpm face, 5fK);beingia commoh^figufe:; ! Delivered Air Temperatjure--ivaries from,about 72F fSr.yentilAtipii only, to about 150 F for complete heating. .... . ;; '!..... Steam Pressure--2 to 10 psig, 5 psig, bcingi.common.' Hot,Water Temperature--150 to 225 F. -' Water Velocity--2 to 6 fps. . ; ,. i I.................. 'i, - , -r . . . , ' . Water Quantity--Based on about .20 deg'temperature drop tbrpjigh! a.bpt-.water coil. i. i....................... . Air Resistance--The total resistance through heating, coils is .Usually, limited to from i to | in. of water gage for public buildings, to about TihVfor 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 ih cooling coilsv i For a given duty, entering airtemperature, and steam pressure; it.is possible to select several arrange^ ments of the same design of coil depending upon the relative impbrtahce of space, crossrsectional,area;:and1air resistance. fn Cooling Coils .. Cooling.&ad dehurriidifying coUs are usually rated within theSe limits: Entering Air Dry-Bulb--60 to 100 Ft / . ... ; Entering Air Wet-Bulb--50 to 80 F. Air Face Velocities--300 to 800 fpm :(s6metiines:asTdw as 200 and as high as-1200)'. ' Volatile Refrigerant Teinperatures- -25 to55K, at coil' Suction Outlct. ; `' . Water Temperatures-^40to65F.. ,, -m, v, Water Quantities--2 to .6 gpmlper tpnj. pr equivalent to ia! watpr temperature rise of from 4 to 12.deg. ... ; Water Velocity--2 to 6 fps. ^ 1 '1 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 Chapter30.) 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 6O0,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 nodehiimidification is desired; for which condition the dew.-point of: the entering air is equal to or lower, than the cpqling, coil surface teniper- ature, the coil selection is made on.the basis Of dry-bulbtemperatures 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 consideratoon ,of both dry- and wet-bulb air temperatures,. It is further compli cated by the fact that the proportional amount of dehumidification required ls highly variable. The methods outlined in the section, Heat Transfer mid 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