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American Society of Heating and Ventilating Engineers GUIDE 1936 FOR HEATING, VENTILATING, AIR CONDITIONING An Instrument of Service prepared for the Profession--Containing a Technical Data Section OF REFERENCE MATERIAL ON THE DESIGN AND SPECIFICATION OF HEATING, VENTILATING AND AIR CONDITIONING SYSTEMS--BASED ON THE TRANS C ACTIONS--the Investigations of the Research Laboratory and o operating Institutions--and the Practice of the Members and Friends of the Society , together with a Manufacturers' Catalog Data Section Containing Essential and Reliable Information Concerning Modern Equipment also The Roll of Membership of the Society WITH Complete Indexes to Technical and Catalog Data Sections Vol. 14 $5.00 Per Copy Published Annually by American Society of Heating and Ventilating Engineers 51 Madison Avenue New York, N. Y. Chapter 15 AIR POLLUTION Sources of Air Pollution, Effects of Air Pollution on Health, Pul monary Effects, Occlusion of Solar Radiation, Industrial Air Pollution, Abatement of Atmospheric Pollution, Smoke Abate ment, Dust and Cinder Abatement THIS chapter considers the hygienic aspects of atmospheric pollution and the methods by which this pollution may be lessened. Infor mation concerning the cleaning of air brought into buildings for ventilat ing purposes will be found in Chapter 16, and a discussion of the exhaust ing of dusts and toxic gases from factories and industrial plants is con sidered in Chapter 21. The impurities which contribute to atmospheric pollution include carbon from the combustion of fuels, particles of earth, sand, ash, rubber tires, leather, animal excretion, stone, wood, rust, paper, threads of cotton, wool, and silk, bits of animal and vegetable matter, and pollen. Microscopic examination of the impurities in city air shows that a large percentage of the particles are carbon. (See Fig. 1, Chapter 16, for size of impurities in air.) Dust, Fumes, Smoke The most conspicuous sources of atmospheric pollution may be arbitrarily classified according to the size of the particles as dusts, fumes, and smoke. Dusts are particles of solid matter varying from 1.0 to 150 microns in size. Fumes include particles resulting from chemical pro cessing, combustion, explosion, and distillation, ranging from 0.1 to 1.0 micron in size. Smoke is composed of fine soot or carbon particles, less than 0.1 micron in size, which result from incomplete combustion of carbonaceous materials, such as coal, oil, tar, and tobacco. In addition to carbon and soot, smoke contains unconsumed hydrocarbon gases, sulphur dioxide, sulphuric acid, carbon monoxide, and other industrial gases capable of injuring property, vegetation, and health. The lines of demarcation in these three classifications are neither sharp nor positive, but the distinction is descriptive of the nature and origin of the particles, and their physical action. Dusts settle without appreciable agglomeration, fumes tend to aggregate, smoke to diffuse. Particles larger than one micron will eventually settle out by gravitation; particles smaller will remain in suspension as permanent impurities unless they agglomerate to sizes larger than one micron. 287 AIR POLLUTION AND HEALTH Many kinds of dusts and gases are capable of producing pathological changes which may cause ill health. The harmful effects depend largely upon the chemical and physical nature of the impurities, and the con centration, length of time, and conditions under which they are breathed. Dust particles must be minute in size to be inhaled at all, although fairly large particles may gain access to the upper air passages. The human body possesses remarkable filtering media for protecting the lungs. Small hairs which line the nasal passages, and a multitude of microscopic hairs, called cilia, in the epithelial lining in the bronchial tubes intercept many of the dust particles before they reach the lungs. The constant inhalation of dusts in city air irritates the mucous mem branes of the nose, throat, and lungs, and eventually may produce dis comfort and a series of minor respiratory disorders. The pigmented lung of the city dweller is an example of the pathological change produced over a period of years. This condition may be of no clinical importance, but an exaggeration of it in the coal miner results in anthracosis or dark spots on the lung due to the presence of pigment in the lymph channels which impairs the functioning of the lung cells under stress. Effects of Solids . Bronchitis is the chief condition associated with exposure to thick dust, and follows upon inhalation of practically any kind of insoluble and non- colloidal dust. Atmospheric dust in itself cannot be blamed for causing tuberculosis, but it appears to have a marked influence in aggravating the disease once it has started. There is, however, quite reliable evidence that carbon pigment, one of the atmospheric dusts, tends to wall off local tuberculosis rather than to further its spread. The sulphurous fumes and tarry matter in smoke are probably more dangerous than the carbon. In foggy weather the accumulation of these substances in the lower strata may be such as to cause irritation of the eyes, nose, and respiratory passages, leading to asthmatic breathing and bronchitis and, in extreme cases, to death. The Meuse Valley fog disaster will probably become a classic example in the history of gaseous air pollution. Released in a rare combination of atmospheric calm and dense fog, it is believed that sulphur dioxide and other toxic gases from the industrial region of the valley caused 63 sudden deaths, and injuries to several hundred persons. Physical examination showed difficult breathing, rapid pulse, cyanosis, cardiac dilation, and a redness and inflammation of the mucosa of the nose, mouth, throat, trachea, and bronchi. . Carbon monoxide from automobiles and from chimney gases con stitutes another important source of aerial pollution in busy cities. During heavy traffic hours and under atmospheric conditions favorable to concentration, the air of congested streets is found to contain enough CO Chapter 15 Air Pollution to menace the health of those exposed over a period of several hours, particularly if their activities call for deep and rapid breathing. In open air under ordinary conditions the concentration of CO in city air is believed to be insufficient to affect the average city dweller or pedestrian. Occlusion of Solar Radiation The. loss of light, particularly the occlusion of solar ultra-violet light due to smoke and soot, is beginning to be recognized as a health problem in many industrial cities. Measurements of solar radiation in Baltimore* by aclinic methods show that the ultra-violet light in the country was 50 per cent greater than in the city. In New York City2 a loss as great as 50 per cent in visible light was found by the photo-electric cell method. The effect of air pollution on the health of city dwellers is difficult to determine, owing to the slowness of its manifestations. The aesthetic and economic objections to air pollution are so definite, and the effect of air borne pollen can be shown so readily as the cause of hay fever and other allergic diseases, that means and expenses of prevention or elimination of (lux pollution have seemed justifiable to the public. AIR POLLUTION IN INDUSTRY In many industrial processes, sufficient amounts of dusts, fumes, and vapors are liberated to be injurious to the health of workers. Some dusts are poisonous (lead, mercury, arsenic, manganese, and cadmium) and some act as irritants (silica, steel, iron, and granite). Certain duste may produce catarrhal conditions and increase susceptibility to such diseases as bronchitis, pneumonia, and tuberculosis. Silicious dust is especially harmful because it has a direct damaging action upon the tissue of the lungs, but organic dusts, both animal and vegetable (hair, pollen, textile, and fiber), do not seem to affect the lungs at all, although they may cause considerable discomfort in the upper respiratory passages to persons sensitive to them. Industrial gases and fumes act specifically upon the mucous mem branes. the lungs, blood, skin, and eyes. Some extremely poisonous gases act after very short exposures. Among these are carbon monoxide, hydrogen sulphide, ammonia, chlorine, bromine, arsine, and cyanogen. The industrial processes which liberate harmful substances are too manifold and the effects too diverse to be considered here, where dis cussion is limited to the commonest and most serious with which the ventilating engineer may be confronted, namely, carbon monoxide, lead, and silica. Fot a more thorough treatise on the subject reference should be made to books by Hamilton2, Rosenau4, and Henderson and Haggard6. Carbon Monoxide Poisoning Carbon monoxide is a common form of poisonous industrial gas, met with in mines, foundries, coke-oven sheds, garages, and houses. Its action `Kfiecls of Mmospheric Pollution upon Incidence of Solar Ultra-Violet Light, by J, H. Shrader. M. H. CoblenU and P. A Korff (American Journal of Public Health, p, 7. Vol. 19, 19291. Studies in illumination, by J. E. Ives (U S Public Health Service Bulletin No. 197, 1930). Industrial Poisons in the United States, by Alice Hamilton. Preventive Medicine and Hygiene, by Milton J Rosenau Voxjous Gases, by V. Henderson and H Haggard. 280 American Society of Heating and Ventilating Engineers Guide, 1936 is due to the fact that the combining power of carbon monoxide with the haemoglobin of the red blood corpuscles is about 300 times greater than, that of oxygen. Since the resulting stable combination destroys the power of the haemoglobin to unite with oxygen in the lungs and to supply it to the tissues, the effects are due to lack of oxygen, and the symptoms are those of anoxemia, namely, dizziness, headaches, sleepiness, fatigue, and, in extreme cases, paralysis and death. The dangerous saturation level of the blood with carbon monoxide is about 50 per cent. Even as little as 0.07 per cent in the air will render, in half an hour, one quarter of the red corpuscles incapable of uniting with oxygen. One to two parts per 10,000 parts of air is set as a safe limit of pollution which may be breathed for a long time without producing perceptible symptoms. Silicosis Silicosis is a chronic disease of the lungs which results from the local physio-chemical action of hydrated silica upon the pulmonary tissue, causing progressive lymphatic fibrosis, and rendering the tissue-suscep tible to tuberculosis. The disease is slow in evolution, requiring usually a number of years of exposure. It occurs principally among granite workers, sand blasters, metal miners, metal polishers, potters, and mill stone workers. Lead Poisoning Lead poisoning is the most insidious and most common of all industrial diseases. It occurs principally among lead workers and smelters, lead miners, potters, painters, typesetters, stereotypers, plumbers, and workers with glass, gold and silver. Lead, in practically all forms, is a cumulative poison which is absorbed by way of the bloo.d stream, chiefly from the respiratory tract, but also from the digestive tract and from the skin. The effect may be either an acute or chronic poisoning. The principal symptoms are colic, constipation, anemia, headache, anorexia, a bluish line along the edges of the gums, rheumatic pains, and, in extreme conditions, paralysis, blindness, insanity, and death. ' It has been found that 2 mg per day is the smallest dose, by inhalation, which in the course of years may result in lead poisoning. Regular inhalation during the usual working hours of air containing less than 0.2 mg of lead per cubic meter does not seem to produce serious lead poisoning in individuals of representative industrial groups7. ' Prevention The prevention of industrial hazards from dusts and poisonous gases is largely a ventilation problem consisting of keeping the impurities in air down to a safe concentration. As yet there are no generally accepted standards on which to base the design of the ventilation -equipment. Approximate data on the toxicity of various gases and fumes met with in industrial establishments are given in Table 1. Column 5, giving the maximum allowable concentrations for prolonged exposures, was com piled from experiments in which most exposures lasted not more than a Lead Poisoning, by Thomas Morrison Legge (Journal Royal Society Arts, 1929, Vol. 77, p. 1023). rWhat is a Dangerous Quantity of Lead Dust in Air, by C. M. Sails (Industrial ffyiiene Bulletin, New York State Department of Labor. 1925). 290 Chapter 15 --Air Pollution week, and it is reasonable to assume that over more prolonged exposures such concentrations would cause pernicious effects. Much is known concerning the physiological and pathological effects induced by various types and concentrations of atmospheric pollutants. In the absence of an accepted standard for safe breathing, and because of the slow, cumulative effects of certain kinds of air contaminants, the best procedure is the periodic medical examination of individuals, and the Table 1. Toxicity op Gases and Fumes in Parts per 10,000 Parts of Air* Vator or Gar fUprDLT Fatal Maximov Concentration roft rnov to I Hotm Carbon monoxide . Carbon dioxide..... Hydrocyanic acid.. Ammonia............... Hydrochloric acid gas ..... Chlorine ........................ Hydrofluoric acid gas...... Sulphur dioxide................ Hydrogen sulphide........... Carbon bisulphide.--........ Phosphene........ ................ Arsine................................. Phosgene.... ...................... Nitrous tumes................... Benzene.............................. Toluene and xylene......... Aniline............................... Nitrobenzene..................... Petrol................................. Carbon tetrachloride........ Chloroform........................ Tetrachlorethane -............ Trichiorethylene....... ........ Methyl chloride................ Methyl bromide................ Lead vapor........................ 40 800-1000 30 50-100 10-20 10 2 4-5 10-30 20 2X Over M 2X-7M 190 190 243 480 250 73 370 1500-3000 200-400 15-20 IX 25 X H Ho x-i 5-7 11 4-6 X M 1-iX 100-220 240 140 200-400 20-40 Maxoauu Concentration ros I Hour 10 X 3 2-3 5 1-2 X 31-47 31--47 1-1X Moo 40 50 70 10 Maximou Allowable roR Prolonqbd Exposure 1 X 1 Mo Mo l X J 00 1X-3 tt, 16 2 ix 6-10 2 6-6 Original data compiled by V. Henderson and H. Haggard. (See Noxious Gases, 1927.) Data revised . by T. M. Legge. (See Lessons Learned from Industrial Cases and Fumes, Institute of Chemistry of Great Britain and Ireland. London, 1930.) routine measurement and study of the concentration and the physical and chemical characteristics of the dusts to which those individuals are exposed. _ ABATEMENT OF SMOKE AND AIR POLLUTION Successful abatement of atmospheric pollution requires the combined efforts of the combustion engineer, the public health officer, and the public itself. The complete electrification of industry and railroads, and the separation of industrial and residential communities would aid materially in the effective solution of the problem. In the large cities where the nuisance from smoke, dust and cinders is 291 American Society of Heating and Ventilating Engineers Gt) the most serious, limited areas obtain some relief by the' heating. The boilers in these plants are of large size designedrai ated to burn the fuel without smoke, and some of them are equippetfi| dust catching devices. The gases of combustion are usually discharged a much higher level than is possible in the case of buildings that c-- their own boiler plants. . In general, time, temperature and turbulence are the essential reqt ments for smokeless combustion. Anything that can be done to inc any one of these factors will reduce the quantity of smoke discha Especial care must be taken in hand-firing bituminous coals. Chapter 27.) . Checker or alternate firing, in which- the fuel is fired alternately ` . separate parts of the grate, maintains a higher furnace temperature andj thereby decreases the amount of smoke. Coking and firing, in which the fuel is first fired close to the firing door and the coke pushed back into the furnace just before firing again, pro- " duces the same effect. The volatiles as they are distilled thus have to pass over the hot fuel bed where they will be burned if they are mixed with sufficient air and are not cooled too quickly by the heat-absorbing surfaces of the boiler. Steam or compressed air jets, admitted over the fire, create turbulence in the furnace and bring the volatiles of the fuel more quickly into contact with the air required for combustion. These jets are especially helpful for the first few minutes after each firing. Frequent firings of small charges shorten the smoking period and reduce the density. Thinner fuel beds on the grate increase the effective combustion space in. the furnace, supply more air for combustion, and are sometimes effective in reducing the smoke emitted, but care should be taken that holes are not formed in the fire. A lower volatile coal or a higher gravity oil always produces less smoke than a high volatile coal or low gravity oil used in the same furnace and fired in the same manner. The installation of more modern or better designed fuel burning equip ment, or a change in the construction of the furnace, will often reduce smoke. The installation of a Dutch oven which will increase the furnace volume and raise the furnace temperature often produces satisfactory results. In the case of new installations, the problem of smoke abatement can be solved by the selection of the proper fuel-burning equipment and furnace design for the particular fuel to be burned and by the proper operation of that equipment. Constant vigilance is necessary to make certain that the equipment is properly operated. In old installations the solution of the problem presents many difficulties, and a considerable investment in special apparatus is necessary. Legislative measures at the present time are largely concerned with the smoke discharged from the chimneys of boiler plants. Practically all of the ordinances limit the number of minutes in any one hour that smoke of a specified density, as measured by comparison with a Ringelmann Chart (Chapter 43), may be discharged. These ordinances do not cover the smoke discharged at low levels by automobiles, and, although they have been instrumental in reducing the 202 Chapter 15--Air Pollution smoke emitted by boiler plants, they have, in many instances, increased the output of chimney dust and cinders due to the use of more excess air and to greater turbulence in the furnaces. Legislative measures in general have not as yet covered the noxious gases, such as sulphur dioxide and sulphuric acid mist, which are dis charged -with the gases of combustion. Where high sulphur coals are burned, these sulphur gases present a serious problem. DUST AND CINDERS The impurities in the air other than smoke come from so many sources that they are difficult to control. Only those which are produced in large quantities at a comparatively few points, such as the dust, cinders and fly-ash discharged to the atmosphere along with the gases of com bustion from burning solid fuel, can be readily controlled. Dusts and cinders in flue gas may be caught by various devices on the market, such as fabric filters, dust traps, settling chambers, centrifugal separators, electrical precipitators, and gas scrubbers, described in later paragraphs. The cinder particles are usually larger in size than the dust particles; they are gray or black in color, and are abrasive. Being of a larger size, the range within which they may annoy is limited. The dust particles are usually extremely fine; they are light gray or yellow in color, and are not as abrasive as cinder particles. Being ex tremely fine, they are readily distributed over a large area by air currents. The nuisance created by the solid particles in the air is dependent on the size and physical characteristics of the individual particles. The difficulty of catching the dust and cinder particles is principally a function of the size and specific gravity of the particles. Lower rates of combustion per square foot of grate area will reduce the quantity of solid matter discharged from the chimney with the gases of combustion. The burning of coke, coking coal, and sized coal from which' the extremely fine c&at has been removed will not as a general rule produce as much dust and cinders as will result from the burning of non-coklrig coals and slack coal when they are burned on a grate. Modern boiler installations are usually designed for high capacity .per square foot of ground area because such designs give the lowest cost of construction per unit of capacity. Designs of this, type discharge a large quantity of dust and cinders with the gases of combustion, and if pollution of the atmosphere is to be prevented, some type of catcher must be installed. Dust and Cinder Catchers8 The various types of dust and cinder catchers available today can be divided into six general classes: 1. Settling chambers. 2. Dust and cinder traps. 3. Centrifugal separators. . . Sec Smoke and Duat Abatement, by M. D. Engle (A.S.H.V.E, Transactions, Vol. 37,1931). 293 ' 4. Electrostatic precipitators. 5. Gas scrubbers. 8. Fabric filters. The selection of the proper type of catcher calls for a careful i the material to be caught ana the draft and space available. Af` installation, constant vigilance is necessary to keep the catchers in pr working condition if satisfactory operation is to be obtained. ; If possible, the dust or cinder catcher should be installed on the I side of the induced draft fans because the dust and cinders in the | seriously erode the wheels of the fans, the inlet connections and scrolls. Where the induced draft fans operate at high tip speeds and nq<| catchers are installed, it is not uncommon for the fans to require major? repairs within one year and complete replacement within five years. Settling Chambers Probably the oldest form of dust catcher is the settling chamber, which generally consists of a large-sized, gas-tight space into which the dust-laden gases are discharged before being delivered to the chimney. The velocity of the gas should be reduced to a point where the larger and heavier particles will be precipitated by gravity. For good operation, the velocity of the gas should be reduced to a maximum of 2 fps. The bottoms of the chambers should be provided with dump plates through which the collected dust can be removed. Because these chambers are not effective in removing the finer dust particles they have been practically superseded by smaller and less costly devices. Traps, Catchers, Precipitators Various types of traps have been devised. In general they all depend upon breaking the gas up into thin strata and subjecting those thin strata to several abrupt changes in direction. The dust is thrown out of the gas stream into specially shaped pockets, or impinged against a roughened surface. The trapping pockets are drained into a hopper below with a small quantity of gas and the dust settles out by gravity due to the low velocity in the hopper. In the roughened surface type, various sections of the trap are closed off at intervals by means of dampers and the dust is shaken off the roughened surface into a hopper below. These devices work very well in catching large size dust and cinders and trap much of the fine dust. They have been used most extensively on stoker-fired installations. They have the advantages of low pressure drop, relatively small space requirements, and low first cost. Centrifugal catchers obtain separation by projecting the particles tangentially out of the gas stream. The effectiveness of this type of catcher varies directly as the specific weight of the dust and as the square of the tangential velocity, and inversely as the radius of rotation. Electrostatic precipitators are used for catching fine dust. These precipitators consist of dust-tight chambers in which are suspended rein forced concrete slabs on about 10-in. centers. Between the slabs are suspended bare metal rods. High-voltage undirectional current _ is applied to the reinforcing rods in the concrete slabs acting as positive electrodes, the bare rods acting as negative electrodes. The dust-laden Chapter. 15--Air Pollution gas flows horizontally through the precipitator and the dust particles migrate toward the concrete slabs to which they adhere and then fall or are scraped off into the dust hoppers below. Gas Scrubbers Wet scrubbers have been used for many years for removing dust from gases. A number of different types of scrubbers are now being built for removing dust from boiler flue gases. One type depends upon saturating the gas and washing the dust out of suspension by a spray of water. For best results with this type, the water should be atomized into as fine a spray as possible. Another type depends upon splitting the gas into thin strata and subjecting these strata to a number of abrupt changes in direction, throwing the dust against the wet surfaces. The main problem in develop ing a satisfactory wet dust catcher is to find suitable materials of con struction that will resist the corrosive action of the wash water for a reasonable length of time. Fabric Filters . Filters of many kinds have been used with variable success. The filter bags are made of cotton, wool or asbestos fabric. The fabrics used in these filters do not withstand the temperatures at which gases are usually discharged from the boilers, and hence the gases must be cooled by some means. Surface coolers or water sprays can be used for reducing the gas temperatures. One of the serious objections to all of these dust catchers is the relatively high cost of installation and maintenance, and the space required for installation. Disposal of Dust and Cinders Even after the dust and cinders have been caught, the disposal of the material caught presents a serious problem. The cinders discharged with the gases from stoker-fired boilers are usually very high in carbon and ' contain from 60 to 80 per cent as much heat per pound as the coal which is being burned. It is possible, and usually economical, to burn these cinders. They cannot be satisfactorily mixed with the coal in the stoker . hopper but they can be blown into the furnace over the stoker fuel bed and burned satisfactorily. If a sufficient quantity of cinders is caught, a small unit pulverizer can be installed to prepare them for burning over the stoker fuel bed. The same pulverizer can be used for coal at times of peak load and will materially increase the capacity of the fuel-burning equipment for the boiler to which it is connected. No satisfactory market has been developed for the dust caught from pulverized coal installations, but the possibilities are being investigated and it seems likely that in the future this material will have a market value that will go a long way toward paying the fixed charges on the cost of catching it. The distribution of dust in the gas entering and leaving the dust and cinder catchers is not uniform and is different in practically every in- 295 American Society of Heating and Ventilating Engineers Guide, 1936 stallation, and varies widely with changes in furnace conditions. In order to obtain a representative sample it is necessary to traverse the inlet and outlet of the catcher with a sampling tube which faces into the gas flow. The velocity of the gas into the sampling tube must be the same as the velocity of the gas in the duct at the instant the sample is taken. The swirls and eddy currents in the ducts make it difficult to obtain consistent readings, but if the test is conducted by some one of experience, an indication of the approximate efficiency can be obtained. Nature's Dust Catcher ' Nature has provided means for catching solid particles in the air and depositing them upon the earth. A dust particle forms the nucleus for each rain drop and the rain picks up dust as it falls from the clouds to the earth. In fact, without dust in the air to form the nuclei for rain drops it would never rain, and the earth would be continually enveloped in a cloud of vapor. PROBLEMS IS PRACTICE 1 Classify the detrimental aspects of air pollution as It effects large industrial communities. Air pollution may be classified (a) medical, as it affects the physiological functions of people; (6) botanical, as it affects vegetation, trees, plants, shrubs and flowers; and (c) physical, as it affects the discolorization and deterioration of buildings, and the nuisance of soiled interior furnishings, clothes, merchandise, 'etc. 2 Distinguish between dusts, fumes, and smokes. Solid particles ranging in size from 1.0 micron to 150 microns are called dusts (micron * f^s.ooo in-)- - Particles resulting from sundry chemical reactions and ranging from 0.1 to 1.0 micron in size are called fumes. Carbon particles less than 0.1 micron in size which generally arise from the incomplete combustion of such materials as coal, oil, or tobacco are called smokes. 3 What are some of the more important physical properties of these various groups of foreign bodies which are of importance in ventilation? In slowly moving air, dusts tend to settle out by gravity without agglomerating to form larger particles; fumes have the tendency to form larger particles which will settle when they attain the size of approximately 1.0 micron; while smokes tend to diffuse and remain in the air as permanent impurities. 4 Why is atmospheric pollution an important engineering problem? a. Certain impurities, when present in too great concentrations, cause ill health or even death. b. High concentrations of solids occlude solar radiations. c. Some materials cause permanent injury to parts of buildings, as sulphur fumes corrode exposed metal. d. Extra cleaning expense is incurred in dusty localities. . e. Internal combustion engines are damaged by abrasive dusts. 5 How may the hazards of dust-producing industrial operations best be curtailed? By providing mechanical exhaust ventilation sufficient to keep dust concentration at a safe level (see Table 1) and then removing foreign bodies to reduce the pollution of out side air. 296 Chapter 15--Air Pollution 6 A How may the pollution of the atmosphere be lessened? By compelling industrial plants to install dust catching and smoke controlling devices. In" many cities the domestic heating plant is one of the most serious offenders, but these plants are too small to justify the installation of dust catchers. Public education in improved firing methods would be of considerable help in this field. 7 Compare the dry and wet types of dust catchers. The dry types are very effective in removing the larger dust particles but the smaller particles generally pass through other kinds than the electric precipitator. The dry types also require considerable space and therefore sometimes introduce resistance to the flow of air. The wet types are effective in removing some of the smaller dusts and the water-soluble gases. The principal disadvantage of the washer is its short life caused by the corrosive action of the wash water. 8 What size particles are detrimental to health? While fairly large particles may enter the upper air passages, those found in the lungs are seldom more than 10 microns in size, and comparatively few of them are more than 5 microns. It is agreed that particles between 3^ and 2 microns may be harmful; some authorities place the upper limit at about 5 microns, and some incline to extend the lower limit to 0.1 of a micron. 9 * Is the shape of the particle of any significance? Hard particles with sharp corners or edges have a cutting effect on the delicate mucous membranes of the upper respiratory tract which may lower the resistance of the nose and throat to acute infections. This is aggravated by the irritating effects of some chemical compounds which may be taken in with the air and which act to reduce resistance. 10 A What are the principal meteorological effects of smoke and dust?. o. The reduction in the amount of light received. Measurements have shown that visible light may be as much as 60 per cent less intense in a smoky section of a city than in a section that is free from smoke. Ultra-violet light is reduced as much or more, and in some cases is cut out entirely for a time. b. Smoke and dust aid in the formation and prolongation of fogs. City fogs accumulate smoke and become darker in color and very objectionable. The sun requires a longer time to disperse them, and when the water is evaporated, there is a rain of smoke and soot particles that have been entrained. 11 A Why has not smoke abatement been more effective? Because communities have not been made sufficiently aware of the possibilities of burning high volatile fuels smokelessly and of separating cinder and ash from the stack gases to a degree that will prevent a nuisance. 12 A Is the abatement of dust and cinders important? Ves. Only a small percentage of the solid emission from stacks is smoke, in the accepted popular sense; the remainder is fly-ash and cinders. While black smoke is disagreeable and its tarry matter and carbon particles soil anything with which they come in contact, the cinders and some of the ash are hard and destructive. They also, together with dusts from industrial processes, make up the hard, sharp, irritating, air-borne solids that are breathed by individuals not worlang in a dusty mill or factory. 13 A Are air*borne impurities causative factors in hay fever, bronchial asthma, and allergic disorders? Yes. Recent medical investigations indicate that 90 per cent of seasonal hay fever and 40 per cent of bronchial asthma are caused by air-borne pollens, tree dusts, and other allergic irritants. 14 A Name some essential requirements for the smokeless combustion of fuels. Time, temperature, and turbulence. A study of these factors is usually of value in overcoming a smoke nuisance. 297 15 What is the Hlngelmann Chart Method of comparing smoke densi See Chapter 43. The Ringelmann Chart consists of four cards ruled with lines ham different degrees of blackness. These cards, together with a white card and a black onare hung in a horizontal row 50 ft from the observer. At this distance the lines becom. invisible and the cards appear to be different shades of gray, ranging from white to black; The observer, by matching the cards against the shades of smoke coming from a stack, is able to estimate the blackness of the smoke as compared with the chart. Chapter 16 AIR CLEANING DEVICES ,Requirements of an Air Cleaner Typest Air Washers and Scrubberst Viscous Type Filterst Dry Air Filterst Air Filter Installations THE removal of impurities from air brought into a building for ventilating or air conditioning purposes is the function of any air cleaning or filtering device. These impurities include carbon (soot) from the incomplete combustion of fuels burned in furnaces and automobile engines, particles of earth, sand, ash, automobile tires, leather, animal excretion, stone, wood, rust and paper, threads of cotton, wool and silk, bits of animal and vegetable matter, bacteria and pollen. Microscopic examination shows that the character of the impurities varies with the locality, but as a rule carbon forms the greater part of them while the total is somewhat proportional to the state of industrial activity and the wind intensity. Additional information on sources of air pollution will be found in Chapter 15. Observations have shown that practically all atmospheric impurities are less than 5 microns in size. (One micron equals 0.001 millimeter or approximately 0.00004 in.) The size and composition of each individual particle determines its buoyancy and consequently the length of time it will remain in suspension. The chart, Fig. 1, shows graphically the sizes of impurities found in the air, and other related data. ' To estimate the probable dust load for air filter installations, the following approximate averages of atmospheric dust concentration may be used (7000 grains equal 1 lb): Rural and suburban districts.............................................. 0.2 to 0.4 grains per 1000 CU ft Metropolitan districts.......................................................... 0.4 to 0.8 grains per 1000 cu ft Industrial districts................................................................ 0.8 to 1.5 grains per 1000 cu ft REQUIREMENTS OF AN AIR CLEANER To fulfill the essential requirements of clean air, an air cleaner should: 1. Be efficient in the-removal of harmful and objectionable impurities in the air, such as dust, dirt, pollens, bacteria. 2. Be efficient over a considerable range of air velocities. 3. Have a low frictional resistance to air flow; that is, the pressure drop across the filter, measured in inches of water, should be as low as possible. . 4. Have a large dust-holding capacity without excessive increase of resistance, or have ability to operate so as to keep the resistance constant automatically. 6. Be easy to clean and handle, or clean itself automatically. 6. Leave the air passing through the cleaner free from entrained moisture or charging liquids used in the cleaner. . 299 Compiled by W. G. Frink and Copyrighted. 1. Sizes and Characteristics of Air-Borne Solids in inches of water at rated capacity, (3) dust arrestance, the percentage relationship expressing dust removal efficiency at rated capacity, (4) reconditioning power, the energy necessary to operate the mechanism of 'Adopted 1934 by A.S.H.V.E. See Chapter 44. 300 Chapter 16--Air Cleaning Devices an automatic air cleaning device, and (5) dust holding capacity, the amount by weight of standard dust which a non-automatic air cleaning device will retain before reconditioning is necessary. TYPES OF AIR CLEANERS According to the Code, the following four classifications are given the devices: Class A. Automatic Type: In general all air cleaning devices which use power to automatically recondition the filter medium and maintain a non-varying resistance to air flow. Class B. Low Resistance Non-Automatic Type: Air cleaning devices for warm air furnaces, unit ventilating machines and similar apparatus and installations in which a maximum of not more than 0.18 in. water gage is available to move air through the air cleaning device. Class C. Medium Resistance Non-Automatic Type: Air cleaning devices for systems in which a maximum of not more than 0.5 in. water gage is available to move air through the air cleaning device. Class D. High Resistance Non-Automatic Type: Air cleaning devices for the air intake of compressors, internal combustion engines, and the like, where a pressure of 1.0 in. or more water gage is available to move air through the air cleaning device. Air cleaners may be also classified as follows: 1. According to principle of air cleaning. a. Air washers. b. Viscous air filters. (1) Unit type. (2) Automatic type. c. Dry air filters. 2. According to application. a. For central fan systems of ventilation and air conditioning. Filters of the automatic or semi-automatic type are usually recommended and are installed in a central plenum chamber. b. For unit ventilators. Filters of viscous unit or dry type, installed at inlet of individual units. c. For window installations. Self-contained units consisting of fan and filter, usually dry type, adapted to be placed in the ordinary window. d. For warm-air furnaces. Unit type viscous or dry filters placed in small plenum chamber of warm-air house heating systems. e. For compressors and Diesel engines. Unit type viscous or dry filters, installed at air intake of compressors and Diesel engines. f. For compressed air lines. Unit type viscous or dry filters. With the growing congestion of large cities and an industrial growth throughout the entire country, the percentages of foreign material in the air, such as soot or carbon, which are unaffected by an air washer type of air cleaner, have increased. This has brought about the development of the viscous and dry type air filters which are part of many ventilating and air conditioning systems. AIR WASHERS AND SCRUBBERS Information on air washers will be found in Chapter 11. Scrubbers have not been used very extensively in the past for cleaning 301 T American Society of Heating and Ventilating Engineers Guide, 1936 air for ventilating purposes. However, new types have been developed which appear to have possibilities for cases where the air to be cleaned is extremely dirty or where a higher degree of cleanliness is desired than can! be obtained with an air washer. VISCOUS TYPE FILTERS The principle of air cleaning used in viscous filters is that of adhesive impingement. Dust and dirt in the air, especially soot and carbons, are trapped and retained by successive impingements on coated surfaces. While the arrangements of filtering media and the kind of materials used are almost unlimited, there are certain rather,definite requirements for a practical commercial filter. / ' Investigations in this country and abroad demonstrate that the first impingement of dust laden air on a viscous coated surface removes about 60 per cent of the dust, the next impingement takes 60 per cent of what then remains--that is, 24 per cent--and the next impingement removes 9.6 per cent. To secure maximum efficiency, it is necessary to divide the air into innumerable fine streams, as the more intimately and freely the air is brought into contact with the viscous-coated media the better will be the cleaning. The binding liquid used with viscous filters should have the following properties: 1. Its surface tension should be such as to produce a homogeneous film-like coating on the filter medium. 2. The viscosity should vary only slightly with normal changes of temperature. 3. It should be germicidal in its action to prevent the development of mold spores and bacteria on the filter media. 4. The liquid should flow freely at low temperatures. 5. Evaporation should not exceed 1 per cent. 6. It should be fireproof. 7. It should be odorless. Viscous Unit Filters In the unit type viscous filter, the filtering media are arranged in units of convenient size to facilitate installation, maintenance, and cleaning. Each unit consists of an interchangeable cell or replaceable filter pad and a substantial frame which may be bolted to the frames of other like units to form a partition between the source of dusty air and the fan inlet. The necessary washing, draining, and recharging equipment should be installed near each group of unit filters, with hot water and sewer con nections provided. To secure greater dust holding capacity and a practically constant resistance and air volume, the filter media are usually placed in the direction of air flow, with progressively finer filter densities determined by the percentage of dust impinged. This arrangement provides relatively large spaces for the collection of dirt in the front of'the filter where the bulk of the dust is taken out without undue increase in resistance, while at the back of the filter the openings are smaller to secure high efficiency in the removal of the finer dust particles. The resistance of a well-designed unit filter of the adhesive impinge- 302 Chapter. 16--Air Cleaning Devices ment type usually depends upon the velocity at which the air is handled and upon whether the unit is clean or dirty. The cleaning efficiency of the unit is usually highest after it has accumulated a certain portion of its maximum load of dirt because some dust collected in the cell acts as an efficient medium for the further seizing of solids from the air. By periodi cally cleaning a predetermined number of cells, the resistance and capacity of a built-up filter may be held at any desired figure. The frequency of cleaning any unit filter installation depends upon the dust concentration Fig. 2. Chart Showing Change in Resistance Due to Dust Accumulation Fig. 3. Resistance to Air-Flow of a Typical Unit Air Filter of air being cleaned, and on the amount of dirt which can be accumulated in the filter medium without causing excessive resistance. (Figs. 2,3 and 4.) Filters consisting of inexpensive frames of cardboard or similar material filled with viscous-coated glass wool or steel wool are available. Because of their construction^ these units may be discarded when dirty and replaced with new units at relatively little expense. They are used in general ventilation work and with warm air furnaces and other installations where first cost and low resistance to air flow are essential. The operating characteristics of these units conform in general with those of the rigid frame type. Viscous Automatic Filters The principle of air cleaning used in the viscous automatic filters is the same as in the unit filters. The removal of the accumulated dust, 303 American Society of Heating and Ventilating Engineers Guide, 1936 however, is done automatically instead of by hand. The automatic clean ing and recoating of these filters is based on the principle that the viscous fluid itself will perform the cleaning function, thereby, eliminating a sepa rate washing agent. The dust collected by' the filter thus is deposited finally in the bottom of the viscous fluid reservoir from which it may. be removed by different methods, depending on the design, of the filter. There are three general types of automatic filters. They are differentiated from each other according to the process of self-cleaning and renewing of the viscous coating used by each type, as follows: 1. The filter medium has the form of an endless curtain suspended vertically, with its lower portion submerged in a viscous fluid reservoir. The curtain rotates slowly through this bath, thus performing the cleaning and recoating of the filter medium. 2. The filter screen is arranged in the form of shelves or cylinders, and the viscous fluid is flushed through all parts of the medium in a direction opposite to the air flow. 3. The filter medium is arranged vertically and is stationary. The viscous' fluid is flushed from above over the medium, while the air flow is stopped. Fig. 4. Maintenance Chart for Unit- Type Viscous Filters The washing and renewing process in automatic filters usually is inter mittent. It is accomplished by an electric motor or by other motive power and is controlled by manual or by automatic timing devices. The operating cycle is of a predetermined frequency and should be so timed as to insure a constant static pressure drop across the filter. The customary resistance to air flow is %-in. water gage at an air velocity of 500 fpm, measured at the filter entrance. Automatic viscous filters are made up in units which are delivered either fully assembled or in parts to be assem bled at the point of installation. DRY AIR FILTERS Dry air filters, in which dust is impinged upon or. filtered through screens made of felt, cloth, or cellulose, are available in various types. These filters require no adhesive liquid, but depend on the straining or screening action of the filtering medium. Because of the close texture 304 Chapter 16--Air Cleaning Devices of the filtering media used in most of the dry filters, the surface velocity, or velocity of the air entering the media, ranges between 10 and 50 fpm, depending on the nature and texture of the fabric. This necessitates a relatively large screen surface, and the filter media are usually arranged in the form of pockets to bring the frontal area within customary space requirements. As in viscous unit filters, an average constant resistance and air volume may be obtained by periodic reconditioning or renewal of the filter screens. Since some materials suitable for dry filtering media are affected considerably by moisture which tends to cause a rapid increase in resis tance, they should be treated or processed to minimize the effect of changes in humidity. Filters using felt and similar materials as filter media depend upon vacuum cleaning for reconditioning. A special nozzle, operated from a portable or stationary vacuum cleaner, is shaped to reach all parts of the filter pockets. Permanent filter media should be capable of withstanding repeated vacuum cleanings without loss in dust removal efficiency. While most dry filters are cleaned by replacing an inexpensive filter sheet, the useful life of these sheets often may be lengthened by vibrating or vacuum cleaning. INSTALLATION METHODS The published performance data for all air filters are based on straight through unrestricted air flow. Filters should be installed so that the face area is at right angles to the air flow whenever possible. Eddy currents and dead air spaces should be avoided and air should be distributed uniformly over the entire filter surface, using baffles or diffusers if neces sary. The most important requirements of a satisfactory and efficiently operating air filter installation are: 1. The filter must be of ample sire for the amount of air it is expected to handle. An overload of 10 to 16 per cent is regarded as the maximum allowable. When air volume is subject to increase, a larger filter should be installed. 2. The filter must be suited to the operating conditions, such as degree of air clean liness required, amount of dust in the entering air, type of duty, allowable pressure drop, operating temperatures, and maintenance facilities. 3. The filter type should be the most economical for the specific application. The first cost of the installation should be balanced against depreciation as well as expense and convenience of maintenance. The following recommendations apply to filters and washers installed with central fan .systems: 1. Duct connections to and from the filter should change size or shape gradually to insure even air distribution over the entire filter area. 2. Sufficient space should be provided in front as well as behind the filter to make it accessible for inspection and service. A distance of two feet may be regarded as the minimum. 3. Access doors of convenient size should be provided in the sheet metal connections leading to and from the filters. < 4. Ail doors on the clean air side should be lined with felt to prevent infiltration of ft:. unclean air. All connections and seams of the sheet metal ducts on the clean air side irf t should be as air-tight as possible. 305 REFERENCES Testing and Ratine of Air Cleaning Devices Used for General Ventilation Work, by Samuel R. Lewis (A.S.H.V.E, Transactions, Vol. 39, 1933). . Fundamental Principles in the Design of Dry Air Filters, by Otto Wechsberg (A.S.H.V.E. Journal Section, Heating, Piping-ana Air Conditioning, April, 1933). Operation and Maintenance of Air Filters, by W. G. Frank (Heating, Piping and Air Conditioning, May, 1931). Sire and Characteristics of Air-Borne Impurities, by W. G. Frank (Heating, Piping and Air Conditioning, January, 1932). Determining the Quantity of Dust in Air by Impingement, by F. B. Rowley and John Bead (A.S.H.V.E. Transactions, Vol. 35, 1929). A Study of Dust Determinators, by F. B. Rowley and John Beal (A.S.H.V.E. Trans actions, Vol. 34, 1928). Design and Application of Oil-Coated Air Filters, by H. C. Murphy (A.S.H.V.E. Transactions, Vol. 33, 1927). Determining the Efficiency of Air Cleaners, by A. M. Goodloe (A.S.H.V.E. Trans actions, Vol. 30, 1924). PROBLEMS IN PRACTICE 1 Assume a fan and duct system which handled 10,000 cfm through clean filters with a system resistance of 0.8 in. of water and that after the filters hare become dirty the system resistance increases to 1.0 in. of water, and that the fan speed remains unchanged. Is there any way of predicting the rolume of air delivered after the filter becomes dirty? Yes. If the performance curves for the particular make of fan are available, the new volume may be determined from the resistance pressure curve. (Figs. 1, 2, 3 and 4, Chapter 17.) 2 What are the advantages of viscous filters? The principal advantage of the viscous filter is its large dust holding capacity. The dust accumulation is distributed through the depth of the filtering medium rather than upon the surface as in the dry types, which makes it possible for viscous filters to handle heavy dust concentrations without excessive resistance. Since its efficiency and resis tance are based on maximum air velocities of from 300 to 500 fpm through the filter, the viscous filter consumes the minimum amount of space for a given air'volume. 3 What are the advantages of dry filters? Dry filters are more efficient in the removal of fine dust particles from the air, and some types will eliminate even as much as 60 pier cent of the smoke particles. Dry filters also are easily and conveniently maintained by vacuum cleaning,_ vibrating, or renewing the filtering medium. ' 4 # If an air washer is used for cooling and humidity control in an air con ditioning system, Is a filter needed? An air filter is desirable in conjunction with an air washer because of the large amount of soot in the air which, due to its greasy and amorphous nature, is not readily trapped in 306 Chapter 16--Air Cleaning Devices an air washer. Kilters should be placed between the washer and the air intake so that all the dirt will be collected at one point to simplify maintenance and to protect all the equipment in the system. 5 Is an air Biter needed with an extended surface type heal exchanger? An air filter is essential with an extended surface heat exchanger in order to maintain its efficiency, for without this protection dust particles will adhere to the exposed surfaces, and gradually build up a deposit to the point where the efficiency will be impaired and the resistance increased by restricting the air passage. 6 * What is the proper location of a filter in relation to the fan? A filter will operate equally well whether placed on the suction or discharge side of the fan. It has become standard practice, however, to locate the filter on the fan inlet side because there it has: (1) simpler duct connections, (2) reduced static pressure losses, (3) more even air distribution over the entire filter area. Where an exceptionally high efficiency in dust removal must be maintained, it is often advisable to place the filter on the discharge side of the fan so there can be no infiltration of unclean air. 7 What instruments and apparatus are required for determining the pollen concentration in air by means of the settling method? A microscope with a field of known area and a glass slide coated with a viscous material. 3 0 Describe the procedure for determining the pollen concentration in air by means of the settling method. A glass slide coated with a viscous material is placed for a period of 24 hours in a hori zontal position in the atmosphere to be tested. The slide is then removed and placed under the microscope, and pollen counts are made of approximately 25 fields over the area of the glass slide. Having determined the count over a definite area, as for example, 1 sq cm, and finding the settling rate of the average particles from the chart, Fig. 1, the concentration in parts per cubic yard can be calculated. 9 The resistance to air flow of a unit air filter is found to be 0.4 in. of water. The volume of air passing through the filter is 1000 cfm at a velocity of 200 fpm. What would be the filter area required in order to reduce the pressure drop across the filter from 0.4 in. of water to 0.16 in. of water? Referring to Fig. 3: The resistance is substantially proportional to the square of the velocity, or R, = }V Rt IV 0.4 _ 2<XF 0.16 IV . tV = 16,000 Vt = 126.5 fpm Q = AV _ 1000 = 126.5 A - . 1000 , a, c. A - 12675 = 7M " ft The filter area would be increased from 5 sq ft to 7.91 sq ft. 10 A ventilating system complete with filters has a fan which, when operating at 400 rpm and delivering air at 1 in. of water total static pressure, requires an input of 3 horsepower. After the system operates for a time, the pressure drop across the filter caused by the clogging action of the collected dust and dirt . increases from 0.1 in. of water to 0.4 in. of water. To maintain the original 307 American Society of Heating and Ventilating Engineers Gun> rate of air delivery with the increased static pressure, at what speed fan be run and what horsepower will be required? ' Static pressure after clogging of filter = 1 + (0.4 -- 0.1) =. 1,3 in. of water! -; The static pressure-varies as the square of the fan speed. Therefore, if X is the faff after the static pressure increases: 1.Z3 ( X \a 1 V 400/ X = 456 rpm. The horsepower varies as the cube of the fan speed. Therefore, if' Y is the horse-* after the static pressure increases: JL f 456 ~\3 3 " \ 400 / Y = 4.44 horsepower. ' To maintain the original rate of air delivery with the increased static pressure,, the fan speed must be increased from 400 to 456 rpm, and the horsepower from 3 to 4.44. ' ^ 308 Chapter 21 INDUSTRIAL EXHAUST SYSTEMS Classification of Systems, General Rules for Design, Suction and Velocity Requirements, Hoods, Design of Duct Systems, Collectors, Resistance of Systems, Efficiency of Exhaust Systems, Selection of Fans and Motors S' OME type of exhaust and collecting system is necessary in almost every industry and the present chapter attempts to give general information relating to the design of factory exhaust systems in order that efficient and economical control of dusts and fumes may be achieved. CLASSIFICATION OF SYSTEMS There are two general arrangements, the central and the group systems. In the central system a single or double fan is located near the center of the shop with a piping system radiating to the various machines to be served. In the group system, which is sometimes employed where the machines to be served are widely scattered, small individual exhaust fans are located at the center of the machine groups. The group arrangement has the advantage of flexibility. Exhaust systems are also classified by the means employed to collect dust or other material handled. The dust or refuse may be collected and controlled by enclosing hoods, open hoods, inward air leakage, or by exhausting the general air of the room. With some classes of machinery it is not feasible to closely hood the machines and in these cases open hoods over or adjacent to the machines are provided to collect as much as possible of the dust and fumes. This class includes such machines as rubber mills, package filling machinery, sand blast, crushers, forges, pickling tanks, melting furnaces, and the unloading points of various types of conveyors. The open hoods should be placed as close to the source of dust or fumes as possible, with due regard to the movements of the operator. When the hood must be placed at some distance above the machine it should be large enough to encompass an area of considerable extent as diffusion is usually quite rapid. Consideration must also be given to the natural movement of the fumes, l'or those that are lighter than air the hood should be over or above the machine and where a heavy vapor or dust-laden air at ordinary temperature is to be removed, horizontal or floor connections are required. If it is attempted to remove heavy dust such as lead oxides by an over head hood the conditions may be worse than if no exhaust were used at all, owing to the rising air current carrying the dust up through the 371 American Society of Heating and Ventilating Engineers Guide, 1936 breathing zones. The objective to keep in mind in all cases is to take advantage of the natural tendency of the material to move upward' or downward. In another class of operation the main objective is to prevent the escape of dust into the surrounding atmosphere, the removal of some dust from the machine or enclosure being merely incidental. The dust-creating apparatus is enclosed within a housing which is made as tight as prac ticable, and sufficient suction is applied to the enclosure to maintain an inward air leakage, thus preventing escape of the dust. While the exhaust system is required to handle only the air which leaks in through the crevices and openings in the enclosure, yet in many installations leakages are very high and great care is required to obtain satisfactory results with a system of this kind. The inward-leakage principle is utilized' for . controlling dust in the operating of tumbling barrels, grinding, screening, elevating, and similar processes. Certain dust and fume producing operations are best carried on by isolating the process in a separate compartment or room and then apply ing general ventilation to this space. The compartment or room in which the work is performed should be as small as is consistent with convenience in handling the work. The ventilating system should be designed so that a strong current of clean air is drawn across the operator, and away from him toward the work, where the dust is picked up and carried from the room. GENERAL RULES FOR DESIGN The first step in the design of an exhaust system is to determine the number and size of the hoods and their connections. No general rules, however, can be given since hood and duct dimensions are determined by the characteristics of the operations to which they are applied. When a tentativ e decision regarding the set-up has been made, it is then necessary to obtain the suction and air velocities required to effect control. At this point the designer must rely upon the prevailing practice and on such physical data relating to hoods, duct systems and collectors as are avail able. Finally, in choosing the fan, the area of the intake should be equal to or greater than the sum of the areas of the branch ducts. The speed, of course, must be sufficient to maintain the estimated suction and air velocities in the system. In general, the most important requirements of an efficient exhaust and collecting system are as follows1: 1. Hoods, ducts, fans and collectors should be of adequate size. 2. The air velocities should be sufficient to control and convey the materials collected. 3. The hoods and ducts should not interfere with the operation of a machine or any working part. 4. The system should do the required work with a minimum power consumption. 5. When inflammable dusts and fumes are conveyed, the piping should be provided with an automatic damper in passing through a fire-wall. 6. Ducts and all metal parts should be grounded to-reduce the danger of dust ex plosions by static electricity. 7. The design of an exhaust system should afford easy access to parts for inspection and care. 'For more detailed requirements see Sale Practice Pamphlets Nos. 32 and 37. published by the National Safely Council. Chicago. 372 f';i a> 11 2! ! -li-mi-v I uivi-r Svi iais SU.TION VM) \ M,OCrn HKQUKKMIiVrS 'I he irmm.i! of du~l pi ,-tr l>v means of an exhaust hood requires a :n<'\rmi11'i. of ah a! (he p<>.: i ol origin sufficient to carry it to a colIcrlma si^p-iv. The aii vcinrmos neccssjry to accomplish this depend upon (ho phvsiral properties of (lie material to be eliminated and the Taiuu-: 1 Si/.i-. or ('o.vM.orioNS for Wood-Working Machinery Tyi'K oi Machine Diameter of Connections in Inches C iri. il u -aus, I UiAtr, v . .............................. I fin sawn. 12-2-f in. di.im. . . ........ ............... f iriailar sas, 2M0-m. di.im. ............................ BaIkI SAMS', hlA'Iv lll.lil't 2 'll. W'l'It All' i SAWS.. bi.l'h- o ,t ill \vi4c ,p , .......... ... ... Bird -amn. IiI.nI' 4-1 in. wi'ie . Band '.ms, Mndi |..'i in. wide ,. . ............ ................ .... ......................... Band s.nis. Iil.i<!<` 5-6 in. wide ... .......... ............................... 'sninll iihmIi-its ............................................................ ................... Silicic- oiiil tenom rs .................................................. ................ ................ Double end t< nom-rs .............. . ............................................................... Double end. double, bead tenoners................................ ..................................... Plaueis, matckeis,, moulders, sticker-, jointers, etc.-- Willi knives, (i-IOin . ................................................................ . ........ Willi knives, 10-20 in............ ... ...................................... Willi knives, 20-30 in. .. ........ ..................................... . Shapers, kelit noth-......................... . .................................. Shiper-. benvv u irk................... .................... Ite'i mpi'ci. hell |cs? th.iii !i in. wa; ............................ Beil s.Uld.T . belt 0-10 111. wi le . ................ Bell s.'iiidei. bell 10-1 t in. wide . . . ....................... Dim- Sander, 21 in .......... . . . ... Iinn san<Ii", 30 in . ......... . .. ... . I h .rn sander, 30 in, . . Drum sander, !x in.................. .... ................................. . Di-ini s.indei. out is in. . ...................... D.-e saikIi'i , Of in. chain. .............................., Di-e saiider, 26-36 in. iliam.., ............................... i Disc snider, 36~IN in. di.iai. . ..................................... Arm sin Vr . ... .......... . ........................ . - 4 5 6 4 5 6 7 8 6 6 7 10 5-6 6-8 6-10 4-5 8 5 6 7 5 6 7 8 10 5 6 7 4 direction and speed u ilh which if if. thrown off. If the dust to be removed i- already in motion, as is the case with high-speed grinding wheels, the h io'I should he. installed in the path of the particles so that a minimum aii -,'du-uo niav !-e used effectively. it is always desirable to design and locale a hood so lliat the volume of air necessary to produce results is as small as possible. I'ho si 'De suciioi: at the (` f a hood is frequently used in practice as a `".easu' e of iho effective com -ol. 'I'llis is of considerable \ aiue where e-hauM m. stems adapt eel to particular operations have been stat'd.i|.-l-/rd !>y pi art ice. T mbm 1 and 2 ptesent the duct sizes usually ei'pb-'.c.i rc> s(i:id.;id u'.v'i <v 'rhiug machinery and for grinding and Hili'mi: wheel-. Static piCF-me- which in practice have been found i't `es-saiv to ef niiol and convex \ -minus materials, are given in Table 3. I1 must be remembered, however, that the suction is merely a rough American Society of Heating and Ventilating Engineers Guide, 1936 Table 2. Size op Connections for Grinding and Buffing Wheels DlAHSTSft or Wkbels Grinding-- 6 in. or less, not over 1 in. thick._____ .......... .... 7 in. to 9 in., inclusive, not over 1A in. thick___ 10 in. to 16 in., " " " 2 in. ...... 17 in. to 19 in., " " " 3 in* a ___ 20 in. to 24 in., " " " 4 in. " ___ 25 in. to 30 in., " " " ' 5 in. " ___ Max. Grinding StlRFAC* So In. 19 43 101 180 302 472 Mm. DtAu. or Branch Pipes m Incurs 3 3A 4 m 5 6 Buffing-- 6 in. or less, not over 1 in. thick.......... 7 in. to 12 in., inclusive, not over \A in. thick___ 13 in. to 16 in., " " " 2 in. " ...... 17 in. to 20 in., " " " 3 in. " ...... 21 in. to 27 in., " " " 4 in. a ...... 27 in. to 33 in., " " " 5 in. " ...... 19 57 101 189 338 518 m 4 m 5 6 7 measure of the air volume handled and consequently of the air velocity at the opening of the hood. The elimination of any dusty condition requires added information concerning the shape, size and location of the hood used with regard to the operation in question. In some states grinding, polishing and buffing wheels are subject to regulation by codes. The static suction requirements, which range from V/2 to 5 in. water displacement in a U-tube, should be followed although in several instances they may appear to be excessive. Frequently, in these operations, a large part of the wheel must be exposed and the dust laden air within the hood is thrown outward by the centrifugal action of the wheel, thus counteracting useful inward draft. This tendency may be diminished by locating the connecting duct so as to create an air flow of not less than 200 fpm about the lower rim of the wheel. Exact determinations of hood control velocities are not available, but it is safe to assume that for most dusty operations they should not be less Table 3. Suction Pressures Required at Hoods Tipk or Installation Static Section in Inches or Water Exhausting from wood-working machinery--light duty..... ................... Exhausting from wood-working machinery--heavy duty................... Conveying bulkv and heavy materials.. ................................................ i .374 1J4-5 2 2 2-4' 2-3 2 2 2 2-4 2-3 2-32 3-5 21 -I sni.`M it:Kxiiaust Systems Ilian 200 f;>ni al llv point of origin. For granite dust generated by pivumaUc devii-os, Hatch ot al-give velooitics from 150 to 200 fpm, depending on the type of hood used, as sufficient for safe control. Con sidering the character of the industry, air velocities of this order may be expended to similai dusty operations. The method for approximately determining t!u"-c velocities m (onus of the velocity at the hood opening is given below. HOODS No sel rule, can be given regarding the shape of a hood for a particular operation, hut i( is well to remembci that its essential function is to create an adequate velocity distribution. The fact that the zone of greatest effectiveness does not extend laterally from the edges of the opening may frequently be utilized in estimating the size of hood required. Where complete enclosure of a dusty operation is contemplated, it is desirable to leave enough free, space to equal the area of the connecting duct. Hoods for grinding, polishing and buffing should lit closely, but at the same time should provide an easy means for changing the wheels. It is advisable to design these hoods with a removable hopper at the base to capture the heavy dusts and articles dropped by the operator. Such provisions are of assistance in keeping the ducts clear. Air volumes used to control many dust discharges may often be reduced by effective baffling or partial enclosure of an operation. This procedure is strongly urged where dusts arc directed beyond the zone of influence of the hood. Axial Velocity Formula for Hoods \\ lien the normal flow of aii into a hood is unobstructed, the following foimula may be used to dclorminc the air velocity at any point along the avis'1. 'cht'rc V - yefoci'y .il point, ieet per minute. I - ,UCfl of opfoinc. wpi.iic feel. v .. (Ji'U.uho .iiong uxK. fpet. t' " volume ol oil li,nulled, cu'iic fee! per minute. \elocilv f.oMtoiirs i! i^ possible, by u-.o of a spcciafly constructed pitot-tube4 to map on pn;r- of equal velonly in any axial plane located in the field of infi'ir-tvc Ii has lw\ found iba: (he portions of these contours for any In,u! ii lip t'\[irrss( f| a-- poi<~cii!.tge~ of I he velocity al the hood opening mi! ,rr putoly fund ions o1' the ili.qx of ihe hood1. Theodor* Druike'. Philip and CIioaIa Varah P. Control of the Silicosis Hazard in the Hard ! \ Laboratory fkuJv of th* Dcsien of Oust Control Systems for Use with Pneumatic ''iiri'i1 Outline Ton.* (f yurnai of Industrial Hygiene, XII, No 3, March. 1930). l'i if' Hav*. b;- J- Methantcal Engineering. Vol. 55, No. 10. October, \- . !1P-ire ivvc i.o*ul Pxh.usi H <* 5>v ) M Dalla Valle ar.d 1 heodore Hatch (Transactions m , f iiaratof Hoods ,ir.<K*t Suct-on.. dv } M. DallaValle (A.S.H.V.E. Transactions, American Society of Heating and Ventilating Engineers Guide, 1936 Further, the velocity contours are identical for similar hood shapes when the hoods are reduced to the same basis of comparison. These facts are applicable to all hood problems so that when the velocity contour distribution is known, the air flow required can be determined. Fig. 1 shows the contour distribution in two axial planes perpendicular to the sides of a rectangular hood with a side ratio of one-half. The distribu tion shown is identical for all openings with a similar side ratio provided the mapping is as shown in the figure. The contours, of course, are expressed as percentages of the velocity at the opening. Fig. 1. Velocity Contours nor a Rectangular Opening with a Side Ratio of ONE-HaIF. CoNIOURS ARE EXPRESSED AS PERCF.NTAGES OF THE . Velocity at the Opening . Air Flow from Static Readings The volume of air flow through any hood may be determined from the following equation : Q -= 4005 f a VTi (2) Q = volume of air flow, cubic feet per minute, u -- area ul connecting duct, square feet. /.( -- static suction at throat of hood, inches of water. f = onlu-e oi restriction coefficient which varies from 0.0 to 0.0 depending on the ot tie hood. 370 i Cm '-;i s. 21 !ni..-siiua: fi\u m.'i Swi-ms \ i.n.i v.diu ol ` is 0 71. .ill hough Ini- a vv ('l|.`.h.i|ic(| opening a value oi ' in iv Ii'-umi!. 1 l;( l,u(oi " is deli rmiiiod liom the equation : v\ li* < si h< vibuiiv head in the connecting duel. I I-< l< i in .'lulwn iv uni ;i good measure of (lie effectiveness of a hood ui.lfv.}- Mk> ,i>( ,j of (lie opining and (lie location of (lie operation with 0|U le, die li..od ,ue known. Tin-- F clearly indit tiled by Kquation 1 u!;-l .'low' 1 inf ! he vel< il \ al anv (ioiif along the axis v aries inversely a.-ii'- .ee,i of (In opening and the .-.quaie ol the distance. However, this lonnubi (laipied vviih l.qtialion 2 should serve to indicate the velocity condition? to In rspci led when operations are conducted external to the h k| oeuiu;: barge Open Hoods bare'- lioorU, mu h as an- used for electroplating and pickling tanks, slum ! Ik subdivided so (lx area of (he connecting duct is nol less than oik -til*ecuth of the open aria of (lie hood. Frequently, it will be found iK-ceM-ary to btanch the main duct in order to obtain a uniform distri1 ml a n of flow. Canofiy hoods should extend 0 in. laterally from the tank fm every 12-in. elevalion, and wherever possible they should have side and rear aprons so as to pie.vent short circuiting of air from spaces not dbcri'v over (Ik vais or I,inks. In most cases, hoods of this type take advar.'age ol llv natural tendency of the vapors to rise, and air velocities mav 1 kept low. ( toss drafts from open doors or windows disturb the rise die vapors and ilierefore provision must be made for them. The ,i-1 v'-l.is itios roquiied also depend upon (he < liuracter of the vapors given ol!. iv inidr fumes, foi examplr. requiring an air velocity of approxi mate,v 7." Ipm on (he suifate of (he tank and add and steam vapors :eqn- mg vi 'ht ii ii s ;t<. low ,1; *jr, (o 50 fpm. The total volume of air flow necc-'.fv to i.lfain ihfse volodlirs may be approximately determined (rein ' e foliovviig simple ftiimula: <o uror (4) llkerr 0 = total volume of air haivilpil hy hood, dm. P ~ geometer of the tank, feet. .* - -hstvr.ir botvo.en tank and hooM opening, feet. 1 - ,i r ve.or tv drvred along edges ami surface of tank, fpm I aleral F,\liatis( `^-v stems I'l'.eiai r-xh.i mi mi ihod, as d'-vr-hped for eliromiuni plating6, is Mi'a .bit m many ir.st,ru es m prr tert-ire to (he canopy type hoods. II >' -ihod makis use of ihaving .tit and fumes laleially across the top i( v.v- - i !.rik- ;> 'osiotifd dutts .if I iie n.p and extending fully along 'p. ,,r pupe ;.if|o. oi i|-,e i.i: |<v. The slots ,ik 2 in. wide and for effective *-<<' I .'I Hit-', < :*** - ' . ; . j J .ir.fl H ;.:n Wm (l* s Pufolir- Health 177 I American Society of Heating and Ventilating Engineers Guide, 1936 ventilation a 2,000 fpm exhaust air velocity at the slot face is advisable. In addition, the duct should not be required to draw the air laterally for a distance of more than 18 in. and the level of the solution should be kept 6 to 8 in. below the top of the tanks. Flexible Exhaust Systems The flexible exhaust tube method may be advantageously used for removing dust or fumes. Flexible tubes having one end connected to an exhaust system and a slotted hood attached to the other end may be shaped at will to fit in with industrial processes without affecting the ease of operation. Efficient dust or fume removal may be had with use of relatively small exhaust volumes. This type of system may be used on swing grinders, portable grinding wheels, soldering operations, stone cutting, rock drilling, etc. Spray Booths In the design of an efficient spray booth, it is essential to maintain an even distribution of air flow through the opening and about the object being sprayed. While in many instances spraying operations can be performed mechanically in wholly enclosed booths, the volatile vapors may reach injurious or explosive concentrations. At all times the con centrations of these vapors, and particularly those containing benzol, should be kept below 100 parts per million. Spray booth vapors are dangerous (o the health of the worker and care should be taken to mini mire exposure to (hem. It is recommended in the design of spray booths that the exhaust duct be located in a horizontal position slightly below the object sprayed. Stagnant regions within the booth should be carefully avoided or should be provided with exhaust. The air volume should be sufficient to main tain a velocity of 150 to 200 fpm over the open area of the booth, and the vapors may be discharged through a suitable stack to permit dilution, but it is better pi act ice to pass the fumes or vapors through baffle type washers oi scrubbers designed for efficient spray fume removal7. Hoods for Chemical Laboratories Hoods used in chemical laboratories are generally provided with sliding windows which permit positive control of the fumes and vapors evolved by the apparatus. Their design should offer easy access for the installation of chemical equipment and should be well lighted. Air velocities should exceed 50 fpm when the window is opened to its maxi mum height. DUCT .SYSTEM DESIGN The duct system should be large enough to transport the fumes or material without causing serious obstruction to the air flow. It is good practice to proportion the ducts to obtain the desired velocities and suction pressures at the hoods, although in many cases only an approxi mation, to an ideal design is possible. Many exhaust hoods, and par- "Knr a iss. spr.n* booths, see Special Bulletin No. 16, Spray Pointing in Pennsylvania, Depart* ment of I ib*r Anri l.vlustrv, 11126. H.i:risbur<, Pa. 1 mi!N 21 Kuiniriai Exhaust Systems (kiiUiIv those used in hurting and polishing, are connected by short luaiuh pipes In the main duct which renders proportioning impractical. (iotislruct ion The, duct leading from the hoods to the exhaust fan should be con structed of -.licet metal not lighten than is shown in Table 4. The piping should he fiee from dents, fins and projections on which refuse might catch. All permanent circular joints should be lap-jointecJ, riveted and sol dered, and all longitudinal joints either grooved and locked or riveted and soldered. Circular laps should be in the direction of the flow, and piping installed out-of-doors should not have the longitudinal laps at the bottom. Every change in pipe size should be made with an eccentric taper flat on the bottom, the taper to be at least 5 in. long for each inch change in diameter. All pipes passing through roofs should be equipped with collais so arranged as to prevent water leaking into the building. The main trunks and Inanch pipes should be as short and straight as possible, strongly supported, and with the dead ends capped to permit inspection and cleaning. All branch pipes should join the main at an Taui ! ! Gao, 'it Sheet Meiai. to be Used eor Various Duct Diameters DuHETfttt Of I)CCT I Qa.GE Of MlTA!> Hi( oi Olo Kin rO to m U\ -'Nc^r . . . . ....................,........ . ........... -............. ......... ........ ....................................... * .. . , . ................................... ................. . . ..... 24 22 20 IS acute angle, the junction being at the side, or top and never at the bottom of <ho main Ihauch pipes should not join the main pipes at points where the material from one branch would tend to enter the branch on the opposite side of (he main. f'leatmut opening'- having suitable covers should be placed in the main and Inanch pipes -> that every part of the system can be easily reached in rase the system r'ogs. Either a large cleanout door should be placed in the main suctk n pipe near the fan inlet, or a detachable section of pipe, held in pla^e by 'eg hands, mav lie provided. Vh'ow- <-h mid 1 e tv.i !< a* lea^t I wo gages heavier than straight pipe of the -amr diai'.clci, tin- hr Iter to enable (hem to withstand the addi tional went chiltI by changing the diieolion of flow. They should prefeiab!\ have a 'hr< ,A radius of at le<i?t one ar.d one-half times the diameter of the pipe. Every pipe should be kept open and unobstructed throughout its entire iengili. and no fixed ^ccen should be placed in it, although the use of a hap at the junction < f the hood and branch pipe is permissible, provided it is not allowed till up completely. The passing o; pq (brough fire-walls should be avoided wherever possible, and sweep.up < oncer i ions should be so arranged that foreign material cannot I e ca-.ily introduced into them. liT'i American Society of Heating and Ventilating Engineers Guidb, 1936 Table 5. Air Speeds in Ducts Necessary to Convey Various Materials Material Am Velocities (rw) 2000 3000 2000' 2000 2000 1500 2200 5000 4000 4000 ' At the point of entrance of a branch pipe with the main duct, there should be an increase in the latter equal to their sum. Some state codes specify that the combined area be increased by 25 per cent. While this is not always necessary and is frequently done at the expense of a reduced air velocity, it is none the less advisable where future expansion of the exhaust system is contemplated. Air Velocities in Ducts When the static suction has been fixed for a given hood, the air velocity in the duct may be determined from Equation 2. Air velocities for conveying a material should be moderate. Table 5 gives the velocities generally employed for conveying various substances. Equations 5a and 5b may be used as tests to determine the conveying efficiency of a system8. Velocities determined from these formulae should be increased by at least 25 per cent since they represent the minimum at which a stated size and density of material can be transported. For vertical ducts: V = 13,300 --f~T s+1 (Sa) For horizontal duels: where V = 6000 ---- d0-3* s+1 (5b) V = air velocity in duct, feet per minute. s = specific gravity of particles. d = average diameter of largest particles conveyed, inches. Example 2. Granular material, the largest size of which is approximately 0.37 in. in diameter, with a specific gravity of 1.40 is to be conveyed in a vertical pipe the velocity of the air in which is 4100 fpm; find whether the material can be transported at this velocity. Substitute data in Equation 5a and multiply by 1.25: V = 1.25 X 13,300 X^X 0.37- Antilog (0.57 X log 0.37) = 0.5GS; the required velocity is, therefore, 5500 fpm. DallaVallc. J. M.: Determining Minimum Air Velocities for Exhaust Systems. (A.S.IT.V.E. Journal Section. Heating, Piping and Air Conditioning. September. 1032). 380 Cm m-ter 21 - 1 nousn;iAi. Exhaust Svstp.ms Tabi.k 6. Loss Through 90-Di;g Elbows KlUOW CkRoTCr IIPIn.l'.KNEl)IiO,MNI.C|TSitIN I'bh Ol.Nl 50 10O 150 200 to 500 I l | Loss in J'kii C:nt or Velocity Head 75 26 17 14 Hence, the duct velocity must be increased either by speeding up the fan or decreasing the diameter of the duct, or both. Duct Resistance The resistance to flow in any galvanized duct riveted and soldered at the joints mat- he obtained from Fig. 3, Chapter 20. The pressure drop through elbows depends upon the radius of the bend. For elbows whose cenferlinc radii vary from oO to 300 per cent of pipe diameter, the loss may be estimated from Table 6. ft is sometimes convenient to express the resistance of an elbow in terms of an equivalent length of duct of the same diameter. Thus with a throat radius equal to the pipe diameter the resistance is equivalent to a section of straight pipe approximately 10 diameters long, while with a throat diameter radius 1J4 times the dia meter, the resistance is about the same as that of seven diameters of straight pipe. COLLECTORS The most common method of separating the dust and other materials from the air is to pass the mixture through a centrifugal or cyclone collector. In this type of collector the mixture of the air and material is introduced on a tangent, near the cylindrical top of the collector, and the whirling motion sets up a centrifugal action causing the compara tively heavy materials suspended in the air to be thrown against the side of the separator, from which position they spiral down to the tail piece, while the air escapes through the stack at the center of the collector. nWOTSTTC4 The diameter of the cyclone should be at least 3)4 times the diameter _ of the (an discharge duct. When two or more separate ducts enter a : cyclone, gales should be provided to prevent any back draft through a i system which may not be operating. Cyclones working in conjunction with two or more fans should be designed to operate efficiently at twothirds capacity rating. The following formula is useful in computing the loss through a cyclone when the velocity of the air in the fan discharge duct is known ;. ^-(i)2 (6) where tic - the pressure drop through the cyclone, inches of water. E " the air velocity in the fan discharge duct, feet per minute. If a cyclone is used to collect light dusts such as buffing wheel dusts, 381 American Society of Heating and Ventilating Engineers Guide, 1936 feathers and lint, the exhaust vent should be large enough to permit an air velocity of 200 to 500 fpm. This will, of course, require a cyclone of larger dimensions than given for the foregoing general case. When a high collection efficiency is desired, or the material is very fine, multicyclones may be used. These are merely small cyclones arranged in parallel which utilize the principle of high, centrifugal velocity to attain separation. The capacities and characteristics of this type of separator should be obtained from the manufacturers. Cloth Filters Filters are used when the material collected by an exhaust .system is valuable or cannot be separated efficiently from the air with an ordinary cyclone. They are also employed when it is desirable to recirculate the air drawn from a room by the exhaust system, which otherwise might entail considerable loss in heat. Bag filters which are properly housed may be operated under suction. Bag houses used in the manufacture of zinc oxide and other chemical products are operated on the positive side of the fan. Wool, cotton and asbestos cloths are commonly used as filtering mediums. When woolen cloths are employed, the filtering capacities vary from H to 10 cfm per square foot of filtering surface, depending on the character of the material collected. The rates for cotton and asbestos cloths are lower. The type of filter cloth and the' rates of filtration depend, of course, on the material to be collected and the fan capacity. The time increase of resistance varies with the amount of material per mitted to build up on the surface of the filter and can be determined only by experiment. The limits of the increase may be regulated by adjust ment of the shaking or cleaning mechanism. These limits may be regulated further according to the capacity of the fan and the effective performance of the hoods and the duct system. For additional information on Dust and Cinders, see Chapter 15, Air Pollution, p. 293. RESISTANCE OF SYSTEM The maintained resistance of the exhaust system is composed of three factors: (1) loss through the hoods, (2) collector drop, and (3) friction drop in the pipes. The loss through the hoods is usually assumed to be equal to the suction maintained at the hoods. The collector drop in inches of water is given approximately by Equation 6, but where possible the resistance of the particular collector to be used should be ascertained from the manu facturer. Friction drop in the pipes must be computed for each section where there is a change in area or in velocity. Find the velocities in each section of pipe starting with the branch most remote from the fan. The friction drop for these sections can be determined by reference to Table 6. Total friction loss in the piping system is the friction drop in the most remote branch plus the drop in the various sections of the main, plus the drop in the discharge pipe. Chapter 21 - Industrial Exhaust Systems EFFICIENCY OF EXHAUST SYSTEMS The efficiency of an exhaust swem depends upon its effectiveness in reducing the concentration of dust<5. fumes, vapors and gases below the safe or threshold limits9. Too much emphasis cannot be placed on the necessity of testing exhaust systems frequently by determining the concentration of atmospheric con tamination at the worker's breathing level. Commonly accepted values of threshold limits for the usual gases and vapors are given in Tabic 7. SELECTION OF FANS AND MOTORS Manufacturers generally provide special fans for the collection of various industrial wastes. These are available for the collection of coal dust, wood shavings, wool, cotton and many other substances. For Taolu 7. Threshold Limits or Common Vapors and Gases3 St BSTANCE 1 1 Spec. Gim\ . INFLAMMABLE { OP C.UJ OR } 1 \apur(AirI) i Limits (%> PnTSTOLOOlCAL Action Maximum Allowable Concentration (ppm) Chlorine....................... ,j Ozone. . . Hvdrogen chloride ,. 1 Sulphur dioxide . .. ! Carbon monoxide . . . Hvdroger. sulphide* .. , 1 Ren/ene . . . Methanol . Carbon tetrachloride ... } I 2.4S0 5.0 1.2078 2.21)38 0.9071 1.190 2 73 1.1 0.3 i non-inflamm. 1 do ! do . do , 12. .5-74 | 4.3-46 ! l. 1-7.0 l 7.5-26.5 j nop infiamm. i irritant do do do asphyxiant do anesthetic do do 0.35 0.80 10.0 10.0 100.0 85-130 100.0 100.0 100.0 Taken from The I'leveiUion of Occupational th/tiucol P.nRtnteriHfi. Vo! o?. l\'o. 1. April, I93o). by R. R. Sayers and J. M. DallaValle (We* particular features concerning special fans, consult the Catalog Data Section of Thf. Guide and manufacturers' data. When substances having an abrasive character are conveyed, the fan blades and housing should be protecied from wear. This may be accomplished by placing a collector on the negative side of the fan or by lining the housing and blades with rubber. if no future expansion of an exhaust system is contemplated, the fan motor should be chosen to provide the calculated air volume. Should, however, the exhaust system be required to handle more air in the future, the motor should be adequate for the maximum load anticipated. Further information regarding the choice of fans and motors is given in Chapters 17 and 42. PROTECTION AGAINST CORROSION The removal of gases and fumes in many chemical plants requires that metals used in the construction of the exhaust system be resistant to 'Criteria for Industrial Exliaust Systems, by J J Bloomfield (A.S.H.V.E. Transactions. VoI. 40, 1934). 383 Rubber lining or chrome-nickel alloys Aluminum coated iron, aluminum, high chrome-nickel alloy* 1 ron or steel High chrome-nickel alloys Rubber lining, chrome-nickel alloys Nickel-chrome alloys ^Condensed from data given by Chilton and Huey (Industrial and Engtneerint Chemistry, Vol. 24,1233), chemical corrosion. A list of the materials which may be used to resist the action of certain fumes is given in Table 8. Hoods and ducts when short, may frequently be constructed of wood and be quite effective. Rubberized paints are available and may be applied as protective coatings in handling such gases and fumes as chlorine and hydrochloric acid. PROBLEMS IN PRACTICE What determines the efficiency of an exhaust system? It is dependent upon its effectiveness in reducing the concentration of dust, fumes, vapors and gases below the safe or threshold limit. ' 2 Are state regulatory requirements as to suction applicable to all sorts of dust collecting installations? . As a rule the regulations refer only to grinding wheel and buffing wheel systems. 3 What is the most common method of reducing total air volumes handled in cases employing large hoods over apparatus covering a large area? The use of the petticoats on large hoods which permits a comparatively high air velocit\ at the rim of the hood and controllably small velocities in the center. 4 What other types of collectors are available for use in the place of cyclonei and filters when chemical and physical conditions obviate the possibility of th< use of them? Devices such as scrubbers and contactors, using water or other contacting liquids, am electrical precipitators. 5 What is the most frequent error made in dust collector system design? The omission of some means of putting into the workroom air having the proper charac teristics to replace that which has been exhausted. 6 Are there available means for testing the performance of dust' collectin systems when they are required to meet high industrial hygienic standards Yes. Such means are set up by the United States Public Health Service'and by th Standard Code, for Testing and Rating Air Cleaning Devices Used in General Vent Intion Work. (Chapter 44). 7 Why is it not permissible to connect up emery wheels and buffing wheels t the same exhaust system? 3KI CHAI'!ER 21 -"-InOUSI K1AI Hxil A VST SYSTEMS Emery wheels and buffing wheels should he handled hy separate systems because of the (ire hazard, as it is possible foi sparks from the emery wheels to ignite the lint and dust from the bulling wheels when both are carried through the same system. 8 Give an important characteristic of centrifugal type dust collectors which should be given consideration when applying this type of collector to instal lations requiring high separating eflicieucics. The separating action of a cyclone or centrifugal type collector depends largely on centrifugal force Reducing the radius of air flow increases the centrifugal force for a given velocity of flow. Accordingly, the smaller size units usually give higher separating factors, and better results can sometimes be obtained by using a number of small col lectors instead of one large unit. 9 Mention some general suggestions relating to the design of efficient in dustrial exhaust systems. a. Endeavor to obtain a maximum degree of effectiveness with a minimum volume of air, by the useol well designed hoods closing in the sources of fumes or material to be removed so located as to take advantage of the natural direction taken by the fumes or materials when leaving their source. b. Give particular care to the velocity of flow. The duct velocities for material con veying systems must be high enough to properly carry/ the material, but they should not be higher than necessary because excessive velocities increase the pressure requirements and result in a waste ol power. e. Select the type of fan best suited to the job. For installations where stringy material is handled do not use a fan wheel which has a shroud. d. When handling the refuse from various machines, study the grouping and operating cycles of the machines. Connecting a large number of machines into one system is frequently very uneconomical. z. Avoid unnecessary distances and bends in laying out the piping system. 10 Tbc static pressure measured at the throat of a buffing wheel hood is 2 in. and the velocity head measured with a Pitot tube is 1,6 in. Calculate the restriction coefficient f. From Equation 2, V = 4005/ a/ ht. From the theory of air flow, V = 4005 v/ hv. Hence, \/ hv = / \/ ht. or Vf = 0.89 11 A tank, 4- ft by 8 ft, contains a fluid which gives off injurious vapors. A targe hood is located 30 in. above the top of the tank and extends slightly over tj* dges Assuming that a velocity of 60 fpm is required to adequately control the vapors uear the edges of the tank* calculate the air flow required. U*ing Equation 4l/3=2x4+2X8=-2ift;> = 30 inches = 2.5 ft: V = 60 fpm. Hence, Q 7= 1.4 X 24 X 2.5 X 60 = 5040 cfm. 12 Silica dust with a specific gravity of 2.65 is being conveyed in a duct system. velocity measured in a vertical portion of the system is found to be 2700 pm. M&hat is tbc maximum diameter particle transported at this velocity? 2 05 Using Equation 5a, 2700 - 13,300 X o.Oo X <f0-S7v from which d = (0.2811*75 = 0.11 in. ' 385