Document p5a44bbeQd83G2Lr5QnEmL4w

American Society of Heating and Ventilating Engineers Guide, 1935 The drop in>pressure in air distributing systems is due to the dynamic losses and the friction losses. The friction losses are those due to the friction of the air against, the sides of the duct. The dynamic losses are those due to the change in the direction or in the velocity of air flow. Dynamic Losses Dynamic losses occur principally at the entrance to the piping, in the elbows, and wherever a change in velocity occurs. The entrance loss is the difference between the actual pressure required to produce flow and the pressure corresponding to the flow produced; it may vary from 0.1 to Fig. 2. Curve Showing Loss of Pressure in Square Elbows 326 Chapter 20---Air Duct Design 0 5 times the velocity head. The pressure loss in elbows must also be flowed for in the design. It is customary to express dynamic losses in terms of the percentage of the velocity head; in other words, the per centage of that pressure corresponding to the average velocity in the duct which is expressed in terms of inches of water gage. Figs. 1 and 2 show the effect of changing the radius of elbows of square and rectangular section. These charts are based on tests of pipe elbows of ordinary good sheet metal construction. For example, a five-piece round pipe elbow having a centerline radius of one diameter has a loss of about 25 per cent of the velocity head. At a velocity of 2000 fpm the corresponding head is 0.25 in. water gage, and at this velocity the elbow just referred to would cause a pressure drop of 0.063 in. water gage. Experience has shown that good results may-be obtained-when the radius to the center of the elbow~ is \Vi times the pipe diameter. The pressure drop will then be approxi mately 17 per cent of the velocity head for round ducts, and 9 per cent for square ducts. Very little advantage is gained in making elbows with a radius of more than two diameters. Friction Losses Friction losses vary directly as the length of the duct, directly as the square of the velocity, and inversely as the diameter. Since length is a fixed quantity for any system, the factors subject to modification are the area and the velocity, which determine the relation between the first cost of the duct system and the cost of the power for overcoming friction. The friction between the moving air and pipe surface causes a loss of head which is numerically equal to the pressure required to maintain a given velocity, and is expressed in the following modification of Fanning's formula: For round pipe and standard air (70 F and 29.92 in. barometer) , ,L , L ( V \2 L ? D hv CD \ 4005 / (3) For rectangular ducts where Hl = loss of head, inches of water. / V \2 v = (40Q5 ) = velocity head, inches of water. V = velocity of air, feet per minute. L = length of pipe D = diameter of pipe all in feet. a. b = sides of rectangular duct / = coefficient of friction. C =~jr = length of pipe in diameters for one head loss. For all practical purposes C varies only with the nature of the pipe surface: C = 60 for perfectly smooth pipe; = 55 for pipe as used in planning mill exhaust systems; = 50 for heating and ventilating ducts; = 45 for 327