Document pBX35vwYXMGQdxdYa6JYop18E

American Society of Heating and Ventilating Engineers Guide, ticularly those used in buffing and polishing, are connected by subranch pipes to the main duct which renders proportioning impracti^ Construction The ducts leading from the hoods to the exhaust fan should be co structed of sheet metal not lighter than is shown in Table 4. The pin;11* should be free from dents, fins and projections on which refuse mieSt catch. s1 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 Table 4. Gage of Sheet Metal to be Used for Various Duct Diameter* ---------- ,_ Diameter of Duct Gaoe or Mtiu 24 22 20 18 . ' 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 where the material from one branch would tend to enter the branch on the opposite side of the main. ^ Cleanout openings having suitable covers should be placed in the main 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 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 352 Chapter 21--Industrial Exhaust Systems f'+ , o be an increase in the latter equal to their sum. Some state codes ><sb0 ' r that the combined area be increased by 25 per cent. While this ?not aiways necessary and is frequently done at the expense of a reduced e. velocity, it is none the less advisable where future expansion of the jj^aust system is contemplated. Table 5. Air Speeds in Ducts Necessary to Convey Various Materials Material flfood chips and shavings. - Sawdust------ ------ ------ Jote dust---...................... Robber dust....... ............... jdrtaTdust (grindings).--. Lead dusts.----- -----......... Brass turnings (hne;----Fine coal----------------------- Am Velocities (fpm) 2000 3000 20002000 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 system8. Velocities determined from these formulae 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-- d.no r+1 (5a) For horizontal ducts: where V = 6000 --d.ras s+1 V -- air velocity in duct, feet per minute, r = specific gravity of particles. i = average diameter of largest particles conveyed, inches. (6b) j. * .* uiutwioi, me, iaigcai ouc ui wuicii is approximately u.g/ m. in TS*tef' ^lth gravity of 1.40 is to be conveyed in a vertical pipe the velocity vdocfty^ m whlch 13 4100 fpm; find whether the material can be transported at this Substitute data in Equation 5a and multiply by 1.25; V = 1.25 X 13,300 X X 0.37-" ZA th.eHence'tffp h!'5? Xi '*? 037) Z '568; re,?uir,ed velocity is, therefore, 5500 fpm. thedameter'o?the'duct.^r^oth.1"*"1^6336^ by Spedi"g "P the r decreasi"S Systema' <ASH-VE' Journal 353