Document 2je47xEnEK797w5KnQ4eDLV3r

756 CHARTER 41 1948 Guide of vanes and gives the resistance, expressed in equivalent length of straight pipe, for a 7-in. x 7-in. elbow of each type. . The pressure, loss through elbows of less than 90 deg may be assumed to be directly proportional to the ratio of the angle through which the turn is made. The resistance will vary widely for the large degree turns depending upon the aspect ratio and the length of straight pipe between the elbows but, for practical purposes, it may be assumed that the ratio remains proportional to the angle through which the turn is made. Reverse 90 deg elbow turns should be avoided wherever possible but, where used, the friction indicated in Fig. 6 should be doubled for the second elbow. Additional tests are needed on the loss of pressure in elbows and other types of duct fittings in order to reconcile the difference between values shown in Fig. 6 and results of more recent tests shown in Table 3. A study of this subject will be undertaken at the A.S.H.V.E. Research Laboratory for the purpose of obtaining such data. PROPORTIONING THE LOSSES The entrance loss through the outside air intake louvers will vary with the design of the louvers and method of connection to the system. The louvers and connecting duct will have a friction resistance of from 0.25 to 1.00 times the velocity pressure. Therefore, the total entrance loss will vary from 1.25 to 2.00 Aw. Common practice is to use 1.5 Aw for a 75 per cent free area louver with connecting duct having 15 deg tapered sides. Wherever air passes through a plenum space having a negligible velocity, allowance must be made for the loss in velocity head. This loss in velocity head may be taken as the difference in velocity heads in the plenum and the duct. Where the ducts are very smooth with long transformation fittings, a regain in static pressure is sometimes allowed, but ordinary construction does not warrant a consideration of this factor, and it is customary to neglect it. When it is allowed, the regain is esti mated at one-half the difference between the velocity pressure at the fan outlet and at the last run of pipe. Other losses of pressure occur through the heating units, at the air 'washer and at air filters. In the design of an ideal duct system, all factors should be considered and the air velocities proportioned so' that the resistance will be practically equal in all ducts regardless of length. DUCT SIZES Ducts and flues for gravity circulation must be sized so that the friction loss will not exceed 50 per cent of the available chimney effect due to the temperature and height of the column of heated air. Duct systems for mechanical circulation may be sized so as to have much higher pressure losses than gravity systems. The total pressure of these systems is limited to the pressure which the fan will produce. The general rules to be followed in the design of a duct system are: 1. The air should be conveyed as directly as possible at reasonable velocities to obtain the results desired with greatest economy of power, material and space. 2. Sharp elbows and bends should be avoided unless turning vanes are used. 3. Transformation pieces should be made as long as possible. The angle between the sides and axis of the duct should never exceed 30 deg and, where possible, 15 deg should be made the maximum. Air Duct Design 757 4. Especial care should be taken to maintain a true cross-section and not to restrict the air flow either in transformation pieces or in elbows. 5. Rectangular ducts or flues should be made as nearly square as possible. Good practice limits the ratio between the long side and the short side to 3 to 1. In no case should this ratio exceed 10 to 1. 6. Wherever possible, ducts should be constructed of smooth material such as sheet metal. Where masonry ducts are used, proper allowance for the surface friction coeffi cient should be made. 7. The use of furred spaces, spaces between Joists, etc., should be avoided unless lined with sheet metal. -Procedure for Duct Design The general procedure for designing a duct system is outlined in the several items listed herewith: 1. Study the plan of the building and draw in roughly the most convenient system of ducts, taking cognizance of the building construction, avoiding all obstructions in steel work and equipment, and at the same time maintaining a simple design. A 2. Arrange the positions of duct outlets to insure the proper distribution of air. 3. Divide the building into zones and proportion the volume of air necessary for each zone. 4. Determine the size of each outlet, based on the volume as obtained in the preceding paragraph, for the proper outlet velocity and throw. 5. Calculate the sizes of all main and branch ducts by either of the following two methods: a. Velocity Method. Select the velocity in the various sections, reducing the velo city from the point of leaving the fan to the point of discharge to the room. In this case the pressure loss of each section of the duct is calculated separately and the total loss found by adding together the losses of the various sections of the continuous run. b. Friction Pressure Loss Method. Proportion the duct for equal friction pressure loss per foot of length. 6. Calculate the friction for the duct offering the greatest resistance to the flow of air, which resistance represents the static pressure which must be maintained at the fan outlet or in the plenum space to insure distribution of air in the duct system. The duct having the greatest resistance will usually be that having the longest run, although not necessarily so. Air Velocities The air velocities given in Table 4 have been found to give satisfactory results in engineering practice. Where the higher velocities are used, the, ducts should be cross-braced to prevent breathing, buckling or vibration, and should be constructed of heavier gage metal. At the higher velocities it is particularly important to design the ducts for minimum resistance. As high velocities at one point offset the effect of proper design in all other parts of the system; emphasis should be placed on the importance of air. velocities, elbow design, location of dampers, fan connections, grille and register approach connections, and similar details. For industrial buildings, noise is seldom given much consideration, and main duct veloci ties as high as 2800 or 3000 fpm are sometimes used but, when these velocities are used, due consideration should be given to duct design, resistance pressure, fan efficiencies and motor horsepower. For-department stores and similar buildings, 2000 to 2200 fpm are sometimes used in main ducts where noise is not objectionable and space conditions warrant it. Where high velocity diffusing outlets are used, the duct velocity should not be less than the throat velocity of the diffusers, as dynamic-losses