US3492378A

US3492378A - Method of operation of a continuous strip heating furnace

Info

Publication number: US3492378A
Application number: US728549A
Authority: US
Inventors: Louis E Simoncic
Original assignee: Bethlehem Steel Corp
Current assignee: Bethlehem Steel Corp
Priority date: 1968-05-13
Filing date: 1968-05-13
Publication date: 1970-01-27
Anticipated expiration: 1987-01-27

Description

Jan. 27, 1970 1 E. sl'MoNclc 3,492,378

METHOD OF OPERTION OF A CONTINUOUS STRIP HEATING FURNACE Filed May 13, 1968 United States Patent O 3,492,378 METHOD OF OPERATION OF A CONTINUOUS STRIP HEATING FURNACE Louis E. Simoncic, Walnut Creek, Calif., assignor to Bethlehem Steel Corporation, a corporation of Delaware Filed May 13, 1968, Ser. No. 728,549 Int. Cl. F27b 9/28, 9/12; C21d 1/26 U.S. Cl. 263-52 2 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF 'II-IE INVENTION This invention relates to an improved method of operating a metal heating furnace and more particularly to a method of operating a vertical furnace for the continuous heating of steel strip by means of open-flame burners.

Cold reduction of steel strip produces a product which is hard, has little ductility, and is unsuitable for further metalworking. Cold reduced steel can be restored to a soft ductile state by the process of heat treating. The heat treating operation comprises the steps of heating the steel to a high temperature and then cooling the strip according to a predetermined time temperature relationship, which must be closely controlled to permit maximum production of high quality strip. Early heat treating furnaces were of the batch type in which steel, in the form of packs of sheets or in coils, was first heated beneath a box-type hood and then cooled slowly in place. Today, a large amount of steel strip is heat treated in continuous lines, which include a heating furnace or zone that is followed by one or more cooling zones. Strip traveling at a high rate of speed continuously passes through the furnace where the steel is heated to the desired temperature and then proceeds through a cooling zone or zones where the steel is cooled in a controlled maner. In most instances, particularly where the product is to be coated with molten zinc or the like, great care is taken to insure that the steel strip leaving a heat treating line has a cleanbright surface, i.e. unoxidized.

By controlling the atmosphere in the several zones of a continuous heat treating line a clean scale-free product can be produced. In the cooling zones of such lines the atmosphere is controlled by introducing a passive gas, such as a 95%-5% mixture of nitrogen and hydrogen, into such zones. In the furnace or heating zone of such lines the manner of controlling the atmosphere depends upon the method by which such zones are heated. When the furnace zone is indirectly fired, as by means of radiant tube heaters, the atmosphere is controlled by introducing a passive gas into the heating zone. When the furnace zone is directly red, as by means of open-ame burners, the atmosphere comprises the products of combustion from the burners. The atmosphere in direct fired furnaces is controlled to maintain it deficient in oxygen, i.e. essentially non-oxidizing for heat treating purposes, by firing all of the burners with an air-gas mixture deficient in oxygen.

rice

Many direct fired heating furnaces fo continuous heat treating lines are vertical structures which are divided into several zones. Each zone is fired by a plurality of burners arranged on both sides of the strip material which passes downwardly through the furnace. Firing all burners in such furnaces with a fuel-air mixture deficient in oxygen has been recognized as an inefficient method of heating. However, this manner of operation has been considered necessary to insure that an essentially non-oxidizing atmosphere was ymaintained throughout the heating zone. In addition to being an ineicient method of heating, this method of operation causes other problems. The oxygen deficient products of combustion pass upwardly through the furnace and into the stack where they cornbine with oxygen from air drawn into the stack and burn, thereby raising the stack temperature with resultant damage to the stack brickwork.

SUMMARY OF THE INVENTION It is an object of this invention to more efficiently operate a direct tired vertical continuous heating furnace `than the method heretofore in use.

Another object of this invention is to operate a direct tired vertical continuous heating furnace in a manner to reduce the temperature in its stack and prolong the life of the stack refractory.

The present invention provides a novel method of operating a direct red vertical furnace for the continuousy DESCRIPTION OF THE DRAWINGS FIGURE l is a transverse sectional view of a heating furnace or chamber of a continuous heat treating line.

-FIGURE 2 is an enlarged fragmentary view illustrating the construction of the burners in one of the heating zones of the furnace of FIGURE 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings there is shown a vertical furnace or heating chamber 10 of a continuous heat treating line 11. Furnace 10 is forme-d of two panels 12 `and 13 which can be separated to permit access to the interior portions for repairs and for other purposes, such as threading strip material through the furnace. Furnace 10 is divided into several heating zones,

zones

14, 15 and 16 in the lower portion of the furnace and zone 17 in the -upper portion of the furnace. Chamber or space 18 between the opposing faces of the zones is made as narrow as practicable for most efficient heating. Each zone is heated by a plurality of opendiame gas burners 19, of the radiant refractory type, arranged in horizontal rows. Each burner 19 comprises a block 20 of refractory material having a concave depression 21 and a central passage 22. A burner tube 23 extends through each passage 22 and terminates in a nozzle 24 the end of which projects into the bottom of its respective block depression 21. The burners 19 of each row receive a mixture of combustible gas and air from `a horizontal manifold 2S which connects with a vertical supply pipe 26 that connects with a supply source, not shown. Each zone has a Separate supply pipe in which there is a mixing valve that controls the ratio of air to fuel of the combustible mixture supplied to the zone burners.

Continuous metal strip 27 to be eat treated passes over guide roll 28 and into furnace 10 through opening 29 in the furnace top. The strip passes vertically downward through chamber 18 between opposite banks of burners 19 in each of

zones

17, 16, 15 and 14 and is progressively heated to the desired temperature. The strip leaves the furnace at the bottom through opening 30, passes under guide roll 31 and continues through the remainder of the heat treating line, not shown. Hot products of combustion from burners 19 rise through furnace chamber 18, and are carried away by ilues 32 to stack 33. Adjacent top opening 29 are pipes 34, one on each side of strip 27, through which compressed air is blown downwardly into the opening. The compressed air acts as a seal and prevents the escape of hot gases through opening 29.

To obtain maximum heat output from a burner there must be a stoichiometric mixture of oxygen and fue-l for the particular fuel used. This ratio will vary according to the particular fuel used. For example, oxygen and methane react as follows:

The reaction is exothermic. A stoichiometric mixture occurs and maximum heat is produced when two molis of oxygen react with one mol of methane, an oxygen-methane ra-tio of 2 to l. When natural gas, which is primarily methane, is used as a fuel, an air to fuel ratio of approximately 10.5 to 1 provides maximum heat.

In the operation of furnace according to my invention burners 19 in the lower portion of the furnace, i.e.

zones

14, and 16, are tired with a combustible air-fuel mixture which has a deficiency of oxygen and which will yield near maximum heat, while producing products of combustion that are deficient in oxygen, i,e, essentially nonoxidizing. As is well known to those skilled in the art, the non-oxidizing nature of such products of combustion is a function of the differential between t-he llame temperature of the burners and the temperature of the strip, as well as the ratio of both CO to CO2 and H2 to H2O. In the upper portion of the furnace, i,e. in zone 17, burners 19 are red with a combustible air-fuel mixture which has an excess of oxygen and which will yield near maximum heat value, while producing an atmosphere having an excess of oxygen. This oxygen in upper zone 17 provides the additional oxygen necessary to complete combustion in this zone of the oxygen deficient products of combustion which pass upwardly from

lower zones

14, 15 and 16. The energy thus released is available to usefully preheat the strip in the portion of the furnace above zone 17.

In practice the burners of furnace 10 are red with a combustible mix-ture of air and natural gas having a heat content of 1035 to 1062 B.t.u./cu. ft. In the lower portion of furnace 10, burners 19 are fired with an air-gas mixture deficient in oxygen. In zone 14 the burners are fired with an air-gas mixture having a ratio of about 71/2 to 1; in zone 15, the burners are fired with an air-gas mixture having a ratio of about 8 to 1; and in zone 16 the burners are red with an air-gas mixture having an air-gas mixture of about 9 to 1. These air to gas ratios are lower ,than the approxi-mate 10.5 to 1 stoichiometric ratio for a mixture of air and natural gas. In the upper portion of the furnace, in zone 17, burners 19 are fired with an airgas mixture of about 11 to 1, i.e. having a slight excess of oxygen and greater than the stoichiometric ratio for a mixture of air and natural gas. The resulting products of combust-ion having an excess of oxygen formed in zone 17, along with air which infiltrates into the furnace, provide oxygen to complete combustion of the oxygen deficient products of combustion that rise from the lower zones.

By operating the furnace by the method of this invention the atmosphere within the furnace is maintained essentially non-oxidizing and substantially all of the gases are burned within the furnace, prior to their passing into ilues 32 and stack 33. The fuel balance of the furnace is more efficient than the efficiency obtained by the prior method of operating such furnaces, and the strip may be passed through the furnace at a faster speed than was previously the case.

What is claimed is:

1. A method of operating a vertical furnace for 'the continuous heating of uncoated metal strip, said furnace comprising an upper portion and a lower portion, each of said portions having a plurality of burners, said method comprising the steps of l(A) passing the strip downwardly through said furnace,

(B) heating the strip by tiring substantially all of said burners in each said upper and lower portions with a combustible mixture of air and fuel,

(l) the combustible mixture supplied to the burners in the upper portion of said furnace having an air to fuel ratio greater than the stoichiometric ratio,

(2) the combustible mixture supplied to the burners in the lower portion of said furnace having an air to fuel ratio less than the stoichiometric ratio.

2. A method of operating a verticai furnace for the continuous heating of uncoated metal strip, said furnace comprising an upper portion having a first heating zone, and a lower portion comprising a second heating zone and a third heating zone located below said second zone, each of said zones having a plurality of burners, said method comprising the steps of (A) passing the strip downwardly through said furnace,

(B) heating the strip by ring substantially all of the burners in each zone With a combustible mixture of air and fuel,

(l) the combustible mixture of air and fuel supplied to the burners in the upper portion rst heating zone having an air to fuel ratio greater than the stoichiometric ratio,

(Z) the combustible mixture of air and fuel supplied to the burners in the lower portion second heating zone having an air to fuel ratio less than the stoichiometric ratio,

(3) the combustible mixture of air and fuel supplied to the burners in the lower portion third heating zone having an air to fuel ratio less than the stoichiometric ratio and less than the air to fuel ratio of the combustible mixture supplied to the burners in the lower portion second heating zone.

References Cited UNITED STATES PATENTS 2,625,387 1/1953 Hess 263-52 JOHN I. CAMBY, Primary Examiner U.S. Cl. X.R. 148-16