US2149447A - Furnace for treating materials at high temperatures - Google Patents

Description

March 7, 1939. u. LAMM ET AL FURNACE FOR TREATING MATERIAL-S AT HIGH TEMPERATURES Filed Dec. 5, 1936 In new Zors. U/wLa/nm Harry Forssell O I 1 fliiomg Patented Mar. 7, 1939 UNITED STATES PATENT OFFICE FURNACE FOR TREATING MATERIALS AT HIGH TEMPERATURES ration Application December 5, 1936, Serial No. 114,318 In Sweden December 7, 1935 1 Claim.

Certain materials, as graphite or several metals or metal alloys, require a treatment at high temperatures, for instance in the neighborhood of 2000 0., often during a rather long period, as

several days, in order to obtain the properties required for certain puposes, as for brushes for electrical machines, anodes for electric valve or discharge vessels, etc. For two distinct reasons, it is advisable to carry out such heat treatment in vacuum, namely for protecting the material treated against the influence of gases, and for affording a better heat insulation and thus reducing the energy losses. The necessary heat is generally supplied in an electrical way.

The carrying out of the heat treating process in Vacuum may also be useful for promoting the purposes aimed at by the heat treating process itself, as for instance the liberation of the material from gases. This may especially be the case, 20 when the heating process is carried to melting, as is well-known for several metals which are melted in vacuo in order to attain a higher state of purity.

A vacuum-tight receptacle for enclosing the material to be treated cannot generally, however, be made to stand such high temperatures as are often necessary for the heat treatment, and it is therefore necessary to provide an efficient heat insulation inside the vacuum-tight receptacle 30 around the material to be treated. The materials generally used for such heat insulation purposes, as porous ceramic or fibrous materials, are,

however, not suitable for use in vacuum, especially not in high vacuum, as their porosity not 35 only retards the general evacuation to much and thereby retards the whole process but also may permit the said material to absorb gases which may have, when liberated, a detrimental influence on the material treated.

According to the present invention, a furnace for treating materials and objects at high temperature in vacuum, is provided with a heat insulation inside the vacuum receptacle, which as distinguished from ceramic or fibrous material is granular in form, the main diameter of the grains being neither too small or too large, but so as to maintain the ratio between the total volume of the material and the total surface thereof between certain reasonable limits. When for instance substantially spherical grains are used, the

main diameter thereof should not be less than 55 and with a certain allowance for porosity and irregularities in form may be taken as one tenth of the diameter, the aforesaid limits of the grain sizes correspond substantially to the limits .01 millimeter and l millimeter for the ratio between volume and surface of the particles. It has been found, that if a sufiiciently high number of particles outside each other, thus connected in series for the fiow of heat, are employed and if the vacuum is suificiently high, as one corre sponding to a pressure of one millimeter of mercury or less, the inner heat conductivity of the particles plays a very unimportant role for the heat transfer, as by far the greatest resistance against the said transfer is encountered at the contact surfaces between the different particles. For this reason, materials of rather high conductivity for heat, as metals or graphite, can be very well used for the heat insulation, and these materials have generally the advantage of being only very slightly porous and free from vaporizable impurities which otherwise may cause damage to the material treated. In many cases, it may be advisable to use as heat insulator a material of the same kind as that treated, so that not even vaporized quantities of the heat insulator itself may reach the treated material and cause damages thereto. It is important that all the grains are nearly of the same size so that the space between bigger grains will not be filled out by smaller ones which might increase the heat transmitting contact surfaces.

One form of furnaces according to the present invention is shown in the accompanying drawing in vertical section.

The drawing shows a furnace which is chiefly intended for the heat treatment of graphite. I is a vacuum-tight receptacle which is preferably provided with a cooling jacket and which, in charging, is first provided with a bottom layer 2 of graphite grains, preferably of a diameter of .5 to 2 millimeters, care being taken that all the grains have about the same diameter which can be accomplished by proper sifting. On this bottom layer, there is placed a heat treating chamber 3, preferably built up from graphite plates which may be joined together by rabbets and grooves for tightness. Holes in the wall of said chamber communicate with corresponding and oppositely situated holes in the wall of the surrounding receptacle by graphite tubes 4 which embrace leading-in conductors 5 to electric heating elements 6 in the heating chamber, said elements preferably consisting of graphite plates cut in zig-zag. If the heating current is three phase, the wall of the chamber may preferably serve as a neutral point, one end of each element being connected to the wall by a graphite bolt 1, while the other end is supported by the leading-in conductor 5. Generally the wall may serve as a conducting connection between the different heating elements. The outer end of each conductor 5 is fixed to a metal plate 8 serving as a terminal and being cooled by a Water jacket 9. Around the chamber 3 there is packed a filling of graphite grains I0, and after the introduction of the charge, the whole is covered with such grains, all of the same character as in the bottom layer. An inspection tube ll may penetrate through the upper granular layer towards a window l2 in the cover l3 of the receptacle. After fixing the said cover in place, the receptacle is evacuated through a tube M.

It has been found that in the arrangement described, when fully evacuated (to about 1 millimeter of mercury), the heat losses will be very small in spite of the high heat conductivity of the graphite, because of the very low heat transfer at the contact surfaces between the different grains. For instance, it has been found possible to keep a heat treatment chamber of a volume of 300 litres at a temperature of 2000 C. by a power supply of 20 kw. At the same time, all parts heated to incandescent or red temperature will be of the same material as the objects treated, namely graphite, and this prevents any deterioration by vaporized and again condensed constituents.

Having now described our invention and the manner of its operation, we declare that what we claim is:

A furnace for treating materials at high temperatures, comprising a vacuum-type receptacle, evacuating means therefor, a heat-treatment chamber centrally located therein, electric heating elements located in said chamber, and heat insulating material separating said chamber from said receptacle, said material consisting of graphite grains of substantially uniform size, each grain having a mean diameter of from 1 to 10 millimeters.

UNO LAMM. HARRY FORSSELI-

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