Document NGV7vBNE8p5YdXNEz972Y42DE

American Society of Heating and Ventilating Engineers Guide, 1934 Chapter 19--Air Duct Design 3. The sides of all ducts or flues should be as nearly equal as possible. (In no case should the ratio between long and short sides be greater than 10 to 1). Procedure for Duct Design The general procedure for designing a duct system is as follows: 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, equipment, etc., and at the same time maintaining a simple design. 2. Arrange the positions of duct outlets to insure the proper distribution of heat. 3. Divide the building into zones and proportion the volume of air necessary to supply the heat 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. 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 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. 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 in 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 following velocities of air are considered standard for public buildings: 1. Through the outside air intakes, 1000 fpm. 2. Through connections to and from heating unit, 1000 to 1200 fpm. - 3. Through the main discharge duct, from 1200 to 1600 fpm. 4. In branch ducts, 600 to 1000 and in vertical flues 400 to 800 fpm. 5. In registers or grilles, 200 to 400 fpm depending upon the size and location. diffusers of proper design are used, 25 per cent higher air velocities are permissible. If, , These duct velocities may safely be increased 20 per cent if first-class construction is used to prevent any breathing, .buckling, or vibration. High velocities at one point in the system neutralize the effect of proper design at all other points; hence the importance of splitters in elbows and similar precautions. For industrial buildings noise is seldom considered, and main duct velocities as high as 2800 or 3000 fpm may be used where conditions will permit. For department stores and similar buildings, maximum velocities with good construction and design may be as high as 2000 or 2200 fpm in main ducts, with suitable reduction in branches and outlets. With these velocities first-class duct construction is essential. Proportioning the Size for Friction By means of Figs. 4 and 5 the diameter of branch pipes necessary to carry a given percentage of the total air in the main pipe with the same friction per foot of the length may be determined. These charts, as well as Fig. 3, are based on the assumption that the coefficient of friction varies inversely as the 1/7 power of the capacity. 261