Document a4rNNmXneQr2d2D83x8R11g89

American Society of Heating and Ventilating Engineers Guide, 1936 The velocity head corresponding to a velocity of 1340 fpm is j = 0,112 in. 77 6 Taking 50 diameters as one head loss, then X 0.112 = 0.174 in. static loss in duct. Ou 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 only is then: 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 system.............:.............................................................. 0.755 in. The fan should be selected from the manufacturer's ratings which, according to the Standard Test Code for Disc and Propeller Fans, Centrifugal Fans and Blowers1, will deliver 22,935 cfm at a static pressure of 0.755 in. and which has an outlet area of 16H 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 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. 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 Table 3. Pipe Sizes for Example 4a ' Volume or Am . ..(era) 16,800 11,550 9,450 5,250 4,200 3,150 2,100 Per'Cent , op Total Volume Diameter or Pipe (Inches) 100.0 68.8 56.2 31.3 25.0 18.8 12.5 47 jzC / 38 / 31 / 28.5 .' / 25.3 / 21.6 Equivalent Size or. Rectangular Duct (Inches) 38 x 48 30x46 30 x 40 24x34 24 x 28 16x34 16x24 aVelocity through intake grilles (not shown) to be approximately 400 fpm. *See Chapters 17 and 44. ` 364 Chapter 2U--^;Air Duct Design v.t: .r.:: 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. All risers will require dampering as in Example 3. The calculation of the friction is as follows: The longest run from the intake grille to fan inlet is 100 ft. (1) Duct friction 100 ft of 47-in. pipe ___ ___--.......... --__-...... 25.6 dia. Two 28H-in., 90-deg elbows in riser ^ ^ ^ X 30 ^.... ............................... 36.4 dia. (Two bad elbows in riser each equivalent to 30 diameters of duct). One 283^-in., 90-deg elbow in horizontal run ...... ........................ 5 2 dia. Total diameter of 47-in. pipe......................... ........ ................................. 67.2 dia. Velocity head corresponding to 1400 fpm is = 0.122 in. Taking 50 diameters as one head loss, then --67:--2 X 01-1-2--2 ........ i..... ... .. .. 0.164 in. 50 (2) Intake loss from grille (1heads at a 400 fpm velocity,1 Yi X 0.01) ..... . 0.015 in. (3) Static pressure required to produce one velocity head at 1400 fpm___ 0.122 in. .(4) Loss occasioned by step-up of velocity (0.20 X 0.122)........ ..... ....___.. 0,024 in. (This loss varies from 0.05 to 0.40 velocity head depending upon the nature of the change, r or average systems 0.20 velocity head is a close approximation.) ` ' Static pressure loss on inlet side... .................. ...... ........... ...... .......... ......... . 6.325 in. 365