US3827854A

US3827854A - Automatic metal protecting apparatus and method

Info

Publication number: US3827854A
Application number: US00410220A
Authority: US
Inventors: W Gildersleeve
Original assignee: Individual
Current assignee: Individual
Priority date: 1973-10-26
Filing date: 1973-10-26
Publication date: 1974-08-06
Anticipated expiration: 1991-08-06
Also published as: CA1002151A; GB1413944A; FR2249170B1; FR2249170A1; DE2406029B2; DE2406029A1; DE2406029C3; ZA74351B; AU6461474A; IT1005371B; BE810642A

Abstract

When a metal strip passing through a furnace is overheated, the furnace is automatically shut down in a special way to prevent outside air from rushing in to cause strip oxidation in critical furnace areas. The fuel-air mixture in the furnace is gradually reduced to a low rate which is substantially above zero, and then continues at that low rate for a definite period of time. Also, a non-oxidizing gas such as nitrogen is gradually introduced into the furnace at a gradually increasing rate, reaching its full flow after a definite period of time, while maintaining positive pressure in the furnace chamber. The nitrogen flows for a definite period of time at full volume, and then the gas-air fuel mixture is cut off, still maintaining positive pressure in the furnace chamber. This avoids leakage of air into the furnace; further, exhaust stack dampers, which are initially open, as required automatically to maintain positive furnace pressure, are gradually and automatically brought to a closed position to assist in preventing outside air from being drawn into the furnace chamber.

Description

llnited States Patent [191 Gildersleeve Aug. 6, 1974 AUTOMATIC METAL PROTECTING APPARATUS AND METHOD [76] Inventor: William E. Gildersleeve, 44 Taylor Rd., Conshohocken, Pa. 19428 22 Filed: Oct. 26, 1973 [21] Appl. No.: 410,220

3.72.1,520 3/1973 Bloom 432/8 Primary Examiner-John J. Camby [57] ABSTRACT When a metal strip passing through a furnace is overheated, the furnace is automatically shut down in a special way to prevent outside air from rushing in to cause strip oxidation in critical furnace areas. The fuel-air mixture in the furnace is gradually reduced to a low rate which is substantially above zero, and then continues at that low rate for a definite period of time. Also, a non-oxidizing gas such as nitrogen is gradually introduced into the furnace at a gradually increasing rate, reaching its full flow after a definite period of time, while maintaining positive pressure in the furnace chamber. The nitrogen flows for a definite period of time at full volume, and then the gas-air fuel mixture is cut off, still maintaining positive pressure in the furnace chamber. This avoids leakage of air into the furnace; further, exhaust stack dampers, which are initially open, as required automatically to maintain positive furnace pressure, are gradually and automatically brought to a closed position to assist in preventing outside air from being drawn into the furnace chamber.

19 Claims, 3 Drawing Figures PATENTED RUG 61914 SHEET 2 BF 3 ZONE 1 ZONE 2 PILOT 2 QUENCH g PILOT N QUENCH FURNACE STRIP TEM PERATU RE CONTROLLER BRIEF DESCRIPTION OF THE INVENTION The present invention relates to strip metal heating, and more particularly to a method and apparatus for controlling a strip heating furnace to prevent harm to the strip due to overheating or oxidation.

In the continuous heating of metal strip for annealing and galvanizing or other purposes, the strip is pulled through a furnace chamber in front of open burners. When, for some reason, the strip slows down or stops, the burners have usually been quickly throttled to a minimum or shut off quickly in order to prevent damage to the strip by oxidation, overheating, or otherwise.

At the same time, it has been founddesirable that a cooling gas of a non-oxidizing type be introduced into the furnace to reduce the temperature of the strip and of the furnace refractory lining below a value that causes harm to the strip. Further, in performing this operation, it is highly desirable to prevent excessive oxidation of the strip surface. When the strip is to be galvanized, substantially all oxidation must be prevented if possible.

The U.S. Patent to Cope et al U.S. Pat. No. 3,396,951, granted Aug. 13, 1968, discloses a purging means for a gas-fuel system for a continuous strip heating chamber which has a control system that is responsive to a reduction in rate of movement of the strip for immediately shutting off the fuel supply and instantaneously introducing a non-oxidizing gas into the heatin g chamber, in an endeavor to prevent oxidation of the strip surfaces. We have found, however, that in many furnaces the simultaneous shutting off of the burners and the introduction of a cooled gas such as nitrogen or fuel gas into the heating chamber causes a sudden cooling of the heated chamber, with consequent sharp reduction in chamber pressure to below atmospheric pressure, causing outside air to be drawn into the chamber, through small unsealable openings in the furnace, tending to cause oxidation of the strip. This strip oxidation sometimes, particularly in the case of rather thin gauge strip, causes strip burn-offs and tearing of the metal upon subsequent start-up.

It is frequently the practice in the continuous heating of metal strip to pull it vertically downwardly through a furnace chamber having open burners, in which event the combustion products from the burners come into contact with the surfaces of the strip, and then to conduct the strip through a long horizontal treatment chamber provided with indirect heating tubes. The horizontal chamber contains a non-oxidizing atmosphere, such as nitrogen, for example. When the strip heating line is shut down for any reason, it is important to maintain a proper protective atmosphere in thehorizontal heat treating section of the process.

Accordingly, any sudden reduction of pressure in the direct fired portion of the furnace, when communicated to the connected horizontal heat treating chamber, immediately causes a sudden reduction of pressure to below atmospheric pressure in the horizontal heat treating chamber, inducing outside air to leak through small unsealable openings in the horizontal heat treating chamber to contaminate the protective atmosphere, causing oxidation and damage to the metal.

This is of particular importance, because such horizontal heat treating chambers are often very long, such as 500 feet in length, for example, and contain a substantial amount of valuable heat treated strip. When such strip is damaged by oxidation as a result of contamination of the protective atmosphere, substantial production losses are incurred. In addition, before production can be resumed, the chamber must be completely purged of all oxygen.

It is necessary when the moving strip is brought to a stop, to cool the chamber of the vertical furnace so that the strip in this section will not become oxidized to the extent that there is an appreciable loss in cross section or that it loses strength from becoming too hot. The initial cooling while the strip is decelerated to a stop is provided by a non-oxidizing cooling medium in the lower portion of the furnace. The upper portions are cooled with air, an oxidizing medium.

Accordingly, it is particularly important to avoid a sudden reduction of pressure in the direct fired portion of the furnace, and in other portions of the strip heating production line.

It is accordingly an object of the present invention to provide a controlled system for a strip heating furnace which operates effectively to prevent damage to the strip upon overheating of the strip for any reason.

It is a further object of this invention to control the temperature and atmosphere of a strip heating furnace so that the strip is at all times properly heated and kept in a bright condition, except for a short section on a line stop.

Modern strip heating furnaces have been developed to such an extent that the strip can be heated rapidly to a predetermined temperature. One type of a difficulty occurs when the strip is slowed down below a minimum operating speed, or is stopped. The furnace temperature must be reduced rapidly to prevent the strip from being burned through, or heated to such an extent that it breaks of its own weight, or tears or breaks when it is subjected to the stresses of the startup operation. In addition, the atmosphere of the furnace must be controlled so that excessive oxidation does not take place.

It is accordingly another object of this invention to provide an automatic shutdown system for a strip heating line, which automatically reduces the amount of heat that is applied to a strip which is in danger of overheating, which prevents the introduction of oxidizing substances which might be harmful to the strip, which maintains a positive pressure in the treating area throughout the period of shutdown and affirmatively eliminates the sudden pressure drop which usually accompanies the cooling of the strip and of the area within the direct fired section of the furnace chamber, and which affirmatively maintains a positive pressure to eliminate the leakage of air into any area where it can damage the surfaces of the strip.

Other objects and advantages of this invention, including the simplicity and economy of the same, and the ease with which it is adapted to existing strip heat treating lines, will further become apparent hereinafter and in the drawings.

DRAWINGS Referring to the drawings: FIG. 1 is a diagrammatic view in side elevation showing a portion of a strip heating furnace and a connected horizontal heat treating chamber illustrating certain features of this invention;

FIG. 2 is a schematic diagram showing the arrangement of the automatic control means comprising one embodiment in accordance with this invention, and

FIG. 3 is an electrical diagram illustrating the time sequence of operations in accordance with one embodiment of this invention.

Referring to the specific form of the invention shown in the drawings, specific terms will be used in this specification for the sake of clarity, but it should be understood that these terms are not intended to limit the scope of the appended claims.

Turning now to FIG. 1 of the drawings, the strip 1 is passed over

guide rolls

2 and 3 to travel downwardly in a vertical path through furnace 4. The furnace may be of the type shown in the U.S. Pat. No. 2,869,846 to Bloom or the U.S. Pat. No. 3,320,085 to Turner, for example. It is provided with radiant cup-type burners 5 which face the strip and fire directly into the furnace chamber. Products of combustion rise in the chamber and are exhausted through ducts 6 at the top of the furnace and are conducted to the stack 6'.

The furnace through which the strip travels is maintained at a minimum critical temperature, and the fuelair ratio is controlled to provide the necessary reducing character of the gases (products of combustion) for effecting proper heating and final strip clean-up. The fuel-air ratio of the furnace is regulated to provide a slight excess of fuel so that there is no free oxygen in the furnace atmosphere, and so that there are from 3 percent to 6 percent combustibles in the form of carbon monoxide and hydrogen.

When the strip leaves the furnace, it has a clean surface suitable for producing an adherent coating that must be protected. To this end, upon leaving the furnace, the strip passes through a throat 7, around a bottom roll 10 and to and through a horizontal treating chamber 9 that is filled with a protective atmosphere which can be neutral or reducing. The delivery end or snout 23 of this horizontal treating chamber 9 is below the level ofthe molten zinc 11 in pot 12. Suitable guide rolls 13 are provided to guide the strip through the horizontal treating chamber 9. Horizontal treating chamber 9 is preferably insulated and may have conventional provisions for heating and/or cooling so that the strip passing through it will be brought to the desired coating temperature. Metals other than zinc, such as aluminum for example, may be used in the pot 12.

The strip may be heated in horizontal treating chamber 9, using radiant tubes 14. A protective gas, such as nitrogen or a mixture of nitrogen with a small percentage of hydrogen, is usually introduced into the chamber 9, preferably at various points designated in the drawing as HNX.

The number 15 designates a temperature sensing device, conveniently of the optical pyrometer type, which is arranged to sense continuously the temperature of the strip 1 as it passes around the bottom roll 10. Its signal is electrically connected to operate important sequences, as will further be described in connection with F IGS. 2 and 3.

The top of the furnace chamber is slightly open at 16 to provide an entry slot for the strip 1.

Air bars

20, 20 are provided above and to each side of the opening 16. Each air bar 20 is an elongated pipe and has a plurality of spaced air jets which are aimed obliquely downwardly into the entry slot 16 in a manner to impede the flow of gases upwardly from the furnace chamber, outwardly through the opening 16.

The number 21 represents a damper in duct 6, which normally controls furnace chamber pressures and is opened and closed by a damper operator, usually in the form of a cylinder 22. The damper 21 is centrally mounted in duct 6, and swings adjustably about its shaft 6(a) and thus controls furnace chamber pressures. It is opened and closed by the air cylinder 22 operating the shaft 6(a) through a mechanical linkage. Damper 21 increases furnace back pressure when it is closed. It is, however, incapable of forming a perfect seal when closed, since leakage space is present all around the edge of the damper in order to allow for expansion.

There are many opportunities for air to leak into furnace 4 or horizontal treating chamber 9 in response to a sudden reduction of furnace chamber pressure. Typical leakage areas include the hearth roll shafts 13 (which extend through chamber 9), access doors, threading doors, bolted and clamped connections, the snout 23 which corrodes in contact with molten zinc, and light and sight ports.

Heretofore, it has been considered desirable, upon encountering a stoppage of the strip, to shut off the burner fuel and air of the direct fired vertical furnace and simultaneously introduce a cold neutral gas such as nitrogen, in order to protect the metal of the strip from overheating and oxidation. However, it has now been found that such action in some cases results in a sudden reduction of furnace chamber pressure, causing air to be drawn strongly into the furnace chamber, oxidizing the strip. Further, such sudden reduction of furnace chamber pressure communicates itself through slot 7 to the horizontal duct 9, which is intended to hold a protective atmosphere, but which is invaded by outside air through the leakage points heretofore mentioned.

In accordance with this invention, the timing and rate of introduction of the cooling gas and of the fuel and air to the burners are controlled in a critical manner to maintain a positive pressure in the furnace chamber throughout the entire shutdown procedure. This is accomplished, in essence, by avoiding a simultaneous introduction of cooling gas and shutting off of the fuel and air to the burners. Preferably, the cooling gas is only gradually introduced, with gradual increase of rate of introduction and the flow of fuel and air is reduced gradually to a predetermined low rate, which is maintained at a constant rate for a period of time. After the cooling gas flow has reached a steady rate and has continued at such rate for a definite period of time, the flow of fuel and air is shut off.

Such a procedure maintains positive pressure in the furnace chamber and in the horizontal treatment chamber 9 as well a factor of considerable importance because a very substantial length of finished strip is maintained in the chamber 9, and would all be subjected to damage by leakage of air into the chamber 9.

As will be apparent from FIGS. 2 and 3 of the drawings, which show one preferred way of carrying the invention into effect, a relay is incorporated into the operating circuit of the equipment which controls the running of the strip through the furnace. This relay is energized when the line is in the Run" condition as opposed to a .log" or Thread condition. When this relay is energized the strip line is intended to run at normal production speeds under control of the line opera- I01, and the furnace IS started Lin anautomatically 56- Valve A4 Nitrogen quench (bottom of Furnace Closed 7' quenced cycle. When this relay' is de-energized the furnace is shut down in an automatically sequenced. cycle. Valve Bl Zone i. Burner Air Shut-Off Closed Valve B2 Zone 1, (Fuel) Gas ShutOff Closed Valve V Re is a re 'ulatin i valve which main- It IS an inherent characteristic of a furnace heating tuins ii proger fuel-tfi-air hint.

Valve B3 Zone 2. Nitrogen for hurner cooling Open Stnp .that there is a low Speed point at which i i 1m Valve Cl Zone 2. Shut-Off valve for fuel-air Closed practical to operate, at which speed even the minimum mixture. heat input tends to cause strip overheating. This speed point varies with the gauge or thickness and width of Turningnow to FIG. 3, which is a simplified electristrip being processed. A contact is incorporated into cal sequencediagram for a furnace of a configuration the control system of the line, which contact is autoas shown in FIG. I and with valves as shown in FIG. 2: matically closed at speeds below the speed at which the In this diagram 011 line 19 a relay (llne P lightest gauge strip can be processed. At such low lay) is shown. This relay IS principally operated by the speeds the line is automatically stopped and the furstrip moving mechanism and is energized when the nace is shut down in an. automatically sequenced cycle, l5 strip moving n m y The deenerglzmg 0f thlS relay when the strip temperature sensor 15 detects an overcauses the moving strip to come to a stop. temperature condition as the. strip leaves the direct A l y n P ay) 1S Shown m lme fi d ti f th f 23; which when energized causes the furnace stop se In describing the automatic controls, a two-zone furquence to be initiated. When relay F.S.R. is deenernace has been selected for convenience; itwill be apgizecl, the furnace start sequence is initiated. preciated that this invention is-applicable to furnaces The furnace stop relay in line 23 has several contacts having any number of zones. F.S.R. which are in the following positions with the line Turning now to FIG. 2, a simplified combustion and stopped. (The relay is energized with the line stopped). cooling system is shown. The valvesshown are typically supplied with operators such as cylinders or diaphragm operators or other types which are designed to operate Line (1 5), (9), & (21) Closed (these all close on line stoppage). alvalv e iltlt a definitz:l fixled o controllable speed. Aln Line (3), (8L (22) Open (these anopen on e ectrica y operate so enoi type va ve is typica y line stoppage) used to cause the valves to function. In this diagram, 7 i A A2 g; g temperature comm: The furnace sequence. timer controls are in the folva ves wit operators e rom t e temperature contro lowing positions with the line Stopped. system. Means are provided to cause this valve to move to a predetermined m nimum flow position by the re- S4 (Line (Line 8), moval of an electric signal regardless of the temperas-2 (Line 13), ture signal. All other valves A1,A3-,A4, B1, B2, B3and (Lme 22) tggz gyf C1 are arranged so that, when an electrical signal is ap- (Line 3), (Line 4) plied the valves open at a controlled rate, and when the (Linc l d h l l t n d S-8 (Line 21) Open (these all open on sigtna IS mterrupte t e va ves c ose at a con ro e line stoppage) ra e.

Referring again to w t e line pp the When the strip line is in the Run" condition the furfurnace control valves are initially in positions as folnace Stop relay on line 23 f FIG 3 i d i d d lows: its contacts reverse position. At this instance the fur- VLIIVC Al Flue Air (Opens when energized) Open nace start sequence begins since timer motor M (line Valve Aiil- Zonle l, Temp. Clontrjol (This: valve is Closed 22) runs h h Contact s 8, to Operate ti contro lngw en energize an is set or f minimum fire when deenergized). contacts 8-1 th-rough S 8' v e i P- 5 2 l g l 10 Zone 1 l 8 The furnace light-off sequence IS as follows (the time alve itrogen quenc ottom o urnace ose Zone l) (opened only on line stop for approxcycle 5 Selecte'd as an mpi y and vanes wlth imately three niinutes) Furnace and line characteristics), see FIG. 3.

Time Contacts Function 0 Sequence timer Motor M starts when relay F.S.R. is deenergized and its contact on line 22 closes. 5 seconds S-l on Line 2 Valve Al Closes.

opens. 10 seconds 5-2 on Line 13 Valve B3 closes and Valve Cl is opens. energized and opens. (Switch b-3 of valve B3, line 14 closes) (Zone 2 lights on minimum). 12 seconds 5-3 on Line 4 Valve A5 opens. (Zone 2 goes closes. on temperature control) 14 seconds 5-4 on Line l0 Valves BI and B2 open.

closes (Zone 1 lights on minimum). l5 seconds S-5 on Line 8 Timer TD 2 is reset.

opens 16 seconds 5-6 on Line 3 Valve A2 opens. (Zone I goes on closes temperature control) 7 58 seconds 5-8 on Line 22 Timer is stopped.

opens. 3-7 on Line 2l closes.

In shutting down the furnace, the line may be stopped at the option of the line operator by pressing the Line Stop push button on line 17. When the Line Stop push button on line 17 is pressed its contact is closed and is maintained closed, this being a conventional type of maintained contact which remains closed until it is manually opened. Time Delay Relays TD3F on line 16 and TD3S on line 16A are energized and begin timing. Relay RC is energized and its contact in line 6 closes, energizing nitrogen purge valves A3 and A4 on

lines

6 and 7. The purge timer on line 5 is also energized. When timer TD3F times out (approximately 5 seconds) its contact on line 24 closes and the furnace Stop Relay F.S.R. on line 23 is energized to initiate the furnace stop sequence. When timer TD3S times out (approximately l seconds) its contact on line 19 opens to deenergize the Line Stop Relay L.S.R. on line 19 to stop strip movement.

On line stop, the furnace start sequence timer motor M resets by running for two seconds and returns all start contacts S-l -8 to their initial positions, as heretofore outlined.

The timer TDl, which was energized when the relay CR contact closed, may be set for approximately a period of 3 minutes, after which it gradually closes valves A3 and A4, shutting off the nitrogen quench.

After a delay of about 3 to 6 seconds, for example, depending on the speed at which the temperature control valves move to their minimum positions, valve Al, controlling the air bleed to the flue 6, has fully opened and the furnace pressure control damper 21 moves under the influence of the furnace pressure controller to its new position to maintain a positive pressure. Time delay relay TD2 operates to shut off the air and gas to Zone 1 completely with gradual movement of valves B1 and B2, and gradually to open the nitrogen valve B3. When this valve B3 is fully open, a limit switch opens to cause the combustible mixture supply valve C1 to close. The introduction of nitrogen into the Zone 2 burner nozzles serves to maintain a burner pressure to prevent backfire, and to keep the burner tips from overheating.

LlNE STOP BY TEMPERATURE CONTROL While the line is operating at production speeds the strip may be overheated due to furnace malfunction or due to some physical characteristics of the strip. Should overheating occur at these speeds, a contact (line in the temperature controller closes and actuates timers TD4F on line 17 and TD4S on line 17(0), and the alarm shown in line 18. The line operator is now alerted and must try to correct the condition operationally. If the temperature is not corrected within one minute, the

timer contact TD4F on line 25 closes to energize the furnace stop relay F.S.R. on line 23 and initiate furnace shutdown. Five seconds later, TD4S contact on line 19 opens to stop strip movement.

When the line is being slowed without the operator pressing the stop button, and after closing of the minimum process speed contact MSC heretofore described, (see Line 15 in FIG. 3), the strip will overheat at some low speed. In this case, timer TD3F on line 16, timer TD3S on line 16(a) and a relay RC on line 15 are energized. The relay contact (line 6) of relay RC closes to energize valves A3 and A4, thus admitting nitrogen quench to the furnace, A-3 at the bottom of Zone 1 and A-4 at the bottom of Zone 1. Time delay relay TD3F is set to delay for 5 seconds, for example, and then shuts down the furnace by energizing relay F.S.R. on line 23. Also, relay TD3S contact in line 19 opens 5 seconds later, deenergizing relay L.S.R. on line 19 to stop strip movement. The time of shutting down the furnace after pressing the stop button and the further time delay for strip travel are adjustable by simply adjusting the settings of time delay relays TD3F, TD3S, TD4F and TD4S.

It will be appreciated that the valves that are used in controlling the flow rates of fuel and air into the burners, and of protective gas into the zones of the furnace chamber, are carefully constructed so that they do not open or close instantaneously. On the contrary, means are deliberately introduced into the valves to limit the rate at which they can open or close. In a conventional air operated diaphragm type valve, for example, the speed of operation of the valve is limited by inserting an orifice plate of predetermined size into the air line which leads to the diaphragm chamber.

Accordingly, when reference is made herein to any of the aforementioned control valves opening or closing, it should be appreciated that such action takes place slowly and gradually over a period of 2 to 5 seconds, more or less, for example. This gradual adjustment of a valve from a closed to an open position provides, a flow rate which gradually increases or gradually decreases over a predetermined period of time.

For example, when the furnace is being shut down due to overheating of the strip, the valves A2 and A5 that control fuel and air to the burners move gradually toward thier minimum fire positions, causing a gradual reduction of feed rate for a considerable period of time (2-5 seconds, for example). Similarly, the valves A3 and A4 controlling the introduction of cooling gas (such as N or HNX, for example) open gradually and slowly, gradually increasing the feed rate of the gas for a considerable period of time (2-5 seconds, for example).

In a shutdown, the introduction of cooling gas and the shutting off of burner fuel and air do not occur simultaneously. The rate of flow of fuel and air is gradually decreased to a predetermined low rate. The protective gas flow rate gradually increases until it reaches its maximum. Steady flow of all materials follows. Then, later on the fuel and air are shut off.

It will be appreciated that various changes may be made in the form of the invention shown in the drawings and described herein. If desired, the protective gas may be introduced at various periods of time before the fuel or air valves are actuated. Further, the rates of movement of the control valves may be varied, provided the pressure in the furnace chamber is maintained above atmospheric pressure. Other changes in the specific form, arrangement and sequencing of the electrical and control elements shown in FIGS. 2 and 3 may be made, and equivalent elements may be substituted, parts may be reversed, and certain features may be used independently of others, all without departing from the spirit and scope of this invention as defined in the appended claims.

The following is claimed:

1. In a method of preventing oxidation of a metal strip in shutting down a furnace chamber having a plurality of burners, the steps which comprise:

a. reducing the fuel supply to the burners from its normal supply rate to a low fuel supply rate which is much lower than said normal rate but substantially above zero,

b. maintaining said low fuel supply rate substantially constant for a period of time,

c. introducing'into said chamber a protective gas which is non-oxidizing to the metal strip,

d. maintaining said feed of said protective gas and concurrently maintaining said fuel at said low fuel supply rate, both for a predetermined period of time, and

e. thereafter shutting off said fuel supply.

2. The method defined in claim 1 wherein positive pressure is maintained within the chamber throughout steps (a) through (e).

3. The method defined in claim 1 wherein steps (a) through (e) are automatically actuated in response to sensing the strip temperature, the temperature of actuation being above a predetermined temperature.

4. The method defined in claim 3, wherein the strip temperature is sensed at a location in the vicinity of the exit of the strip from the furnace chamber.

5. The method defined in claim 4, wherein the furnace chamber is direct fired and the strip temperature is sensed after the strip has exited from the direct fired portion of the furnace chamber.

6. The method defined in claim 1, wherein step (c) occurs prior to step (a).

7. The method defined in claim 1, wherein a time delay is provided after any of steps (a) (e) following which the strip movement is stopped.

8. In the method of claim 1, wherein said chamber includes a passageway for exhaust gases, which passageway has an opening provided with a controllable closure for said opening, the step which comprises closing said opening during the introduction of said protective gas into said chamber.

9. The method defined in claim 1, wherein said chamber has adjacent zones one above the other, and wherein protective gas is separately introduced into each of said zones.

10. The method defined in claim 1, wherein the protective gas is introduced into said chamber at a gradu- 10 ally increasing rate over a predetermined period of time.

11. In the method of claim 1, wherein said furnace chamber has an outlet that is in communication with a strip treating chamber containing a protective gas and also having openings to the atmosphere, the step of maintaining the pressure in said furnace chamber higher than the'pressure in said strip treating chamber to prevent any substantial inward flow of said atmosphere into the protective gas in said chamber.

12. In an apparatus for heat treating metal strip, which apparatus includes a furnace having a chamber containing a plurality of burners, the fuel feed to which is valve controlled, means for passing said strip through said furnace, and means for introducing a protective gas into the furnace chamber, an automatic sequence control having a turning device connected to actuate said control valve for reducing the rate of fuel to the burners, time delay means and means for introducing said protective gas into said furnace chamber after a predetermined time delay, and in response to said time delay means.

13. The apparatus defined in claim 12 wherein means are provided for maintaining positive pressure in said chamber when the rate of feed of fuel is reduced.

14. The apparatus defined in claim 12, wherein a temperature sensing means is arranged in the vicinity of the strip exit from the chamber, and wherein said temperature sensing means is calibrated to actuate said control valve to initiate a reduction of said rate of feed of fuel.

15. The apparatus defined in claim 12, wherein said chamber has a flue for exhaust gases, provided with a controllable flue damper wherein means are provided for signalling a furnace shutdown, and wherein control means is provided for closing the flue damper in response to a furnace shutdown signal.

16. The apparatus defined in claim 12, wherein said chamber has adjacent zones one above the other, and wherein separate means are provided for introducing protective gas into each of said zones.

17. The apparatus defined in claim 12, wherein said furnace chamber has an outlet that is in communication with a strip treating chamber containing a protective gas and also having openings to the atmosphere, said protective gas introducing means having sufficient capacity, after said time delay means has operated, to maintain a pressure within said furnace chamber which is greater than the gas pressure within said strip treating chamber.

18. The method defined in claim 1 wherein concurrently with the furnace shutdown sequence, air is introduced into the flue of said furnace chamber.

19. The method defined in claim 1 wherein air is introduced in a selected area of the furnace chamber to provide continuing cooling.

UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION Patent 3.827.854 AUGUsT 6, 1974 Inventor WILLIAM E GILDERSLEEVE It is certified that error appears in the aboveidentified paterlt and that said Letters Patent are hereby corrected as shown below:

SELAS CORPORATION OF AMERICA .[73] Assignee:

' Dresher, Pennsylvania to be added to the Title Page.

Signed and sealed this 3rd day of December 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. A c." MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-DC 60376-P69 us. GOVERNMENT PRINTING omcs: I969 o-ass-su FORM Po-1 050 (10-69)

Claims

1. In a method of preventing oxidation of a metal strip in shutting down a furnace chamber having a plurality of burners, the steps which comprise: a. reducing the fuel supply to the burners from its normal supply rate to a low fuel supply rate which is much lower than said normal rate but substantially above zero, b. maintaining said low fuel supply rate substantially constant for a period of time, c. introducing into said chamber a protective gas which is nonoxidizing to the metal strip, d. maintaining said feed of said protective gas and concurrently maintaining said fuel at said low fuel supply rate, both for a predetermined period of time, and e. thereafter shutting off said fuel supply.

6. The method defined in claim 1, wherein step (c) occurs prior to step (a).

7. The method defined in claim 1, wherein a time delay is provided after any of steps (a) - (e) following which the strip movement is stopped.

10. The method defined in claim 1, wherein the protective gas is introduced into said chamber at a gradually increasing rate over a predetermined period of time.

11. In the method of claim 1, wherein said furnace chamber has an outlet that is in communication with a strip treating chamber containing a protective gas and also having openings to the atmosphere, the step of maintaining the pressure in said furnace chamber higher than the pressure in said strip treating chamber to prevent any substantial inward flow of said atmosphere into the protective gas in said chamber.