Document OEr66npR3OOejjDx0gGEJV6Jp

HEATINC VENTILATING AIR CONDITIONING GUIDE 1941 avoided to prevent objectionable noise. Three typical types of vanes are shown in Fig. 5 which gives the approximate number of duct widths recommended to be used in estimating the resistance 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 of the elbows indicated in Fig. 4 should be doubled for the second elbow. Fig. 5. Design and Corresponding Loss of Pressure in Elbows with Turning Vanes 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 hy to 2.00 ky. Common practice is to use 1.5 hT 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 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, but generally ordinary construction does not warrant a consideration of this factor, and it is customary to neglect it. When it is allowed, the regain is estimated at one-half the difference between the velocity pres sure 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 ordinary practice in ventilation work1 it is usual to keep the sum of the duct losses one-third to one-half and the loss 580 CHAPTER 31. AIR DUCT DESIGN through the other units at less than one-half of the static pressure. The remainder is then available for producing velocity. In the design of an ideal duct system, all factors should be taken into consideration and the air velocities proportioned s<5 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 aspirating 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 available pressure from the fan used. General Rules The general rules to be followed in the design of a duct system are enumerated herewith: 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. 4. Especial care should be taken to maintain a true cross-section and not to restrict the air now 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 coefficient 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. 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. Arbitrarily fix the velocity in the various sections, reducing the velocity from the point of leaving the fan to the point of discharge to the roonvr 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. 581