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. ..-v' American Society of Heating and Ventilating Engineers Guide, 1937 Table 5. Air Speeds in Ducts Necessary to Convey Various Materials Grain dust Wood chips and shavines Sawdust Jute dust................ Rubber dust- Lint Metal dust (erindinvsV . Lead dustR Brass turnings (finel Fine coal_ .. Material Ani y(mnoic) rnta 2000 3000 2000 2000 2000 1500 2200 5000 4000 4000 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 exhaust system is contemplated. 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 --f-- d.n s+1 (5a) For horizontal ducts: where V = 6000 --s + 1r- d. (5b) V = air velocity in duct, feet per minute, s = specific gravity of particles. d = average diameter of largest particles conveyed, inches. Example . 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 velocity. Substitute data in Equation 5a and multiply by 1.25: V = 1.25 X 13,300 X ~ X 0.37- Antilog (0.57 X log 0.37) = 0.568; the required velocity is, therefore, 5500 fpm. "Determining Minimum Air Velocities for Exhaust Systems, by J. M. DallaValle. (A.S.H.V.E. Journa Section. Heating, Piping and Air Conditioning, September. 1932). 392 Chapter 21--Industrial Exhaust Systems Table 6. Loss Through 90-Deg Elbows Cbntkb Lind Radius in Peh Cent or Pipe Diahdtdb 50 100 150 200 to 300 Loss in Pbb Cent or Vklocitt Head 75 26 17 14 Hence the duct velocity must be increased either by speeding up. the fan or decreasing the diameter of the duct, or both. Dud Resistance The resistance to flow in any galvanized duct riveted and soldered at the joints may be obtained from Fig. 3, Chapter 20. The pressure drop through elbows depends upon the radius of the bend. For elbows whose centerline radii vary from 50 to 300 per cent of pipe diameter, the loss may be estimated from Table 6. It is sometimes convenient to express the resistance of an elbow in terms of an equivalent length of duct of the same diameter. Thus with a throat radius equal to the pipe diameter the resistance is equivalent to a section of straight pipe approximately 10 diameters long, while with a throat diameter radius 1J4 times the dia meter, the resistance is about the same as that of seven diameters of straight pipe. COLLECTORS The most common method of separating the dust and other materials from the air is to pass the mixture through a centrifugal or cyclone collector. In this type of- collector the mixture of the air and material is irftroduced on a tangent, near the cylindrical top of the collector, and the whirling motion sets up a centrifugal action causing the compara tively heavy materials suspended in the air to be thrown against the side ,of the separator, from which position they spiral down to the tail piece, while the air escapes through the stack at the center of the collector. The diameter of the cyclone should be at least 3j/ times the diameter of the fan discharge duct. When two or more separate ducts enter a cyclone, gates should be provided to prevent any back draft through a system which may not be operating. Cyclones working in conjunction with two or more fans should be designed to operate efficiently at two- thirds capacity rating. The following formula is useful in computing the loss through a cyclone when the velocity of the air in the fan discharge duct is known: . *.-*(*) where (6) " ha = the pressure drop through the cyclone, inches of water. V = the air velocity in the fan discharge duct, feet per minute. If a cyclone is used to collect light dusts such as buffing wheel dusts, 393