Document ZBGd3jzO1Kx8xNDq5BDdbEBwp

Heating Ventilating Air Conditioning Guide 1939 The pressure at the outlets nearest the fan will be greater than at the pipes fartl, along the run so that the former will tend to deliver more than the calculated amour* air. To remedy this condition, volume regulating dampers should be located at the h of each riser and adjusted for proper distribution. At points where branches leave th* main it may be advisable, depending upon the nature of the installation, to inst n adjustable splitters similar to that shown in Fig. 6 where the main duct divides intvTic 58 in. X 30 in. and 50 in. X 30 in. branches. "e The rectangular equivalents are selected from Table,!; the width to depth proporti will be determined by construction requirements and ease of fabrication. The cal D Iation of the friction is as follows: cu- The longest run from the fan outlet to diffuser is 150 ft 0 in.; 150 ft of 56-in nm. 150X12 Pipeu equivalent to----- gg---- ------------------------------------------------------------- ------..........-. 32.2 diam 45 , Two 45-in., 90-deg elbows (2 X ^oo X 8.5)----------- -------------.:.-..-.-.--.-..-.-.-.-.-..-.-......... -- 13.7 (];am (Assume each elbow equivalent to 8.5 diameters of duct, Fig. 1). <y> Two 23-in., 90-deg elbows (2 X X 8.5).--..............-............. .....................7.0 diam oo ' Two 23-in., 90-deg elbows in riser (2 X oo X 30).^------------- -------- (Two bad elbows in riser, each equivalent to 30 diameters of duct). --- 24.7 diam Total diameter of 56-in. pipe.______________________ !_______ ....... 77.6 The velocity head corresponding to a velocity of 1340 fpm is (/14304005\)2 = 0.112 in. Taking 50 diameters as one head loss, then 77.6 50 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)4 heads X. 0.0625)............ (2) Tempering coil loss (from manufacturer's tables)________________ (3) Air washer loss (from manufacturer's tables)_______________________ (4) Reheating coil loss (from manufacturer's tables)__________________ (5) Allowance for regulating dampers and diffusers__________________ .0.094 in. 0.100 in. 0.250 in. 0.100 in. 0.100 in. Static pressure loss of system______ ________________________ 0.755 in. The fan should be selected from the manufacturer's ratings which according to the Standard Test Code for Centrifugal and Axial Fans1, will deliver 22,935 cfm at a static pressure of 0.755 in. and which has an outlet area of 16)4 sq ft. The method of design used in Example 3 is the equal friction method described under the heading Procedure for Duct Design. This involves *See Chapters 27 and 45. 594 Chapter 29. Air Duct Design '3H 24x16 34x16, 16,800 cfm 11.550 cfm J 9.450 cfm J | 148x38 > 46x30 40x30 3,150 cfm 1^4X16^50 gke Fig. 7. Exhaust System Layout the arbitrary reduction of velocity from the fan outlet to the point of discharge to the room, and the friction is calculated by adding the pressure losses of each section of duct. This method requires dampering in the risers and supply branches in order that equalization of air flow can be attained. Example 4- Fig. 7 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 charts, Figs. 4 and 5, the pipe sizes.are as indicated in Table 3 for both round and rec- Table 3. Pipe Sizes for Example 4a VOLUME or Aib (ctm) 16,800 11,550 9,450 5,250 4,200 3,150 2,100 Pb&Cbnt or Total Volume 100.0 68.8 56.2 31.3 25.0 18.8 12.5 Diameter of Pipe (Inches) 47 41 38 31 28.5 25.3 21.6 Equivalent 8m or Rectangular Duct (Inches) 38x48 30x46 30x40 24 x 34 24x28 16 x 34 16x24 Velocity through intake grilles (not shown) to be approximately 400 fpm. 595