System For Accelerated Cooling Of Loads In Controlled Atmosphere Forced Circulation Type Furnaces

Abstract

A system for the accelerated cooling of loads in controlled atmosphere forced circulation type furnaces having a central ventilator includes a gas cooling chamber at the base of the furnace equipped with a water cooled coil. The chamber is between the refractory insulator and the metal plate which constitutes the base platform in such a way that gas can enter the chamber through collection valves. An annular space for the return of cooled gas is provided between the chamber and the protective bell.

Description

BACKGROUND OF THE INVENTION

The cooling system of the invention relates principally to cold rolled steel plate annealing furnaces. The importance of such systems is great since they are basic to the manufacture of automobile bodies, tinplate and other products, and since they continuously replace other systems customarily utilized in the manufacture of steel plate. Consequently, this occasions a growing number of installations complementing the major iron and steel plants. This type system can also be applied, naturally in bell furnaces at all times, for annealing other ferrous and non-ferrous materials and, on occasion, in other types of furnaces.

In order to better understand the application of this new cooling system and to more clearly grasp the advantages it offers in relation to those currently employed, we will briefly recall the features of a bell furnace, its operation, and summary of the more frequently used cooling processes.

Bell furnaces are units of varying size which can take loads from some hundred kilograms up to 50 tons and more. As stated, many of these are used for annealing cold rolled steel plate.

Each unit is constituted by: the so-called heating bell, because of its shape, which is a movable component containing the electrical resistors, radiating or burning tubes, heat generators, and thermal insulation, lining, metal frame, etc.

Each unit also includes one or more bases on which the load is placed, and which when operational are covered by means of a so-called protective bell insulating the load from outside air. Air is replaced inside the protective bell by a protective environment the purpose of which is to prevent oxidizing of the load during the process.

In order to have uniform temperature at all points of the loads and to accelerate the heating and cooling processes, there is a ventilator fan or propeller which causes gas circulation, interchanging the heat with the protective bell, transmitting it to the load during the heating period or, inversely, transmitting that of the load to the outside during the cooling stage. To this interchange there is added, of course, that heat generated by radiation.

The number of bases required to obtain maximum performance of each unit depends on the cycle to be followed, heating and cooling time, loading and unloading, container cleansing, etc., since the heating bell must be continuously heating certain of the bases while the others go through the indicated stages successively.

As is deduced, cooling time is of importance for utilization of the bases, for which reason the major specialized enterprises in the world devote much effort to the development and study of procedures for accelerating such cooling. Rapid cooling enables the reduction of the number of bases for the same production, with consequent advantages with respect to size of plants, reduced costs, etc.

Also of interest is the possibility of modernizing existing plants, increasing production merely by adding heating bells while maintaining the number of bases.

In sum, performance of a unit is as follows: The load to be treated, generally several coils, is deposited on a base. The load is then covered by means of a protective bell limiting the space to be occupied by the controlled atmosphere, the corresponding cleansing is effected, and the complete system is covered with the heating bell. This latter is energized and the base temperature, as well as that of the protective bell and load, commences to rise until it reaches that necessary for the treatment. This temperature is maintained for sufficient time or, if a special treatment is involved, it can be varied according to an established plan.

The necessary thermal energy to effectively carry out the desired treatment having been furnished, the heating bell is removed and the cooling process is initiated in the protective bell. When cooling terminates, the base is unloaded following removal of the protective bell. A first base is now ready to restart the cycle.

Meanwhile, a second base will be in one of the earlier stages of heating, cooling, etc., and in order that the heating bell may always be functional, a number of bases will be necessary. This number will be dependent on the duration of the successive operations, and may be two, three, four, five or more in special cases.

Cooling of bell furnaces in conventional installations is effected by interchanging heat between the load and the exterior. This is effected by conductivity through the plate of which the protective bell is formed.

Load heat passes to the bell by radiation and convection and, thus to the outside. As stated, cooling is accelerated as a result of forced circulation maintained inside the protective bell. In order to achieve the same effect, it is possible to accelerate outside air circulation to the bell by means of the corresponding installations and at times the exterior of the bell is even sprayed with cold water.

As previously noted, world specialists seeking increasingly rapid cooling to improve the efficiency of the furnaces indicated, studied and patented several procedures which have been used with varying degrees of success.

Certain of these prior cooling procedures will now be considered.

One procedure consists in extracting through the base a part of the gas circulating inside the furnace by means of a pipe, cooling it in a coil submerged in water and subsequently returning it to the regular system. Naturally, this entire cycle is impelled by the base ventilator fan and normally there should be two coolers per base.

Another system consists in a similar operation. However, gas movement and its passage through the auxiliary cooling system is effected by means of an independent ventilator fan.

Finally, a rather well known third procedure consists in placing a given number of refractory steel coils in the gas current proper arranged in the base and on the most appropriate points of the gas distribution diffuser. Water is conveyed through these coils during the cooling phase.

All of the procedures described suffer from some drawback not presently resolved and which include the following:

The first procedure collects only a part of the circulating gas which part can be estimated to be one fourth to one eighth of the entirety. This requires voluminous installations under the base which in turn requires special fondations, it being necessary to have a true basement rather than a simple foundation.

From the foregoing it is clear that this procedure is costly to install and can well be prohibitive when it is proposed to modernize plants where such a circumstance has not been foreseen.

Under the second procedure a greater amount of circulating gas can be collected and recycled and it requires less space underneath the base, but it leads to increased consumption of electric power and does not prevent the drawbacks noted in the first solution. Since part of the cooling system operates at negative pressure, it is susceptible to air intake which deteriorates the atmosphere and oxidizes the load.

The third arrangement has the advantage over the others that all the circulating gas passes through the coils, but these have the drawback that given the space available, which is restricted, the size of the coils is limited.

On the other hand, the stated coils are subjected to the furnace operating temperature, approximately 700.degree. C throughout the heating and annealing period, so that when the cooling stage is reached, cold water is caused to circulate through the pipes. This creates a heavy thermal shock which can be the cause of accidents but, above all, it reduces the life of same. Further, coils located in the current, within the diffuser of the bases, constitute serious obstacles to gas circulation, with a resultant loss of yield.

The arrangement of the present invention obviates the drawbacks of these prior procedures and constitutes a simple and effective assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the system of the invention, reference will be made to the following detailed description taken with the accompanying drawings wherein:

FIG. 1 is a sectional view of bell furnace in which components constituting the new cooling system are installed. The axis of symmetry of the drawing divides the figure into two parts:

The right half shows the gas circulation circuit during the heating phase and annealing of the load, and the left half shows the system during cooling phase.

FIG. 2 is a Schematic plan view of the diffuser and valve arrangement of the system of the present invention.

FIG. 2a is a perspective sectional view of the device taken approximately from the circular area of FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION

The furnace and cooling system of the invention is organized in the following manner:

A cooling chamber (11) is situated at the base of the furnace, between the metal base (15) of the furnace and a refractory hearth of lining (4) having a suitable insulating capacity. A water cooled coil (6) is installed in this chamber. Other complementary cooling tubes (12) are welded to the metal base (15) exteriorly of chamber 11. A series of a suitable number such as eight collecting valves (2) are arranged closely near the periphery of a centrally positioned ventilator fan (1). The valves 2 extend through aligned openings in hearth 4 and a diffuser 3 positioned immediately thereabove. The valves 2 have cut-out portions forming gas collecting surfaces 2' facing the periphery of fan 1. The valves 2 can selectively be moved to an open (ascend) position or to a closed (descend) position by means of the activating mechanism (5). When the valves ascend or open, gas impelled by the ventilator fan (1) is intercepted by surfaces 2' of the valves and passes through the valves to the chamber (11) where it loses heat. The cooled gas returns to the load area of the furnace through the annular space 11' between the chamber (11) and the protective bell (9).

The location of the collecting valves closely adjacent the ventilator fan periphery is a fundamental feature of the invention to insure that the gas will reach the valves with the greatest speed, thus necessitating its passage through only a small area. The most desirable arrangement for the diffuser 3 is to have the vanes or dividers thereof for channeling the gas alternately extend to the ventilator fan periphery, the other vanes being shorter and interrupted by valves 2. This arrangement is schematically illustrated in FIG. 2. This is similar to American Rolec-type distributors.

Performance of the furnace in its two phases of heating and cooling is as follows:

During the heating and annealing phase (the right section of FIG. 1), the heating means such as resistors (14) situated on the heating bell (13) radiate heat to the protective bell (9). Arrows indicate that air impelled by the ventilator (1) passes through the diffuser (3) and is deflected upwardly by the guide plate 3' thereof into the space between the load 8 and bell 9 and is heated on its vertical course by contact with the protective bell. The thus heated gas enter convectors (10), communicating its heat to load (8) and then descends through the inner space centrally of the annular shape coils of the load 8, finally returning to the fan (1) to reinitiate the cycle. During this entire phase the collecting valves (2) have remained in their closed or lowered position.

When annealing, the circulating cycle of which has been stated, is finalized, the load cooling stage as shown in the left part of FIG. 1 commences, it consisting of the following steps:

In the first place, naturally, the heating bell (13) is removed and the collecting valves (2) are raised. Consequently, gas impelled by the ventilator fan (1) is collected or intercepted by the surface 2' of the open valves and passes through the valves to the lower cooling chamber (11) where it is guided by deflectors (not shown) through a coil (6) and cooled thereby. The gas also losing heat by contact with the lower base (15) which is cooled by water pipes (12) welded thereto. The gas which has given off part of its heat as it passes through chamber (11), exits by space 11' into the space surrounding load 8 and takes heat from the load, thereafter following the same path as during the heating cycle, to reenter the fan, and then pass again to the chamber (11) to lose additional thermal energy.

The basic advantages of this installation for cooling gas in a closed circuit in bell furnaces can be summarized by the following points:

1. Gas collection or interception by means of the valves 2 can be effected due to the type of diffuser utilized in all gas distribution canals in such a way that, despite losses through joints, 100 percent of the circulating gas can be diverted and made to pass through the cooler. Even considering the joint losses, more than 80 percent of the gas can be diverted to the cooler.

2. By activating greater or fewer of the valves it is possible to control the amount of diverted gas in as many ways as there are valves. Consequently cooling speed may also be controlled.

It is likewise possible to open the valves to a greater or lesser extent, thus causing the aforesaid control to be continuous.

3. As noted in FIG. 1, when the valves are closed there is nothing to hamper free circulation in the base. Thus there is no interference with the operation of the furnace during the heating period or when annealing is in progress.

4. The coil and complementary cooling system are maintained with water during the entire procedure, only the flow varying in accordance with the need. In this manner the thermal shock so detrimental to the life span of the cooling pipes is avoided.

5. Use of very modern refractory and insulating materials in the construction of the bases offers a new perspective for the system. It is thereby possible to reduce by 50 percent the conventional thickness of the refractory lining and insulation of the base, whereby it is possible to mount a fully refrigerated base having the same thickness as present non refrigerated bases. It is consequently possible to modernize existing plants at minimum cost since construction work such as foundations, basements, etc., is not necessary, nor is it required to change the essential components of the furnaces since neither the course of the bells nor the constitution of the bases vary. Modernization is then restricted to replacing the base lining with a new one in addition to the refrigerating components which can occupy exactly the same space.

6. A detailed mathematical study of load losses occurring in the two heating-annealing and cooling phases show that it is easy to proportion the various components of the systems in such a way that pressure losses may be exactly the same in both directions, whereby operating conditions of the motor-ventilator unit are maintained with optimum results.

Claims

I claim:

1. A system for accelerating the cooling of a load in a furnace having a centrally located ventilator fan for recirculating gas through a main furnace chamber containing said load, said system comprising:

an insulating-refractory layer positioned beneath said main furnace chamber, said fan being positioned on top of said insulating-refractory layer centrally thereof in said main furnace chamber, whereby gas recirculated by said fan is directed radially outwardly from said fan above said insulating-refractory layer;

a bottom metal plate forming the base of said furnace spaced below said insulating-refractory layer;

said insulating-refractory layer and said plate forming therebetween a cooling chamber;

cooling coils positioned within said cooling chamber;

an annular space continuously connecting the outermost extent of said cooling chamber with said main furnace chamber;

a plurality of openings extending upwardly through said insulating-refractory layer from said cooling chamber, at positions inwardly of said outermost extent thereof, to said main furnace chamber at positions thereof immediately adjacent the periphery of said fan;

collecting valve means vertically movable in each of said openings, from a first lower position not interferring with said gas directed radially outwardly from said fan, to a second upper position interferring with said gas directed radially outwardly from said fan; and

each of said collecting valve means having thereon means directed toward said periphery of said fan for intercepting substantially all of said gas directed radially outwardly from said fan, when said collecting valve means are in said second position, and for directing said thus intercepted gas through said openings into said cooling chamber, whereafter said gas is cooled in said cooling chamber, passed through said annular space into said main furnace chamber thereby cooling said load, and then returned to said fan.

2. A system as claimed in claim 1, wherein said means for intercepting on each of said collecting valve means comprises a cut-out portion forming a curved concave gas collecting surface.

3. A system as claimed in claim 1, further comprising a gas diffuser positioned in said main furnace chamber on top of said insulating-refractory layer and peripherally surrounding said fan; said diffuser having an outer guide plate means for directing said gas directed radially outwardly from said fan upwardly into the outer extent of said main furnace chamber, when said collecting valve means are in said first position; said diffuser having substantially radially inwardly extending vanes for directing said gas outwardly from said periphery of said fan to said outer guide plate means.

4. A system as claimed in claim 3, wherein alternate of said vanes extend inwardly to positions immediately adjacent said periphery of said fan, the remainder of said vanes extending inwardly to positions spaced from said periphery of said fan, said openings being located between the innermost ends of said remainder of said vanes and said periphery of said fan.

5. A system as claimed in claim 1, further comprising additional cooling coils rigidly attached to said bottom metal plate exteriorly of said cooling chamber.

6. A system as claimed in claim 1, wherein said collecting valve means are vertically movable to positions intermediate said first and second positions.