Document ba7boX4ZNvVgJ1D9MELygoJg3

American Society of Heating and Ventilating Engineers Guide Velocity at collector inlet: y - 3|33 = 3 120 V 1.069 Collector Drop = C ^ oooy =0145 (S)2 = 14 in- f water- Table 8. Total Resistance of the System Shown in Fig. 1 1. Loss at hoods......... ............................................................... 2:00 in. 2. Loss through piping...... :.................. 3.49 in. 3. Loss at collector.................... 1.40 in. ofwater ofwater ofwater Total resistance of system..................... -.............................. 6.89 in. of water The total resistance of the system as indicated in Table 8 is 6^89 m-: of water. Therefore an exhaust fan to handle 3,330 c.f.m. at 7 in. static pressure will be required. SELECTING THE FAN The usual types of ventilating fans are unsuitable for exhaust systems* which are required to handle materials such as shavings, sawdust, emery dust, etc. Higher pressures are required than in ventilating work and in addition housings and blast wheel must be so constructed that the" materials handled do not deposit in them. While the fans used in different exhaust systems are more or less of the same general type, modifications are frequently necessary to fit them for handling such materials as long' shavings, strips of paper, cotton, pulverized coal, etc. Where considerable quantities of explosive dust or inflammable materials pass through the exhaust fan, the blast wheel should be constructed of brass composition, copper or other soft metal and in all cases ample clearance should be provided between blast wheels and housings. Where stringy or fibrous material is to be handled through the fan, a. fan wheel especially designed for that purpose should be employed. For further data on the selection of fans, see Chapter 32. 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 introduced 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 it spirals down to the tail piece, while the air escapes through the stack at the center of the collector. For most systems, the inlet size of the collector may be the same as the diameter in inches of the main pipe leading to it. The larger the collector within certain limits the better will be the separation, and the less will be the back pressure on the fan and the power consumed. 392 '-GhapTER 25-Exhaust Systems for Dust and Refuse '*P ^G ^hsatprutecrtiokn>--is s-o--m---etimes requiredj fco__rAfine,4dnuestt.,aalslsoo some bb!lowpea1 T cturers use a special type of collector for furnace feed, the pipe.wf".11 t0 deliver the material to furnaces as uniformly as possible. gJjlSw ore than one fan delivers into a single collector a back pressure Mffien *n ^ to prevent one fan blowing back through the other in ^the stecDoinadntfsa,nwshheoreuldwosotodprefofursaeniys ruesaesdonfo. r fuel, it is delivered by In- m0jjrgCtly from the collector to the furnace. The discharge pipe frQin tfte bottom of the collector is divided and the junction If^shed with a switch or valve so arranged that when the material jT,rrlj00 fast for the fires it can be diverted into a reserve bin. vrlT furnace feeder should be hinged where it is attached to the lower f the discharge pipe, in order that it may be disconnected from the when the fan is shut down. Also great care must be taken to '^vide an absolutely tight switch. Otherwise, when discharging refuse Uvthe storage bin, fine sawdust will sift through this valve and settle ^ the furnace feed pipe, and, in case the fireman has neglected to dis^Shnect the feeder from the furnace, the flame may flash back, following ?h:i6stthraeirnfoorfmfisneosfacwodlluesctt,oirnstoorthseepcaorllaetcotrosr.are: settling chambers, cloth treen and bag collectors, bag houses, air washers and electric precipitators. p-/'.: . DESIGN OF HOODS iSlf the material to be moved is already in motion, as are the chips thrown off from wood-working machines, the hoods should be arranged in the path of the particles so that the velocity of the particles assists the air in caHrroyiondgstshheomuldatbeeriaalrrtaongtheed tthorodaratwofdtuhset haonoddf.umes away from the face hf the operator. They should be placed as close as possible to the source ;bf dust or waste material and wherever practical, the hoods should '.'eHntoireoldysenacrelousesuthaellydcuosnt sptrruocdtuecdinogf ogpaelvraantiiozne.d sheet iron or other equally ^substantial and durable material. The material should be heavy enough ;ito withstand the abrasive action of the dust and refuse. The hoods should jbe of sufficient mechanical strength to keep their shape and should be :twell braced and substantially supported. Galvanized iron used should :'f.-neIvferabceidligohr tecor rtrhoasniveNofu.m2e2sgaagre. present heavy material painted with acid resisting paint should be used, or the hoods may be made of non- - coTrrhoseiveexpmoasetedrieadl.ges of all sheet metal hoods should be bound with wire or band iron, not only to give the necessary stiffness, but also to prevent .* the operator from being cut by the raw edges of the sheets. CONVEYOR PIPES The conveyor pipes leading from the hoods to the fan and thence to i,, m3rle 0f galvanized iron, the gage of which *-- THio ninine