Document JJn93wn3VEa9jMz5Ry2RO42dZ

746 CHAPTER 41 , 1946 Guide 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, Air Duct Design 747 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 haying 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 may be taken as the .velocity head corresponding to the difference in velocities in the plenum and the duct. Where the ducts are very smooth with long transformation fittings, a regain in static pressure is sometimes allowed, Fig. 4. Rectangular Equivalents op RouNd Ducts where used, the, friction indicated in Fig. 5 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 difference between values shown in Fig. 5 and results of more recent tests shown in Table.2. A study of this subject will be undertaken at the A.S.H.V.E. Research Laboratory for the purpose of obtaining such data. . CENTERLINE RADIUS IN PER CENT OF PIPE DIAMETER OR WIDTH Fig. 5. Loss of Pressure in Elbows 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 the1 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 t<? 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.