Document QMgqyrxjRzYRzq7YdML03593k

HEATINC VENTILATING AIR CONDITIONING GUIDE 1940 Fig. 1. Curve Showing Loss of Pressure in Round Elbows The drop in pressure in air distributing systems is due to the dynamic losses and the friction losses. The friction losses for turbulent flow (which occur in all practical air flow problems) are due to the friction of air against the sides of the duct and to internal friction between air molecules. The dynamic losses are those due to the change in the direction or in the velocity of air flow. 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 0.5 times the velocity head. The pressure loss in elbows must also be allowed for in the design. It is customary to express dynamic losses in Ccntol-Unc Radius in Pczccnt or Pipe Width Fig. 2. Curve Showing Loss of Pressure in Square Elbows 544 CHAPTER 30. AIR DUCT DESICN 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 90 deg elbows of round' and square section1. These charts are based on tests of pipe elbows of ordinary good sheet metal construction. For .example, a fiye-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. The pressure loss through an elbow, may be assumed to be directly proportional to the angle through which the turn is made. This is approximately true for 90 deg elbows or less but varies considerably for higher degrees of turn. Experience has shown that good results may be obtained when the radius to the center of the elbow is 1% times the pipe diameter. The pressure drop will then be approximately 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 centerline radius of more than two diameters2. Friction Losses Theoretically 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 and internal friction between air molecules cause a loss of head which is numerically equal to the pressure required to maintain a given velocity, and is ex pressed in the following modification of Fanning's formula: For round pipe and standard air (70 F and 29.921 in. barometer) For rectangular ducts Al =/-*v = ~ (4653.) (3) where &L ~ loss of head, inches of water. (V \2 4QQg 1 = velocity head, inches of water. V = velocity of air, feet per minute. L = length of pipe D = diameter of pipe all in feet, o. b = sides of rectangular duct / = coefficient of friction, or friction factor. (4) C =y- = length of pipe in diameters for one head loss. 'Loss of Pressure Due to Elbows in the Transmission of Air Through Pipes or Ducts, by F. L. Busev (A.S.H.V.E. Transactions, Voi. 19, 1913, p. 366). y 'Pressure Losses in Rectangular Elbows, by R. D. Madison and J. R. Parker (Healing, Pining and Air Cottdtttoning, July, p. 365, August, p. 427, September, p. 483, 1936). 545