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Guide, 1934 An Instrument of Service prepared for the Profession-- and Containing - reference data . on the design and specification of heating ' and ventilating systems-- Based on the Transactions--the Investigations of the Research Laboratory and Cooperating Institutions-- ' and tNe Practice of the Members and Friends o'f 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 Vol. 12 $5.00 Per Copy Published Annually by American Society of Heating and "Ventilating Engineers 51 Madison Avenue New York 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 TH IS 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. SOURCES OF AIR POLLUTION 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). Oust, 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 laigcr 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. American Society of Heating and. Ventilating Engineers Guide, 1934 Fly-Ash, Cinders The term fly-ash is usually applied to the extremely small particles of ash, and the term cinder to the larger particles of coke and ash which are discharged with the gases of combustion from burning coal. EFFECTS OF AIR POLLUTION ON 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 epithelium lining in the bronchial tubes intercept many of the dust particles before they reach the lungs. PULMONARY EFFECTS 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 phagocyted 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 noncolloidal 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- CiiAi'TP.it l.i Aik Piii.i.i'TiON stiltiies another important source of aerial pollution in busy cities. Ouring heavy traffic hours and under atmospheric conditions favorable to concentration, the air of congested streets is found to contain enough CO 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 Baltimore1 by actinic 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 this pollution have seemed justifiable to the public. INDUSTRIAL AIR POLLUTION 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 dusts 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. For a more thorough treatise on the subject reference should be made to books by Hamilton2, Rosenau*. and Henderson and Haggard5. la if Miwvjulurw Rotlutnm upon Itn id^ftce of Solar Ultra-Violet Light, by J. H. Shrader. M. H. Cob/enu and P A Korfl (-1 mencan Journal of Publu Health, p 7. Vol. 19. 1929). 'Studies in Illutumation. by J. E. Ives (V. 3. Publtc Health Service Bulletin No. 197, 1930). 'Industrial Pensions in the United States, hy Alice Hamilton. '1'ieventive Medicine and Hyg.cne. by M.Hon J. Rosenau. 'N'-ixi us (as<*<. by V Henderson and II. Haggard. 209 American Society of Heating and Ventilating Engineers Guide, 1934 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 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 posioning 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 blood 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, rheumantic pains, and, in extreme conditions, paralysis, blindness, insanity, and death. It has been found5 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 groups'. 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. toad Poisoning, by Thomas Morrison Legge {Journal Royal Society Arts, 1929. Vol. 77. p. 1023). What is a Dangerous Quantity of Lead Dust in Air, by C. M. Sails (Industrial Hygiene Bulletin, New York State Department of Labor, 1925). 210 Chapter IS--Air Poli-CIion 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 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' routine measurement, and study of the concentration and the physical and chemical characteristics of the dusts to which those individuals are exposed. ABATEMENT OF ATMOSPHERIC 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 the most serious, limited areas obtain some relief by the use of district heating. The boilers in these plants are of large size designed and oper ated to burn the fuel without smoke, and some of them are equipped with dust catching devices. The gases of combustion are usually discharged at a much higher level than is possible in the case of buildings that operate their own boiler plants. SMOKE ABATEMENT In general, time, temperature and turbulence are the essential require ments for smokeless combustion. Anything that can be done to increase any one of these factors will reduce the quantity of smoke discharged. Kspecial care must be taken in hand-firing bituminous coals. (See Chapter 27). Checker or alternate firing, in which the fuel is fired alternately on separate parts of the grate, maintains a higher furnace temperature and 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 passover the hot fuel bed where they will be burned if they are mixed with sullirient air and are not cooled toe quickly by the heat-absorbing surfaces of the boiler. Steam or rompressed 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 211 Ami.ru an Society of Heating and Ventilating Engineers Guide, 1934 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 Tadi.e 1. Toxicity of Gases and Fumes in Parts per 10,000 Parts of Air-1 Vapor or Gas Rapidlt Fatal Maximum Concentration POR PROW H to l Hour Maximum Concentration por ! Hock Maximum Allowable por Prolonosd EXPOOORE Carbon monoxide.............. Carbon dioxide.................. Hydrocyanic acid.......... . Ammonia............................ Hydrochloric acid gas Chlorine............................. Hydrofluoric acid gas....... Sulphur dioxide................ Hydrogen sulphide.......... Carbon bisulphide............ Phosphene........................... Arsine......... .......'................. Phosgene............................ Nitrous fumes................... Benzene..................... ......... Toluene and xylene........ Aniline..................... .......... Nitrobenzene..................... Petrol.................................. Carbon tetrachloride........ Chloroform......................... Tetrachlorethanc............... Trichlorethylene................ Methyl chloride................ Methyl bromide................ Lead vapor......................... 40 800 1000 30 50-100 10-20 10 2 4-5 10-30 20 2H Over M 2 Mr/ M 190 190 243 480 250 73 370 1500-3000 200-400 15-20 m 25 M M. fo 5-7 11 4-6 M M l-l K 100-220 240 140 200-400 20-40 10 H 3 2-3 5 1-2 31-47 31-47 1-1M Moo 40 50 70 10 l M l Mo Moo Mo Mo Ml Moo M 1M-S Mo Hoo 16 2 m 5-10 2 5-6 "Original data compiled by Y. Henderson and H. Haggard. (See Noxious Cases, 1927). Data revised bv T. M, Legge. (Sec Lessons Learned front Industrial Cases and Fumes, Institute of Chemistry of Great Britain and Ireland. London, 1930). 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 212 Chapter 15--Air Hoilltiox 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 40), 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 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 CINDER ABATEMENT 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 the following 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 coal has been removed will not as a general rule produce as much dust and cinders as will result from the burning of non-coking 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. 213 American Society of Heating and Ventilating Engineers Guide, 1934 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. 4. Electrostatic precipitators. 5. Gas scrubbers. 6. Fabric filters. The selection of the proper type of catcher calls for a careful study of the material to be caught and the draft and space available. After installation, constant vigilance is necessary to keep the catchers in proper working condition if satisfactory operation is to be obtained. If possible, the dust or cinder catcher should be installed on the inlet side of the induced draft fans because the dust and cinders in the gases seriously erode the wheels of the fans, the inlet connections and the scrolls. Where the induced draft fans operate at high tip speeds and no 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 advantage of low pressure drop, relatively small space requirements, and low first cost. `See Smoke and Dust Abatement, by M. D. Engle (A.S.H.V.E, Transactions, Vol. 37.1031). Chaimrr 1.5--Air Pou.ition 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 unidirectional 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 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 50 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 215 American Societv of Heating and Ventilating Engineers Glide, 1934 peak load and will materially increase the capacity of the fuel-burning equipment for the boiler to which it is connected. N'o 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 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. 216 Chapter 16 AIR CLEANING EQUIPMENT Reguirctnett ts of an Air Cleaner, Types, Air Washers and Scrubbers, Viscous Type Fillers, Dry Air Filters. Air Filler Installations IR cleaning devices are intended lo remove impurities in air brought into a building for ventilating or air conditioning purposes. 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.............. Metropolitan districts .... Industrial districts .............0.2 to 0.4 grains per 1000 cu ft .......................0.4 to 0.8 grains per 1000 cu ft .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. ;i. Have a low frictional resistance to air flow, that is, the pressure drop across the filter, measuied 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 lo operate so as to keep the resistance constant automatically. 5. Be easy to clean and handle, or clean itself automatically. 6. I.eave the air passing through the cleaner free from entrained moisture or charging liquids used in the cleaner. 217 Amkrhan Soitlh of Heating and Ventilating Engineers Ginde, 1934 The A.S.H.V.E. Standard Code for Testing and Rating Air Cleaning Devices I'sed in General Ventilating Work1 explains how such devices are rated by (1) capacity in cubic feet of air handled per minute, (2) resistance OtAM | Of PAR[TtatsJ IN iMMWSf 5CALE OF ATMOSPHERIC IMPURITIES RATE Of (NUMBER Surface I lOKC <0/1scttuhg| OF PARIticus m Apia m SOUAAt IM f.P.M INCHES LAW5 OF SETTLING IN RELATION TO FO{SRPHEREAS0I^0R04C0ORNATAININSINOGF PARTICCE SIZE OF (OCNJirt l|.|cIHuf.tFNi.U(oTcINtftmFI*R0 {UWt* OF OCHAACATION AP*0.) | AT TO* f. PARTICLES pall with IHCREASWG VELOCtTV fisr* mm? E a afe- 5 --d PARTICLES MOVE LIKE OAS MOLECULES Oiatart o( | in time t ilSxIO'NSTfcs.oOTtTf q1 W'2^HAi |R"Go* constant) 6.316 x I01 Ifljr (T TAebmspoelurateture | [hi* Humber of Oojj hsMOl,|l molecules in ontmolL06*10*>l Compiled by W. G. Frank and Copyrighted. Fig. t. 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 `Submitted at the Semi-Annual Meeting of the A.S.H.V.E., June. 1933. 218 Charier 16--Air Cleaning Kocipviknt 'an automatic air cleaning device, and (5) dust holding capacity, the amount bv weight of standard dust which a non-automatic air cleaning '-device will retain before reconditioning is necessary. TYPES OF AIR CLEANERS According to the (`ode, the following four classifications are given the devices: Class A. Automatic Typo: 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- Lou' Resistance N'on-A ulontatic Type: Air cleaning devices for warm air furnaces, unit ventilating machines and similar apparatus and installations in which a maximum of not more than 0.1S 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. .Von-Automatic Type: Air cleaning devices for the air intake of compressors, internal combustion engines, and the like, where a pressure of t.O 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, 0) Unit type. (2) Automatic type. c. Dry air filters. 2. According to application. a. For central fan systems of ventilation and air conditioning. Kilters 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 tvpe 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 ns soot or carbon, which are unaffected by an air washer type of air cleaner, have increased. This has brought about the development of the v iscous and dry type air filters which arc pa-t of many ventilating and ait conditioning systems. AIR WASHERS AND SCRUBBERS Information on air washers will be found in Chapter II. Scrubbers have not been used very extensively in the past for cleaning 219 1 American Society of Heating and Ventilating Engineers Guide, 1934 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 arrangement 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 progressive 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- 220 Chapter 10 -Air Cleaning Equipment 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. ;5. Resistance to Air-Flow of a Typical Fnit Air Filter of air being cleaned, and on the amount of dirt which can be accumulated in Lhe filter medium without causing excessive resistance. 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 urjits 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, A 221 American Society of Heating and Ventilating Engineers Guide, 1934 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 where 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 rccoating 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 222 Chai-tlk Hi- Air (.'i.cvninc. livcir'iGM' 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. AIR FILTER INSTALLATIONS 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 size for the amount of air it is expected to handle. An overload of 10 to 15 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 [or 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. All doors on the clean air side should be lined with felt to prevent infiltration of unclean air. All connections and seams of the sheet metal ducts on the clean air side should be as air-tight as possible. 223 Chapter 21 INDUSTRIAL EXHAUST SYSTEMS Types, Design of Systems, Suction and Velocity Requirements, Design of Hoods, Design of Duct Systems, Collectors, Resistance of Systems, Selection of Fans and Motors EXHAl'ST and collecting systems are found in almost every industry and are a vita! adjunct in maintaining safe and hygienic conditions. 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. TYPES 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. For 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 Ami'KU'an Sni >ktv of Hr.ATtMr. ami Vkntii.atikc; Encikkkrs Guide, 1934 all, owing to the rising air current carrying the dust up through the 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 only required to handle 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. DESIGN OF SYSTEMS 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 tentative 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. `For more detailed requirements see Safe Practice Pamphlets Nos. 32 and 37, published by the National Safety Council. Chicago. 280 Ompipr 21--Industrial Exhaust SvstruN 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. SUCTION AND VELOCITY REQUIREMENTS The removal of dust or waste by means of an exhaust hood requires a movement of air at the point of origin sufficient to carry them to a col lecting system. The air velocities necessary to accomplish this depend upon the physical properties of the material to be eliminated and the Table 1. Size of Connections for Wood-Working Machinery Type of Machine Diameter op Connections in Inches Circular Saws, 12-in. diam..,,............................................... Circular Saws, 12-24-in. diam............................................. Circular Saws, 24-40-in. diam............................... ............. Band Saws, Blade under 2 in. wide--................................ . Band Saws, Blade 2-3 in. wide .......................................... Band Saws, Blade 3-4 in. wide............................................ Band Saws, Blade 4-5 in. wide................. ,,....................... Band Saws, Blade 5-6 in. wide ......................................... Small Mortiscrs.............................. .............................. ........ Single End Tenoners............................................................. Double End Tenoners ........................................................ Double End. Double Head Tenoners................................ Planers, Matchers, Moulders, Stickers, Jointers, etc.-- With Knives, 6-10 in. _.............................. ............... With Knives, 10-20 in...... .................................. With Knives, 20-30 in...................... ........................... Shapers, Light Work............................................................ Shapers, Heavy Work.................................................... ..... Belt Sander, Belt less than 6 in. wide........ ...................... . Belt Sander, Belt 6-10 in. wide............................,........... Belt Sander, Belt 10-14 in. wide...................................... Drum Sander, 24 in.....-.............. ........... .............................. Drum Sander, 30 in...... .................................................. . , Drum Sander, 36 in.....,,.................... ................................... Drum Sander, 48 in................ .. fc............... ...... ............. Drum Sander, over 48 in........................... .. .................. Disc Sander, 24 in. diam................... ................... ........... Disc Sander, 26-36 in. diam............ ........................... ........ Disc Sander, 36-48 in. diam______ _____ ,,____ _______ Arm Sander............................................................. ........... direction and speed with which it is thrown off. If the dust to be removed is already in motion, as is the case with high-speed grinding wheels, the hood should be installed in the path of the particles so that a minimum air volume may be used effectively. It is always desirable to design and locate a hood so that the volume of air necessary to produce results is as small as possible. The static suction at the throat of a hood is frequently used in practice as a measure of the effectiveness of control. This is of considerable value where exhaust, systems adapted to particular operations have been standardized by practice. Tables 1 and 2 present the duct sizes usually American Socii.i \ of Heating and Ventilating Engineers Guide, 1934 employed for standard wood-working machinery and for grinding and buffing wheels. Static pressures which in practice have been found necessary to control and convey various materials, are given in Table 3. it must be remembered, however, that the term suction is merely a rough Table 2. Size of Connections for Grinding and Buffing Wheels Diameter of Wheels Grinding-- 6 in. or less, not over 1 in. thick......... 7 in. to 9 in., inclusive, not over 1 *A in. thick..... 10 in. to 16 in., u " " 2 in. M ...... 17 in. to 19 in., 20 in. to 24 in., u " " u "*43 in. " ...... in. u ...... 25 in. to 30 in., " " " 5 in. u ...... Buffing-- 6 in. or less, not over 1 tn. thick......... 7 in. to 12 in., inclusive not over lH in. thick...... 13 in. to 16 in., " " " 2 in. " ...... 17 in. to 20 in., " " " 3 in. " ...... 21 in. to 27 in., " " 4 in. u ...... 27 in. to 33 in., " " " 5 in. u ...... Max. GSruirnfdaicneg Sq tN 19 43 101 180 302 472 19 57 101 189 338 518 oMfinB. rDaniacmh. Pipes in Inches 3 3H 4 4J4 5 6 314 4 4J4 5 6 7 Tablf, 3. Suction Pressures Required at Hoods Static Soction in Inches or Water Exhausting from pottery processes.............................................. .. ........... IK-5 2 2 2-4 2-3 2 2 2 2-4 2-3 2-3 2 3-5 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 1)4 to 5 in. water displacement in a tf-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- 282 Cihpti:r '21 --Industrial, Exhaust S\$tkms 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 than 200 fpm at the point of origin. For granite dust generated by pneumatic devices, Hatch2 gives velocities from 150 to 200 fpm, depending on the type of hood used, as sufficient for safe control. Considering the character of the industry, air velocities of this order may be extended to similar dusty operations. The method for approximately determining these velocities in terms of the velocity at the hood opening is given below. DESIGN OF HOODS No set rule can be given regarding the shape of a hood for a particular operation, but it is well to remember 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 fit 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 are directed beyond the zone of influence of the hood. Axial Velocity Formula for Hoods When the normal flow of air into a hood is unobstructed, the following formula may be used to determine the air velocity at any point along the axis: V _ (U.4 100 V ' x1 (1> where V = per cent of yelocity at opening. A = area of opening in square inches (or square feet). x = distance outward from opening in inches (or feet). It is important to note that the velocity function varies in direct proportion to the area. Hence, under certain conditions, a large opening may function more effectively than a small one for the same volume of ^Control of the Silicosis Hazard in the Hard Rock Industries. (Journal of Industrial Hygiene, Vol. XII. No. 3. March, 1930). American Society of Heating and Ventilating Engineers Guide, 1934 flow. The formula, of course, presumes that the air velocity distribution across the hood opening is uniform3. Example 1. A small liood 64 sq in. in area handles 400 cfm. What will be the air velocity at a point 5 in. outward along the axis if the flow is unobstructed? Solution. Substitute in Equation 1 and solve for Y, thus Y 100 - Y 0.1 X 64 5X5 from which Y - 20.4 per cent of the velocity at the opening of the hood. Velocity at open.ing == -4-0--0-- Xxj-1--4--4- ** 900 f.pm Hence, the velocity at the point in question is 900 X 0.204 -- 184 (pm Air Flow from Static Readings The volume of air flow into any hood may be determined from the following equation: where <3 - 4005 fa Vh, (2) Q -- volume of air flow in cubic feet per minute. a = area of connecting duct in square feet. At = static suction at throat of hood in inches of water. / = orifice or restriction coefficient which varies from 0.6 to 0.9 depending on the shape of the hood. An average value of/is 0.71, although for a well-shaped opening a value of 0.8 may be used. If it is assumed that the entrance loss of a hood is proportional to the velocity head, / can be determined by the relation: (3) where hv = the velocity head. For duct ends and abrupt openings hi = hv and for flared openings hi - 0.5hv. The term sialic suction is not a good measure of the effectiveness of a hood unless the area of the opening and the location of the operation with respect to the hood are known. This is clearly indicated by Equation 1 which shows that the velocity function at any point along the axis varies directly as the area of the opening and inversely as the square of the distance. However, this formula coupled with Equation 2 should serve to indicate the velocity conditions to be expected when operations are conducted external to the hood opening. Large Open Hoods Large hoods, such as used for electroplating and pickling tanks, should be subdivided so that the area of the connecting duct is not less than one- `Velocity Characteristics of Hoods under Suction, by J. M. Dallavalle (A.S.H.V.E. Transaction's Vo!. 38. 1932). 284 ClIAI'II.R J1-- lNDlVTKI.U. EvHAOT SvSTIi.MS fifteenth the open area of the hood. Frequently, it will be found necessary to branch the main duct in order to obtain a uniform distribution of flow. Canopy hoods should extend 6 in. laterally from the tank for every 12-in. elevation. In most cases, hoods of this type take advantage of the natural tendency of the vapors to rise, and air velocities may be kept low. Cross drafts from open doors or windows disturb the rise of the vapors and therefore, provision must be made for them. The air velocities required also depend upon the character of the vapors given off, cyanide fumes, for example, requiring an air velocity of approximately 75 fpm on the surface of the tank and acid and steam vapors requiring velocities as low as 25 to 50 fpm. The total volume of air flow necessary to obtain these velocities may be approximately determined from the following simple formula: Q = VAPDV (4) where Q - total volume of air handled by hood in cfm. P -- perimeter of the tank in feet. D = distance between tank and hood opening in feet. I" > air velocity desired along edges and surface of tank in fpm. 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 benzene, should he kept below 100 ppm. Spray booth vapors are dangerous to the health of the. worker and care should be taken to minimize exposure to them. It is recommended in the design of spray booths that the exhaust duct be located in a horizontal position slightly above the object sprayed. Stagnant regions within the booth should be carefully avoided or should be provided with a vertical exhaust. The air volume should be sufficient to maintain a velocity of 150 to 200 fpm over the open area of the booth and the vapors should be discharged through a suitable stack to permit dilution^ 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. `For a discussion of spray booths, see Special Bulletin No. 16. Spray Painting in Pennsylvania. Depart ment of Labor and Industry. 1926. Harrisburg. Pa. 285 American SoriFTv of Hi.mini. and Ventilating Enginkf.ks Gchie. 1934 DESIGN OF DUCT SYSTEMS The duct system should he 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 ticularly those used in buffing and polishing, are connected by short branch pipes to the main duct which renders proportioning impractical. Construction The ducts leading from the hoods to the exhaust fan should be con structed of sheet metal not lighter than is shown in Table 4. The piping should be free from dents, fins and projections on which refuse might' catch. Table 4. Gage of Sheet Metal to ue Used for Various Duct Diameters orDiameter Duct Gaoc or Metal S in. or less....... 9 to IS in. . 19 to 25 in............ 21) in. or more...... 24 22 20 IS All permanent circular joints should be lap-jointed, 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 oti 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 collars so arranged as to prevent water leaking into the building. The main trunks and branch 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 acute angle, the junction being at the side or top and never at the bottom of the main. Branch pipes should not join the main pipes at points such that the material from one branch tends to enter the branch on the opposite side of the main. Cleanout openings having suitable covers should be ptaced in the main and branch pipes so that every part of the system can be easily reached in case the system clogs. Either a large cleanout door should be placed in the main suction pipe near the fan inlet, or a detachable section of pipe, held in place by lug bands, may be provided. Elbows should be made at least two gages heavier than straight pipe of the same diameter, the better to enable them to withstand the addi tional wear caused by changing the direction of flow. They should pref erably have a throat radius of at least one and'one-half times the diameter of the pipe. Every pipe should be kept open and unobstructed throughout its entire 286 Ciurn-u `21 Inim'stkiai, Sistkvis length, and no fixed screen should be placed in-it, although the use of a trap at the junction of the hood and branch pipe is permissible, provided it is not allowed to fill up completely. The passing of pipes through fire-walls should be avoided wherever possible, and sweep-up connections should be so arranged that foreign material cannot be easily introduced into them. 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. Table 5 Am Speeds in Ducts Necessary to Convey Various Materials Material Grain dust............................................ . Wood chips and shavings........................ Saw dust................................................. Jute dust............... ............................... Rubber dust......... ........... .......... Lint................................................. .... Metal dust (grindings' ........................ Lead dusts............... i.. ..................... brass tui lungs (line)........................... Fine coat.......... Am Velocities {tpm) 2000 3000 2000 2000 2000 1500 2200 5000 4000 4000 Air Velocities in Ducts When the static suction has been fixed for a given hood, the air velocity in the duet may be determined from Kquation 2. Air velocities for conveying a material should be moderate. Table 5 gives the velocities generalU employed for conveying various substances. Equations 5a and 5b may be used as tests to determine the conveying efficiency of a system. Velocities determined from these formulas 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: r = 13,300 s , </"'> s1 I (5a) For horizontal duels:- where. V = fiOOO ? , d'M98 s -t- 1 (5b) V = air velocity in duc(. in feet per minute. i = specific gravity of particles. d = average diameter of largest particles conveyed, in inches. Example . 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. American Society of Heating and Ventilating Engineers Guide, 1934 Substitute data in Equation 5a and multiply by 1.25. V -- 1.25 X 13,300 X 2--.4 X 0.37." Antilog <0.57 X log 0.37) = 0.50S; the required velocity is, therefore, 5500 fpm. Hence, the duct velocity must be increased either by speeding up the fan or decreasing the diameter ol the duct or both. Duct Resistance The resistance to How in any galvanized duct riveted and soldered at the joints may be obtained from Fig. 3, Chapter 19. The pressure drop through elbows depends upon the radius of the bend. For elbows whose centerline radii vary from 50 to 300 percent of pipe diameter, the loss may be estimated from Table 6. It 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 1% times the dia meter, the resistance is about the same as 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. The diameter of the cyclone should be at least 3times the diameter of the fan discharge duct. When two or more separate ducts enter a cyclone, gates should be provided to prevent any back draft through a 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: where kc = the pressure drop through the cyclone in inches of water. V - the air velocity in the fan discharge duct in feet per minute. If a cyclone is used to collect light dusts such as buffing wheel dusts, 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 288 Chapter 21 -Industrial Exhaust Systems separation. The capacities and characteristics of this type of separator should be obtained from the manufacturers. Cloth Filters Filter bags are used when the material collected by an exhaust system is valuable or cannot be separated 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 con siderable 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 bags are employed, the filtering capacities vary from Y 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 slightly 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 lime increase of resistance varies with the amount of material permitted to build up on the surface of the filter and can only be determined by experiment. The limits of the increase may be regulated by adjustment of the shaking or cleaning mechanism. These limits may further be regulated according to the capacity of the fan and the effective performance of the hoods and the duct system. 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 G, 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 G. Total friction loss in the piping system is the ftiction drop in the most remote branch plus the drop in the various sections of the main, plus the drop in the discharge pipe. 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 particular features concerning special fans, consult the Catalog Data Section of '['he Guide and manufacturers' data. When substances having an abrasive, character are conveyed, the fan blades and housing should be protected from wear. This may be accomplished by placing a 290