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Burning issues impact kettle purchases
Burning Issues Impact Kettle Purchases Energy consumption and production capability are among the prime considerations. By Thomas H. Cook, Consultant, Hot Springs, S.D.
Editor's note: This is the first of a multi-part series.
ecause of rapidly rising natural gas prices, the information in this article can help galvanizers make better purchasing decisions on kettle furnaces. Future kettle furnaces will have to operate using less energy and have higher production capabilities to be competitive. A hot dip galvanizing kettle is made of nearly pure iron, initially has two-inch thick walls, and is usually operated at 840 to 850°F. When exceeding 890°F, the iron kettle walls rapidly react with zinc, causing premature failure. A properly constructed kettle in a proper furnace will normally have a useful lifetime of five to 10 years. Many have a lifetime of seven years without difficulty. Some information in this article may apply to steel heat treating furnaces.
B
BASIS DATA FOR CALCULATIONS The parameters used are as follows: • Average specific heat for iron (between 20 and 500°C) = 0.135 cal./g. C deg (raise temperature of iron)* • Average specific heat for zinc (between 20 and 500°C) = 0.117 cal./g. C deg (raise temperature of zinc)* • Delta H (enthalpy of fusion, at melting point for zinc) = 23 cal./g (melt zinc)* • Room temperature: 70°F (21.1°C); kettle zinc ternperature: 850°F (454.4°C) • Delta H (enthalpy of latent heat of vaporization for water) = 540 cal./g.* • 100,000 BTUs = 1 therm* • H e a t lost at zinc/air interface (convection and radiation for "open" kettle) = 6,700 BTUs/ft. 2.* • Heat lost at zinc/air interface (convection and radiation for "enclosed" kettle) = 4,000 BTUs/ft.2/hr. *** • Heat lost at zinc/air interface when kettle is fully covered with 4 inches of insulation = 358 BTUs/ft.2/hr. *** • Safe sidewall h e a t t h r o u g h p u t = 10,000 BTUs/ ft.2/hr. ** • The percentage of total b u r n e r h e a t passing through kettle salls heating zone (side-fired-flatflame) = 50%*** • The percentage of total b u r n e r h e a t passing through kettle walls heating zone (high-velocityend-fired) = 67%*** 46
• To protect the kettle from early failure: outside top (protects zinc/air interface) insulation six vertical inches downward from kettle top; and (protects at zinc/dross line) bottom insulation 9 inches upward from kettle bottom. (*Chemistry Physics Handbook; **Zinc Corp. (now Horsehead Corp.); ***author's best estimates from experience.) The heat required to galvanize one short ton of steel (2,000 lb.) is as follows: 210,570 BTUs (to raise temperature of steel from 70 to 850°F, 2,000 lbs. of steel); 18,571 BTUs (to raise temperture and melt 7% GZU, 140 lbs. of zinc for makeup); 6,317 BTUs (to raise temperature of racking, 3% wieght of steel product); 2,313 BTUs (to vaporize water in wet flux in capillary crevices); 771 BTUs (to vaporize water in wet flux on flat steel surfaces); 1,458 BTUs (to heat up and remelt zinc from splash-out of kettle during steel entry, 11 lbs. of zinc); and equally 240,000 BTUs total to galvanize one short ton of steel (2.4 therms). FLAT-FLAME-SIDE-FIRED KETTLE (45 ft. l o n g x 5.25 ft. w i d e x 8 ft. d e e p ) This kettle is h e a t e d by several flat-flame (also called spiral) burners mounted perpendicular to and along the two long sides of the kettle. The ends of the kettle are not heated. The heat of these burners convect and radiate through the long side walls and then exhaust at the flue near one end. The average contact time between the hot gases and side walls is quite short because the average distance of contact is only half the length of the kettle. For a shallow kettle (such as six feet deep or less), normally a single horizontal row of flat-flame burners on each long side is used. For a deep kettle, two staggered rows are usually used. To prevent early kettle failure, all kettles require horizontal insulation attached on the outside of the kettle sides. In this case, both long-side walls from the top edge six inches downward must be insulated to p r e v e n t b u r n - t h o u g h at the zinc/air interface. Similarly, both long-side walls from the bottom, nine inches upward, must be insulated to prevent early burn-through at the zinc/dross boundary. The vertical height of the heating zone along the two long-sides is 96 in. (8 ft.) - 6 in. (top insulation) www.metalfinishing.com
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- 9 in. (bottom insulation) = 81 in. (6.75 ft., vertical height of heating zone). The entire heating zone is the vertical height times the horizontal length of the long side times two sides: 6.75 ft. x 45 ft./side x 2 sides = 607.5 ft. z. The safe h e a t t h r o u g h p u t t h r o u g h the h e a t i n g zone is 607.5 ft. z x 10,000 BTUs/ft.Z/hr./100,000 BTUs/therm = 60.75 therms/hr. This kettle is taken to not have a kettle enclosure and loses 6,700 BTUs/sq. ft./hr, at the zinc/air interface due to convection and radiation losses. For the whole zinc surface (no enclosure), the radiated and convected losses are 6,700 BTUs/ft. z x 45 ft. x 5.25 ft./100,000 BTUs/therm = 15.83 therms/hr. Thus, the net h e a t available for galvanizing is 60.75 therms/hr. Total through heating walls zone minus 15.83 therms/hr, convected and radiated heat losses at zinc/air interface equals 44.92 therms/hr. net for galvanizing. The heat required to galvanize one short ton is 2.4 therms. Thus, this flat-flame-sidefired kettle has a capacity of 44.92 therms/hr./2.4 therms/ton = 18.72 short tons/hr, continuous for flatflame-side-fired. Experience shows t h a t with flatflame-side-fired heating, about 50% of the burner heat passes through the kettle walls heating zone with the r e m a i n i n g energy (50%) passing up the exhaust flue. So the required total burner capacity is 60.75 therms/hr./0.50 = 121.5 therms/hr. (total required burner capacity). If 18 flat-flame burners (nine on each side) are used, then each burner will need to have an output of 121.5 therms/hr./18 burners = 6.75 therms/hr./burner. HIGH-VELOCITY-END-FIRED ( 4 5 ft. l o n g x 5 . 2 5 ft. w i d e
KETTLE x 8 ft. d e e p )
With this type heating system, the kettle sits within the furnace walls such that (as viewed from above) there is a one- to two-foot space between all four outside kettle walls and inside the four furnace walls. The high-velocity burners are mounted within this one- to two-footwide space normally on two diagonal (opposite) kettle corners. The burners are directed to fire lengthwise along the long walls and then to go around the far corner, so end walls are heated. The kettle corners adjacent to the burners are insulated, or have a metal stand-offshield to prevent corner burnout. As viewed from above, the hot gases from the burners pass around and around the kettle in a continuous loop, giving a long period of time to exchange their heat to all four vertical kettle walls. Ultimately, the cooler exhaust gases pass out the flue near the bottom of the furnace at one end. The vertical height of the heating zone is the same as the previous example: 81 in. (6.75 ft., vertical height of heating zone). The entire heating zone is the vertical April 2005
height times the horizontal perimeter of the kettle: 6.75 ft. x 45 ft. + 45 ft. + 5.25 ft. + 5.25 ft. = 678.4 ft.2). The safe h e a t t h r o u g h p u t t h r o u g h the h e a t i n g zone is 678.4 ft. 2 x 10,000 BTUs/ft.Z/hr./100,000 BTUS/therm = 67.84 therms/hr. This kettle is taken to have a kettle enclosure, an enclosure e x h a u s t fan t h a t operates only d u r i n g steel entry into the zinc, and only loses about 4,000 BTUs/ft.Z/hr. at the zinc/air interface due to convection and radiation losses. A kettle enclosure is a fixed steel rectangular structure above the kettle, up to the ceiling and made of steel sheet. The skimming is accomplished t h r o u g h sliding doors. The lifts enter and exit the enclosure through end doors. If the sliding doors t h r o u g h which s k i m m i n g is done come downward to close, t h e n safety equipmerit m u s t be adequate to prevent an accident in which these doors become a guillotine. Horizontally sliding doors or u p w a r d closing doors provide a much safer system. An exhaust fan sucks the smoke out of the enclosure and through a bag house. For the whole zinc surface, this is 4,000 BTUs/ft.2/hr. x 45 ft. x 5.25 ft./100,000 B T U s / t h e r m = 9.45 therms/hr. The net heat available for galvanizing is 67.84 therms/hr, total heat through heating walls zone minus 9.45 therms/hr, convected and radiated heat losses at the zinc/air interface equals 58.39 therms/hr, net for galvanizing. This high-velocity-end-fired kettle has a capacity of 58.39 therms/hr./2.4 therms/ton = 24.33 short tons/hr, continuous for high-velocity-end-fired. Experience shows t h a t with high-velocity-end fired heating, about 67% of the burner heat passes t h r o u g h the kettle walls h e a t i n g zone w i t h the remaining 33% passing up the exhaust flue. So the required total burner capacity is 67.84 therms/hr./0.67 = 101.25 therms/hr, total required b u r n e r capacity. If six high-velocity burners (two groups of three burners each and these groups on diagonal kettle corners) are used, then each burner will need to have an output of 101.25 therms/hr./6 burners = 16.88 therms/hr./burner. This high-velocity-end-fired kettle furnace is modeled after an actual furnace. The burners are pulsed until a heat demand of about 97% is reached, then fire fully and continuously (pulsing stops). If in a different furnace design, where the burners are pulsed (even at high heat demand), then higher capacity b u r n e r s would be required to achieve the 24.33 short tons/hr, this kettle is capable of. For example, if the burners (16.88 therms/hr./burner) are pulsed "on" for three seconds and "off" for one second (at the highest heat demand), then the m a x i m u m capacity is lowered to 17.26 tons/hr. To restore the capacity of 47
the kettle to 24.33 tons/hr., burners with a capacity of 22.51 therms/hr./burner are required.
ECONOMICS High-Velocity-End-Firing vs. Flat-Flame-SideFiring (48,660 tons steel/yr.) The end-fired kettle, as described in the previous section, operated eight hours/day for 250 working days/calendar year, can galvanize: 24.33 tons/hr, x 8 hr./day x 250 days = 48,660 short tons/yr. The total heat used during production hours is 8 hours/day x 250 days x 101.25 therms/hr. -- 202,500 therms/year (production). The total non-production hours/calendar year are [365 days x 24 hr./day] - (8 working hr./day) x 250 working days] = 6,760 idle hr./yr. During this non-production time the kettle is fully covered with a four-inch thick insulated cover. The heat lost at the zinc/air interface for the year during non-production is (the 0.67 in the denominator is required because 67% of the burner energy is passing through the heating zone of the walls): 6,760 hr./yr, x 45 ft. x 5.25 ft. x 358 BTUs/ft.2/hr./0.67 = 8,533 therms/yr. (conv./rad.). The total heat consumed for the year is 202,500
therms/yr, for production + 8,533 therms/yr, for convected and radiated heat losses at the zinc/air interface during idle= 211,033 therms/yr, total heat consumed by the high-velocity-end-fired furnace. For the side-fired kettle to galvanize the 48,660 short tons in 250 working days, the hr./day for production must be 48,660 tons/18.72 tons/hr./250 days = 10.4 hr./day for flat-flame-side-fired. The heat used for the side-fired kettle for production is 10.4 hr./day x 250 days x 121.5 therms/hr. = 315,900 therms/yr, for production. The total nonproduction hours/calendar year are [365 days x 24 hr./day] - [10.4 hr. x 250 days] = 6,160 idle hr./yr. During this non-production time the kettle is taken to not be covered. Many kettles cannot be covered because the burners cannot be "turned-down" sufficiently or turned off. The convection and radiation heat loss at the zinc/air interface for the year during non-production is (the 0.50 in the denominator is required because 50% of the burner energy is passing through the h e a t i n g zone of the walls): 6,160 hr./yr, x 45 ft. x 5.25 ft. x 6,700 BTUs/ft.2/hr./ 0.50 = 195,010 therms/yr. (conv./rad.). The total heat consumed by the flat-flame-side-
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$408,728/yr. for flat-flame-sidefired. On a per short ton basis this is $408,728/48,660 short tons = $8.40/short ton for flat-flame-sidefired. The total energy cost for galvanizing 48,660 short tons/yr, in the high-velocity-end-fired kettle operating eight hr/work day is 211,033 therms/yr, x $0.80/therm = $168,826/yr. for high-velocityFigure 1: Kettle energy savings: flat-flame-side-fired versus high-velocity-end-fired: end-fired. On a per short ton 22,500 tons/yr. basis this is $168,826/48,660 = fired furnace for the year is 315,900 therms/yr, for $3.47/short ton for high-velocity-end-fired. production + 195,010 therms/yr, for convection and The kettle energy savings by using high-velocityradiation losses at the zinc/air interface during idle end-fired r a t h e r t h a n flat-flame-side-fired is - 510,910 therms/yr, total heat consumed by the flat$408,728/yr. for flat-flame-side-fired - $168,826/yr. for high-velocity-end-fired = $239,902/yr. kettle flame-side-fired furnace. Some galvanizers are paying $8/thousand ft. 3 for energy savings using high-velocity-end-fired versus n a t u r a l gas. For gas with a h e a t value of 1,000 flat-flame-side-fired. BTUs/ft. 3, this is $0.80/therm. On a per short ton basis this is $8.40/short ton for flatThe total energy cost for galvanizing 48,660 short flame-side-fired - $3.47/short ton for high-velocity-endtons/yr, in the flat-flame-side fired kettle operating 10.4 fired = $4.93/short ton kettle energy savings using hours/work day is 510,910 therms/yr, x $0.80/therm = high-velocity-end-fired versus flat-flame-side-fired.
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This $239,902/yr. total energy savings is a result of three items: higher efficiency of high-velocity-endfiring; fully covering the kettle during idle periods; and having an enclosure around the kettle. For a galvanizer having a 45-ft. long x 5.25-ft. wide X 8-ft. deep kettle that is flat-flame-side-fired, not fully covered during idle and not having a kettle enclosure, which is only able to use one of these energy saving methods, the individual savings are: switching from flat-flame-side-fired to a high-velocity-end-fired kettle is a $103,707/yr. savings; fully covering the kettle during idle periods (all hours except the 10.4 hours of production/250 work days) is a $147,840/yr. savings; and using a kettle enclosure is a $89,405/yr. savings. The sum of these individual savings is greater (by $101,050/yr.) t h a n the total for all three ($239,902/yr.) because after using one, you're saving energy. The remaining energy to save is proportionately smaller. Mathematically, a reasonable "term" for this is "geometric decay." These individual savings are presented so galvanizers who are able to use only one energy savings method can d e t e r m i n e cost/payback figures. This enables the best "bang for the buck" and enables better future planning for predictably higher natural gas prices. Besides the $239,902/yr. kettle energy savings in galvanizing 48,660 tons of steel using a high-velocityend-fired furnace, fully covering the kettle during idle periods, and having a kettle enclosure, substantial savings are likely by reducing overtime pay. About 50 workers would be required to galvanize 48,660 tons/yr. Because 10.4 hr./work day are required using a flatflame-side-fired furnace (versus 8 hr./work day using high-velocity-end-firing), an overtime of 2.4 hr./work day for 50 workers is likely. If the differential overtime cost is $9/hr. then: $9/hr. x 50 workers x 250 work days/yr, x 2.4 hr./work day = $270,000/yr. savings for less overtime For this overtime savings to materialize, the thermal capacity of the flat-flame-side-fired kettle would have to be the "bottle neck" of the galvanizing plant, but normally it is. Lowering energy costs by capturing waste h e a t from the kettle flue to h e a t the caustic and flux tanks is becoming popular. Normally, there is enough waste heat available to heat both the caustic and flux to about 150 to 160°F, provided suds (bubbles) are m a i n t a i n e d on the surface of these process tanks. The h e a t t r a n s f e r method usually consists of a large steel coil in a horizontal section of the flue, large exchange coils in the caustic (steel) 50
and flux (titanium), and a high-capacity pump, pumping somewhat pressurized hot water. The process coils are protected by sturdy barriers. The large heat-exchange coils in the caustic and flux tanks must be in one end only, to facilitate bubble generation and distribution, and enable oil removal at the end opposite the coil. The yearly savings at $0.80/therm for natural gas is between $47,000 and $119,000, depending on the efficiency of the heating systems that the recovery system is replacing and the ability to maintain suds on the caustic and flux. This heat recovery system is not unique to highvelocity-end-firing and can be used on flat-flame-sidefired furnaces. The reason to consider heat recovery from the kettle flue is if a change from flat-flame-sidefired is under consideration.
High-Velocity-End-Firing vs. F l a t - F l a m e S i d e - F i r i n g (22,500 t o n s steel/yr.) The galvanizing production of 48,660 short tons/yr. in the previous section would be characteristic for some captive shops but not usual for general jobshop North American galvanizers. Typically, job shop companies galvanize about one million lb./yr. for each foot of length of kettle. Thus, for a North American job-shop galvanizer with a 45-ft.-long kettle, the expected production would be about 22,500 tons steel/yr. Figure 1 shows the results of the analogous calculations in the previous sections but applied to 22,500 short tons/yr. Consider the individual energy savings for each method: switching from flat-flame-side-fired to a high-velocity-end-fired kettle is a $75,910/yr. savings (less t h a n the previous case because less gas is burned); fully covering the kettle during idle periods (all hours except the eight hours of production/250 work days) is a $162,240/yr. savings (greater than the previous case because the kettle is covered more hr./yr.); and using a kettle enclosure results in a $89,405/yr. savings (same as previous case because enclosure savings are independent of production). The sum of these individual savings do not add up to the total savings because w h e n one method is started, the remaining methods become less significant (e.g. "geometric decay"). Thomas H. Cook has taught chemistry, physics, and math at Nebraska Western College and Black Hills State University for a combined 20 years. He has consulted, p u t on workshops, and published about 25 articles on hot dip galvanizing during the last 30 years. He can be contacted at (phone~fax) 605-7454567; (e-mail) galvecon@gwtc, net. www.metalfinishing.com
Editor's note: This is the first of a multi-part series.
ecause of rapidly rising natural gas prices, the information in this article can help galvanizers make better purchasing decisions on kettle furnaces. Future kettle furnaces will have to operate using less energy and have higher production capabilities to be competitive. A hot dip galvanizing kettle is made of nearly pure iron, initially has two-inch thick walls, and is usually operated at 840 to 850°F. When exceeding 890°F, the iron kettle walls rapidly react with zinc, causing premature failure. A properly constructed kettle in a proper furnace will normally have a useful lifetime of five to 10 years. Many have a lifetime of seven years without difficulty. Some information in this article may apply to steel heat treating furnaces.
B
BASIS DATA FOR CALCULATIONS The parameters used are as follows: • Average specific heat for iron (between 20 and 500°C) = 0.135 cal./g. C deg (raise temperature of iron)* • Average specific heat for zinc (between 20 and 500°C) = 0.117 cal./g. C deg (raise temperature of zinc)* • Delta H (enthalpy of fusion, at melting point for zinc) = 23 cal./g (melt zinc)* • Room temperature: 70°F (21.1°C); kettle zinc ternperature: 850°F (454.4°C) • Delta H (enthalpy of latent heat of vaporization for water) = 540 cal./g.* • 100,000 BTUs = 1 therm* • H e a t lost at zinc/air interface (convection and radiation for "open" kettle) = 6,700 BTUs/ft. 2.* • Heat lost at zinc/air interface (convection and radiation for "enclosed" kettle) = 4,000 BTUs/ft.2/hr. *** • Heat lost at zinc/air interface when kettle is fully covered with 4 inches of insulation = 358 BTUs/ft.2/hr. *** • Safe sidewall h e a t t h r o u g h p u t = 10,000 BTUs/ ft.2/hr. ** • The percentage of total b u r n e r h e a t passing through kettle salls heating zone (side-fired-flatflame) = 50%*** • The percentage of total b u r n e r h e a t passing through kettle walls heating zone (high-velocityend-fired) = 67%*** 46
• To protect the kettle from early failure: outside top (protects zinc/air interface) insulation six vertical inches downward from kettle top; and (protects at zinc/dross line) bottom insulation 9 inches upward from kettle bottom. (*Chemistry Physics Handbook; **Zinc Corp. (now Horsehead Corp.); ***author's best estimates from experience.) The heat required to galvanize one short ton of steel (2,000 lb.) is as follows: 210,570 BTUs (to raise temperature of steel from 70 to 850°F, 2,000 lbs. of steel); 18,571 BTUs (to raise temperture and melt 7% GZU, 140 lbs. of zinc for makeup); 6,317 BTUs (to raise temperature of racking, 3% wieght of steel product); 2,313 BTUs (to vaporize water in wet flux in capillary crevices); 771 BTUs (to vaporize water in wet flux on flat steel surfaces); 1,458 BTUs (to heat up and remelt zinc from splash-out of kettle during steel entry, 11 lbs. of zinc); and equally 240,000 BTUs total to galvanize one short ton of steel (2.4 therms). FLAT-FLAME-SIDE-FIRED KETTLE (45 ft. l o n g x 5.25 ft. w i d e x 8 ft. d e e p ) This kettle is h e a t e d by several flat-flame (also called spiral) burners mounted perpendicular to and along the two long sides of the kettle. The ends of the kettle are not heated. The heat of these burners convect and radiate through the long side walls and then exhaust at the flue near one end. The average contact time between the hot gases and side walls is quite short because the average distance of contact is only half the length of the kettle. For a shallow kettle (such as six feet deep or less), normally a single horizontal row of flat-flame burners on each long side is used. For a deep kettle, two staggered rows are usually used. To prevent early kettle failure, all kettles require horizontal insulation attached on the outside of the kettle sides. In this case, both long-side walls from the top edge six inches downward must be insulated to p r e v e n t b u r n - t h o u g h at the zinc/air interface. Similarly, both long-side walls from the bottom, nine inches upward, must be insulated to prevent early burn-through at the zinc/dross boundary. The vertical height of the heating zone along the two long-sides is 96 in. (8 ft.) - 6 in. (top insulation) www.metalfinishing.com
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- 9 in. (bottom insulation) = 81 in. (6.75 ft., vertical height of heating zone). The entire heating zone is the vertical height times the horizontal length of the long side times two sides: 6.75 ft. x 45 ft./side x 2 sides = 607.5 ft. z. The safe h e a t t h r o u g h p u t t h r o u g h the h e a t i n g zone is 607.5 ft. z x 10,000 BTUs/ft.Z/hr./100,000 BTUs/therm = 60.75 therms/hr. This kettle is taken to not have a kettle enclosure and loses 6,700 BTUs/sq. ft./hr, at the zinc/air interface due to convection and radiation losses. For the whole zinc surface (no enclosure), the radiated and convected losses are 6,700 BTUs/ft. z x 45 ft. x 5.25 ft./100,000 BTUs/therm = 15.83 therms/hr. Thus, the net h e a t available for galvanizing is 60.75 therms/hr. Total through heating walls zone minus 15.83 therms/hr, convected and radiated heat losses at zinc/air interface equals 44.92 therms/hr. net for galvanizing. The heat required to galvanize one short ton is 2.4 therms. Thus, this flat-flame-sidefired kettle has a capacity of 44.92 therms/hr./2.4 therms/ton = 18.72 short tons/hr, continuous for flatflame-side-fired. Experience shows t h a t with flatflame-side-fired heating, about 50% of the burner heat passes through the kettle walls heating zone with the r e m a i n i n g energy (50%) passing up the exhaust flue. So the required total burner capacity is 60.75 therms/hr./0.50 = 121.5 therms/hr. (total required burner capacity). If 18 flat-flame burners (nine on each side) are used, then each burner will need to have an output of 121.5 therms/hr./18 burners = 6.75 therms/hr./burner. HIGH-VELOCITY-END-FIRED ( 4 5 ft. l o n g x 5 . 2 5 ft. w i d e
KETTLE x 8 ft. d e e p )
With this type heating system, the kettle sits within the furnace walls such that (as viewed from above) there is a one- to two-foot space between all four outside kettle walls and inside the four furnace walls. The high-velocity burners are mounted within this one- to two-footwide space normally on two diagonal (opposite) kettle corners. The burners are directed to fire lengthwise along the long walls and then to go around the far corner, so end walls are heated. The kettle corners adjacent to the burners are insulated, or have a metal stand-offshield to prevent corner burnout. As viewed from above, the hot gases from the burners pass around and around the kettle in a continuous loop, giving a long period of time to exchange their heat to all four vertical kettle walls. Ultimately, the cooler exhaust gases pass out the flue near the bottom of the furnace at one end. The vertical height of the heating zone is the same as the previous example: 81 in. (6.75 ft., vertical height of heating zone). The entire heating zone is the vertical April 2005
height times the horizontal perimeter of the kettle: 6.75 ft. x 45 ft. + 45 ft. + 5.25 ft. + 5.25 ft. = 678.4 ft.2). The safe h e a t t h r o u g h p u t t h r o u g h the h e a t i n g zone is 678.4 ft. 2 x 10,000 BTUs/ft.Z/hr./100,000 BTUS/therm = 67.84 therms/hr. This kettle is taken to have a kettle enclosure, an enclosure e x h a u s t fan t h a t operates only d u r i n g steel entry into the zinc, and only loses about 4,000 BTUs/ft.Z/hr. at the zinc/air interface due to convection and radiation losses. A kettle enclosure is a fixed steel rectangular structure above the kettle, up to the ceiling and made of steel sheet. The skimming is accomplished t h r o u g h sliding doors. The lifts enter and exit the enclosure through end doors. If the sliding doors t h r o u g h which s k i m m i n g is done come downward to close, t h e n safety equipmerit m u s t be adequate to prevent an accident in which these doors become a guillotine. Horizontally sliding doors or u p w a r d closing doors provide a much safer system. An exhaust fan sucks the smoke out of the enclosure and through a bag house. For the whole zinc surface, this is 4,000 BTUs/ft.2/hr. x 45 ft. x 5.25 ft./100,000 B T U s / t h e r m = 9.45 therms/hr. The net heat available for galvanizing is 67.84 therms/hr, total heat through heating walls zone minus 9.45 therms/hr, convected and radiated heat losses at the zinc/air interface equals 58.39 therms/hr, net for galvanizing. This high-velocity-end-fired kettle has a capacity of 58.39 therms/hr./2.4 therms/ton = 24.33 short tons/hr, continuous for high-velocity-end-fired. Experience shows t h a t with high-velocity-end fired heating, about 67% of the burner heat passes t h r o u g h the kettle walls h e a t i n g zone w i t h the remaining 33% passing up the exhaust flue. So the required total burner capacity is 67.84 therms/hr./0.67 = 101.25 therms/hr, total required b u r n e r capacity. If six high-velocity burners (two groups of three burners each and these groups on diagonal kettle corners) are used, then each burner will need to have an output of 101.25 therms/hr./6 burners = 16.88 therms/hr./burner. This high-velocity-end-fired kettle furnace is modeled after an actual furnace. The burners are pulsed until a heat demand of about 97% is reached, then fire fully and continuously (pulsing stops). If in a different furnace design, where the burners are pulsed (even at high heat demand), then higher capacity b u r n e r s would be required to achieve the 24.33 short tons/hr, this kettle is capable of. For example, if the burners (16.88 therms/hr./burner) are pulsed "on" for three seconds and "off" for one second (at the highest heat demand), then the m a x i m u m capacity is lowered to 17.26 tons/hr. To restore the capacity of 47
the kettle to 24.33 tons/hr., burners with a capacity of 22.51 therms/hr./burner are required.
ECONOMICS High-Velocity-End-Firing vs. Flat-Flame-SideFiring (48,660 tons steel/yr.) The end-fired kettle, as described in the previous section, operated eight hours/day for 250 working days/calendar year, can galvanize: 24.33 tons/hr, x 8 hr./day x 250 days = 48,660 short tons/yr. The total heat used during production hours is 8 hours/day x 250 days x 101.25 therms/hr. -- 202,500 therms/year (production). The total non-production hours/calendar year are [365 days x 24 hr./day] - (8 working hr./day) x 250 working days] = 6,760 idle hr./yr. During this non-production time the kettle is fully covered with a four-inch thick insulated cover. The heat lost at the zinc/air interface for the year during non-production is (the 0.67 in the denominator is required because 67% of the burner energy is passing through the heating zone of the walls): 6,760 hr./yr, x 45 ft. x 5.25 ft. x 358 BTUs/ft.2/hr./0.67 = 8,533 therms/yr. (conv./rad.). The total heat consumed for the year is 202,500
therms/yr, for production + 8,533 therms/yr, for convected and radiated heat losses at the zinc/air interface during idle= 211,033 therms/yr, total heat consumed by the high-velocity-end-fired furnace. For the side-fired kettle to galvanize the 48,660 short tons in 250 working days, the hr./day for production must be 48,660 tons/18.72 tons/hr./250 days = 10.4 hr./day for flat-flame-side-fired. The heat used for the side-fired kettle for production is 10.4 hr./day x 250 days x 121.5 therms/hr. = 315,900 therms/yr, for production. The total nonproduction hours/calendar year are [365 days x 24 hr./day] - [10.4 hr. x 250 days] = 6,160 idle hr./yr. During this non-production time the kettle is taken to not be covered. Many kettles cannot be covered because the burners cannot be "turned-down" sufficiently or turned off. The convection and radiation heat loss at the zinc/air interface for the year during non-production is (the 0.50 in the denominator is required because 50% of the burner energy is passing through the h e a t i n g zone of the walls): 6,160 hr./yr, x 45 ft. x 5.25 ft. x 6,700 BTUs/ft.2/hr./ 0.50 = 195,010 therms/yr. (conv./rad.). The total heat consumed by the flat-flame-side-
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$408,728/yr. for flat-flame-sidefired. On a per short ton basis this is $408,728/48,660 short tons = $8.40/short ton for flat-flame-sidefired. The total energy cost for galvanizing 48,660 short tons/yr, in the high-velocity-end-fired kettle operating eight hr/work day is 211,033 therms/yr, x $0.80/therm = $168,826/yr. for high-velocityFigure 1: Kettle energy savings: flat-flame-side-fired versus high-velocity-end-fired: end-fired. On a per short ton 22,500 tons/yr. basis this is $168,826/48,660 = fired furnace for the year is 315,900 therms/yr, for $3.47/short ton for high-velocity-end-fired. production + 195,010 therms/yr, for convection and The kettle energy savings by using high-velocityradiation losses at the zinc/air interface during idle end-fired r a t h e r t h a n flat-flame-side-fired is - 510,910 therms/yr, total heat consumed by the flat$408,728/yr. for flat-flame-side-fired - $168,826/yr. for high-velocity-end-fired = $239,902/yr. kettle flame-side-fired furnace. Some galvanizers are paying $8/thousand ft. 3 for energy savings using high-velocity-end-fired versus n a t u r a l gas. For gas with a h e a t value of 1,000 flat-flame-side-fired. BTUs/ft. 3, this is $0.80/therm. On a per short ton basis this is $8.40/short ton for flatThe total energy cost for galvanizing 48,660 short flame-side-fired - $3.47/short ton for high-velocity-endtons/yr, in the flat-flame-side fired kettle operating 10.4 fired = $4.93/short ton kettle energy savings using hours/work day is 510,910 therms/yr, x $0.80/therm = high-velocity-end-fired versus flat-flame-side-fired.
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This $239,902/yr. total energy savings is a result of three items: higher efficiency of high-velocity-endfiring; fully covering the kettle during idle periods; and having an enclosure around the kettle. For a galvanizer having a 45-ft. long x 5.25-ft. wide X 8-ft. deep kettle that is flat-flame-side-fired, not fully covered during idle and not having a kettle enclosure, which is only able to use one of these energy saving methods, the individual savings are: switching from flat-flame-side-fired to a high-velocity-end-fired kettle is a $103,707/yr. savings; fully covering the kettle during idle periods (all hours except the 10.4 hours of production/250 work days) is a $147,840/yr. savings; and using a kettle enclosure is a $89,405/yr. savings. The sum of these individual savings is greater (by $101,050/yr.) t h a n the total for all three ($239,902/yr.) because after using one, you're saving energy. The remaining energy to save is proportionately smaller. Mathematically, a reasonable "term" for this is "geometric decay." These individual savings are presented so galvanizers who are able to use only one energy savings method can d e t e r m i n e cost/payback figures. This enables the best "bang for the buck" and enables better future planning for predictably higher natural gas prices. Besides the $239,902/yr. kettle energy savings in galvanizing 48,660 tons of steel using a high-velocityend-fired furnace, fully covering the kettle during idle periods, and having a kettle enclosure, substantial savings are likely by reducing overtime pay. About 50 workers would be required to galvanize 48,660 tons/yr. Because 10.4 hr./work day are required using a flatflame-side-fired furnace (versus 8 hr./work day using high-velocity-end-firing), an overtime of 2.4 hr./work day for 50 workers is likely. If the differential overtime cost is $9/hr. then: $9/hr. x 50 workers x 250 work days/yr, x 2.4 hr./work day = $270,000/yr. savings for less overtime For this overtime savings to materialize, the thermal capacity of the flat-flame-side-fired kettle would have to be the "bottle neck" of the galvanizing plant, but normally it is. Lowering energy costs by capturing waste h e a t from the kettle flue to h e a t the caustic and flux tanks is becoming popular. Normally, there is enough waste heat available to heat both the caustic and flux to about 150 to 160°F, provided suds (bubbles) are m a i n t a i n e d on the surface of these process tanks. The h e a t t r a n s f e r method usually consists of a large steel coil in a horizontal section of the flue, large exchange coils in the caustic (steel) 50
and flux (titanium), and a high-capacity pump, pumping somewhat pressurized hot water. The process coils are protected by sturdy barriers. The large heat-exchange coils in the caustic and flux tanks must be in one end only, to facilitate bubble generation and distribution, and enable oil removal at the end opposite the coil. The yearly savings at $0.80/therm for natural gas is between $47,000 and $119,000, depending on the efficiency of the heating systems that the recovery system is replacing and the ability to maintain suds on the caustic and flux. This heat recovery system is not unique to highvelocity-end-firing and can be used on flat-flame-sidefired furnaces. The reason to consider heat recovery from the kettle flue is if a change from flat-flame-sidefired is under consideration.
High-Velocity-End-Firing vs. F l a t - F l a m e S i d e - F i r i n g (22,500 t o n s steel/yr.) The galvanizing production of 48,660 short tons/yr. in the previous section would be characteristic for some captive shops but not usual for general jobshop North American galvanizers. Typically, job shop companies galvanize about one million lb./yr. for each foot of length of kettle. Thus, for a North American job-shop galvanizer with a 45-ft.-long kettle, the expected production would be about 22,500 tons steel/yr. Figure 1 shows the results of the analogous calculations in the previous sections but applied to 22,500 short tons/yr. Consider the individual energy savings for each method: switching from flat-flame-side-fired to a high-velocity-end-fired kettle is a $75,910/yr. savings (less t h a n the previous case because less gas is burned); fully covering the kettle during idle periods (all hours except the eight hours of production/250 work days) is a $162,240/yr. savings (greater than the previous case because the kettle is covered more hr./yr.); and using a kettle enclosure results in a $89,405/yr. savings (same as previous case because enclosure savings are independent of production). The sum of these individual savings do not add up to the total savings because w h e n one method is started, the remaining methods become less significant (e.g. "geometric decay"). Thomas H. Cook has taught chemistry, physics, and math at Nebraska Western College and Black Hills State University for a combined 20 years. He has consulted, p u t on workshops, and published about 25 articles on hot dip galvanizing during the last 30 years. He can be contacted at (phone~fax) 605-7454567; (e-mail) galvecon@gwtc, net. www.metalfinishing.com