Document LJNBY6oB598onrwLnLGEYKmDz

American Society of Heating and Ventilating Engineers Guide i9jj Further information regarding the choice of fans and motors is m Chapter 17. glVenin PROTECTION AGAINST CORROSION The removal of gases and fumes in many chemical plants requires ti, ^ metals used in the construction of the exhaust system be resistant Table 6. Loss Through 90-Deg Elbows Elbow Center Line Radius in Peb Cent of Pipe Diameter 50 100 150 200 to 300 Loss in Per Cent of Velocity Head 75 ' 26 17 14 chemical corrosion. A list of the materials which may be used to resist the action of certain fumes is given in Table 7. Hoods and ducts when short, may frequently be constructed of wood arid be quite effective. Table 7. Materials to be Used for the Protection of. Exhaust Systems Against Corrosion3 1 Ttpb of Fume Conveyed _______ ' Protective Material to bb Used : : :________ ________ _______________ ______'_____________ __________________________ > Rubber lining or chrome-nickel alloys Hydrogen sulphide--------------------- Aluminum coated iron, aluminum, high chrome-nickel alloys Iron or steel High chrome-nickel alloys -i Rubber liningj-chrome-nickel alloys ' Nickel^chrortie alloys ' J ' . J Condensed from data given by Chilton and Huey {Industrial and Engineering Chemistry, Vol. 24,1932). Rubberized paints are available and may be applied as protective coatings in handling such gases' and fumes as chlorine and hydrochloric acid. PROBLEMS IN PRACTICE 1 /Should individual operations be served by an individualized dust collector system? Yes, if operations are usually kept individual in a group of machines. 2 Are state regulatory requirements as to suction applicable to all sorts of dust collecting installations? 1 .As a rule the regulations refer only to -grinding wheel and buffing wheel systems. They, Are needed for .many other industrial processes. 356 Chapter 21--Industrial Exhaust Systems 4 the most common- method of reducing total air volumes handled |P|i^,aeinployn,6 large hoods over apparatus covering a large area? , .tg petticoat or double hood which permits a comparatively high air velocity jffifas*" jiJe"hood and controllably small velocities in the center. Wk t other types of collectors are available for use in the place of cyclones when chemical and physical conditions obviate the possibility of the -jph as scrubbers and contactors, using water or other contacting liquids, jkcttl precipitators, and dynamical precipitators. \fhat s the most frequent error made in dust collector system design? _ ` of some means of putting into the workroom air having the proper characreplace that which has been exhausted.-------------- . uj there available means for testing the performance of dust collecting ^tcms when they are required to meet high industrial hygienic-standards? guch means are set up by the United States Public Health Service and by the Sandard Code for Testing Centrifugal Fans (Chapter 41). 7 Why is it not permissible to connect up emery wheelsand buffing wheels to fffie sroe exhaust system? ............... * *'. '1 * 1 Tmery wheels and buffing wheels should be handled by separate systems because of the *-6rt hazard, as it is possible for sparks from the emery wheels to ignite the lint and dust from the buffing wheels when both are carried through the same system. I dye an important characteristic of centrifugal type dust collectors which tMold be given consideration when applying this type of collector to instal-Utiona requiring high separating efficiencies. The separating action of a cyclone or centrifugal type collector depends largely on 'centrifugal force. Reducing the radius of air flow increases the centrifugal force for a Jgiveri velocity of flow. Accordingly, the smaller size units usually give higher separating , (adore, and better results can sometimes be obtained by using a number of small coll lectors instead of one large unit. d I Mention some general suggestions relating to the design of efficient in dustrial exhaust systems. ^Endeavor to obtain a maximum degree of effectiveness with a minimum volume of air, ,by the use of well designed hoods closing in the sources of fumes or material to be removed so located as to take advantage of the natural direction-taken by the fumes or materials when leaving their source. Give particular care to the velocity of flow; The duct velocities for material confflnng systems must be high enough to properly carry the material, but they should not De signer than necessary because excessive velocities increase the pressure requirements, fpd result in a waste of power. the type of fan best suited to the job. For installations where stringy material "handled do not use a fan wheel which has a shroud; handling the refuse from various machines, study the grouping and operating 'iS? i TMe machines. Connecting a large number of machines into one system'is trequently very uneconomical. `'.Avoid unnecessary distances and bends in laying out the piping system. Static pressure measured at the throat of a.buffing wheel hood is 2 in. fe ."*? velocity head measured with a'Pitot tube is: 1.6 in. Calculate the ffiWtion coefficient f.