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Heating Ventilating Air Conditioning Guide 1939 Chapter 29. Air Duct Design F ic . 6. T y p ic a l L a y o u t of A ik D is t r ib u t io n System MAIN TRUNK DUCTS A main duct with branches is generally used to convey tempered air { r ventilation purposes only. In place of individual ducts, a compara tively large main duct supplies air by branches to the room or rooms. The elocities vary according to the nature of the installation and the degree of v ietness required. At the start of the run a velocity as high as 2000 fpm may be used, but this is considered the maximum for public building work, and is reduced to from 400 to 800 fpm in the risers. This duct system may be designed so that the loss of pressure in the branches is equalized in a manner similar to that previously described. Equal Friction Method Example S. Fig. 6 shows a typical layout of an air distribution system which is applicable for ventilation of hotel dining rooms and offices. The volume of air in cubic feet per minute for the room is determined on the basis of the number of air changes per hour required. In the example shown, the room ventilated is a hotel dining room 135 ft x 85 ft x 15 ft. A 7^-minute air change (8 air changes per hour) is assumed for proper ventilation, giving 22,935 cfm as the air required. 22 935 The clear area of the fresh air inlet is based on a velocity of 1000 fpm or = 22.94 sq ft. If the air washer is provided with automatic humidity control, the tempering coil should raise the temperature of the entering air to 32 F. The washer with its auto matic control will then raise the temperature from 32 F to 42 F. If the washer is not provided with automatic humidity control, the tempering coil must raise the temperature of the entering air to at least 55 F to allow for some temperature drop in the washer due to evaporation. The reheating coil is selected to raise the temperature of the air from that leaving the air washer to 70 F. The air washer should have a maximum velocity of 500 fpm through the dear area, which, in this case, is 46 sq ft. For more detailed infor mation on tempering coil and air washer control, see Chapter 37. Since the plan shows a moderately short run of main duct with no risers near the fan outlet, a fan should be selected which will have the required capacity of 22,935 cfm with a maximum velocity through the fan outlet of 1400 fpm. The outlet area, therefore, should be 16H sq ft. The main pipe size should be selected to give a velocity equal to or less than the . velocity at the fan outlet. Choosing a 56-in. pipe with a cross-sectional area of 17.1 sq ft, the velocity in the main pipe will be 1340 fpm. Using the friction pressure loss method this 56-in. main pipe will be taken as the basis of calculation. Fig. 6 shows the amount of air to be handled by each section of pipe. Expressing the volume handled by each section as a percentage of the total volume and using the charts. Figs. 4 and 5, the pipe sizes are as shown in Table 2. Volume or Am (cfm) 22,935 12,510 10,425 8,340 6,255 4,170 2,085 Table 2. Pipe Sizes for Example 3a Pee Cent or Total Volume 100.0 54.6 45.4 36.3 27.2 18.2 9.1 Diamstbb or Pipe (Inches) 56 45 42 39 35 29K 23 Equivalent Stsb or Rectangulab Duct (Inches) 60 x 44 58x30 50x30 42x30 42x24 30x24 30x15 592 Velocity through diffusers (not shown) to be approximately 300 fpm. 593 t