Document gbbrmpG6DRywEjoYrRvzLY7GL

HEATING VENTILATING AIR CONDITIONING GUIDE 1940 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 7J^-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 clear area, which, in this case, is 46 sq ft. For more detailed infor mation on tempering coil and air washer control, see Chapter 38. 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 basisx)f calculation. Fig. 4 shows the amount of air to be handled by each section of pipe. Using the chart. Fig. 3, the pipe sizes are as shown in Table 2. The pressure at the outlets nearest the fan will be greater than at the pipes farther along the run so that the former will tend to deliver more than the calculated amount of air. To remedy this condition, volume regulating dampers should be located at the base of each riser, or in each branch duct, and adjusted for proper distribution. At points where branches leave the main it may be advisable, depending upon the nature of the installation, to install adjustable splitters similar to that shown in Fig. 4 where the main duct divides into the 58 in. X 30 in. and 50 in. X 30 in. branches. The rectangular equivalents are selected from Table 1; the width to depth proportion will be determined by construction requirements and ease of fabrication. The calcu lation of the friction is as follows: The longest run from the fan outlet to diffuser is 150 ft 0 in.; 150 ft of 56-in., pipe is equivalent to --............................................................. ................. 32.2 diam Two 45-in., 90-deg elbows (2 X j4g5 X 8.5)._..................................................... 13.7 diam (At 1.5 centerline radius, pressure loss is 17 per cent of velocity head, Fig. 1, = 0.17 X 50 = 8.5 diame ters of duct.) Two 23-in., 90-deg elbows (2 X 2jg3 X 8.5),...................................................... 7.0 diam Volume or Ara (era) 22,935 12,510 10,425 8,340 6,255 4,170 2,085 Table 2. Pipe Sizes for Example 3a Diameter or Pipe (Inches) 56 45 42 39 35 29K 23 Equivalent Size or RboTiNomxB Ducr (Inches) 60 x 44 58x30 SOx 30 42 x 30 42 x 24 30 x 24 30 x 15 Velocity through diffusers (not shown) to be approximately 300 fpm. 554 CHAPTER 30. AIR DUCT DESIGN Two 23-in., 90-deg elbows in riser (2 X X 30)------------------- ---------------- 24.7 diam (Two bad elbows in riser, each equivalent to 30 diameters of duct.) Total diameter of 56-in. pipe.77.6 (1340\2 4qqJ = 0.112 in. Taking 50 diameters as one head loss, then 77 6 X 0.112 = 0.174 in. static loss in duct. Where the connection pieces are made with long easy slopes and the general work manship is good, a regain in static pressure may be deducted from the foregoing pressure loss. This can be taken as approximately two-thirds the difference in velocity pressures at the fan outlet and the last run of pipe. The velocity in the riser is 667 fpm with a corresponding velocity pressure of 0.027 in. The fan outlet velocity is 1400 fpm with a corresponding velocity pressure of 0.122 in. The regain equals % (0.122 -- 0.027) = 0.063 in. / The net static pressure loss in the duct is: 0.174 in. -- 0.063 in-----------------------------------------------------------------------------.0.111 in. Other friction losses are as follows: (1) Fresh air intake 1000-fpm velocity (1% heads X 0.0625)0.094 in. (2) Tempering coil loss (from manufacturer's tables)__0.100 in. (3) Air washer loss (from manufacturer's tables)-------------- ---------------------------0.250 in. (4) Reheating coil loss (from manufacturer's tables) 0.100 in. (5) Allowance for regulating dampers and diffusers- ________ ____________ 0.100 in. Static pressure loss of system0.755 in. The fan should be selected from the manufacturer's ratings which according to the Standard Test Code for Centrifugal and Axial Fans*, will deliver 22,935 cfm at a static pressure of 0.755 in. and which has an outlet area of 16J sq ft. Example 4- Fig. 5 shows an exhaust system layout for exhausting from buildings of the same type as in Example 3. Assume the air requirements based on the number of air changes per hour to be 16,800 cfm. Using a velocity of 1400 fpm in the main duct at the fan inlet, which is an average velocity for this type of system, the area of the main is 12 sq ft, which corresponds to a 47-in. pipe. Referring to Example 3, and using the chart, Fig. 3, the pipe sizes are as indicated in Table 3 for both round and rectangular ducts. .Volume or Air (era) 16,800 11,550 9,450 5,250 4,200 3,150 2,100 Table 3. Pipe Sizes for Example 4 Diameter or Pips (Inches) 47 41 38 ( 31 28.5 25.3 21.6 Equivalent Size or Rec tangular Duct (Inches) 38 x48 30x46 30 x 40 24 x 34 24 X 28 16x34 16x24 Velocity through intake grilles (not shown) to be approximately 400 fpm. *See Chapters 28 and 46. 555