US1947612A - Electric furnace - Google Patents

Description

Feb. 20, 1934. w RE 1,947,612

ELECTRIC FURNACE Original Filed Oct. 24, 1931 Fg z Elmo/whorl I Nov/e Patented Feb. 20, 1934 PATENT OFFICE ELECTRIC FURNACE William a. mom, Pittsburgh, Pa, assignor to Pittsburgh Research Corporation, Pittsburgh,

Original application October 24, 1931, Serial No. 570,957. Divided and this application May 10, 1932. Serial No. 610,441

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My invention relates to improvements in electric furnaces and more particularly to furnaces adapted for annealing or similar heat treating operations.

l An important object of my invention is the provision of a novel resistor support for use in this type of furnace.

Another object of my invention is to provide a novel and efilcient distribution of the resistor elements about the walls of the furnace.

Other objects and advantages of my invention will be apparent during the course of the following description.

This is a division of my co-pending application 570,957 for electric furnaces filed October 24, 1931. V

In the accompanying drawing, which forms a part of this specification, and wherein like characters of reference denote like or corre sponding parts throughout the same,

Figure 1 is a diagrammatic elevation of one of the side walls of my furnace showing the distribution of resistor elements thereon,

Figure 2 is a similar view of one of the end walls of the furnace,

Figure 3 is a perspective view showing the resistor element mounted on its supports,

Figure 4 is a vertical sectional view through the resistor support,

.Figure 5 is a similar view of a slightly modified form of resistor support, and,

. Figure 6 is a detail of the resistor supporting spool showing two forms thereof.

In the drawing, wherein for the purpose of illustration is shown a preferred embodiment of my invention, the numeral 10 designates a heating furnace or hood of any suitable construction and having electric resistance elements 44 therein. I

In the heat treating of material piled vertically on the hearth of this type of furnace, if the heater elements are uniformly distributed over the wall surfaces the upper parts of the charge will attain a higher temperature than the lower parts of the charge due to the effect of convection and re-radiation from the arched roof and other factors. Heretofore it has been the practice to avoid this overheating of the top of the furnace by the use of two or more zones of heater elements each separately controlled so that the ribbons in the higher zones are carried at a lower temperature than ribbons in the lower zone to compensate for this tendency to overheating of the top of the furnace. Such an arrangement is disadvantageous, however,

and I propose to obtain uniform heating of the charge by a novel distribution of a single zone of heating elements. a

An ordinary and advantageous use of my invention is for heat treating sheets of metal 50 stacked horizontally on the furnace base with the edges to the side-walls. The heat conductivity of such charge is very much greater horizontally than vertically, and for that reason I preferably place my heating elements principally on the side walls of the furnace with a small portion on the end walls to compensate for equal heating there. I also preferably locate the major portion of the resistors of the heating elements below the horizontal center of the charge so that the radiation and convection had of the charge is approximately the same in the bottom portions of the charge and the top portions of the charge. I also prefer to so distribute the heat on the side-walls to avoid overheating the corners. Irregular heating tends to buckle the sheets or over-oxidize the edges of the corners which by my invention I have avoided.

- In Figure 1 I have shown one method of accomplishing this result. The heating element in the form of a resistor ribbon 44 is supported by suitable insulating supports 45 arranged in upper, lower and middle rows. In the arrangement shown in Figure 1, for every resistor loop that extends from the lower to the upper row of supports, two loops extend only to the middle row of supports. As a consequence, over two thirds of the heating element surface'is arranged below the horizontal center line of the furnace. It is obvious that other arrangements of the resistor loops may be used to obtain the desired distribution.

The usual practice is to use the same proportion of heating elements on the end walls as on the side walls but this has been found to be impracticable, due to overheating of the ends of the charge in this type of furnace. If any appreciable amount of heat is supplied by the end walls, the end portions of the charge are sup- 0 plied with heat from three sides, whereas the central portions of the charge are supplied with heat from only two sides, resulting in very unequal heating of the charge. To remedy this defect, I propose to heat'the charge from two sides only and produce at the end walls only enough heat to compensate for the heat losses of the end walls. This is accomplished by providing only a few resistorloops on the end walls, as seen in Figure 2 where the loops are arranged in no the center and at the bottom of the end walls where most of the heat losses occur. Any desired distribution could be used, however.

In Figures 3, 4, 5 and 6 I have shown a resistor support which is well adapted for this type of furnace. Insulating and refractory blocks 46 are set into the walls of the furnace cover and carry heat resistant metal pins 47 which project beyond the inner surface of the furnace cover and slidably receive electrical insulating spools 48 which are held in place by suitable fasteners such as U-shaped clips 49 fitting in suitable notches in the ends of the pins 47. In the form of the invention shown in Figures 3 and 4, the block 46 is split horizontally and provided with a bore to receive the pin and communicating with a vertical bore 50. The pin 47 is provided with a bent end which extends into the bore 50 to prevent removal of the pin when the block sections are assembled. Additional bores 51 may be provided for securing the block sections together or to receive the pin if a different position of the pin is desired.

The spool may be formed in one piece as shown in Figure 5 and at the right of Figure 6, or it may be split at its center to facilitate assembling, as seen in Figure 4 and at the left of Figure 6. The center portion of the spool is polygonal in cross section to present knife edge contacts to the resistor ribbon, and thereby reduce conduction losses to the support and permit a less hampered radiation from the resistor.

It will be seen that the spools 48 may be readily removed or replaced, by removing the fastener 49 and sliding the spool from the pin, without removing the block 46 from the wall.

In Figure 5 the insulating block 46' is formed in one piece and a lag screw 47' is used in place of the pin 47. The screw 47' may be removed and replaced without removing the block 46 from the wall. The head of the screw 47' retains the spool in position.

While I have shown and described the preferred embodiment of my invention it is to be understood that various changes in the size, shape and arrangement of parts may be resorted to without departing from the spirit of my invention or the scope of the subjoined claims.

Having thus described my invention what I claim and desire to protect by Letters Patent is:

1. In a heat treating furnace, resistance element supports arranged in three spaced substantially horizonal rows about the walls of said fur-' nace, and a resistance element looped about said supports, and extending from the bottom row of supports to the intermediate and top rows of supports, some of the resistance loops terminating at the intermediate row of supports, whereby a greater part of the heating surface of said resistance element is arranged below the horizontal center of the charge of said furnace.

2. The method of heating a charge of material in a closed space which consists in heating from two opposite sides of the material, and compensating for heat losses on the other sides of the material.

3. A resistance support for electric furnaces comprising a pin secured to the furnace wall,

and an insulating spool removably arranged on said pin, said spool being split intermediate its ends.

4. A resistance support for electric furnaces comprising a pin secured to the furnace wall, and an insulating spool removably arranged on said pin, said spool having its intermediate portion polygonal in cross section, and being split intermediate its ends.

5. The method of heating a charge of material in a closed space which consists in heating from two opposite sides of the material, and compensating for heat losses on the other sides of the material, while supplying the greater portion of heat below the horizontal center of the charge.

6. In a heat treating furnace, a resistance element supported in loops which extend from points adjacent the bottom of the furnace towards the top thereof, some of the loops extending to points adjacent the top of the furnace while other of said loops terminate at points adjacent the horizontal center of said furnace whereby the greater portion of the heating surface of said resistance element is arranged below the horizontal center of the charge in said furnace.

'7. In a heat treating furnace, a resistance element supported in loops of varying lengths in said furnace, the greater portion of the heating surface of said resistance element being arranged below the horizontal center of the furnace.

8. In a heat treating furnace having a generally rectangular shape, a resistance element arranged about the four walls of the furnace, there being a greater amount of the heating surface of the resistance element on the side walls per unit wall area than on the end walls of the furnace.

9. In a heat treating furnace having a gen erally rectangular shape, a resistance element arranged in loops upon the walls of the furnace, there being a greater portion of the heating surface of the loops below the horizontal center of the furnace than above it, and there being .a greater amount of the heating surface of the resistance element on the side walls per unit wall area than on the end walls of the furnace.

WILLIAM E. MOORE.

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