Document Gqw7b8ymb4zndGbDerKbVr6n

FILE NAME: National Safety Council (NSC) DATE: 1953 Feb DOC#: NSC024 DOCUMENT DESCRIPTION: NSC - National Safety News - Practical Aspects of Dust Suppression  r 1; r. ; I kite - it I? " S i!1 J 1 ifwii' ,'TL-L fi;\ Wt? & 0 IMi f i By W . B. LAW RIE M.Sc., F.K.M.S., A.I.M. INCIDENTAL g en eratio n of -- large quantities of dust is a feature of many industrial proc esses. In some, the dust is danger ous to health; in others, although perhaps innocuous, it appears in objectionable quantities. H ig h er output often increases the density of the dust cloud generated, so the modern factory may aggravate the dust problem. The term "dust supression" in cludes all methods by which at mospheric dust concentrations are reduced. These methods fall into two main groups. One approach aims at prevention of dust genera tion. The other is directed to the control of dust clouds which have heen allowed to form. The former, which may be de scribed as dust elimination, is al ways the better and often the more difficult method. The latter, referred to as dust control, is never so good, although it may be the only practical ex pedient. It is, however, of great practical importance to keep in mind these fundamental differ ences in technique. Dust supression can be applied in three ways. First and best is elimination of the dusty process. The second is alteration of the process giving rise to the dust, so that less dust is produced. This is not so good as complete elimina tion, but it reduces the amount of dust to be controlled, and so facil itates application of control meth ods. Finally, the dust cloud may he controlled a fte r it has heen al lowed to form in the atmosphere. Where no alternative is avail able, control methods will have to be applied. In the over-all design of a dust supression system, however, they should always be considered last. Success often'depends on the thoroughness with which the first two methods have been used, and the degree to which the actual for mation of the dust cloud has been restricted before the control meth ods were applied. Although local exhaust ventila tion is often necessary, it is only one way of controlling dust, and in certain cases may not even rep resent the major feature of a good dust-suppression system. Local exhaust vend, ation is not always successful. It com m only fails to achieve its en 1 because it has been reg ard ed as the only method, instead of being used in conjunction with other methods of dust suppression. One other general observation must be made. Local exhaust ven tilation is employed to remove dust--not air. This requires some knowledge of the properties of the dust to be removed, and also of the manner in wrich it might be expected to behave. It also means that the norm tl method of estimating the requirt ments of the ventilating system, with reference to the volume of air extracted, may give an inadeouate impres sion of the amount cf dust it will remove. This article, which, appeared original ly in the British Journal of Industrial Safety, is presented in condensed jorm through courtesy of the Royal Society for the Prevention of Accidents, London. Illustrations are from industrial plants in the V. .S'. A. Part 1 of this article, which deals mors specifically with local exhaust ventilation, will appear In. an early issue. Dust and smoke collector hoods behind shakeouts have a capacity of 18,000 cubic reef of air per minute. (Aliis-Chaimers Manufacturing Co.) .M. No/ionai Safety News Fe b r u a r y , 1953 'iffi MWSlk'liliiid outgoing air will be high. The building should always be con structed of materials having good thermal properties to ensure min imum heat loss through walls and roof. Heating should, therefore, be considered part of the ventilat ing problem. If the processes involve furnaces or other heat sources, the general ventilation will be taken up from floor to roof, and dust and fumes will rise. It is necessary, there fore, to have a roof which is well insulated and warm. This is be cause the velocity with which hot dust and fumes rise is a function of their temperature. As the hot column rises, it cools, its u p w ard velocity de creases, and as it moves more slowly it becomes more susceptible to' stray drafts which tend to dissipate it throughout the room. If it meets a cold roof or cold air strata, it may be cooled sufficiently to start falling again. In this case, the roof fans will be trying to extract fumes and dust in an upward direction at a level where these fumes are natu rally falling. If the roof is both high and cold, the hot rising column may be chilled to the tem perature of the surrounding air before it reaches the roof. Then it will cease to rise, and will level out to form a horizontal layer of dust which will float in the air. It is very difficut ~to extract this layer if the fan inlets are a few feet above it. With hot dusty processes the roof should not be flat, because rising columns of hot dust and fume impinging on a flat ceiling will billow o u tw ard and spread rapidly. A ridge roof, which will convey the fumes to a high point at which the fans can be placed, will offer far better ventilation. It is not possible to lay down rules for general application and much more information is still needed.' For example, a careful survey of the temperature gradi ents throughout a building which contains furnaces might well show the ventilating engineer where to place his fans to assist the natural flow of hot, dust-laden air. In new construction, ventilating engineers should work in consulta tion with the architects. Coopera tion of this kind enables the ventilating engineer to control the incoming air as well as the ex tracted air; whereas if the ven tilating system is added to- an existing building he can often con trol only the outgoing air, which imposes limitations on the whole system. Only buildings designed for hot processes and upward ventilation have been discussed. Different considerations may be present if the processes are cold, or if the general ventilation system is de signed to operate downward from roof to floor. Nothing has been . said about arranging the building so that a process needing local ex haust ventilation is near an outside wall to permit short ducts between process and dust arrester. G eneral V entilation General ventilation should not be considered until ail specific points of dust or fume production have been dealt with individually, either by local exhaust ventilation or by other methods. Cheap and effective general ventilation be comes possible only when large concentrations of impurities have been prevented from diffusing throughout the atmosphere. General ventilation should not be used to scavenge a polluted atmosphere. It is highly improb able that it would perform such a function, and even if it did, the dust would have been breathed by the occupants of the room before being extracted. In addition, large quantities of air have to be han dled, and ventilation costs rise steeply. The heating of the build ing will becom e progressively more expensive and less efficient. It follows, therefore, that gen eral ventilation should be designed to deal only with small quantities of residual atmospheric impurities after the bulk of the dust has been disposed of at source. If this has been done by local exhaust ventila tion the general ventilation should be designed to balance the local systems. Upward Ventilation The commonest form ox general ventilation is upward ventilation, extracting at the roof. It is also probably the m ost practicable form in buildings containing scat tered sources of heat. In these cases, however, it is desirable that the ventilating engineer should know the method of generation of the dust or fume, its initial tem perature, its natural method of propagation or flow, the path it will take ard its velocity alon this path. It is evident that the upward extraction of rising air demands a vertical patf. from the point of origin of the impurity to the fair on the roof. Quite apart from theundesirability of allow ing the fumes to spread throughout the room, less ventilation air will h< needed to extract a concentrate! column which is rising naturally and rapidly to the fan. Impurities can usually be most M easily prevented from spreading i\ by segregating dusty processes ii $' ' separate buildings. If this is no Vi possible, partitions may be hurt; till from the roo : and extended down ward as low as possible. Thi will ensure t. tat. warm rising dus and fumes' puss the lower edge o the partitions' before they cool' sufficiently to lose their velocity. They will s ibsequently be con tained, physically, by the partitions as they rise more slowly toward! the roof fans To prevent dust from spreading, it is essential to control eddy cur rents and stray drafts. Drafts which are .flowed to dissipate | rising columns of air may com- | pletely vitiate the general ventila tion system. This point is of still greater im portance in buildings containing furnaces, w .iere p a rtic u la rly Q strong eddy currents may be set up by n a tu ra l convection. It should not b; forgotten that fur naces may >et up such strong cross currents that a badly de- -To page 101 68 al S a f i t y N e w s , F e b r u a r y , 1953 3||j tested as they are turned: before being issued for tise.5. tool proves defective it is me diately tagged and turned inE the electrical department for f" pair. Since a tool can be com pletely tested in seven seconds, additional work places no burdl on the tool crib. Weighing 1 than six pc unds, it may also g used as a portable instrument ',L test tools st the point of opertion. Most electrical departments haaccepted tl is new testing devf because it aids them in spott'^ the hazard. It means that after tool crib attendant has found tlr the equipment ground is brotf or that there is a power grouii the tool is then tagged and to the ele ;trical department gives the electrician a clue what to look for rather tha spend time tearing down the er equipment wear a DUPOR No. 40 the Respirator that is U.S.B. of M. Approved !or Type A Dusts More than 40 sq. in, twin filter area. Sanitary face cloth. Soft rubber face mask. Controlled breathing . . . check valves guard against re-breathing stale air . . . exhalation valve exhausts breathed air. Greater visibility . . . no blind spots! Does not interfere with goggles or glasses. $ 1 2 0 0 frfr d a tt ^ ~~ R 5. Cover Fog^Proof/ Gas-Tight Gogg/es for use with ab o v e respirator . - $ 2 . 0 0 p p Dust Suppression -- From pig'e 68 signed ventilating system mere serves to pollute the whole roo; rendering conditions worse dissipating impurities which wop otherwise have polluted only air above their point of origin" Flat roofs offer the worst corf tions for upward ventilation, rising air does not converge top, point at which the fan is plat but spreads horizontally on ril ing the ceiling. The best way; dealing with such-a state of-afFa" is to inssrt a false ceiling. The frise ceiling is perfora; all over .ts surface and the exth tion fan; are installed in the qg ing above it. There is, thereby an even extraction rate over r -whole a-ea of the ceiling and rising a.r will be extracted at point il meets the ceiling, d method is more efficient if harjj ing partitions are placed Cl areas where large volumes,^ fumes are generated. ,t Downward Ventilation 1 I Rooms which do not coni|j hot processes may be ventila!! downward, from roof to flh' This method is not so cominoqj| British industry, but there se1 tg ^ a growing tendency to use it in Sweden. Its advantages are ob.ous and it extracts dust and fumes without lifting them past breathing level in the process. The general method is to blow in suit ably heated air at roof level. The incoming a ir m ay be blown through distributors or through a perforated false roof. In the latter method a second, ,,,.ru>rated, ceiling is placed below |l,,. reding and air is introduced nihier pressure between the ceiling mid the false ceiling, so that it enters the room through the per forations in the false ceiling. The air is extracted below breathing level, and if the processes in the room are provided with local ex haust ventilation this may serve to extract all or part of the air. The whole system needs careful balancing. Supply of O utside A ir All the air extracted from a ` building must be replaced by fresh air from outside, and this incoming air should be under con trol. This is because the uncon trolled ingress of air may short circuit the general ventilation sys tem and give rise to drafts. In hot processes they may be strong enough to vitiate the whole ven tilating system. The incoming air should be clean, and great care should be . taken to avoid contaminating this air before it enters the building. Air intakes should be arranged so that they cannot draw dusty air from outlet ducts which are dis charging from other parts of the building or from particular proc esses. One method of preventing this is to place outlet ducts high up over the buildings. These outlet ducts should be designed to dis charge at high velocity, so that the used air will be carried away from the factory. Outlet ducts should never be fitted with covers. These . covers restrict the discharge velocity and baffle the air stream, so that the spent air is turned down over the factory roof, instead of being de livered high up into the outside atmosphere. Ideally, i n c o m i n g a i r s h o u l d h e delivered to- the place at which it .b_^a i<"ana/ Sa/efy N e w s , F e b r u a r y , 1953 if"!;: $$$ uY `i . M- m HI fi }!H{f; fii. (Mho S N A K E BITE MIT V when the snake strikes ...you hove 10 minutes! . .. this requires dependable, controlled suction to prevent the poison from spreading. 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Mark all your containers, drums, carboys, bottles, barrels, etc., con taining hazardous chemicals and solvents with safety approved Brady Container Markers. Mounted on. handy Dispenser Cards -- quick to apply -- quick to identify. Bold black letters on caution yellow back ground provide instant visibility. Strong and durable, made of sturdy cotton cloth with Silicone Plastic coating. Self-adhesive, stick to any clean, dry surface. Send for FREE samples today. ______ 1 pint MARKERS 1 SMtT sIwS I6 RWULIlH.TTtAOPE -llmKsTsiwiLs MAHUFCTUIlHtS OF S i l F - S T I C K I N O P R O D U C T S i D T l 1 WH: l& 1 1 I^MPftNY ?80 W, GLENDALE AVENUE MILWAUKEE 12. WtS. SAFETY EQUIPMENT FOR Al l QUICKLY APPLIED OR REMOVED SDMMHTCH) AGAINST BREAKAGE ! is needed, because if it is intro- i duced at some distance from this' 1l point it wiE mix with fumes from il other source', as it passes through V ; the building. It will, in any case, li! lose its -velocity if it has to travel long distance s. If velocity can be maintained, i smaller volume will be needed. drafts must On be the other hand, avoided so that 1id incoming ai] should not be del ered in the form of a jet, which might impir ge on the occupa: of the lost its room. When the air velocity in transit it 1 may still be controlled by reason c temperature % In plenum systems, it has be u found in Sweden that the incorn-t mg air can be introduced at a: lower temperature than that oi the air in the room. As the air I blown in at a height of about feet, this coo] air falls to the floor. The angle at which-it falls cam controlled ry regulating the inlei temperature and the whole systei gives a sweeping effect as the cool fresh air falls to the floor before warming no and rising to the roo" extractors. This kind of control1 offers real possibilities, alrhou further information is still need to obtain the correct air moverneiv by judicious use of velocity' and; temperature. One final point might be notice in general ventilation techmcme: Certain incustries produce lar volumes o' dust and fumes-raj particular stages in the .proc; In these cases it may represent considerable savi ng in head costs if the general ventilation s; tem is designed so that the traction volume can be increai to meet the short periods at wh: the maximum dust or fume pn duction occurs. I I P C O M M R A I L C L A M P S ! and I ^ F o r car stops, on loading platforms, temporary sidings, 5 zcars on grades, and traveling cranes . . . Used and recom% mended by leading Steel Companies, Manufacturing Plants, % & Mines, Grain Elevators, Cement Plants and Quarries. WRITE FOR BULLETIN N O . 44 2850 N. FOURTH STREET P H ILA D E LP H IA 33, PA. % iW ///////////M /////////^ no Tact is giving a person a sno in the arm without letting bin feel the nerdle. "Smithers," said the boss m df meek eirnloyee, "I understand! you've been going over my head| "Not that I know of, sir/' f| plied the employee. "Well, isn't it true that you'd been praying for a raise?" National Safety News, February, jf s& M i ii tfr