Document MGZqQwdDoebvqDODM7qKwagv7

American Society of Heating and Ventilating Engineers Guide, 1925-26 The cubic feet of air of standard density taken into the system at each connection is given by the formula: Q 4000 A fJ~* where Q = Cubic feet of air per minute; A = Area of connection in square feet; / = Orifice or restriction coefficient; i = Static suction measured in inches of water. The orifice coefficient / is dependent upon the shape and construction of the hood and will range from 60 to 90 per cent. An average value is 70 per cent. Knowing the suction at each hood and the diameter of each connection, the volume of air passing up each branch can be taken from the accom panying Table 107. The sum of all these volumes gives the total volume to be handled by the exhaust fan. Table 107. Cubic Feet of Air Handled Per Minute Through Average Collecting Hoods Based on Coefficient of Orifice of 0.71 with 10 Per Cent Addedfor Leakage Diameter of Maintained Suction--In. Water Gace . Connection Pipe In, m1 2 2H 3 4 5 \y2 2 2}4 3 1)4 4 AVi 5 6 7 8 .9 10 38 68 107 153 209 273 345 427 614 835 - 1092 1381 1705 47 84 131 188 256 334 423 523 751 1023 1337 1694 2090 54 97 161 217 296 386 488 605 867 1181 1546 1953 2409 61 108 168 243 330 431 546 676 970 1322 1727 2184 2695 67 118 185 266 362 473 598 741 1062 1448 1892 2387 2959 76 136 214 306 418 546 690: 854 1228 1670 2184 2762 3410 86 153 238 343 466 609 . 775 955 1373 1870 2440 3091 3806 , Common practice fs to provide a main suction pipe having an area 20 to 25 per cent in excess of the sum of the areas of the branches enter ing it between the point in question and the dead end of the main. Similarly the discharge pipe leading from the fan outlet to collector is frequently made the same diameter as the large end of the main suction pipe. The reason for this increase in size is that a considerable power saving results from the lower air velocity; However, there is no technical reason why mains should be a certain percentage greater area than the Sumof the connections, and still lower power consumption can be Ob tained by using larger branches and mains of equal area. While the rule 250 American Society of Heating and Ventilating Engineers Guide, 1925-26 of thumb method of determining size of mains works very well in many cases, yet it is always desirable to. figure the mains and branches of the proper size to give the velocity which has been found best suited to the work to be done. In certain special cases where explosive or poisonous dusts such as aluminum buffings, grain dust, powdered sugar, or lead dust are handled, increasing the size of the mains unduly would introduce a serious hazard. An exhaust system to be effective must remove a certain amount of air from each hood or other connection, and in addition must maintain sufficient velocity throughout the piping system to convey the dust or refuse material to the separator. Any system which is mechanically well constructed and handles the requisite air at the connections and maintains sufficiently high velocities, is an effective system from the standpoint of the work done. However, to keep the operating cost low it is advantageous to do the work with as low velocities as the character istics of the material will permit. The skilled designer will keep both of these requirements in mind and produce a system which is both effectiveand economical of power. The maintained resistance of the exhaust system is composed of three factors: (1) Loss through the hoods; (2) Collector drop; and (3) Fric tion drop in the pipes. . A. Suction at the various hoods must be chosen from experience. Loss through the hoods can be calculated by an experienced engineer but may be taken very roughly at one-half the suction. B. Collector drop in inches of water is given by the following formula: Drop = C Y \ iooo ) where C = a constant which depends upon the type of collector and is found to range from 0.25 to 0.75; V = velocity in ieet per minute of air entering the collector. C. Friction drop in the pipes must be computed for each section where there is a change in area or in velocity. Find the velocities in each sec tion of pipe starting with the branch furtherest from the fan. The friction drop for these sections can be determined by reference to Table 108. Total friction loss in the piping system is the friction drop in furthest branch plus the drop in the various sections of the main, plus the drop in the discharge pipe. The total maintained resistance of the system-7-or static head re quired at the fan = A -{- B + C. SELECTING THE FAN Having determined the volume Of air and static head required, the size of exhaust fan,- speed and horse-power can be found by reference to the manufacturers performance tables or charts covering the type of exhaust fan selected. 251