Document 934zop1wJb4Dnbq85LxR0qGZD

FILE NAME: &'ZZ&' DATE: DOC#: &' DOCUMENT DESCRIPTION:^E^^W 1 NATIONAL SAFETY COUNCIL INCORPORATED 20 NORTH WACKER DRIVE CHICAGO 6, ILLINOIS Safe Practices Pamphlet. No. 32 Exhaust Systems Published by National Safety Council, Inc. 20 North Wacker Drive, Chicago 6 Exhaust Systems 1. One of the dangers encountered by workers in many industries is the exposure to dusts, gases, vapors and fumes that are generated by various machines and industrial operations. It is difficult to enumerate all the different potentially harmful substances that are met with in modem industry, but the following general classification may suggest many with which we are more or less familiar: a. Some dusts have a mechanical or irritating action affecting the parts of the body which may be directly exposed. For instance, sharp-pointed particles of metal or rock may harm the eyes, nose, throat and skin. If inhaled into the lungs some dusts may cause fibrosis or chronic inflammation. b. Corrosive dusts from such substances as soda, lime, and others cause inflammation of the skin and other exposed parts of the body. c: Poisonous dusts such as lead, arsenic or mercury may, upon entering the body, cause general poisoning, or they may attack certain parts of the body, such as blood, bones or nerves. d. Dusts from fur, feathers and hair may carry disease germs which may infect the worker. e. Irritating fumes from substances such as ammonia or acids act locally upon the eyes and upon the mucous membrane of the throat, nose and lungs. f. Poisonous fumes from benzol, wood alcohol, aniline, lead, and other substances often affect the blood, heart, eyes, or nerves. This pamphlet is one of a series of more than 150 Safe Practices and Health Practices pamphlets. As a compilation of experience from many sources, it should not be assumed to include every acceptable procedure in the field covered. It must not be confused with federal, state or insurance requirements, or with American Standard safety codes. Attention is called to the American Standard Regulations for the Installation of Blower and Exhaust Systems for Dust, Stock and Vapor Removal, to which reference should be made; the code was formulated under the sponsorship of the National Fire Protection Association and under the procedure of the American Standards Association, 29 W. 39th Street, New York City. The suggestions contained in this pamphlet do not conflict with the requirements of that code. Price: 35c each to members; non-member and quantity prices on request. d. Natural ventilation. (See N.S.C. pamphlet No. 37.) e. Isolating in a separate room or building the process or machine that creates dust, gas, vapors or fumes. f. Providing and requiring exposed workers to wear respiratory protective equipment. (See N.S.C. pamphlet No. 64.) 3. Methods b, c, and e are possible in many cases, but they require special engineering revision in each case, and 2. Dusts, gases, vapors and fumes are the direct or indirect cause of many of the so-called "occupational diseases" and they can be removed or rendered harmless by employing such methods as the following: a. Mechanical exhaust system. b. Installing automatic or closed machinery methods. c. Using wet processes. Figure 1. Tumbling barrel, properly guarded equipped with exhaust system. (See paragraph cannot, therefore, be discussed in detail in this pamphlet. 4. Natural ventilation in eliminating dusts, gases, vapors and fumes is practically limited to the installation of vertical flues or pipes that convey to the outside those substances which are lighter than air or which are warm enough to rise. In some cases, where the natural draft is not sufficient, steam pipes run through such a flue may be a simple and effective means of producing the necessary draft without the use of a mechanical fan. It is important, however, to have a suitable arrangement at the top of the flue, perhaps turning with the wind, to prevent the wind from reversing the current of air in the flue. For more detailed information see N. S. C. pamphlet No. 37, "Industrial Ventilation." 5. Each installation of a mechanical exhaust system presents special problems that may require expert and technical knowledge as well as an understanding of existing conditions, but there are a number of important factors, common to all installations, that are discussed in this pamphlet at least in outline form. 6. There are two general types of exhaust systems, the general and the local. 7. General exhaust systems provide for drawing the air (and dust, gases, vapors or fumes) from a room by means of screw-propeller fans operating at windows or at other openings in the wall. One manufacturer, instead of purchasing the ordinary type of fan usually employed for this purpose, has installed a large electric motor and fitted with an aeroplane propeller. 10.) This is used at a point where (Copyright, 1920, 1926, 1939, National Safety Cou ncil, Inc. All rights reserved. Printed in U. S. A.) 2--S.P.P. 32 SAFE PRACTICES PAMPHLET NO. 32 entering any other place where their presence would be objectionable. Courtesy, General Electric Company Figure 2. Exhaust system on multiplespindle drill for slate panels. A false top is mounted over the regular table; this has holes to provide dearance for the drills after they go through the slate; the space under the false top is connected to a special exhaust hood. Note the dust trap at "A"; heavy partides drop into this trap and are removed at regular in- tervals. (See paragraphs 14 and 37.) dense fumes may be evolved suddenly, and is capable of dearing the air in an emergency in about one minute. In another plant where fumes that are heavier than air are liberated, the floor in a certain room is made of cast-iron grids. Ventilation is secured by downward displacement. 8. The general exhaust system usually involves low initial cost and comparatively low cost of maintenance. It may be especially desirable in laundries, foundries and other places where there is a great amount of heat and where large quantities of steam are created. The general system however, is not always satisfactory particularly where harmful dusts, fumes, gases and vapors are involved. In many cases it is necessary to install local exhaust systems instead of, or in addition to, the general systems. 9. Fans used for general exhaust should be placed well out of reach and if there is any possibility of persons coming into contact with the revolving blades of the fan, the blades should be enclosed with wire mesh or other guards that will exclude a hand or finger. The exhaust from such fans is usually direct to the outside atmosphere, and it is necessary that care be exercised to prevent the exhausted dust, gas, vapor or fumes from returning into the room or from 10. Local exhaust systems require the installation of hoods for the collection of the sub- stance to be removed, a fan and metal pipes or ducts for convey- ing the contaminated air to the outside or to some properly pro- vided place for disposal. Such systems. if correctly designed and operated, can be installed to remove objectionable dust s, gases, vapors or fumes at the ex- act point of origin, with uniform and satisfactory results. If such substances are allowed to dis- Figure 4. One large hood over a series of tanks. perse in the workroom, it usu- (See paragraph 19.) ally becomes practically impos- sible to reduce the concentration to sat- 14. Sometimes conditions are such isfactory limits. (Figure 1.) that hoods or pipes cannot be installed The Hood directly over vats, machines, or other sources of dust, gas, vapors or fumes 11. Much of the success of any local because they would interfere with over- exhaust system depends upon the design head cranes or some other operation. It and installation of the hood. then becomes necessary to provide lat- 12. For removing heavy gases, fumes, vapors and such substances as metal partides and sand, the downward system of ventilation is usually the most effec- eral hoods or those in which the substances to be removed must travel in a horizontal direction instead of vertically. (Figure 2.) tive. In such installations the hood may 15. Frequently, the lateral hood may be a hopper placed directly beneath the be of the suction box type in which the work-table. The pipe leading to the ex- front of the box is perforated with nu- haust fan is usually led from the side merous small-sized intake openings pro- of this hopper (not the bottom) and is viding a high velocity of air, thus forced to handle only the lighter particles which do not drop into the trap at the bottom. (Figure 2.) 16. If the particles to be removed are already in motion, as is the dust thrown from a grinding wheel, the hood, 13. An upward system of ventilation if possible, should be installed in the is usually used to remove fumes, gases, path of the waste particles so the veloc- and vapors that are lighter than air or ity of the particles assists the force of which have a tendency to rise. The the exhaust draft. (Figure 3.) hood may then be flared, or shaped like an inverted funnel, the small end of which is connected to the pipe leading to the exhaust fan. 17. It is important, in general, that every exhaust system be designed to draw the dust or fumes away from the face of the operator. In some installa- tions an additional fan is placed in the open room to direct an air current away from the worker and toward the exhaust hood. Such installations, however, should be made with care, otherwise the air to be exhausted may rebound from the surface of the hood or blow past the hood entirely. Figure 3. Exhaust on small grinder. The hood catches dust panicles at the point of greatest velocity. (See paragraph 16.) 18. For most processes, hoods are constructed of galvanized sheet metal, the gage of which, depending upon the size of the hood and the strength and rigidity required, should ordinarily be 2 EXHAUST SYSTEMS gages heavier than the gage of the exhaust pipe to which it is connected. (See paragraph 29.) For grinding wheels, the exhaust hood should be made of heavy enough material that it will at the same time serve as a protection hood in case the wheel breaks. (See N. S. C. pamphlet 13.) If acid fumes are present and there is danger of the metal of the hood being corroded, the hood may be made of aluminum or lined with lead, or coated with special non-corrosive paint. Or, it may be made of wood properly pitched and joined together with noncorrosive wood screws, or of some other material that will not corrode. 19. In most instances each hood serves a single machine or a single unit; in other installations large hoods extend over groups of such apparatus. (Figure 4.) Mast authorities agree that in the majority of installations the individual type is more satisfactory. 20. To be most effective the hood should be as close as possible to the source of dust, gas, vapor or fumes. Wherever possible, the hood should entirely enclose the operation that is producing the objectionable substance. If it is necessary to have one side open, it may be possible to close this side, at least part of the time, by a sliding door. It might be still more convenient if this door were made of shatterproof or wire glass. Even work that must be done by hand can sometimes be enclosed; this may be accomplished by making a boxlike hood with padded armholes and with the top or one or more sides made of glass. Ventilated tables can also be used for this purpose, having mesh tops through which the waste materials are drawn into a hood underneath the table. 21. If it is impossible or impracticable to enclose completely the point of origin of the air contaminant, the mouth of the hood should extend over the machine or operation at least 6 inches in every direction if the hood is not elevated more than 2 feet. For each additional 2 feet of elevation, the size of the hood should be increased 6 inches in all directions. 22. One type of hood that is effective for some installations is one made with double walls, in which the space between the two walls is about 1 inch. The outer wall of the hood should be extended about 1% inches below the inner wall; rising fumes are then more readily caught than if both walls are of the same length. Air currents from windows and doors do not affect the upward movement of the air in these hoods as much as in single-wall hoods. 23. Hoods with telescopic pipe are convenient and desirable in many instal- S.P.P. 32-3 Figure 6. Exhaust system applied to a cutter grinder. Exhaust pipe is a largesize flexible metallic hose. (See para- graph 24.) Figure 5. Hoods with counterweighted telescopic pipes. Hoods can be readily raised and lowered like a window. (See paragraph 23.) lations. The pipe directly attached to the hood can be of slightly smaller diameter than the section above it. The smaller portion slips into the larger and is held in position by a sliding ring clamp. The hood if provided with counterweights can easily be raised or lowered, like a window. (Figure 5.) 24. Some employers have made use of a "universal" ("knuckle" or "ball and socket") joint which, being placed in the pipe directly over the hood, enables the operator to change the location or direction of the hood whenever necessary. Flexible metallic hose can frequently be used to advantage as shown in Figure 6. 25. In some installations the effectiveness of the hood can be increased by providing covers over vats, tanks or other equipment. Covers may be made 4--S.P.P. 32 SAFE PRACTICES PAMPHLET NO. 32 31. All pipes should be designed to conduct the air with as little resistance as possible. They should, therefore, be made as short and straight as possible. 32. Frequently pipes are reduced to a minimum by installing a separate or unit exhaust system for each machine or operation. The spray booth with its built-in fan and connected motor is a good example. 33. If bends must be provided in exhaust pipes, elbows should be used that are made on a radius in the center line of at least 1% times the diameter of the elbows. Figure 7. A tank with telescopic cover. Lateral exhaust hood removes fumes horizontal ly from between cover and surface of liquid. (See paragraph 25.) in sliding sections which telescope each other, as shown in Figure 7. The exhaust is so arranged that it removes the air from below the cover. 26. Small hoods provide greater suction than large ones (with the same size of fan), but all hoods should be large enough to catch the greatest possible part of the contaminant to be exhausted. Ordinarily, the area of the hood face for natural draft hoods should not be greater than 10 times the area of the pipe to which it is connected. The hood face area for mechanically exhausted hoods may be somewhat greater, depending upon other factors, but it should not exceed 16 times the area of the pipe to which it is connected. 27. In some instances, where the hood provides for downward ventilation, a portable cover or screen has been suspended from the ceiling and so arranged that it can be raised or lowered. This may increase the suction in a horizontal direction as well as deflect some of the contaminant that might otherwise escape into the atmosphere of the workroom. 28. All exposed edges of metal hoods should be rolled and wired. This strengthens the hood and also prevents the workers from receiving cuts and scratches from raw edges of the metal. If the sheet metal of the pipe is heavy enough, raw edges,may be protected with split rubber hose held in place by wire encircling the hose and going through the holes punched for the purpose near the edge of the metal pipe. Pipes or Ducts 29. The pipes or ducts leading from the hood to the exhaust fan are usually made of galvanized sheet metal; the thickness of the pipe walls according to pipe diameter should be not less than that recommended by the National Fire Protection Association, as shown in Figure 8. If pipes are to carry flammable vapors, the thickness of the pipe walls should be not less than that shown in Figure 9. 30. In exhaust systems that carry materials having abrasive or wearing characteristics, the elbows should be made of metal that is heavier than the pipes to which they are connected. Exhaust pipes should be of such strength and fastened securely enough that in case they become clogged and filled to their capacity with the dust they are designed to handle, they will not fall or be damaged by the additional weight. Size of pipes in greatest dimension 8" or less 9" to 20" 21" to 30" 30" or more U. S. gage of metal 24 22 20 18 Figure 8. If exhaust pipes are used to convey dust, starch and refuse, the thickness (in fractions of an inch) of the pipe walls should be not less than that corresponding to the U. S. Metal gage as in- dicated. (See paragraph 29.) 34. Some engineers state that approximately 10 per cent of the power loss in exhaust systems is due to air leakage. Thus, all straight seams and elbows should be made as air-tight as possible by welding, riveting or soldering. If rivets are used, they should be tinned. 35. If pipe joints are riveted or welded, the rivets or welds should be spaced as indicated in Figure 10. The spacing between spot welds or rivets on longitudinal lap joints should not exceed 3 inches, and such joints should also be soldered and edge coated. When locked seams are used they should be double locked. 36. Pipes should never be restricted in area. They should be free from dents, and the inner surfaces should be smooth so the dust will not adhere to the sides and clog up the pipe. At joints, all piping should have at least 1 inch lap made in the direction of the flow of the air current. This further prevents dust clogging and friction loss. 37. In general, vertical runs are undesirable and should be avoided if possible. Where vertical runs are unavoidable, it may be desirable to install a trap at the bottom of the vertical pipe; then all material that is not elevated by the suction of the fan, falls into this trap Size of pipes in greatest dimension 20" or less 21" to 29" 30" or more U. S. gage of metal 20 18 16 Figure 9. If pipes are to carry flammable vapors, the thickness (in fractions of an inch) of the pipe walls should be not less than that corresponding to the U. S. metal gage as indicated. (See paragraph 29.) EXHAUST SYSTEMS S.P.P. 3Z-5 Pipe diameter 8" or less More than 8" to 12" More than 12" to 18" More than 18" to 24" More than 24" Minimum number of spot welds or rivets per joint 4 5 6 7 7 to 9 Figure 10. If the exhaust pipe joints are riveted or welded, the number of rivets or spot welds per joint should be not less than that indicated. (See paragraph 35.) from which it can be removed at regular intervals. (Figure Z.) 38. If an exhaust system has a main duct and a number of branch pipes, dampers or blast gates are sometimes installed. Experiments indicate, however, that exhaust systems are most efficient if all dampers and blast gates are left open. 39. All telescopic slip-joints and ends of metal pipe should be made with a wired outer edge, or protected as explained in Paragraph 28. Some persons use felt packing between the two pipes to prevent air leakage. This requires upkeep and can be dispensed with if the pipe is sufficiently heavy and if, when in operating position, the two pipes overlap a great enough distance. 40. At the point where the piping connects to the suction side of the exhaust fan, there should be a detachable sleeve, not less than 18 inches long, so that ready access is provided to the interior of the fan. The far end of the main duct should be a dead end, closed with a removable cap. 41. Where piping passes through a fire-wall, it is desirable to install an automatic fire damper or fire door in the pipe, especially if the exhaust system contains flammable dust or gases which upon ignition might spread a fire. (Figure 11.) 42. No branch pipe should enter a main duct at an angle greater than 45 degrees; 30 degrees usually gives satisfaction, but a smaller angle is still better. Openings for branch pipes should be made at the top or side of the main duct, never at the bottom. Branch pipes should not enter a main duct directly opposite one another; openings in the main duct for branch pipes should be staggered. Each branch pipe should be riveted and soldered or welded at the point where it enters the main duct. Flange Collar Damper and Wall sleeve Noncorrosise B Cer1114 Use No 8 U.S. Gauge sieel For Damper, Wall sletye and Collar. Courtesy, National Fire Protection Association Figure 11. Automatic fire damper. This is a suggested type of automatic damper for exhaust duct not over 12 inches in diameter, to be installed where exhaust duct passes through firewall. (See paragraph 41.) 43. At certain places in the pipe, traps should be provided to catch the heavier dust which may settle and never reach the exhaust fan. Hand holes with tight-fitting covers should be provided for the purpose of cleaning out the pipe and for removing any solid material that may lodge and obstruct the air flow. 44. To facilitate the cleaning out of dust that may settle behind, underneath or on top of pipes, the distance between the pipe and the wall, floor or ceiling should at no point be less than 6 inches. If the pipes are near the floor, it is wise to provide openings and hoods for the purpose of removing floor sweepings. Size of Pipe 45. The diameter of the pipe is another important detail that must be determined in exhaust system design. A few of the points to be considered in determining pipe sizes are: (a) the size and weight of the particles to be removed; (b) the volume of air to be exhausted; (c) the type and size of the fan and the amount of suction that must be maintained; (d) the size of the hood. 46. In general, pipes of large diameter are costly, but they offer less resistance and therefore require less motive power for the exhaust of air and Diameter of Pipe Inches 2 21/2 3 31/2 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Area of Pipe Square Inches 3.141 4.908 7.068 9.621 12.56 19.63 28.27 38.48 50 26 63.61 78.54 95.03 113.1 132.7 153.9 176.7 201.0 ..226.9 254.4 283.5 314.1 346.3 380.1 415.4 452.3 Diameter of Pipe Inches 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 42 44 46 48 50 52 54 56 58 Area of Pipe Square Inches 390.8 530.9 572.5 615.7 660.5 706.8 754.7 804.2 855.3 907.9 962.1 1017.0 1075.0 1134.0 1194.0 1256.0 1385.0 1520.0 1661.0 1809.0 1963.0 2123.0 2290.0 2463.0 2642.0 Figure 12. Pipe areas for corresponding pipe diameters. (See paragraph 46.) 6--S.P.P. 32 SAFE PRACTICES PAMPHLET NO.' 32 Diameter of Buffing Wheels 6 in. or less, not over 1 in. thick 7 in. to 9 in., inclusive, not over PA in. thick 10 in. to 16 in., inclusive, not over 2 in. thick 17 in. to 19 in., inclusive, not over 3 in. thick 20 in. to 24 in., inclusive, not over 4 in. thick 25 in. to 30 in., inclusive, not over 5 in. thick Maximum Minimum Grinding Diameter Surface of Pipe Sq. Ins. Inches 19 3 43 31/2 101 4 180 41/2 302 5 472 6 Figure 13. Minimum diameters of exhaust pipes for buffing wheels. (See paragraph 47.) Diameter and Thickness of Wheel in Inches 6 in. or less, not over 1 in. thick 6 in. to 9 in.-11/2 in. thick 9 in. to 16 in.-2 in. thick 16 in. to 19 in.-3 in. thick 19 in. to 24 in.-4 in. thick 24 in. to 30 in.-5 in. thick Maximum Minimum Grinding Diameter Surface of Branch in Square Inches Pipe Inches 19 3 43 31/2 101 4 180 41/2 302 5 472 6 Figure 14. Minimum diameters of exhaust pipes for grinding wheels. (See paragraph 47.) 49. In determining sizes of pipe and in designing other parts of an exhaust system, many engineers make a certain allowance for any normal increase in the number of machines or operations installed. 50. Exhaust pipes conveying heated air from certain machines, vats and furnaces may radiate so much heat that they become objectionable. It may then be necessary to cover the pipes with material that will decrease the radiation. Some of the coverings in common use are mineral wool, magnesium carbonate, aluminum foil, cork and asbestos. Purchasers have a choice of plastic coverings or those made in sections. Such coverings may also be necessary to prevent condensation in the pipes when exhausting high temperature air containing quantities of steam or moisture. dust through them. Pipes should not be too small, for they unduly increase the resistance and the cost of motive power, and decrease the efficiency of the exhaust system. The cross-sectional area of a pipe connected directly to an exhaust hood should never be smaller than of the area at the mouth of the hood. If the diameter of a pipe is given, Figure 12 may be used to find the area, or vice versa. 47. In some states, certain sizes of exhaust pipe are specified for certain operations, and manufacturers should comply with the regulations of their respective states. Figures 13, 14 and 15 give a few typical requirements of New York State.. 48. The area of any main duct into which a number of branch pipes enter is usually equal to the combined areas of the branches entering, plus 20 per cent. Following this general practice, Figure 16 gives the diameter in inches of the main duct at any point for any number Self feed table rip saw up to 16" diameter ...... Swing saw up to 20" diameter Disc sander up to 12" diameter Jointer up to 6" knives Single planer up to 20" knives {41/2" pipe on bottom 4" pipe on top 4, 4" 4" 5" Figure 15. Minimum diameters of exhaust pipes for various woodworking opera Lions. (See paragraph 47.) of uniform size branch pipes. Enlarge- The Fan ments in the size of a pipe should be 51. Many makes and varieties of fans made on a taper, 5 inches long for each are used for ventilating purposes. They inch change in diameter, and not by an may, however, all be divided into two abrupt change. principal types; (a) the propeller or Diameter of Branch Pipes in Inches Number of branch pipes 3 13',41 414',4 1515'/21616'/217 Area of Each Branch Pipe in Square Inches 7.07 I 9.62 112.560 I 15.9 I 19.635 I 23.758 I 28.274 I 33.183 I 38.485 Area of Each Branch Pipe Plus 20 Per Cent (Square Inches) 1 2 3 4 5 6 7 8 9 10 11 12.... ....... - 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 8.484 3% 434 534 63/8 7% 81/8 834 934 97/4 101/2 11 111,4 117/g 12% 1234 13% 1334 14 14% 1434 151% 15% 1534 16/8 161/2 16g 17% 171/2 1734 18 11.544 37/8 51/2 63/8 744 8% 91/2 101/, 10% 111/2 12% 1234 133/8 13% 1434 147/8 1534 157/8 1634 1634 17% 1754 18 181/2 18% 191% 1958/ 20 2034 2034 21 15.08 434 61/, 7% 834 9% 1034 1134 1234 131/8 13% 145% 151/2 15% 161/2 17 17% 181/8 183/8 191/8 1954 20% 20% 211/8 211/2 22 227% 227/8 231/2 2334 24 19.08 5 7 83/8 97% 11 121/8 131/8 14 147/8 15% 1634 171/8 17% 181/2 191/8 1934 2034 21 211/2 221/8 22% 23% 2334 241/2 2434 251/8 2534 26% 2658/ 27 23.562 51/2 7% 91,4 11 121/, 131/2 141/2 151/2 161/2 1734 181/2 19 19% 201,4 211/, 22 2238/ 231/2 237/8 241/2 251/8 2534 2634 2678/ 271/2 28 28/ 29 291,4 30 28.51 6 85/8 101/2 12% 131,4 1434 16 17%, 181/8 191/8 20 207/8 2134 2258/ 2334 241/8 247% 2554 261/, 27 2734 2834 29 29% 30% 3034 31% 32 321,4 33 33.93 6% 91A 111/2 13% 1434 161/8 171/2 18% 1934 2034 217g/ 2234 2334 2434 251/2 2634 27% 277/8 2834 291/2 301/8 30% 311/2 21/2 27/8 33% 341/8 3434 351/2 6 9.82 71/8 1034 12% 141% 16 171/2 187/8 20% 21% 221,4 2334 2434 2534 2634 27% 281/2 29% 301/2 311/8 317/8 3234 331,4 3414 341/s 355/8 36g 37 3734 387% 39 46.182 7% 107A 131/2 15g 1734 1834 201A 2134 23 241/, 2534 2634 2734 2834 2934 30% 3154 3234 331/2 347% 351% 36 3634 3754 3834 391,4 39% 40% 41% 42 Figure 16. Diameter in inches of main exhaust pipe at any point for any number of branch pipes. (See paragraph 48.) t. rV f EXHAUST SYSTEMS Feel ,1017! I) ., 1E _a !,. SUCTION 1 VELOCITY 1 5 'eche. re,147.7t. SUCTION I VELOCITY 1.75 lochee p2er0M5 inute SUCTION 2.0 'Aches I VELOCITY 3 70 Feet per S2U* 3CwTItOhN:. 1 V3eE5rL4M0OiFCneIueTtteY S3U.0CITncIOheNs Vpe,r8m0inute S3U.6CITncIOheNs VELOCITY S4U.0CInTcIhOeNs Yflay.UlY S4U.5C1T.0IbOeNe VpEprLNOiCbI,T,,l,Y. S5U,0C'eTcIhOoNs YD5Ee0Tr,1M?iM nutteY Minute p per pflpute per Minute It 7 O Revolu- Cubic Roos pc, Feet per Minute MAU, ,, Revolu- Cubic -.re.' Meer !loos per Minute FMeinnuPteer ,H,,orse- !lose tbloe Feet Power kIngupte /Lig 12 1060 2012 2.11 1146 2172 2.80 1227 2320 13 1056 2346 -210 -1111 2539 3.1 1220 2710 30 14 102 2680 2.74 1138 2890 2.63 1216- 3096 -16 -15511 3012 313 313132644.14 1216 3480 _ 15-1553- ---330, - 3.37- -11. 53---'3649--4771-- -Mir 070 14 15 ' 35 18 17 18 908 2730 2.87 984 2955 3.80 1054 3160 906 7193--317 -9-81 3455 -439- 1050 8690 02-- 1345- 3.73 97-3945 4.94_ ,_1-013- 4210 901 4059 -4:23- 976 4425 6.63 194-4- 4730 903 4553 -41-4- 980 492.5- 6.41 1048 6260 16 797 3675 3.76 860 3860 4.97 _17 794 -170 -4-.27- 857 -530 6.65 40 ir 791 4770 z1.87- 854 6160 6.45 19 790 5350 6.56 853 5780 7.35 20 792 6960 6.33 865 .6430 8.37 18 19 45 21_ 22 _ 23 20 21- 50 23 24- 26 708 4520 706 5280 703 6030 702 6780 704 7530 638 5582 63V 6530 633 7442 632 0 634 9300 4.75 765 -1.41 763 -117 760 7.04 769 8.02 762 4880 6.29 6730 7.16 6610 . 8.16 7310 9.30 8130 10.57 5.86 689 6030 6.67 687 7048 401- 8040 --8-.66- 683 9040 9.89 686 10040 7.76 8.83 10.07 11.47 13.06 920 4125 917 4820 9-T1-- 5500 913 6180 916 6875 818 5220 815 6100 811 6970 810 7830 813 2700 '736 6450 734 7540 7213600 76 9671 733 10740 22 579 6750 7.09 23 77 7900 8.07 55 25 -- 78-6665 9.21 27 674 10130 10.52 22- 7 -1121 11.97 626 624 7290 8620 9.40 10.68 ---a670r 7800 9120 622 9720 12.18 665 10400 621 0920 13.89 e64 11700 623 12160 15.82 666 13000 24 526 8030 8.43 674 8680 23- 524 9390 9.60 ---572 10130 60 27 -52 39700 10.95 570 11570 -24-1- 521 12036- 12.51 -66- 2 13000 30 123- 13370 1.4.21- 6771 14410 11.18 614 9280 12.72 611-- 10840 14.62 609 12370 16.53 608 13900 18.83 -610 16460 28 456 10930 11.50 493 11820 15.22 30 41 12760 13.10 992 13820 .17.30 70 33 763 14680 -1479-3 490 16760 19.76 34 432-100- -17:23- 490 17700 22.60 36 452 -18220 .19.40 491 196680 25.64 526 524 ---6-22 622 623 12630 .14760 1681 1890- 2101 49 31 398 14280 -15.00 430 15490 19.87 -34- -397 -13701-17.10- 428 18030 22.60 50 36 19030 12.47 428 -2-(1-6-30 -25.80 88 39811960 22.25 427 23100 29.40 90 396 23800 25.30 -125- '26160- 33.50 460 16500 459 19300 457 22000 457 24720 458 27500 35 38 90 41 43 1 45 3541 18080 18.97 382 19520 262 1.21100---21.56 ---881-22-80 ',18F 2115W---2-71-65- 379 26030 - 351- 27100- 2815 -878-29260 362 30130 32.00 380 32530 25.15 409 20900 28-.W 408 24400 32.60 407 27900 37.20 407 31300 42.35 -407 34800 nor._ power Revolt,- '13= FCacutble 3.26 1368 3.70 1363 4.22 1360 4.82 1366 5.48 ..1362 2596 3035 3461 3890 4330 4.43 1175 3530 6.05 1168 4126 6.74 -1165 4710 6,67 1 1163 6290 7.47 1161 6890 5.79 1028 4610 6.66 1025 -5390 7.50 1-0-22- 6160 8.58 3020 6910 9.76 1024 7690 7.34 8.36 9.60 10.87 12.36 9.06 10.30 1-1-77213.40 16.22 915 6830 910 6820 907 7780 905 8740 909 9730 7200 : I 8420 6 9610 : -15760 : , 12000 10.90 12.4114.20 16.20 18.46 8716 j 10180 11620 I 13050 14530 13.00 14.80 16.90 19.30 21.90 10370 12120 .7 19830 675 15530 680 17300 17.70 20.20 23.00 26.30 29.80 23.20 26.40 30.00 34.40 39.00 588 14076 685 16470 683 18780 532 21100 684 23600 614 18430 612 21560 610 24600 509 27640 511 30760 29.30 467 23300 33.30 456 27260 3817I 963 31100 43.50 452 34950 49.40 466 38900 Bor.. Bees. Cu. Ft. wwer M. 33g4t3 4.66 1496 2843 5 181492-8320 6.91 1487 3796 6.76 HU 4262 -T.33.- 1490 4749 6.20 1285 3866 7.04 1282 4620 8.04 1279 6160 9.20 1277 6796 -111.101M0 6466 8.10 1125 5050 9.20 1122 5900 10.60 1119 6740 12.00 1170 7670 13.60A-11 2. 8430 10.24 4.000 6385 11.63 997 7460 13.28 994 8616 16.18 1 993 9670 17.20 996 10630 12.64 900 7890 14.36 897 9220 16.40 TN 10600 18.76 3.03 11820 ' 21.26 TR 13160 15.32 918 9540 17.38 816 11160 19.85 10 12730 ' 22.70 812 14300 26.70 -814 16900 18.22 760 11360 20.70 748 18270 23.63 -743 16160 27.00 744 17020 30.60 74118930 24.80 643 16426 28.20 641 18020 32.16 TN 20670 36.76 65823100 41.66 1471 26700 32.40 663 20200 36.80 16123600 42.00 559 26920 48.00 658 30260 54.40 --555 33660 41.04 500 25550 46.55 499 29890 63.10 497 34080 60.80 496 38250 68.80 498 42560 no... power Rey.. or aCu. Ft. g,,,,,, , p=r- Revs. Cu. Ft. D". /Jaw 6.00 1619 6.24 1614 7.76 1610 8.90 107 10.10 1611 3072 3685' 4100 4606, 6126 7.43 1723 3280 8.56 1724 -38109.80 1717 4380 11.20 1715 4920 12.80 MD 6478 8.16 1390 4180 10.10 1486 4460 9.26 ,1886 4876 11.63 1482 -3220 10.67 1381 6636 13.33 1-4;7-8 5960 12.08 1380 6260 15.15 1376 6690 13.70 103-7 - 17.30 140 7450 10.66 1217 6460 13.20 1300 6830 12-13. 1214 .37016.20 1233 6820 13.80 1210 7280 '1-7111 1292 7780 15.80 1208 8180 19.80 120 8740 -1730-1-21-2 9100 22.60 10-4 9730 13.47 1082 6895 16.76 1155 7370 16.30 078 8060 19.27 riTI -85-6-1/ 1-71-5-rus sew 22.07'1147 9840 -21-1(11072-19330 26.10 1-14-3 11040 22.76 107611600 28.65 1-1-6-0 12300 16.63 974 8510 20.62 1040 9110 -1-810-9-1-6 6665 23.76 1036 10630 21.56 96711561 27.17 1033 12130 24.66 96612750 30.90 1-62212630 -27.9-6" 969 14200 _35.60 1036 16170 20.13885 10300 24.95 946 11020 22.86 881 12030 28.73 941 12876 26.08 -8-7-112730 32.90 -9111-4-68` 29.86 675 16440 37.40 936 16500 33.83 878 17180 42.70 -14-0 18360 23.95 811 12260 29.70 866 13120 27.25 808 14330 34.20 -83113830 31.05 805 16360 39.16 86117600 35.66 864 18380 44.66 862 19650 40.30 807 20450 50.90 -864 -21850 32.60 695 16650 40.40 743 17800 37.05 693-0W 46.30-740 20820 42.25 691-MDT 63.30 738 23750 . UT -6-11. 9 /4360 60.60 -11716630 13115 692 27760 69.20 739 29700 42.60 609 21800 62.80 650 23320 48.40 60726170 60.80 648 27260 65.20 604 29070 69.60 646 31100 63.20 603 32670 79.10 645 34920 71.60 605 36350 90.30 647 38900 63.90 542 27550 66.80 678 29500 617211- 5-30" 32200 76.90 -57-6 34500 69.80 537 36750 88.10 -574 39350 -8-0.00 -536111-300 -1- 00 573 44200 .019. 538 45970 114.30 -5-13 49200 , pone Rev8 Pu. Ft. gif,. mg. Horse. Power RPeevrs.i4C11u:r.uFt.t.., Horse_ 90.- 9.18 1833 3486 11.00 1931 3670 10.40 1827-4070- -IUD- 1M 4285 12.00 1821 4645 14.30 1918 4896 13.60 1820 6220163-6 1916 6603 16.60 1826 6830 18.60 1-4-29 6138 12.47 1592 4740 14.92 1658 4990 19.15 iT7-2 6636 17.00 13-6-4 6830 16.23 1-50-0271-19.36 1651 -03T 18.62 103-71100 22.10 1649 74902113 1568 7926 25.20 1652 8340 12.90 14.70 16.70 19.10 21779 17.45 19.90 22.70 25.95 29.45 16.30 1378 6190 19.50 1452 6620 22.80 -13.-5T) 1374 -1230 22,20 1-4-47 732(T-23.0 2120- 1370 8250 25.30 1442 8690 29.70 24.20 1368 9270 28.90 1440 9780 33.90 2-1;6112-72 10350 32.90 1444 10900 38.50 20.70 1225 7820 24.75 1291 8250 28.90 23.50 1221 9140 2518 1287 9640 32.93 26.80 1217 10475 32.05 Mil 10980 37.63 1-6-.711-213 11720 36.60 1282 12360 42.95 34.90 1219 13030 X1286 1373 4813 25.46 1102 9660 30.45 1162 10170 35.60 --28-2-0 719-8-11T7-0 34.0-1158 11880 ---40.33 33.20 1096 12870 39.60 1156 13670 46.40 77 0 1094-1-4460- 21111154 16260 62.95 42.94 1097 16100 51.40 1167 -f10-61 60.10 30.80 1002 11680 36.85 1056 12320 43.10 ---3-4.96 919 13650 41.95 1052 144-00 49.13 40.10 -06-6 16580 41.80 1050 16430 56.15 96.73 995 17500 54.60 1049 18460 64.10 62.00 998 19450 62.20 Wel 20600 72.80 36.70 918 13920 43.90 968 14670 51.30 41.66 -91-51-6-230 60.00 966171.-61 68.50 47r70 912 18660 66.95 963 19550 66.85 54.60911 -2-H30 --6670-ci 962 21980 76.30 -61-30914 23080 74.10 965 24400 86.70 49.90 788 18870 59.70 830 19900 69.80 56.60 787 22080 68.00 828 23270 79.60 64.90 785 25200 -77-.E0f 826 26550 91.00 -- 4 -10- 784 28270 88.0 825 29850 1931.0 -1.25 78-6 TRW 100.80- -827 331130118.00 65201 689 24730 78.00 726 26070 91.20 74.00 ,,_687 28900 88.80 -IN -2017-0- 104.00 84.80 '685 33000 101.20 -1-211-4130 118.80 96.80 684 37050 115.60 722 39070 135.60 Tam 686 31200 131.60 724 43370 154.06 82.50 613 31300 98.70 645 32000 115.30 93.60 611 36550 112.30 643 38550 131.50 167.30 609 41720 128.00 I-411-337W 160.30 12-2-10- 608 46850 166.20 640194-6117E0139.20 610 52100 1.66.50 642 64900 195.00 Figure 17. Typical performance tables for centrifugal fans. These tables are compiled, assumi lig that the area of the main duct is equal to the sun of the areas o all the branch pipes, plus 25 per cent; and assuming that the total resistance of the exhaust system is 21/2 times the suction at the hood. Fan sizes usually indicate the diameter in inches of the fan casing. (See paragraph 55.) 8--S.P.P. 32 SAFE PRACTICES PAMPHLET NO: 32 low-pressure fan, and (b) the centrifugal or high-pressure fan. 61. Fan bearings should not extend into the fan housings or exhaust pipes. 52. Propeller fans have curved screwshaped blades that produce a low pressure draft and move the air in a direction parallel to the axis of the fan. The current of air set in motion travels at a relatively low speed, but such fans are capable, if properly designed and installed, of moving large volumes of air. 53. With centrifugal fans, the air enters at one end of the axle, and is thrown by the rapid rotation of the fan, between numerous straight or concave radial spokes or blades to the outside or box-like casing where the outlet is located and from which the current of air passes. As this type of fan moves a volume of air with considerable velocity and at high pressure, it operates effectively and efficiently against high resistances. 54. Fans should preferably be electrically unit driven. 55. The type and size of fan selected in practice for certain exhaust systems depend upon the pressure in inches of water of the system, the volume of air or gas to be handled in cubic feet per minute (CFM), and the size of the header pipe adjacent to the fan. Figure 17 shows the average performance of centrifugal fans and may be used in selecting a fan that will meet the requirements just outlined. 56. In general a branch pipe velocity of 3,000 feet per minute will satisfactorily exhaust most fumes, vapors, gases and light dusts; a velocity of 4,500 feet per minute is usually needed for grinding and polishing systems, and about 5,000 for casting tumbling barrels. A fair average for dust-collecting systems is from 3,000 to 5,000 feet per minute. (Volume of air moved equals velocity multiplied by the area of the pipe measured in square feet.) 57. There are several methods for determining pressure in inches of water in exhaust systems. In discussing pressure, it is necessary to differentiate between resistance pressure, velocity pressure, and total pressure. The most common method of measuring is by means of a U-tube. The tube is made of glass and is about 8 or 10 inches long. If pressure is to be measured in a branch pipe, a hole of an inch in diameter is made in the pipe approximately 10 di- :14:\z Figure 18. Determining the amount of suction in exhaust pipe. Suction, in inches of water, is measured at the point indicated by letter "X." (See paragraph 57.) 62. If any flammable materials are drawn through the fan, the fan blades and spider should be of bronze, aluminum, or similar material to avoid the possibility of sparks and fire or explosion, or the fan casing should consist of or be lined with such material. ameters of the pipe from the hood, but in no case less than 1 foot from the hood or an elbow. One end of a rubber hose is placed over this hole and the other end attached to the U-tube, which has been half filled with water. The pressure in inches of water, is determined by measuring the difference in height between the surfaces of the water in the two branches of the tube. (Figure 18.) Such tests should be made with all branch pipes open. The majority of regulations call for sufficient static pressure in every branch pipe to produce a difference of level of at least 2 inches of water. This is usually satisfactory for most industrial operations, although a difference of 4 inches is required for removing dust created by granite working tools or machines. 58. Some dust when comparatively dry, and all dusts if moist, will stick to the fan blades. This not only decreases the efficiency of the exhaust system but it may also ruin the bearings and in time destroy the fan. It is necessary, therefore, to clean the fan at regular intervals. If, however, it is necessary to clean the fan at too frequent intervals, it may be an indication that the fan should be placed on the discharge end of the dust or fume collector. Discharge Pipe 63. The discharge pipe leading from the fan to the place of disposal should be as short as possible so the resistance of the system may not be unnecessarily high. In practice, this pipe is usually larger than or of the same size as the duct leading to the fan. Dust and Fume Disposal 64. The method of disposing of the dust or fumes gathered through an exhaust system depends largely upon the character of the waste materials. In a few cases they may be exhausted directly to the outside atmosphere. Great care, however, should be observed to prevent such materials from returning into the workroom or any place where they would be harmful. 59. It is sometimes advantageous to install two small single fans instead of one large fan. Double fans are particularly desirable where one fan would take up too much head room, or where main pipes come from opposite directions. In the latter case, elbows and bends are eliminated, as the main pipes are run directly to the fan inlet. 60. Exposed openings in the motor housing should be covered with substantial wire mesh. Courtesy, Ford Motor Company Figure 19. Fresh air supply. At points where operations generate excessive heat, fresh air is drawn in by blowers through ducts, directly upon the workmen. The cold air supply can be controlled by regulating the damper indicated by the white arrow. The ex haust pipe is installed at the rear of the furnace (See paragraph 79.) EXHAUST SYSTEMS S.P.P. 32-9 65. Cyclone collectors may be used to separate solid particles from the exhausted air. In general, the air is led into the cyclone on a tangent and as it swirls around in the cylindrical chamber the solids are centrifuged or driven to the chamber walls where they slide by gravity into the conical shaped bottom. The clean air escapes from the top of the chamber through a central exhaust tube. Simple cyclones, skimmer cyclones, high efficiency cyclones and multiple cyclones are various types of dry material collectors, all of which use adaptations of the simple cyclone principles. 66. Dust, fumes, viscous matter, fog and mist may be collected by electrical precipitators which utilize high voltage direct current for ionizing and electrically charging the collected materials. Liquids drain from the electrodes, while solids are removed from the electrodes by gravity or by washing, blowing, scraping, or rapping devices. 67. Dry dust may be separated from the air by cloth arrestors. Means must be provided to shake or otherwise remove the dust that builds up to form a mat on the surface of the cloth. Usually this is done only when the exhaust EXHAUST SYSTEM SURVEY FORM Plant Location-- -- -- Building--______-----...... Date......-- ........ -_-____ Surveyed by.___._.........._ _--- GENERAL DATA: Exhaust System No.__ Departments served by system (1)._....__.--....-._.-- (2).........._ _ (3) Changes made since last survey ..... _____ (4) ---- __ _______-- AIR FLOW DATA: Point of measurement Duct diam. (inches) Pitot or static reading (in. water) Air velocity (FPM) Air flow (CFM) Remarks 2. -- ___--_--- _--.. --........ EXHAUST SYSTEM SURVEY FORM (Reverse side) ENVIRONMENTAL DATA: Point of sampling Material sampled Concentration found (State units) Remarks 1. -- 2. -----.-- -- CONSTRUCTION DATA: (Cite parts of system needing attention) Hoods -- ----- -- --------- Ducts -- -- Elbows Collectors (check filters if used) -- ----- ---------- ----- Fan and Motor Figure 20. A suggested form for report of the inspector who makes the surveys of exhaust systems. (See paragraph 81.) system is shut down. It is sometimes advisable to install dampers just ahead of the arrestors, so the dust will not back up into the exhaust system. 68. Dynamic precipitators have fan blades so designed and arranged that dust particles in the exhausted air are rotated so rapidly that they are ejected at the tips of the blades into a by-pass and from there into a hopper. A small percentage of air goes along with the dust particles and this is drawn back into the fan casing. Some dynamic precipitators are so arranged that the exhausted air first passes through a water spray. 69. In impingement collectors the exhausted air is subjected to several changes in direction and velocity and impinged against wet surfaces. Here the waste matter is washed away by a continuous flow of liquid. 70. Fumes and mists are sometimes removed by passing the exhausted air through one or more water sprays. 71. Some vapors can be condensed. In condensing apparatus, the vapor is passed through coiled channels or pipes which are artificially chilled, usually by means of water sprays. 72. Absorption is another means of disposing of some dusts and fumes. Absorption chambers and vessels are constructed in various ways and are often arranged in series. Gas or fumes may be allowed to bubble up in a vessel filled with water or some other absorbing liquid. In absorption towers, the air to be purified or the dust to be absorbed is admitted below, while water trickles down from above, acting on the countercurrent principle. The introduction of coke, lime, activated alumina or other materials may also aid in the absorbing process. 73. It is possible to neutralize some objectionable gases, fumes or vapors by the addition of certain substances which change their chemical composition and render them harmless. Furthermore, useful by-products may sometimes be recovered in this way. 74. Separators should be installed outside the building when conditions permit. They should be constructed of and supported by non-flammable materials. They should have clean-out doors to permit examination of the interior. Vent pipes should extend above 10--S.P.P. 32 SAFE PRACTICES PAMPHLET NO. 32 Roof Line VIEW. Roof Line Diameter Nat leis Then X LEGEND. a-Weather Hood. b- Cover Plate. 3/4" Felt Gasket. d- Non-Coi-rosive Hinge. e- Counter-weight. f - Angle Iron-Pin3,, J- Vent. h- From ApFiaratus. - Disc harcsc Diameter dx SECTION. Courtesy, National Fire Protection Association Figure 21. A suggested design of safety relief vent for exhaust system. (See paragraph 82.) adjacent buildings, especially if located less than 10 feet from them. 75. In general, the recirculation of air after it has been cleaned is not recommended. General 76. Many manufacturers succeed in utilizing the dust or fumes that are collected by one of the methods herein described. For instance, wood dusts may be burned under the boilers; or blast furnace gases and others containing carbon monoxide may be used to heat "stoves," or be burned for other purposes. One manufacturer in the lead industry states that he has saved enough lead to pay more than the entire cost of installing and operating his exhaust systems. 77. Care should be exercised to pre- vent different fumes, dusts, vapors and gases from entering a common main where they may chemically react on each other and form explosive mixtures. It is sometimes advisable to install a compact, individual exhaust system for each room or building, especially where the system handles flammable dusts or fumes. Otherwise the exhaust ducts may simply serve as a fuse or conductor to lead a fire to a number of different points. 78. If an exhaust system is used for flammable dusts, fumes or gases, the metal parts of the system should be electrically grounded. This and other specifications to eliminate or reduce the fire and explosion hazards inherent in exhaust systems and to prevent exhaust systems from spreading fires are discussed in "Blower Systems," by the National Fire Protection Association and approved by the American Standards Association. This publication should be referred to for detailed data. 79. It must always be remembered that sufficient openings should be provided in the workroom to allow for the entrance of additional air. Removal of air through an exhaust system creates a partial vacuum in the workroom which must be offset by the free entrance of fresh air from the outside. Figure 19 illustrates one method of securing a fresh air supply that has proved successful. In some cases, before admitting this incoming air, it may be necessary first to have it purified by filtering and washing, or its temperature may have to be modified; perhaps its humidity should be regulated. These are problems more nearly associated with general ventilation than with exhaust systems, and are therefore discussed in detail in the pamphlet on "Industrial Ventilation." 80. It is sometimes necessary to remove dusts locally by portable apparatus without the installation of permanent hoods, pipes, fans, etc. The cleaning of dusty places by blowers, dry rags, brushes and feather dusters may be harmful to health. There are at present various kinds of portable vacuum cleaning apparatus with instruments fitted for every special kind of cleaning. 81. Systems that were originally installed satisfactorily have been known to fail because hoods became detached from ducts, holes were broken into pipes and pipes were found clogged with dust and all sorts of debris. It is, therefore, necessary in every plant where there is an exhaust system to appoint an inspector or supervisor whose duty it is to see that the entire installation is inspected at regular intervals and that all defects are speedily remedied. Figure 20 shows a suggested form to be filled out by the inspector each time he makes a survey of an exhaust system. 82. If an exhaust system is used to convey refuse or stock, it should be vented to the outside of the building, either directly by flues or separators, or indirectly by bins or vaults into which it discharges. If the refuse or stock is combustible, vertical safety relief vents should be installed in such a manner that they do not deviate more than 22% degrees from the direction of the pipes from which they lead. (Figure 21.) EXHAUST SYSTEMS S.P.P. 32-11 from plugging when used in measuring the velocity of dust-laden air. (Pitot tubes of special design have been used for this purpose.) In certain cases it is desirable to provide a flowmeter of some type in the main line to indicate the total volume of air handled by the system. A pitot tube may be employed for this purpose although a meter section of the venturi type provides a more permanent measuring apparatus." Figure 22. Measuring static, velocity and total pressures. (See paragraph 83.) Testing Exhaust Systems for Effectiveness 83. The ultimate means of determining the effectiveness of any exhaust system is to measure the concentration of the contaminant in the workroom atmosphere. In addition, measurements of air-flow through the system should be made at regular, frequent intervals to make sure it is functioning in accordance with the original design. The following suggestions are quoted from the A.S.A. report entitled, "Fundamentals Relating to the Design and Operation of Exhaust Systems": "Measurement of static suction (pressure) at the throat of the exhaust hood provides the simplest index of operation but it may be safely employed for this purpose only when the coefficient of restriction is not changed by damage to the hood. The measurement of the velocity pressure by means of a pitot tube (see Figure 22) in the branch pipes and in the main pipe as well provides a more exact means of determining the volume of air flowing into the various hoods and through the system and in addition gives direct information concerning the transporting velocities in the system. The accurate determination of average velocity at any cross-section, however, requires more skill and time than is required for the determination of static suction. Care must be exercised to keep the pitot tube References "Fundamentals Relating to the Design and Operation of Exhaust Systems"-- American Standards Association. "Fundamentals of Design, Construction, Operation and Maintenance of Exhaust Systems"--A merican Foundrymen's Association. The American Society of Heating and Ventilating Engineers Guide. Publications of various companies that manufacture exhaust equipment New York State "Rules Relating to the Removal of Dust, Gases and Fumes." "Regulations for the Installation of Blower and Exhaust Systems for Dust, Stock and Vapor Removal"--National Fire Protection Association and American Standards Association. ACKNOWLEDGMENT This pamphlet was prepared by the Industrial Division, National Safety Council. It has also been reviewed by the Safe Practices Conference Committee, manufacturers of exhaust system equipment, consulting engineers and others. The Council gratefully acknowledges their assistance. Rep. 844-3M