Dust and Fume Control in a Modern Melt Shop

THE requirements of fume control in melt shops and their possible solutions has been thoroughly discussed in many papers. This one, therefore, will be limited to experiences with fume-collecting equip- ment on electric furnaces a t the A. Finkl and Sons Company melt shop, Chicago.

The Finkl melt shop contains two 20-ton Moore Lectromelt furnaces. Fume orig- inally was controlled with seven 24,000- cfm roof ventilators operating between

per hour and return i t to the boilers for make-up. The make-up pays for the ad- ditional 80 electrical fan horsepower to run the heaters. The heated air close to the floor line helped the fine particles of dust to rise to the exhaust fans and louvers but the heavier particles would still pre- cipitate on the equipment and visibility was poor on humid days. Dust was being discharged all over the neighborhood and the atmosphere in the shop was not suffi- ciently safe or healthful. Hoods for the fur- naces and dust collectors were the next move.

two corrugated sheet-metal curtains, 75 f t EQUIPMENT apart, extending from the bottom of the truss to the top of the roof. These exhaust American Air Filter, of Louisville, Ken- fans consumed only 14 hp and exhausted tucky, apparently had pioneered the field the fume reasonably well during the sum- in the design of hoods, and the hood could mer 'months, with clean scrap and single be selected from their catalogue. A suit- slag heats. However, during the cold able dust collector appeared to be a much weather; when the windows and doors are greater problem. Rotary collectors, elec- closed a t the base of the building, the roof tronic precipitators, wet collectors, and ventilators would sidetrack air around the bag-type collectors were investigated. curtains from the overhead louvers, and Because of the extremely small particle not from the furnaces. When all-heavy- size, large volumes, and capital investment scrap charges were no longer available and required, the bag-type collector was chosen. double-slag heats were adopted, the smoke The final selection was made from col- condition became serious. Visibility between lectors manufactured by the Wheelabrator crane cab and floor was practically nil. The Corporation, Mishawaka, Indiana. No exhaust fans seemed useless and i t became doubt there are many other suppliers of evident that some means must be provided this type of equipment, but the dust col- to bring heated air in a t approximately floor lector chosen would fit nicely on the Finkl line in order to carry the smoke up to the firebrick storage building, the pro rata bag- roof fans. life guarantee of two years was one of

The next step was to install eight the best, and the price was competitive. 25,000-cfm fresh air heaters, providing Several other manufacturers refused to 200,000 cfm of 60°F air a t a little over 6 f t quote a t the time because of the unknown above floor line. These heaters can con- effluent temperature. dense close to ~o .ooo lb of waste steam The hood is in three sections. The first

272

FUME CONTROL A1 W SCRAP SOURCES 273

section, over the charging door, contains a sliding companion flange, plenum cham- ber, and distribution damper. The center section has three nonmagnetic stainless- steel flappers around the electrodes to facilitate maintenance of the cooler rings and help reduce the hysteresis losses in the hood. The front section has a slot for collecting fume from around the spout. All three sections are supported on the roof-supporting arms and the 13-ft 6-in. roof ring. Each complete hood requires 31,500 cfm a t 4-in. water column suction.

Two separate dust collectors of six baghouses each are assembled into one continuous unit IOO f t long, mounted on top of an adjacent building (Fig 5). Only five of the baghouses are in operation a t the same time; one of them being valved off for shaking into its 60-deg hopper. Rotation from one baghouse to another for shaking is automatic, and duration of baghouse shutdown and bag shaking is adjustable. The bag ratio is 2.92 cfm per square foot with one baghouse off the line for shaking, and 2.44 cfm per square foot of bag area with all the units in operation. Each furnace has its own exhaust system but the two 60-hp exhaust fans can be cross-connected if one should fail. These fans are on the clean-air side, permitting the use of backward-curving blades instead of the long-shavings type, as would be necessary if the fans were on the dusty or 'inlet side of the baghouses. This method of installation is a little more expensive because of additional mani- folding but i t has the following advantages:

I. More efficient nonoverloading type of fan.

2. Easier starting, lighter fan wheel. 3. An impeller that stays clean and lasts

longer. 4. Less vibration for overhead .installa-

tions. 5. Baghouses are under suction instead

of pressure and will not leak dust. 6. Effluent can be discharged back into

the shop on cold days and save the waste heat.

Temperature of the fume into the bag- houses is limited by a motor-operated IS-in. butterfly valve that vents fresh cool air into the system if the duct temperature reaches 27s°F. If this should prove insufficient, or fail, a 12-in. square gravity-operated drop-out damper in the shop will open a t 3o0°F Each hopper has a rotary valve discharging into a common conveyer, so the hoppers can be emptied during operation of the dust collectors.

No matter what kind of hoods are used, they do not fit the measurements, and so cannot be installed between heats. Some- times a week-end may even be too short a time. I t all depends on how accurate the measurements are and how much the fur- nace has warped and distorted between the time of measurement and assembly. I t is like fitting a brand new roof ring on an old furnace, only worse.

The assembly of the knocked-down American Dustube Collector a t the Fink1 plant bordered on the ridiculous. Sup- porting structures had been designed and installed on concrete-capped parapets with little trouble and assembling of the sheet- metal panels to angle-iron uprights was started with the help of very necessary electric impact wrenches, when a repre- sentative of the Iron Workers Union arrived. He requested that iron workers be used instead of the company's maintenance men, so work was started again. Shortly, a sheet-metal worker appeared and sug- gested that sheet-metal workers be em- ployed, so peace was maintained by em- ploying both iron workers and sheet-metal workers to do the erection. The cost was not out of line, however, and American Wheelabrator Co. kept a field man on the job until the equipment was ready for operation.

Chicago Blow Pipe designed and erected

274 PROCEEDINGS OF ELECTRIC FURNACE STEEL CONFERENCE, 1956

the ductwork according to the Fink1 speci- hcations, which accommodated both the hood and dust-collector requirements, and the ductwork as usual, when properly in- stalled, proved to be a maintenance-free item.

The plant's own "A" electricians wired all of the electrical equipment, including 1 2 damper motors, 12 shaker motors, two 60-hp fan motors, one screw conveyer motor, two motorized airbleeds and two solenoid-operated air bleeds, the timers, and Leeds and Northrup duct-temper- ature recorders.

Probably the most interesting, if not the most important item, is the cost of the installation and \rhat it has done to the cost of operating the electric furnaces. About the first part, there is no doubt. The cost was:

. . . . . . . . Structurals.. $ 891.70 Electrical equipment

. . . . . not supplied. 2,701. j3 Ilust collector. . . . . . . 5 2,770.00 lidison, excess facili-

. . . . . . . . . . . . ties.. 946 .oo Iron ant1 sheet-metal

workers. . . . . . . . . . 10,609.40 Leeds & Northrup re- .

corder.. . . . . . . . . . . I , I 58. o j American Air Filter

. . . . . . . . . . hoods.. I 1, j j6.68 . . . . . . . . . . Painting.. 2,687 .oo

Chicago Rlotv Pipc . . . . . . . ductwork.. 17,499 .oo

Plant's own labor. . . . 6,374.32 . . . . . Miscellaneous.. 465.78

$107,659.46

Taking an accelerated five-year de- preciation and adding for ' a n estimated two-year bag life, power consunlption and maintenance, will probably tome close to a very surprising $1.00 per ton cost for dust collection. However, the effect on operation is not as discouraging and hoods and dust collectors are not as hard to live

with as they were expected to be. Operation of the dust collectors began on Oct. 18, 1954. For a 13-week period prior to that date, kilowatt-hours per ton had averaged 524, and for a similar period afterivard they averaged 520. There was a similar negligible comparison in electrode con- sumption, although now it is necessary to carry a higher electrode column so that the electrode clamps will clear the addi- tional hood height without changing opera- tions appreciably. Likewise, the refractory life of the roofs has remained relatively unchanged, although changing a roof takes longer with a hood installed. \\'hen- ever it is possible roofs are changed on \\leek-ends, when the shop is down. Re- moving the cooler rings takes much longer than before and it is very difficult to change them with the hood in the way.

Between 3 and 6 Ib of dust per ton is collected, depending on the cleanliness of scrap and operating conditions. The dust is one of the messiest waste materials to handle-it seems to get all over everything. T o minimize this condition, the dust from the screw conveyer is collected into paper bags, each containing about 80 lb of dust. Even this was a filthy operation until a little bagging booth was made and kept under the suction from the dust-collec- tor manifold (Fig I). An attempt was made to charge the bagged dust 'in order to re- claim some of'the'alloys and save the pay- ment to the scavenger for hauling it away, but the pounds of dust per ton kept going up, so the practice was discontinued.

The analysis of the dust is: FeO, 4.2 pct; FezOs, 35.04; CaO, 14.9; A1203, 12.9; SiOz, 8.8; MnO, 12.1; MgO, 7.9; C, 2.3; Ca&, 1.2; NiO, o . ~ ; Pz06, 0.1; S, 0.26;plus traces of molybdenum oxiae and vanadium.

Probably the greatest unknown factor confronting the plant engineer in installing a dust collector is the temperature of the effluent coming from the hoods. These

is accentuated in the lower, less harmful, that does escape hangs around the shop on frequencies. Audiometer tests are run on humid days and leaves something to be all employees a t least once a year, and so desired. far no one showing an increased loss of The dust collector extracts the dust

Octove bonds,cycles per second

hearing has been found in the melt shop. Therefore, the noise level must be below the critical (Fig 2).

'I'he answer to the question of how much of the dust is collected is a t best a guess, as there is no way available to measure how much escapes. Visually, under normal operating conditions, and comparing one furnace with the hood on while the other is operating with no hood, indications are that between 80 pct and go pct of the dust is collected. The amount escaping from the hoods seems to be greater when the furnace is under high pressure during exothermic reactions, but then there is more smoke. and percentages of collection seem to be about the same (Figs 3, 4, 6). The system is completely satisfactory for the work for which i t was intended. The shop is a much cleaner and safer place in which to work, and the crane operator does not lose visual contact with the floor; however, the small percentage of smoke

efficiently. A man with a white shirt on can stand in front of the discharge from the fans and the shirt will not get dirty. Piping the effluent into the shop did not prove practical because during a carbon block the amount of carbon monoxide in the shop became too high and some of the men on the floor complained of headaches.

During the operating periods of the fur- naces, the shop is fairly free from smoke; however, during tapping and teeming, the smoke in the shop again becomes a prob- lem. An attempt has been made to catch some of the smoke emitted during tapping with the lateral type of hood shown in Fig 7 and probably about half of i t is collected. The smoke is then piped into the dust collector of the furnace, which is tilted, as its hood is inoperative during tapping. An attempt to improve this system was made by adding an axial-flow blower in the opposite side of the pit. I t discharged into a long slot blowing 6400 cfm across the ladle and above the tapping stream, to force the smoke into the lateral hood.

FUME CON'I'l1OL Ah [I> SCllAP 'SOURCES 279

The gas is then sucked through two Pease-Anthony venturi throats, where it is again sprayed with 48 water sprays - o n 2-in. centers in each throat. The water rate a t the throats is 550 gpm per unit, or 1100 gpm for both. The drop in pressure through these veilturi is kept a t 30 in. of \vater.

The gas is then drawn through to cen- trifugal separators, where the dust-laden water is separated from the dust stream and runs off with the overflo\v. Next the gas passes through two 70,000-cfm fans driven by two 500-hp motors, thcn illto a common stack 150 f t high.

The outlet dust loading on our system is 0.02 to 0.03 grains.

We arc getting ready to install this system on two of our foundry electric furnaces.

Getting back to the two systems de- scribed in the papers by Mr. Peterson and Mr. Finkl, there are several questions in my mind.

Mr. Finkl quotcs a dust volume of 3 to 5 lb pcr ton of steel processed and Mr. Peterson quotes 30 Ib per ton, or approxi- mately six times as much Can this differ- ence be explained? The amount coming off our oxygen furnaces is 15 lb per ton and we do not yet have any figures for our electric furnaces.

Secondly, there is a very large difference in the amounts of iron and lime analysis of the two dusts. Can this be explained?

Third, has Mr. Finkl given any consider- ation to installing a hole in his electric furnace roofs as a means of taking a\i7ay the smoke? This sccms to bc finding favor in several places.

W. hl. PATTERSON, CHAIRMAN-Would you care to take a shot a t that, Mr. Finkl?

C. FINKL-AS for the discrepancy in the pounds per ton, I think that is purely a function of the method of operation and the

type of scrap used. We remelt 25 pct of the steel we make. Our scrap is fairly clean and that is probably the reason for it. We do not use any old painted I-beams or things of that nature.

As for the hole in the roof, we gave that serious consideration, and we were afraid of oxidizing the slag. Tha t is the reason we chose the type of closure we use.

DUST I~OLUME W. M. PATTERSON, CHAIRMAN-Thank

you, Mr. Finkl. I think the question about th; amount of dust residue is something that will have to wait to be determined, probably for some time-or shall we say it is more a matter of difference in the oper- ation? We are running behind time, bu t .1 would like to allow five minutes for ques- tions. Would anyone like to ask these gentlemen some questions? Who will be first?

P . E. LINDBERG-I should like to clear up one point. I think the figure for dust is a little misleading. We combine our total dust-that is, blast furnace, open-hearth, and electric furnace. I believe the dust and thc electrics actually check about 25 lb on spasmodic checks; the open hearth, 35 lb, and that is all mixed with our blast. furnace, which runs about 170 lb. The question of whether or not 5 lb or 25 lb is right probably is a matter of how accu- rately this figure can be checked. Our scrap is a clean No. I scrap out of Detroit. Wc do not use any No. z scrap, and the figures I have seen indicated from Bethlehem on the west coast check very closely thc figures that Mr. Petersoil quoted.

W. M. PATTERSON, CHAIRMAN-Thank you, Mr. Lindberg. Are there any other questions or comments?

H. F. WALTHER-These papers con- tained excellent descriptions. One impor- tant point that might be discussed is

whether there is any effect on reducing slags with either of these systems. I am thinking mainly of the possible effects on maintenance of carbidic slags and also silicon slags. I t appears that single oxidizing slags would not be affected much. An opin- ion would be appreciated.

W. h'1. PATTERSON, CHAIRMAN-I\~~ YOU

directing your question to either one of these gentlemen or to both?

H. F. WALTm~-Either one or both, Mr. Peterson or Mr. Finkl.

C. FINKL-First, I would like to apolo- gize. I do not mean that Mr. Lindberg was using poor scrap.

We do double slag and i t is a reducing slag, a carbide slag. I mentioned in the paper though, that the suction with or without the exhaust fan in operation was exactly the same. We do not get any air infiltration a t the door. I n fact, I feel that our slag is unaffected by the hoods.

H. F . WALTHER-That is very interesting.

W. M. PATTERSON, CHAIRMAN-I wish we had more time to go into the effect of smoke and fume collection on reducing slags. I think we will have to recommend that information be brought into our meet- ing next year on that subject. Many people are pondering the very question that you ask, Mr. Walther, and I do not think any- body has any more information than you have. We might take one more question, if there are any.

SAMUEL ARNOLD, ~ R D - A ~ o u ~ three years ago I thought I knew something about removing gases from a furnace. Now

I am not so sure. M y question is this: How much free air is bled into the hood, as i t is my understanding that these hoods have been remodeled and made higher than they were originally? I t is my further under- standing that considerable atmospheric air is drawn into the hood and in that way the negative pressure in the furnace is some- what reduced. I s that correct, Mr. Finkl?

C. FINKL-The previous paper men- tioned generating 6000 c f n ~ per furnace, did i t not?

SAMUEL ARNOLD, ~RD-Something like that.

C. FINKL-That meant the generative gases in the furnace. We are pulling 31,500 cfm through the hood and we have a much smaller furnace, which generates about 1000 or zoo0 cfm of gas in the bath. We are drawing the excess air from around the furnace.

SAMUEL ARNOLD, ~ R D - I ~ other words you have openings in the hood to permit introduction of sufficient atmospheric air for dilution, ant1 that in turn reduces the negative pressure in the furnace?

C. FINKL-Yes, in fact we have difficulty in keeping the openings closed. We have more openings than we know what to do with.

W. M. PATTERSON, CHAIRMAN-I know we could go on indefinitely-it is a very interesting subject-but I am afraid we will have to pass on.

Mr. P. E. Cavanagh, Director of the De- partment of Engineering and Metallurgy, Ontario Research Foundation, Toronto, Ont., Canada, will present a paper on melting stocks. Mr. Cavanagh.