REFERENCES.
[ [1] Goadby, K. W.: Journ. of Hygiene, vol. ix., 1909.
[ [2] Hunter, John: Observations of Diseases of the Army in Jamaica. London, 1788.
[ [3] Drissole and Tanquerel: Meillère’s Le Saturnisme, p. 164.
[ [4] Hoffmann: Journ. de Méd., October, 1750.
[ [5] Weill and Duplant: Gazette des Hôpitaux, lxxix., 796, 1902.
[ [6] Briquet: Bull. Thérap., Août, 1857.
[ [7] Peyrow: Thèse de Paris, 1891.
[ [8] Stevens: Bulletin of Bureau of Labour, U.S.A., No. 95, p. 138, 1911.
[ [9] Zinn: Berl. Klin. Woch., Nr. 50, 1899.
[10] Serafini: Le Morgagni, No. 11, 1884.
CHAPTER XII
PREVENTIVE MEASURES AGAINST LEAD POISONING
Amount of Lead Fume and Dust in the Atmosphere Breathed.
—Lead fuses at 325° C. and boils at between 1450° and 1,600° C. It is volatile when heated to a cherry-red colour—about 550° C.
Experiments[A] carried out in the laboratory of a lead smelting works in London to determine the temperature at which leady fumes rise from the surface of open baths of molten lead, showed that unless pure lead is heated to about 500° C., and at the same time stirred, no appreciable fume comes off, and that from lead, at the same temperature, under ordinary working conditions, little or no lead in the form of oxide passes into the air. From lead that has been unrefined or which contains zinc—that is, lead in the earlier stages of its manufacture (in the reverberatory furnace)—leady fume was not given off at temperatures less than 760° C. even when stirred, because at a temperature of 600° C. the surface of the molten metal became covered with fluid slag, which will not allow any oxide to be given off. Impurities such as tin or antimony prevent the oxidation of molten lead at lower temperatures, and give it a bright, shiny colour. When heated to about 600° C., these impurities form a slag on the surface of the lead containing antimoniates and stannates of lead, which do not evolve lead fumes unless heated to temperatures never likely to be reached in open lead pots. The reason why molten refined lead can give off lead fume more readily than those named is because the oxide formed on the surface is a dry powder and not in the form of slag. Hence, when the bath is stirred, some of the dry oxide is broken up and may rise into the air. When a bath of molten lead is not stirred at all, it can be heated to over 740° C. without finding oxide in the air aspirated—a temperature not obtained under ordinary working conditions.
[A] In these experiments air was aspirated through an iron funnel having an area of 113 square inches (12 inches diameter), placed at a height of 1¹⁄₂ inches above the molten metal, and connected to an iron tube 3 feet in length and ¹⁄₂ inch in diameter. Inside the iron tube was a glass tube, one end reaching own to the top of the funnel and the other connected with a tube containing pure loose asbestos wool, and continued down to a tightly stoppered bottle holding dilute sulphuric acid. Another glass tube connected this bottle with an aspirator. The asbestos tube was weighed before and after each test, and the asbestos then treated with nitric acid, and the lead determined volumetrically. In none of the tests made was lead found in the bottle containing sulphuric acid.
Were there nothing else to consider but escape of lead fume from a pot or bath of molten metal, obviously hooding over of the bath and removal of the fume from the atmosphere of the workroom would be unnecessary until this temperature was reached. Usually, however, the bath is kept standing exposed to the air, and the oxide which forms on the surface has to be skimmed off periodically, and whenever the ladle is emptied a small cloud of dust arises. Or at times, in certain processes, chemical interaction takes place in the bath, as in the dipping of hollow-ware articles previously cleaned in hydrochloric acid, with evolution of fume of volatile chloride of lead. Any vessel, therefore, of molten metallic lead in which skimming is necessary, or in which chemical action gives rise to fume, requires a hood and exhaust shaft, even although the temperature is little, if at all, above the melting-point—unless, indeed, a separate exhaust can be arranged for the removal of the dust immediately above the point where the skimmings are deposited.
Of many samples of dust collected in workrooms where there are baths of molten lead, it is impossible to say definitely how much of the lead present is due to fume, and how much to dust. Thus, a person tempering the tangs of files was attacked by plumbism, and a sample of dust collected from an electric pendent directly over the pot, at a height of 4 feet from the ground, was found to contain 15·6 per cent. of metallic lead. Similarly, a sample taken above a bath for tempering railway springs contained 48·1 per cent. metallic lead[1]. And, again, a sample collected from the top of the magazine of a linotype machine contained 8·18 per cent. Such analyses point to the necessity of enclosing, as far as possible, the sources of danger—either the fume or the dust, or both. Determination of the melting-point of the molten mass will often help in deciding whether there is risk of fume from the pot, and, if there is not (as in the sample of dust from the linotype machine referred to), will direct attention to the sources of dust in the room. Proceeding on these lines, S. R. Bennett[2], using a thermo-electric pyrometer which had been previously standardized and its rate of error ascertained, and checking the results in some cases by a mercury-in-glass thermometer (the bulb of which was protected by metal tubing), determined the temperature of the various pots and baths of molten lead used in the Sheffield district. As was anticipated, temporary cessation of work, stirring up of metal, recoking of furnaces, and other causes, produced fluctuations of temperatures from minute to minute in the same pot, and in its different parts. The compensated pyrometer used gave for file-hardening pots a maximum of 850° C., and a minimum of 760° C., the average mean working temperature being about 800° C. The variations of temperature of lead used for tempering tangs of files and rasps was found to be high, and largely unrestricted from a practical standpoint. The maximum was 735° C., and the minimum 520° C., the average mean working temperature being 650° to 700° C., varying more than this within a few hours in the same pot. Spring tempering is carried out at some comparatively constant temperature between a maximum of nearly 600° C. and a minimum of 410° C., depending on the kind of steel and the purpose for which the steel is to be employed. Generally, the temperature required rises as the percentage of carbon in the steel is diminished. As these baths are larger than file-hardening pots, the temperature range is higher at the bottom than at the top unless well stirred up. Some lead pots are set in one side of a flue, and the temperature in the mass is then greater on the furnace side. From further observation of these pots during experiments, he was inclined to believe that the lead did not volatilize directly into the atmosphere, as heated water does, but that the particles of coke, fused oil, etc., which rise from the surface, act as carriers of the rapidly oxidized lead particles which cling to them.
Similar experiments were carried out in letterpress printing works. The average temperature was 370° C. in the stereo pots, and in the linotype pots at work 303° C. Scrap lead melting-pots when hottest registered 424° C., but registered as low as 310° C., according to the amount of scrap added, the state of the fire underneath, etc. The best practical working temperature depends largely on the composition of the metal used. That at some factories is the same for stereo drums as for lino pots—viz., 81·6 per cent. lead, 16·3 per cent. antimony, and 2·0 per cent. tin, added to harden the lead. On the other hand, some printers use a higher percentage of antimony in the lino than in the stereo metal. Lead melts at 325° C., and antimony at 630° C., but by adding antimony to lead up to 14 per cent. the melting-point is reduced at an almost uniform rate to 247° C., after which further addition of antimony raises the melting-point. This explains why temperatures as low as 290° C. are practicable for linotype pots. The molten eutectic has a specific gravity of about 10·5, whereas the cubic crystals average 6·5 only; therefore in these pots the latter float on the top, and excess of antimony is to be expected in the skimmings or on the surface.
Administration of certain sections of the Factory and Workshop Act, 1901, would be simplified were there a ready means available for determining the extent of contamination of the air—especially of Section 1, requiring the factory to be ventilated so as to render harmless, as far as practicable, all gases, vapours, dust, or other impurities, generated in the course of the manufacturing process, that may be injurious to health; of Section 74, empowering an inspector to require a fan or other means if this will minimize inhalation of injurious fumes or dust; of many regulations having as their principal object removal of dust and fumes; and of Section 75, prohibiting meals in rooms where lead or other poisonous substance is used, so as to give rise to dust or fumes. Unfortunately, owing to the difficulty hitherto of accurate collection, only a very few determinations of the actual amount of lead dust and fume present in the atmosphere breathed have been made. This lends peculiar value to a series of investigations by G. Elmhirst Duckering, which have thrown much light on the amount of lead fume present in the air of a tinning workshop, and the amount of lead dust in the air during certain pottery processes, and the process of sand-papering after painting. Incidentally, also, they help to determine the minimal daily dose of lead which will set up chronic lead poisoning[3]. Aspirating the air at about the level of the worker’s mouth for varying periods of time, he determined the amount of lead in the fume, or in the dust, per 10 cubic metres of air, and from knowledge of the time during which inhalation took place he calculated the approximate quantity inhaled per worker daily. We have summarized some of his conclusions in the table on [pp. 204, 205]:
Duckering’s experiments as to the presence of fumes containing compounds of lead in the atmosphere breathed were carried out in a workshop for the tinning of iron hollow-ware with a mixture consisting of half lead and half tin. The process of manufacture and the main sources of lead contamination in the air (knowledge arrived at from these experiments) are explained on [p. 59]. As the result of laboratory experiments designed to show the effect of the violent escape of vapour produced below the surface of molten metal in causing contamination of the air, and the nature of the contaminating substances, he was able to conclude that the chemical action of the materials (acid and flux) used, and subsequent vaporization of the products of this action, was a much more important factor than the mechanical action of escaping vapour. Subsequently, experiments carried out on factory premises gave the results which are expressed in the table as to the relative danger, from lead, to (a) a tinner using an open bath; (b) a tinner working at a bath provided with a hood and exhaust by means of a furnace flue; and (c) the nature and extent of air contamination caused by the operation of wiping excess of metal (while still in a molten state) from the tinned article. In all three experiments aspiration of air was made slowly: it was maintained at the rate of 3 to 4 cubic feet an hour in the first experiment for between seven and eight hours; in the second for twenty-eight to twenty-nine hours; and in the third for twenty-four to twenty-five hours. The person engaged in tinning at the open bath was shown to be exposed to much more danger than one working at a hooded bath, while the wiper was exposed to even more danger than the tinner using an open bath, since not only was he inhaling fume from the hot article, but also fibre to which considerable quantities of metallic lead and tin adhered.
Analysis of samples of dust collected in different parts of the workroom bore out the conclusions derived from analysis of the fumes. Thus, samples collected from ledges at varying heights above the tinning bath containing the mixture of tin and lead contained percentages of soluble lead (lead chloride) in striking amount as compared with samples collected at points in the same room remote from any source of lead fume, while the insoluble lead present, as was to be expected from the fact that it consisted of lead attached to particles of tow floating in the air, was less variable.
TABLE XII., SHOWING QUANTITIES OF LEAD (Pb) IN THE ATMOSPHERE AT BREATHING LEVEL.
(G. E. Duckering’s Experiments.)
Dust.
—Reference to the [table] shows that the conditions in the pottery workrooms, as stated in Column 7, are reflected in Columns 3 and 5. Further details from his experiments may be useful. Thus, in a dipping room where low-solubility glaze was in use, the amount of lead in the dust collected per 10 cubic metres of air was 0·70 milligramme. The average of four experiments where there were no dipping boards was 1·80 milligrammes, and where dipping boards were used, 3·75; i.e., 1·95 milligrammes of lead in the dust per 10 cubic metres of air is added by the use of dirty dipping boards. As the result of his experiments, Duckering believes that approximately 1·95 milligrammes of lead per 10 cubic metres of air was due to the fine spray given off in the shaking of the ware. In bright sunlight, he says, the spray can be seen dancing high above the dipping tub. In a dipping house where work was done slowly by two occupants only, the proportion of lead in the measured quantity of air was also low—0·58 milligramme per 10 cubic metres. Where, in the absence of special provision made for admission of fresh air to a fan, the air was drawn from a neighbouring room in which lead processes were carried on, the amount of lead rose to 5·76 milligrammes at the level breathed by the gatherer at a mangle. In ware-cleaning the average of all his observations where lead was used (eleven) was 3·44 milligrammes; and he concluded that “wet cleaning of ware causes less direct contamination of the atmosphere, even where no local exhaust is applied. A still more important result of wet cleaning, however, is that the overalls keep much freer of dust.” The highest results were obtained when the process of ware-cleaning was done outside the influence of the exhaust draught. In one instance, where the ware was cleaned at a distance of 6 feet from the exhaust opening, 13·34 milligrammes per 10 cubic metres of air were found. Subsequently at the same point, after the exhaust system of ventilation had been remodelled, 0·95 milligramme only was present. Even in a stillage room in which no work was done other than the placing on and removal of the boards from the racks, the lead content per 10 cubic metres of the air was 1·08 milligrammes. In glost-placing, the average of four experiments was 1·83 milligrammes—no doubt the result of glaze on the boards. As much as 9·11 milligrammes of lead was found per 10 cubic metres of air in the centre of a large majolica-painting room, with wooden floors and much traffic in it. Wooden floors generally appeared to influence the results, as determinations of the lead present were higher in rooms with them than with tiled floors.
In coach-painting the proportion of lead found by Duckering in the air breathed during the actual time of sand-papering explains the severe incidence of poisoning in this class of work. The [table] shows the amount of lead in the air to be enormous, and in many cases much in excess of the amount found in the air when wiping off in the tinning of hollow-ware. The work of sand-papering is, however, very rarely continuous, the time occupied in it being, for the painter, about one to two hours daily; for the brush hand, two to three and a half hours; and for the painter’s labourer, four to five hours.
Knowing intimately the processes at which the estimations recorded in the table were made, the relative frequency of cases of plumbism reported among those employed at them, and the duration of employment prior to attack, we believe that, if the amount of lead present in the air breathed contains less than 5 milligrammes per 10 cubic metres of air, cases of encephalopathy and paralysis would never, and cases of colic very rarely, occur. And this figure is a quite practical one in any process amenable to locally-applied exhaust ventilation. Somewhere about 2 milligrammes, or 0·002 gramme, of lead we regard as the lowest daily dose which, inhaled as fume or dust in the air, may, in the course of years, set up chronic plumbism.
Local Exhaust Ventilation.
—In considering preventive measures against lead poisoning, precedence must be given to removal of fumes and dust by locally-applied exhaust ventilation, as, unfortunately, the wearing of a respirator is neither in itself a sufficient protection, nor, if it were, could the constant wearing of one be enforced. A respirator is of no use against lead fume. In the case of dust, the conditions which it must fulfil to be effective are, first, that the air breathed is freed from dust, and, secondly, that it should not incommode the wearer. Further, it should be simple in construction, easily applied, and allow of frequent renewal of the filtering medium. No existing respirator of moderate price conforms quite satisfactorily with these requirements. The more closely to the face it is made to fit, and the more effectually the air is filtered, the greater is the inconvenience experienced when it is worn. This inconvenience is due to the exertion (showing itself in increase of the respiratory movements and pulse-rate) caused in aspirating the air through the filtering medium, and rebreathing some portion of the expired breath, containing a much greater proportion of carbonic acid gas and of moisture at a higher temperature than are present in fresh air. Respirators, therefore, except for work lasting a short time—half an hour to an hour—cannot be considered an effective or sufficient means of protecting the worker against dust. If a respirator must be worn, the simplest form is a pad of ordinary non-absorbent cotton-wool (absorbent wool quickly becomes sodden and impervious), about 3 inches by 4 inches, placed over the mouth and nostrils, and kept in position by elastic bands passed round the ears. The pad should be burnt after use.
With a smooth, impervious floor, however, and ventilation designed to remove the fumes and dust at, or as near as possible to, the point of origin, lead poisoning would become very rare in most of the industries to be described. The essential points of such a system are—(1) The draught or current of air set in motion either by heat or by a fan; (2) the ducts along which the current travels; (3) the hoods or air-guides designed to intercept and catch the fumes and dust at the point of generation; (4) inlets from the outside air into the room to replace continuously the air extracted, and, in many cases, (5) a suitable dust filter or collector.
Exhaust by Heat.
—Processes giving rise to fumes or to dust liberated on stirring or skimming, which can be dealt with by the draught created in the furnace flue or over a bath of molten metal provided with adequate hood and duct up which the heated air travels, are—Smelting, refining, spelter manufacture, and the numerous operations necessitating the melting of lead, such as tinning with a mixture of tin and lead, sheet lead and lead piping, stereo pots in letterpress printing, pattern-making, tempering springs, file-hardening, etc. The dusting of red-hot metallic surfaces, as in vitreous enamelling, might possibly also be dealt with in the same way. The disadvantage of the exhaust by heat is the uncertainty and inequality of the draught, and the size of the duct necessary to cope with the volume of rarefied air from above the molten vessel.
The closer the hood is brought down over the point where the fumes escape, the less risk is there of cross-currents deflecting them into the workroom. Hence all baths of molten metal should have the sides and back closed in, leaving as small a space open in front as is practicable in view of necessary skimming or other operations.
In the case of tinning baths, Duckering[4] describes completely successful results when from the top of the hood a shaft at least 24 inches in diameter was carried vertically upwards into the open air to a height of 18 feet, and the top of the shaft fitted with a wind screen in the form of a very large cone, having its lower edge below the upper edge of the shaft, and its nearest point at least 8 inches from the top of the shaft. Smoke produced in large quantity at any point 6 inches outside the front of the hood was entirely drawn into it. As, however, the inrush of air caused an eddy of the fumes at the upper edge of the opening, the edges of the hood were turned inwards, so that the operation of wiping was done in a sort of short tunnel. In general, it may be said that the diameter of pipes leading from hoods to the outer air (on the efficacy of the draught in which success depends) is much too small. Frequently mere increase in size will convert an indifferent draught into a good one. The height of the hood also—i.e., the distance between its lower border and the point where it joints the duct—is of importance. The shorter this distance is, the less serviceable does it become for the removal of fume. Indeed, it may even retain the fume which, were the hood not present, would rise to the roof. Sometimes safety is increased by making the hood double, leaving a space between the two sheets, and so concentrating the draught at the centre and at the margin. With a fan, ducts of less diameter can be used than when dependence is placed on heat alone. A duct carried into a chimney-stack has the advantage of dispersing the fume at a safe distance from the workroom.
The variableness of the draught produced by heat makes it unsuitable for removal of dust, except such as arises from skimming. The receptacle for the skimmings should always be kept inside the canopy of the hood. We have, however, seen the dust given off in the heading of yarn dyed with chromate of lead successfully carried away under hoods connected up by branch ducts with the main chimney-stack.
Fig. 1.—Davidson’s Sirocco Propeller Fan.
Exhaust by Fans.
—The draught for removal of dust, and frequently also of fumes, is produced by a fan, of which there are two types: (1) low-pressure volume fans and (2) high-pressure centrifugal fans. In the first the draught is created by the rotation of a wheel with inclined vanes, causing the air to be driven transversely through the wheel parallel to the axis of rotation ([Fig. 1]). During a revolution a portion of the air is cut off from one side of the wheel, and transferred through the wheel to the other. Such fans are light, run easily, and are cheap. They are of many forms, both with regard to the number of blades—from two to eight—and general manner in which they are arranged. Some closely resemble the screw-propeller of a ship, while others have blades turned over and fastened on an outer rim. Their main defect is inability to overcome any but slight resistance in the course of suction behind, as from constriction in, or friction along the sides of, the ducts and right-angled bends, or of outflow in front, as from wind-pressure. Under favourable conditions, however, and when carefully fitted, a volume fan will exhaust dust and fumes through a system of ducts several feet in length, as, for example, from mono and linotype machines and electro melting-pots in letterpress printing works. But, in order to avoid resistance from friction, the ducts have to be somewhat larger in diameter than when a centrifugal fan is used. With nine[A] linotype machines connected up to a 14-inch propeller fan, the branch ducts should be about 4 inches in diameter, and the main duct 12 inches, increasing from 12 to 15 inches within 2 feet of the fan-box. The shorter and straighter the course of the duct to the propeller fan, the more efficiently it works. Wind-guards are necessary to overcome resistance from this source in front, but their position requires to be carefully considered, so as to prevent the screen itself crippling the outflow.
[A] If gratings are also inserted in the same duct for general ventilation the number of machines must be decreased pro ratâ.
All fans require frequent cleaning, and in this respect propeller fans have the advantage over centrifugal, in that they are usually more accessible.
Fig. 2.—Davidson’s Dust Centrifugal Fan.
Centrifugal Fans.
—Generally, in the removal of dust, a strong suction has to be set up in a system of narrow ducts by means of a centrifugal fan—i.e., a fan-wheel formed by a number of vanes attached to an axle mounted in a spiral-shaped casing—so that when the wheel rotates air is carried along by the vanes, and flies off tangentially into the space between the blades and the casing, and thence to the outlet ([Fig. 2]). The air inlet or junction of the fan with the exhaust duct is at the centre of the fan, an arrangement by which the kinetic energy created by the rapid motion of the air leads to increase of draught instead of being wasted in production of eddies in the surrounding spaces. They are made in many different patterns, according to the nature of the work to be done. Their advantage over the propeller type in the removal of dust lies in the fact that they overcome greater internal resistance, and a uniform high velocity in a complicated system of pipes can thus more easily be maintained.
Fig. 3 shows adjustable hoods and ducts fitting closely over rollers for mixing coloured inks, and serving not only to prevent inhalation of lead dust by the workers, but also the colour from one machine affecting that on another. In the particular room where the installation is fitted there are thirteen separate sets of rollers; the diameter of the branch duct of each machine is about 5 inches, and that of the main duct close to the fan about 20 inches. The special points we have considered as to entrance of all branch ducts into the main duct tangentially, gradual tapering of the main trunk, and collection of the dust in filter-bags, are noticeable. Further, when one set of rollers is not in use the raising of the hood automatically cuts off the draught through it. (Drawing supplied by the Sturtevant Engineering Company, Limited, London.)
Ducts.
—The main duct should be of metal (steel, sheet-iron, or zinc); it should be circular in shape, have as straight and short a course as possible, and be tapered in such manner that the area of cross-section at any point shall equal the combined areas of all the branch pipes which have entered it at that point ([Fig. 3]). Proper dimensions must be studied in relation to the size of the fan and the work to be done. Wooden ducts, unless chosen for specific reasons, such as the presence of acid in the fumes to be removed, are very unsatisfactory, as it is difficult to maintain them in an air-tight condition or to make branch pipes enter with rounded junctions. Where several branch ducts enter a main duct, situation of the fan midway between them has advantage, not only in saving metal in piping, but also in causing the distance of the fan from the farthest branch duct to be only half what it would be were the fan placed at the end of the system (see [Fig. 7], p. 217). Further, the sectional area of the two collecting ducts will be less than that of one main duct, and greater uniformity of flow thereby secured. Where the two ducts join up into the single duct of the fan, the bends must be easy; otherwise the draughts would collide and neutralize one another. Branch ducts, if they cannot be made tangential to a rounded curve, should enter the main duct at an angle of 30 degrees, as by so doing equalization of the draught at different openings is made fairly uniform. The very common defect of a right-angle joint diminishes the draught by nearly one-half. Branch ducts should never be made to enter a main duct on the outer side of a bend, because at this point the pressure of the current of air inside the duct is increased. They should join up on the inside of a bend, where the pressure is reduced.
Hoods and Air-Guides.
—As the object of hoods is to concentrate the draught on the fumes or dust to be removed from the worker, position in regard to origin of the fumes or dust requires first consideration. The more restricted the opening consistent with unimpeded work, the more effective is the draught, and the less disturbed will it be by cross-currents in the workroom. Pendock lays it down as a useful principle that the area of the front opening into the hood should not be more than four times that of the exhaust throat—i.e., the point of junction of the hood and duct ([Fig. 4]). Not less important is it that the draught should operate below the breathing level. Preference as to the direction to be given to the exhaust current should be in the order named: (1) Downwards; (2) downwards and backwards combined; (3) backwards and upwards combined; and (4) upwards only. Use should be made, for the removal of the fumes or dust, of any initial current of hot air set up from a bath of molten metal or from a heated metallic surface, as in vitreous enamelling. Hence under such circumstances only (3) and (4) need be considered. Generally hoods applied err in having too wide an opening, or they are placed too far away from the source of danger. They require sometimes to be adjustable to suit different-sized articles. Care is necessary to see that, when a hood has been adjusted for large articles, it is readjusted for smaller-sized articles. The principle of ventilation downwards and backwards is recognized as right for grinding and polishing on a wheel, since the tangential current set up by the wheel in its rotation is utilized. Pug-mills in paint-works are perhaps best ventilated by applying the exhaust to a dome-shaped hood covering the posterior half of the mill. Edge-runners must be encased, with an exhaust pipe attached to the casing and sliding doors or shutters for introduction or removal of material ([Fig. 5]). A small negative pressure inside the casing is all that is necessary, so as to insure passage of air inwards and not outwards. Branch ducts must protect the casks out of which material is scooped, and the receptacle into which it is discharged. In scooping out dry colour from a barrel, it is unwise to attempt to remove the dust created at every displacement of air on removal of a scoopful by means of a hood suspended over the barrel. Instead, the last joint of the duct should be a telescopic one, so that it can be lowered into the barrel, and be kept at a distance of about 6 inches above the material. The air is thus drawn downwards into the barrel ([Fig. 6]).
Fig. 4 shows a well-designed arrangement of hoods, duct, and fan, in the packing of white lead, and the filter-bags for collecting the dust so removed. An additional safeguard is introduced, as the casks stand upon grids through which a down-draught is maintained by connecting the space underneath with the exhaust system. (Drawing supplied by the Sturtevant Engineering Company, Limited, London.)
Processes such as colour-dusting, aerographing, ware-cleaning, enamel-brushing, and the like, are best carried out at benches under hoods with glass tops. Air will enter from in front, and carry the dust or spray away into the exhaust duct placed at the back of the bench.
Fig. 5 shows a pan mill with edge runners fitted with casing (partially open). The casing is connected to a powerful fan, and branch ducts with telescopic terminal sections control the dust in scooping out from the barrel, in feeding into the mill, and at the point where the ground material is discharged.
Collection of Dust.
—Frequently no heed is paid to the collection of the dust. Sometimes a dust chamber is arranged to intercept it on the far side of the fan, or attempt is made to blow the dust into a tank of water. The fine dust of which we are speaking cannot be satisfactorily collected by either of these methods, nor even by a cyclone separator, so useful for the collection of many kinds of dust. In lead works generally, the dust removed by the fan is best collected in filter-bags made of some porous fabric. Various efficient filters constructed on these lines by Messrs. Henry Simon, Ltd.; Messrs. Beth and Co., Ltd.; and the Sturtevant Engineering Company, Ltd., are on the market.
Fig. 6 shows an arrangement of piping with balanced telescopic joints fitted to a Sirocco dust fan for removal of dust, in an electric accumulator works, when scooping out litharge from a cask into the receptacle prior to emptying the weighed quantity into the mixing machine, also under a hood connected with the exhaust system. (Illustration supplied by Davidson and Company, Limited, Belfast.)
In collecting the dust, care must be taken to provide an adequate outlet for the spent air, so as to prevent creation of a source of friction in front which might destroy the effectiveness of the installation.
Fig. 7.—Exhaust Ventilation on the Patent “Pentarcomb” Principle applied to Linotype and Monotype Machines in Printing Works, as installed by the Zephyr Ventilating Company, Bristol.
P, Patent “pentarcomb” for equalizing exhaust; V, patent “pentarcomb” for general ventilation; D, main and branch ducts; F, fan; U, upcast from fan; M, hoods over metal-pots of monotype machines, constructed to raise and lower, and swing out and in with metal-pot; L, hoods over metal-pots of linotype machines, constructed to raise and lower.
In the illustration “pentarcomb” grids connect the branch ducts over the metal-pots of mono and linotype machines with the main duct. The “pentarcomb” grids are arranged also elsewhere in the main duct itself to assist in the general ventilation of the workroom. The hoods over the metal-pots are constructed to be raised and lowered, and to swing out and in radially with the melting-pot arm. (Drawings supplied by the Zephyr Ventilating Company, Bristol.)
In order to secure equality of flow from a number of branched ducts, the Zephyr Ventilating Company apply a special grating of curved and slanting inlets—the “pentarcomb”—to each branch duct. The air passing through the comb is split up into numerous small columns, and the inclination of the curve which each is made to take is such as to reduce friction to a minimum. By means of this device we have found, in a trunk with twenty branches, the draught at the one farthest from the fan as serviceable as that next to it. The method is illustrated applied locally to remove the fumes from linotype machines, and generally in the main duct for removal of foul air near the ceiling.
Where electricity is available as a motive power for driving the fan, some modification in the views expressed as to the curvature of the pipes and system of installation can be allowed. In a red lead plant, for instance, it may be desirable to have the pipes leading to the sifter or packing machine with sharp angles, so as to prevent tendency of such heavy dust to collect in them. The electric current allows a fan to be installed at any point desired; and if applied with knowledge that the increased friction due to an acute angle has to be overcome, the result may be quite satisfactory.
The various forms of vacuum cleaning apparatus with mouthpieces designed to aspirate the dust from different surfaces are sure to be increasingly used. In our opinion, wherever electric power is available, they will obviate barbarous methods involving use of hand-brushes to collect dust from machines, such as those for litho-dusting or for sweeping lead dust from benches and floors, or use of bellows to blow out the dust from compositors’ cases.
Finally, the carrying out of lead processes by automatic methods and with the interior of the casing under a negative pressure, so that the material is transported from one process to another by means of worms or conveyors, is everywhere to be aimed at. Or, again, it has been found possible on a commercial scale, by means of compressed air in a closed system of receivers and pipes, to force material in very fine state of division from one place to another, as, for instance, of litharge from the cask into the mixing machine for preparation of the paste for manufacture of accumulator plates, without risk of contact.
Indication of the efficiency of the draught may be gained by holding smoke-paper at the orifice of the hood. The definition of efficient exhaust in some regulations for the removal of fumes, as in the Tinning Regulations, is that it shall not be deemed to be efficient unless it removes smoke generated at the point where the fume originates. Accurate gauging, however, of the draught can only be done with an anemometer, so as to determine the number of linear and cubic feet passing through the throat per minute. Only rarely does one find an occupier alive to the value of the use of such an instrument. The importance of this point has been recognized in the Regulations for Heading of Yarn, by the requirement that the speed of each exhaust opening shall be determined once in every three months at least, and recorded in the general register. We prefer to use Davis’s[A] self-timing anemometer, which gives readings in feet per second without the need of a watch. Other useful anemometers—Casella’s or Negretti and Zambra’s—require to be timed.
[A] It is not available for velocities exceeding 1,200 linear feet per minute.
The details of all routine observations on localized exhaust ventilation might well be entered on a card hung up in the workroom. Such a card drawn up by our colleagues, Miss Lovibond and Mr. C. R. Pendock, has the following headings:
| FIRM .......... | PROCESS .......... | ||
| Fans: No. .......... | Kind .......... | Size .......... | Maker .......... |
| Motive power .......... | H.P. .......... | Method of driving .......... | |
| Other load .......... | Condition of driving .......... | ||
| Screen .......... | Dust collection .......... | ||
| Direction .......... | |||
| Periodic cleaning .......... | |||
| Hoods: No. .......... | Kind .......... | Size .......... | |
| Structure .......... | |||
| Distance between each .......... | |||
| Ducts: No. .......... | Kind .......... | ||
| Size .......... | Length .......... | Section .......... | |
| Structure .......... | |||
| Periodic cleaning .......... | |||
| Fresh-Air Inlets: No. .......... | Kind .......... | ||
| Position .......... | |||
| Size .......... | |||
| Fixed or temporary .......... | |||
| Hood: | Position of Anemo- meter. | Date ..... | Date ..... | Remarks. | ||||
|---|---|---|---|---|---|---|---|---|
| Reference Number. | External Conditions ..... | External Conditions ..... | ||||||
| Area of Throat. | Speed F. p.m. | Volume C.F. p.m. | Area of Throat. | Speed F. p.m. | Volume C.F. p.m. | |||
Frequent cleaning and inspection of exhaust installations are very important, as accumulation of dust greatly impedes the flow of air at all points of the system. The person employed in cleaning the fan should wear a respirator. Hoods and ducts should always be cleaned with the exhaust in full action.