Document 6Be24NB84byBjoDRenaaJdxo3

figu,* 4. $ itiii iinglro"** grind** i vd in a oarltty at |k local dhoUll * tm mvii b* odjviWbU, Tk* fl*kibl* dvct (A) permit* noHxtl at lh vthavit hood (6) at needed. ^Courtti* Amaricon Fovndryman'i So- octcrthclcxn. should not be' con sidered .is a universal substitute tor' adequate local exhaust re moval. elimination of the con taminant. or containment. 129. Many states and municipal* ilics have dust control codes or or* dinances with which employers must comply. In a few states, for instance, written approval of plans must be obtained before a local exhaust system . is installed. Each employer should therefore know his . slate and municipal dust control re* quirements. 130. Each type of exposure must be considered separately. For ex ample. a hval exhaust system suit able for welding or cutting of steel might not be satisfactory for weldinc or cutting steel coated with red lead. local exhaust systems 131. A local exhaust system for the control of an industrial dust or funic traps the air contaminant near its source mi that an operator standing at the process is not ex posed to harmful concentrations. The system should be designed to enclose the process as completely as possible. This method usually is preferred to ' general ventilation, hut should be used only when the contaminant cannot he controlled by isolation, process revision, or substitution of less harmful mate rials. Even though a process hav been isolated, it may still require a local exhaust Aystem. 132. A local exhaust system con sists of four principal parts: .i. Hoo<l\ or other inlets, into which the air-home contaminant is draw n. h. Quels, to carry the contaminated air to a central point. c. Dust and fume collectors, to dean the air before it is discharged. d..A fan and motor to keep the air moving through the system. 133. While each of .these parts should be.designed and installed to perform its required function with respect to the system us a whole, design of the exhaust hood demands the greatest care. The degree of control of dust at the point of gen eration or dispersion is determined by the shape of the hood or. degree of enclosure, the location of' the hood and its distance from the dust source, and the rate of How of air into the hood. A poorly designed hood can make an exhaust system ineffective. 134. There is no standard hood. In every .ease, the hood must be designed to fit the specific opera tion and to make the exhaust effec tive without interfering with the operation (Figure 4). Among the factors to be considered are the natural air currents in the room and other exhausts or windows in the area. 135. The hood should be shaped to conform to the shape of the area of dust production-so as to secure reasonably uniform air velocity over this area. A hood which does not en close the process should be placed with its opening as dose as possible to the point of generation of the dust or fume (Figure 5) because the velocity of the air in the zone of the hood influence is inversely proportional to the square of the distance from the fa'ce of the hood. 136. The hood opening, or part of it. should be located so as to re ceive directly dust that is thrown off along u well-designated path (Figure 6). The directional energy of the material cun thus be used for its own capture. Air movementmust always be past the employee, then oyer the dust source, and di rectly into the face qf the hood. 137. The fan should be of suffi cient capacity to maintain the re quired air capture velocity at the point of generation of the dust. In ternal baffles should be installed to guide the air flow where it is most needed. Flanges should be provided wherever possible to reduce the air flow from areas where no dust is produced: that is. air-flow' contours should be controlled. 138. Enough air must be sup plied to the room from the outside to replace the air that is removed by the exhaust system. Otherwise, there will be interference with other exhaust systems in the area or with gas or oil (lames in nearby furnaces. Great difficulty has occurred where an exhaust system caused a slightly negative - pressure in a room con taining a gas furnace. As a result air came down the furnace flue, and (he area became contaminated witMearbon monoxide from the fur nace. 139. With small exhaust systems, air that is removed usually can be replaced by infiltration flow, but larger exhausts may'need'a positive air supply (Figure 7). An adequate supply of make-up air, tempered when necessary, is one of the most frequently overlooked fundamentals of ventilation. Air always should be supplied ;-in quantities equal to or slightly in excess of the- amounts exhausted. 140. The si2e of -the ducts, the type and size of the dust collectors, aod the type and size of the fan and motor (explosion-proof where necessary) arc among the other fac tors which must be considered in the design of an exhaust system. Pre venting ignition of a combustible figor* i. To ochirr* fho prop*' nhowi* otr lUlf, tho hood* fa' rtxu borrl (urnoc*i <on b* potit1on*d ot dot* to tho fvrnoc* ipouft oi procticol. SveH petitioning I, mod* ptuibl* by mounting tho hoodt on a trollty tutpandod Iron, on ovorhoad track. (Courttty Atntricon 8rok She* Co.] 103 Notional .SefityiNews, Junt 1963 contaminant is a prime safety con sideration. Since discussion of the subject of exhaust system design is beyond the scope of this data sheet, an experienced ventilation engineer should be consulted. 141. - Also, information can be secured from severaj excellent pub lications, one of which is the current edition of Industrial Ventilation-* A Manual of Recommended Pracl dee, published by the American Conference of Governmental Indus trial Hygienists. Another is the chapter entitled "Local Exhaust Systems and Ventilation" in the Accident Prevention Manual -foe Industrial Operations, published by the National Safety Council. . General ventilation 142. Where it is impossible, im practical, or too expensive to con trol dust entirely by local exhaust - systems, general ventilation must be used as a supplement or a substitute. It should be noted, however, that where local exhaust systems can be used, they will always do a better job than general ventilation. 143. General ventilation requires the introduction of enough dean air from the outside to dilute the con taminated atmosphere to a safe level. This method requires larger volumes of air than local exhaust systems to accomplish the same con trol. and will not be effective uni formly over a large room. It should be considered only when local ex haust systems require such assis tance. This is usually where the sources of dust are widely dispersed and each is small. 144.. General room exhaust and supply fans must be carefully in stalled. Eddy currents, which will interfere with local exhaust systems, must be avoided. Employees must not be exposed to .excessive con centrations of dust as the dust-laden air moves away from the point of generation, and sufficient makeup air from the outside must be sup plied to the room to replace the air that is exhausted. The location and design of the source of supply are . important. It is becoming common practice to supply clean, tempered air to the work zone for controlled - dilution. Wet methods 145. Wet dust is not dispersed as readily as dry dust--advantage of this fact should be taken whenever possible.'Carloads of dry minerals in some cases may be wetted down before they are unloaded. Aisles in foundries should be wetted down to prevent dispersal of the dust by traffic. Water sprays can .be used at some operations. Wet drilling methods can be used for rock drill ing to wet the dust as it is formed. Personal protective equipment 146. Respirators of various de-. signs are available which will give protection against toxic and pneu moconiosis-producing dusts by Al tering out the contaminant from the gir. The U. S. Bureau of Mines has set up performance standards for dust respirators and gives approval to respirators that meet these stand ards. it is important that a respirator be used only for the particular dust exposure for which it has been approved. 147. Although approved respira tors will give excellent protection when properly flited, they should be used only as supplements to other methods of control or for short or occasional exposures and not as primary controls. 148. Proper fitting of a mechani cal Alter respirator to the face of the individual is most important because a small .space between the facepiece and the face will permit dustladcn air to bypass the Alter. 149. Respirators must be in spected ' and cleaned daily. Filters should be replaced before they be come so plugged with dust as to seriously increase resistance to breathing. Proper Alters for replace ment should be available. Medical program 150. An effective medical con- trol program will help prevent cases of occupational disease. Such a program can also serve as.a check on the engineering controls because symptoms of exposure in a .group of workers will indicate a failure that must be corrected. The extent of the medical program will depend .upon the seriousness of the expo sures. 151. An industrial hygiene pro gram should parallel the medical program. Both arc essential to pro tect the health of employees. 152. The physical examination for hew employees,should include a thorough pre-employment history, with the occupational background given in detail. Chest X-rays should be made of all new employees who will be working in dust exposures that could produce disabling pneu moconiosis. The examining physi cian should decide on placement of those who have pneumoconiosis, active or significant past tubercu losis. abnormally low timed vital capacity, or serious pulmonary dis eases. 153. Periodic physical examina tions. including chest X-rays, should be made of employees exposed to toxic dusts, fumes, and mists. Such checks can help And incipient cases of poisoning in which symptoms are slight, and the X-rays can pick up early symptoms of lung condi tions. Suitable preventive measures can then be taken. 154. Routine periodic clinical examinations, stipple cell counts, porphyrin determinations, and propperly evaluated blood and urine lead level measurements, are prac tical methods for checking em ployees exposed to lead. If unsafe exposures are found, further en vironmental control is mandatory. Affected employees - should, of course, be given proper medical treatment. 155. Medical controls for cm-' ployces who work with radioactive dusts must be more stringent than the medical controls for most dusts. An extensive bioassay sampling program is nearly always required. Other control measures 156. Although the most effective method of control is to prevent con tamination of workroom air and thus prevent inhalation of harmful rtgun e. Tho ca-tnfvfel fort* croonod Ur grinding lml wvw rk# g*norot*d dull H r,ev*l in a w*tW*finod ptk. To proronf diiporlion of tho dull, Ik* **Ho* hood 1* ptoeod diWorfyin lh* dud rtrocm, do-- * it* touroo. (Court*** A/nrfcon Foundry**"1! Sodtfy) Nettonol Setaty News, June 1963 103