Document rejjz22BXKOLJ06kE3eaVOGVr

American Society of Heating and Ventilating Engineers Guide, 1934 DESIGN OF DUCT SYSTEMS The duct system should be large enough to transport the fumes or material without causing serious obstruction to the air flow. It is good, practice to proportion the ducts to obtain the desired velocities and suction pressures at the hoods, although in many cases only an approxi mation to an ideal design is possible. Many exhaust hoods, and par ticularly those used in buffing and polishing, are connected by short branch pipes to the main duct which renders proportioning impractical. Construction The ducts leading from the hoods to the exhaust fan should be con structed of sheet metal not lighter than is shown in Table 4. The piping should be free from dents, fins and projections on which refuse might catch. Table 4. Gage of Sheet Metal to be Used for Various Duct Diameters Dumxtsb or Duct Gaoe o? Mbtal 24 22 20 18 All permanent circular joints should be lap-jointed, riveted and sol dered, and all longitudinal joints either grooved and locked or riveted and soldered. Circular laps should be in the direction of the flow, and piping installed out-of-doors should not have the longitudinal laps at the bottom. Every change in pipe size should be made with an eccentric taper flat on the bottom, the taper to be at least 5 in. long for each inch change in diameter. All pipes passing through roofs should be equipped with collars so arranged as to prevent water leaking into the building. The main trunks and branch pipes should be as short and straight as possible, strongly supported, and with the dead ends capped to permit inspection and cleaning. All branch pipes should join the main at an acute angle, the junction being at the side,or top and never at the bottom of the main. Branch pipes should not join the main pipes at points such that the material from one branch tends to enter the branch on the opposite side of the main. Cleanout openings having suitable covers should be placed in the jnain and branch pipes so that every part of the system can be easily reached in case the system clogs. Either a large cleanout door should be placed in the main suction pipe near the fan inlet, or a detachable section of pipe, held in place by lug bands, may be provided. Elbows should be made at least two gages heavier than straight pipe of the same diameter, the better to enable them to withstand the addi tional wear caused by changing the direction of flow. They should pref erably have a throat radius of at least one and one-half times the diameter of the pipe. Every pipe should be kept open and unobstructed throughout its entire 286 Chapter 21--Industrial Exhaust Systems length, and no fixed screen should be placed in it, although the use of a trap at the junction of the hood and branch pipe is permissible, provided it is not allowed to fill up completely. The passing of pipes through fire-walls should be avoided wherever possible, and sweep-up connections should be so arranged that foreign material cannot be easily introduced into them. At the point of entrance of a branch pipe with the main duct, there should be an increase in the latter equal to their sum. Some state codes specify that the combined area be increased by 25 per cent. While this is not always necessary and is frequently done at the expense of a reduced air velocity, it is none the less advisable where future expansion of the i------------1-j.___ 1 Table 5. Air Speeds in Ducts Necessary to Convey Various Materials Grain dust......................... Wood chips and shavings. Saw dust---------- ----- ------Jute dust----------------------Rubber dust............ ......... Lint.................................... Metal dust (grindings)-- Lead dusts........................ Brass turnings (fine)--.... Fine coal....... .................... Material Air Velocitier (fmi) 2000 3000 2000 2000 2000 1500 2200 5000 4000 4000 Air Velocities in Ducts When the static suction has been fixed for a given hood, the air velocity in the duct may be determined from Equation 2. Air velocities for conveying a material should be moderate. Table 5 gives the velocities generally employed for conveying various substances. Equations 5a and 5b may be used as tests to determine the conveying efficiency of a system. Velocities determined from these formulas should be increased by at least 25 per cent since they represent the minimum at which a stated size and density of material can be transported. For vertical ducts: V = 13,300 s + 1 rfO.670 (5a) For horizontal ducts: V = 6000 d.* J +1 (5b) where V = air velocity in duct, in feet per minute. ,, s = specific gravity of particles. d = average diameter of largest particles conveyed, in inches. -- Example S. Granular material, the largest size of which is approximately 0.37 in. in diameter, with a specific gravity of 1.40 is to be conveyed in a vertical pipe the velocity of the air in which is 4100 fpm; find whether the material can be transported at this UpllV'if-V 287