Document 3N9p7xN6e4MRoQ7DDxQ2gDx2n

EXHAUST SYSTEMS, GENERAL <r COMFORT VENTILATION TABLE 28-C RECOMMENDED MINIMUM DUCT VELOCITIES Nature of Contaminant Examples Duct Velocity (fpm) Vapors, gases, smokes, fumes, and All vapors, gases, and smokes, zmc very light dusts...................... and aluminum oxide fumes, wood flour and cotton bnt Medium density dry dusts........ Cotton, buffing and rote lint, wood, gram, rubber, and bakehte dust Average industrial dust.............. Wool, wood, sand blast, grinding, and shoe dust, wood shavings, foundry dust Heavy dusts............................... Lead, and heavy foundry dusts, metal turnings, pite butts Large particles, of heavy moist materials................................. Moist lead and other heavy dusts 2,000 3,000 4,000 5,000 5,000 and over Adopted from Allen D Brandt "A Sommer; of Baun Data for Exhaust Systems " Beatrng and Ventilating (May 1M11. example, a 10-id branch leading to a hood only 12 ft from the fan will short-circuit a second branch 4mm diameter coming from a hood 25 ft away--If the system Is not balanced This, of course, is because the friction loss in a long length of smalldiameter pipe is far greater than the loss m a short length of large pipe. Friction loss (which in turn causes pressure drop) is far more important m air systems than it is, for instance, in water piping Whether a system operates under positive (blowing) or negative (suction) pressure makes no difference so for as duct design features are concerned Usually, however, exhaust systems have high air velocities to keep dust m suspension in the duct Posi tive pressure systems, those that supply ven tilating air to a given space, operate with relatively low duct velocities. Table 28-C shows recommended minimum duct veloci ties for handling different types of con taminants Energy losses occur in ducts ui the following ways Velocity pressure A definite negabve pressure or suction must be created by the fan to get air moving at a certain velocity. This suction, known as velocity pressure or velocity head, depends on the square of the velocity: where VP = velocity pressure in inches, water gauge V = velocity in fpm High velocity systems, such as are need ed to convey dusts, are more expensive to operate than low velocity systems Table 28-D shows examples of how much nega tive pressure an exhaust fan must create to get air to move at the given velocities. Air horsepower depends directly on pres sure and rate of air flow, as follows py o Air horsepower = where P = pressure in inches of water Q = air flow in ofm Therefore, the horsepower to make 1,000 cfm move at a velocity of 5,000 fpm, for example, would be six tunes the horsepower needed to make it move at 2,000 fpm. Hood entrance loss This is determined by the shape of the hood, varying from a bellshaped entrance with negligible loss (rare), to an abrupt open-end pipe where the loss is 0.93 VP (undesirable). An additional reason for putting flanges on, besides their 28-24