Document Em0XznXxZ7BzkOzn4aKV0X2pV

of andAmerican Society Heating Ventilating Engineers Guide, 1934 SECTION I TOP SHELL ' SIDE SHEET Bottom sheet these crossbreaks ' ARE NEVER = . SHOWN DMA PUN ELEVATION REINFORCE!) CROSS SEAMS SEAMS BETWEEN ADJACENT PANELS OR PUIN CROSS SEAMS Fig. 9. Details of Seams Fig. 10. Method of Installing Heating Unit < 270 19--Chapter Air Duct Design Where there are one or more ducts with branches, the velocity of air in the ducts may be either chosen arbitrarily or calculated for friction losses. When arbitrary values are assigned, a certain amount of dampering should be provided for; this will be small when the method chosen permits a drop in velocity as the quantity of air is reduced. After the total air quantity and the size of fan are ascertained, the main duct is usually fixed as being at least equal in area to the fan outlet, or perhaps 10 per cent greater. From this main pipe all others are propor tioned. For example, if the main duct is 30 in. in diameter, a branch to carry 10 per cent of the total capacity should be 12.7 in. in diameter (see Fig. 4) in order to have the same friction per foot of length, while one carrying one-half the total capacity of a 30-in. main with the same friction loss per foot would be 23.4 in. in diameter. By this method of equalizing friction it is unnecessary to consider the resistance of each section of pipe independently, but only to know the distance from the fan outlet to the end of the longest run of pipe, the number and size of elbows, and the diameter and velocity in the largest pipe. Example 5. If the greatest length of piping in a system is 130 ft with a 26 in. diameter main pipe and one 20-in. elbow, the piping having been designed for equal friction per foot of length, the friction would be the same as for 130 linear feet of 26 in. pipe, or 60 diameters. To this should be added the friction loss in elbows, in this case one 20 in. elbow, which has a loss equivalent to one-fifth of a velocity head or ten diameters of 20-in. pipe. This in turn is 20 X 10 = 7.7 diameters of 26 in. pipe. The total equivalent length of the system will then be 60 + 7.7, or 67.7 diameters. Since 50 diameters is equivalent to one velocity head, the loss is--gg- = 1.35 times the velocity head. If the velocity is, for example, 2200 fpm, corresponding to 0.3 in. pressure, the friction loss of the system will be 1.35 X 0.3 = 0.405 in. Frequently the prevention of sound in a heating or ventilating system imposes more severe restrictions than the prevention of excessive pressure drop. This question is highly involved and requires consideration of many factors. The air velocities to be used will vary with the standard of construction used in the ducts themselves as well as with the nature of the occupancy and the construction of the building. In general, architects and engineers who leave the details of duct construction to the contractor must, of necessity, design for lower velocities than might be required for quiet operation if proper construction details were always followed. The contractor may be expected to-build the ducts by the least' expensive methods, and the engineer must anticipate this. For further information on noise reduction, see Chapter 18. Details of Duct Construction If panel construction is used with standing seams or similar reinforce ment, and the panels are cross-broken to give rigidity, there is less like lihood of vibration due to air flow, or deflection due to- air pressure. Elbows made without splitters, and improperly shaped transformation sections produce high local velocities which are the cause of noise in duct work. The use of first-class duct construction with well designed trans formation sections and splitters in elbows tends to maintain relatively uniform velocities with decrease in turbulence and in the noise produced. 271