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FILE NAME: BF Goodrich (BFG) DATE: 1939 DOC#: BFG037 DOCUMENT DESCRIPTION: Article - Exhaust Systems - NSC Safe Practices NATIONAL SAFETY COUNCIL INCORPORATED 2 0 N O R T H WACKER DRIVE C HICA GO 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 ex posure to dusts, gases, vapors and fumes that are generated by various machines and industrial operations. I t is difficult to enumerate all the different potentially harmful substances that are met with in modem industry, but the following gen eral 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. I f inhaled into the lungs some dusts may cause fibrosis or chronic in flammation. b. Corrosive dusts from such sub stances as soda, lime, and others cause inflammation of the skin and other ex posed parts of the body. c: Poisonous dusts such as lead, ar senic 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 in fect the worker. e. Irritating fumes from substances such as ammonia or acids act locally upon the eyes and upon the mucous mem brane of the throat, nose and lungs. f. Poisonous fumes from ben zol, 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. Atten tion 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 As sociation, 29 W . 39th Street, New York City. The suggestions contained in this pamphlet do not conflict with the re quirements of that code. Price: 35c each to members; non-member and quantity prices on request. d. Natural ventilation. (See N.S.C. pam phlet No. 37.) e. Isolating in a separate room or build ing the process or machine that cre ates dust, gas, vapors or fumes. f. Providing and requiring exposed work ers 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 "oc cupational 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 prac tically limited to the installation of ver tical 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, how ever, to have a suitable arrangement at the top of the flue, perhaps turning with the wind, to prevent the wind from re versing the current of air in the flue. For more detailed information see N. S. C. pamphlet No. 37, "Industrial Venti lation." 5. Each installation of a mechanical exhaust system presents special prob lems that may require expert and tech nical knowledge as well as an under standing of existing conditions, but there are a number of important factors, common to all installations, that are dis cussed in this pamphlet at least in out line form. 6. There are two general types of exhaust systems, the general and the local. 7. General exhaust systems pro vide 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 purchas ing the ordinary type of fan usu ally 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 nett, 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 objec tionable. 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 clearance 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 particles drop into this trap and are removed at regular in tervals. (See paragraphs 14 and 37.) dense fumes may be evolved sudden ly, and is capable of clearing 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 down ward displacement. 8. The general exhaust system usu ally involves low initial cost and com paratively 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 al ways satisfactory particularly where harmful dusts, fumes, gases and vapors are involved. In many cases it is neces sary to install local exhaust systems in stead 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 nec essary 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 d u s t 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. perse in the workroom, it usu ally becomes practically impos sible to reduce the concentration to sat isfactory limits. (Figure 1.) The Hood 11. Much of the success of any local exhaust system depends upon the design and installation of the hood. 12. For removing heavy gases, fumes, vapors and such substances as metal par ticles and sand, the downward system of ventilation is usually the most effec tive. In such installations the hood may be a hopper placed directly beneath the work-table. The pipe leading to the ex haust fan is usually led from the side of this hopper (not the bottom) and is thus forced to handle only the lighter particles which do not drop into the trap at the bottom. (Figure 2.) 13. An upward system of ventilation is usually used to remove fumes, gases, and vapors that are lighter than air or which have a tendency to rise. The 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. Figure 3. Exhaust on small grinder. The hood catches dust particles at the point of greatest velocity. (See paragraph 16.) One large hood over a series of tanks. (See paragraph 19.) 14. Sometimes conditions are such that hoods or pipes cannot be installed directly over vats, machines, or other sources of dust, gas, vapors or fumes because they would interfere with over head cranes or some other operation. It then becomes necessary to provide lat eral hoods or those in which the sub stances to be removed must travel in a horizontal direction instead of vertically. (Figure 2.) 15. Frequently, the lateral hood may be of the suction box type in which the front of the box is perforated with nu merous small-sized intake openings pro viding a high velocity of air, 16. If the particles to be removed are already in motion, as is the dust thrown from a grinding wheel, the hood, if possible, should be installed in the path of the waste particles so the veloc ity of the particles assists the force of the exhaust draft. (Figure 3.) 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. 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 ex haust 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. pam phlet 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 non corrosive 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.) Most 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 en tirely enclose the operation that is pro ducing 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 box like 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 impractica ble to enclose completely the point of origin of the air contaminant, the mouth of the hood should extend over the ma chine or operation at least 6 inches in every direction if the hood is not ele vated more than 2 feet. For each addi tional 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 l J/2 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 di ameter 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, ena bles the operator to change the location or direction of the hood whenever neces sary. Flexible metallic hose can fre quently be used to advantage as shown in Figure 6. 25. In some installations the effec tiveness 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 ex haust pipes, elbows should be used that are made on a radius in the center line of at least 1Yi times the diameter of the elbows. in sliding sections which telescope each other, as shown in Figure 7. The ex haust is so arranged that it removes the air from below the cover. 26. Small hoods provide greater suc tion 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 sus pended 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 es cape into the atmosphere of the work room. 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 Fig ure 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. Ex haust 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 m etal 24 22 20 18 Figure 8. If exhaust pipes are used to convey dust, starch anti refuse, the thick ness (in fractions of an inch) of the pipe walls should be not less than that cor responding to the U. S. Metal gage as in dicated. (See paragraph 29.) 34. Some engineers state that ap proximately 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 solder ing. 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 ex ceed 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 un desirable and should be avoided if pos sible. Where vertical runs are unavoid able, 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 m etal 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.) Pipe diam eter 8" or less More than 8" to 12" More than 12" to 18" More than 18" to 24" More than 24" M inimum num ber o f 8 P O t welds o r 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.) EXHAUST SYSTEMS S.P.P. 32-- 5 from which it can be removed at regular intervals. (Figure 2.) 38. If an exhaust system has a main duct and a number of branch pipes, dampers or blast gates are sometimes installed. Experiments indicate, how ever, that exhaust systems are most effi cient 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 ex plained 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 over lap a great enough distance. 40. At the point where the piping connects to the suction side of the ex haust fan, there should be a detachable sleeve, not less than 18 inches long, so that ready access is provided to the in terior 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 sys tem 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 satis faction, but a smaller angle is still bet ter. 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. 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 an other important detail that must be de termined in exhaust system design. A few of the points to be considered in de termining 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 main tained; (d) the size of the hood. 46. In general, pipes of large di ameter are costly, but they offer less resistance and therefore require less mo tive power for the exhaust of air and D iam eter of Pipe Inches 2 ................... 2'A................. 3 ................... V /i................... 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 ............... 4523 D iam eter 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 7068 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 D iam eter of Buffing W heels Maximum G rinding Surface Sq. Ins. 6 in. or less, not over 1 in. thick............................................... 19 7 in. to 9 in., inclusive, not over 1 in. thick.......................... 43 10 in. to 16 in., inclusive, not over 2 in. thick........................... 101 17 in. to 19 in., inclusive, not over 3 in. thick........................... 180 20 in. to24 in., inclusive, not over 4 in. thick...........................302 25 in. to30 in., inclusive, not over 5 in. thick...........................472 M inim um D iam eter of Pipe Inches 3 3 4 4 5 6 . . . . , . . ,_ Hgure 13. Minimum diameters of exhaust pipes for buffing wheels. (See paragraph 47.) Diameter and Thickness of W heel in Inches Maximum Grinding Surface in Square Inches 6 in. or less, not over 1 in. thick....................... ....................... 19 6 in. to 9 in.--1 in. thick.............................. ....................... 43 9 in. to 16 in.--2 in. thick.............................. .......................101 16 in. to 19 in.--3 in. thick.............................. .......................180 19 in. to 24 in.--4 in. thick.............................. .......................302 24 in. to 30 in.--5 in. thick.............................. .......................472 M inim um D iam eter of Branch Pipe Inches 3 3 4 4 5 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 in stalled. 50. Exhaust pipes conveying heated air from certain machines, vats and fur naces may radiate so much heat that thgy beCQme objectionable. It may then be necessary to cover the pipes with ma terial that will decrease the radiation. Some of the coverings in common use are mineral wool, magnesium carbonate, aluminum foil, cork and asbestos. Pur chasers have a choice of plastic coverings or those made in sections. Such cover ings may also be necessary to prevent condensation in the pipes when exhaust ing 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 ex haust system. The cross-sectional area of a pipe connected directly to an ex haust hood should never be smaller than tV 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 op erations, and manufacturers should com ply with the regulations of their respec tive 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" di ameter ................... Swing saw up to 20" diameter ............... Disc sander up to 12" diameter .............. Jointer up to 6" knives Single planer up to 20" knives ............. (4 pipe on I bottom 4" pipe on l top 4" 4" Figure 15. Minimum diameters of exhaust pipes for various woodworking opera tions. (See paragraph 47.) of uniform size branch pipes. Enlarge ments in the size of a pipe should be made on a taper, 5 inches long for each inch change in diameter, and not by an abrupt change. The Fan 51. Many makes and varieties of fans are used for ventilating purposes. They may, however, all be divided into two principal types; (a) the propeller or Diameter of Branch Pipes in Inches 3 Number of branch 7.07 pipes 354 1 4 1 454 1 5 1 554 1 6 I 654 7 Area of Each Branch Pipe in Square Inches 9.62 1 12.560 1 15.9 | 19.635 | 23.758 | 28.274 I 33.183 138.485 A rea of E ach Branch P ipe P lu s 20 P e r 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............ 30...... _.... 8.484 354s 4 5 6 7 854 8 9 m 1054 11 11 11 12 12 1354 13 14 14 14 1554 1554 13 16 16 16 17 17 17 18 11.544 37 5 6 7 8 9 10 107 11 12 12 13 137 14 14 15 157 16 16 17 17 18 18 18 19 19 20 20 20 21 15.08 4 6 7 8 9 10 11 12 13 13 14 15 15 16 17 17 18 18 19 19 20 20 21 21 22 22 22 23 23 24 19.08 5 7 8 9 11 12 13 14 14 15 16 17 17 18 19 19 20 21 21 22 22 23 23 24 24 25 25 26 26s 27 23.562 5 7 9 11 12 13 14 15 16 17 18 19 19 20 21 22 22 23 237 24 25 25 26 267 27 28 28 29 29 30 28.51 6 85 10 12 13 14 16 17 18 19 20 207 21 22 23 24 247 25 26 27 27 28 29 29 30 30 31 32 32 1 33 33.93 6 9 11 13 14 16 17 18 19 20 21 22 23 24 25 26 27 277 28 29 30 30 31 2 2 33 34 34 35 6 9.82 7 10 12 14 16 17 187 20 21 22 23 24 25 26 27 28 29 30 31 317 32 33 34 347 35 s J6 37 37 38 39 46.182 7 10 13 15 17 18 20 21 23 24 25 26 27 28 29 30 31 32 33 34 35 36 36 37 38 39 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.) S.P.P. 32 EXHAUST SYSTEMS Figure 17. Typical performance tables for centrifugal fans. These tables are compiled, assuming that the area of the main duct is equal to the sum 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, ran sizes usually indicate t e diameter in inches of the fan casing. (See paragraph 55.) S--S.P.P. 32 SAFE PRACTICES PAMPHLET NO. 32 low-pressure fan, and (b) the centrif ugal or high-pressure fan. 61. Fan bearings should not extend into the fan housings or exhaust pipes. 52. Propeller fans have curved screw shaped blades that produce a low pres sure draft and move the air in a direc tion 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 in stalled, 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 vol ume of air with considerable velocity and at high pressure, it operates effec tively and efficiently against high re sistances. 54. Fans should preferably be elec trically 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 (C FM ), 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 se lecting a fan that will meet the require ments just outlined. 56. In general a branch pipe velocity of 3,000 feet per minute will satisfac torily exhaust most fumes, vapors, gases and light dusts; a velocity of 4,500 feet per minute is usually needed for grind ing 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 meas ured in square feet.) 57. There are several methods for determining pressure in inches of water in exhaust systems. In discussing pres sure, it is necessary to differentiate be tween resistance pressure, velocity pres sure, 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 ]A of an inch in diameter is made in the pipe approximately 10 di Figure 18. Determining the amount of suc tion 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, alumi num, or similar material to avoid the possibility of sparks and fire or explo sion, 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 meas uring 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 pres sure 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 re moving dust created by granite working tools or machines. 58. Some dust when com paratively dry, and all dusts if moist, will stick to the fan blades. This not only decreases the efficiency of the exhaust sys tem but it may also ruin the bearings and in time destroy the fan. It is necessary, there fore, to clean the fan at regular intervals. If, however, it is nec essary 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 ex haust system depends largely upon the character of the waste materials. In a few cases they may be exhausted di rectly 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 advan tageous to install two small single fans instead of one large fan. Double fans are particu larly 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 cov ered with substantial wire mesh. Courtesy, Ford Motor Company Figure 19. Fresh air supply. At points where opera tions 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 66. Dust, fumes, viscous matter, fog to separate solid particles from the ex and mist may be collected by electrical hausted air. In general, the air is led precipitators which utilize high voltage into the cyclone on a tangent and as direct current for ionizing and electrical it swirls around in the cylindrical cham ly charging the collected materials. ber the solids are centrifuged or driven Liquids drain from the electrodes, while to the chamber walls where they slide solids are removed from the electrodes by gravity into the conical shaped bot by gravity or by washing, blowing, tom. The clean air escapes from the scraping, or rapping devices. top of the chamber through a central exhaust tube. Simple cyclones, skim mer cyclones, high efficiency cyclones and multiple cyclones are various types 67. Dry dust may be separated from the air by cloth arrestors. Means must be provided to shake or otherwise re of dry material collectors, all of which move the dust that builds up to form use adaptations of the simple cyclone a mat on the surface of the cloth. Usu principles. ally this is done only when the exhaust EXHAUST SYSTEM SURVEY FORM P lan t_____________________________________ Location-----------------Building_________________ D ate.............. ....... ---------- Surveyed by. GENERAL DATA: E xhaust System No._________ D epartm ents served by system (1)_______ (2)___ :-------------------- (3)............. ....................... ...... (4)--............... . Changes m ade since last survey.................. ........................ ............. .............. -- AIR FLO W DATA: Point of measurement Duct diam. (inches) Pitot or static reading (in. "water) Air velocity (FPM ) i ...... ......... -- -- -- ? A ir flow (C FM ) Remarks EXHAUST SYSTEM SURVEY FORM (Reverse side) ENVIRONM ENTAL DATA: Point of sam pling Material sampled Concentration found (State units) 1 Remarks C O N ST R U C T IO N DATA: (Cite parts of system needing attention) Elbow s ........................--................... Collectors (check filters if used)-. Fan and M otor........ - .... ............................. -- ................. ...................... .............. 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 in to the fan casing. Some dynamic pre cipitators are so arranged that the ex hausted air first passes through a water spray. 69. In impingement collectors the ex hausted 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. Ab sorption chambers and vessels are con structed 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 counter current principle. The introduction of coke, lime, activated alumina or other materials may also aid in the absorb ing 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 w a y . 74. Separators should be installed outside the building when conditions permit. They should be constructed of and supported by non-flammable ma terials. They should have clean-out doors to permit examination of the in terior. Vent pipes should extend above 10-- S.P.P. 32 SAFE PRACTICES PAMPHLET NO. 32 approved by the .American Standards Association. This publication should be referred to for detailed data. 79. I t must always be remembered that sufficient openings should be pro vided 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 suc cessful. 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 prob lems more nearly associated with gen eral ventilation than with exhaust sys tems, and are therefore discussed in de tail in the pamphlet on "Industrial Ventilation." 80. It is sometimes necessary to re move dusts locally by portable appa ratus without the installation of per manent 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 instru ments fitted for every special kind of cleaning. Courtesy, National Fire Protection Association Figure 21. A suggested design of safety relief vent for exhaust system. (See paragraph 82.) 81. Systems that were originally in stalled satisfactorily have been known to fail because hoods became detached 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 rec ommended. General 76. Many manufacturers succeed in utilizing the dust or fumes that are col lected by one of the methods herein described. For instance, wood dusts may be burned under the boilers; or blast furnace gases and others contain ing 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 ex haust systems. 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. 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 78. If an exhaust system is used for convey refuse or stock, it should be flammable dusts, fumes or gases, the vented to the outside of the building, metal parts of the system should be either directly by flues or separators, or electrically grounded. This and other indirectly by bins or vaults into which specifications to eliminate or reduce the it discharges. If the refuse or stock fire and explosion hazards inherent in is combustible, vertical safety relief exhaust systems and to prevent exhaust vents should be installed in such a man systems from spreading fires are dis ner that they do not deviate more than cussed in " Blower Systems," by the Na 22 Yi degrees from the direction of the 77. Care should be exercised to pre tional Fire Protection Association and 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 pur pose although a meter section of the venturi type provides a more permanent measuring apparatus." Testing Exhaust Systems for Effectiveness 83. The ultimate means of deter mining the effectiveness of any exhaust system is to measure the concentration of the contaminant in the workroom at mosphere. In addition, measurements of air-flow through the system should be made at regular, frequent intervals to make sure it is functioning in ac cordance with the original design. The following suggestions are quoted from the A.S.A. report entitled, "Funda mentals Relating to the Design and Op eration of Exhaust Systems" : "Measurement of static suction (pres sure) 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 re striction 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 trans porting velocities in the system. The ac curate 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 m e r i c a n 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 Amer ican Standards Association. ACKNOWLEDGMENT This pamphlet was prepared by the Industrial Division, National Safety Council. It has also been reviewed by the Safe Practices Conference Com mittee, manufacturers of exhaust sys tem equipment, consulting engineers and others. The Council gratefully acknowledges their assistance. Rep. 844--3M