Isothermal Furnace

Abstract

A furnace providing an isothermal heating all along its height comprising a liquid which is heated for filling an envelope surrounding the volume to be heated with the vapor of this liquid and a system for filling the upper part of this envelope with a gas, not mixing with this vapor, for varying the operating height of the furnace.

Description

The present invention relates to heating furnaces of the isothermic type.

In number of industrial applications it is necessary to raise bodies to determinate temperatures, under perfectly isothermal conditions throughout the whole of the heated volume. In the field of the electrical energy, it is known, for example, to employ resistance furnaces operating by the Joule effect, electron bombardment furnaces, where the calorific energy is derived from an electron beam striking an anode which is formed by the body to be heated, and high-frequency furnaces in which the energy is supplied to the body to be heated, through the medium of a circuit wound around this body and wherein high-frequency alternating current is flowing.

Furnaces of this type have two drawbacks; they are hardly capable of achieving uniform temperature all along their structure and, on the other hand it is difficult to adjust the length of the heated zone, which is a fundamental necessity in certain applications.

It is an object of this invention, to avoid such drawbacks.

According to the invention there is provided a furnace comprising an envelope defining the enclosure to be heated; within said envelope an amount of liquid; means for heating said liquid for providing saturated vapor of said liquid within said envelope; and means for modifying the volume of the portion of said envelope filled with said vapor.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made to the drawing accompanying the ensuing description and in which:

FIG. 1 shows a furnace according to the invention; and

FIG. 2 shows a modification.

The furnace shown in FIG. 1, comprises a cylindrical volume surrounded by a double wall 2 defining a closed volume 2a in which a vaporizable liquid 3 has been enclosed. A heating device 4 using the Joule effect, is placed at the bottom of the cylindrical body 2a and the volume 1 is closed at its two ends if need be by screwed plugs 5 and 6. An envelope 7 forming a "heat shield" improves the efficiency of the assembly by preventing external thermal energy losses.

A duct from a source of gas under pressure 8 with an expansion valve 9, a pressure gauge 10 and a cock 11, communicates with said volume. The gas used should not be capable of reacting chemically, at the operating temperatures, with any other of the bodies it contacts, and may for example be argon or helium.

The operation of the furnace of the invention is as follows:

The liquid 3 is raised to boiling point by the heating device 4 and its vapor fills the whole of the closed volume 2. The volume and the pressure at the operating temperature should be so related that the vapor is always saturated, i.e., in a state of equilibrium with the liquid.

The physical laws governing the states of fluids as a function of temperature and pressure, indicate on the one hand that at a given pressure there is a maximum temperature of operation above which the whole of the fluid is evaporated and at which the mechanism described, which implies the simultaneous presence of the liquid and the vapor phases, can no longer take place, and on the other hand that at a given temperature there is a minimum pressure of operation below which the whole of the fluid is evaporated, with the same consequence.

If the temperature and pressure ranges are selected so that the vapor and liquid phases are present simultaneously, volume 2a and the walls are the seat of a heat exchange phenomenon resulting from the equilibrium between the thermal energy lost to the exterior across the walls, and the thermal energy supplied at the same point which compensates the heat lost by the condensation of an appropriate quantity of vapor. This liberation of heat corresponds to the thermal energy expended at the time of evaporation. By way of example, and in order to give some idea of the orders of magnitude involved, a furnace 40 cm. long according to the invention, using liquefied lithium as a fluid has isothermally produced a temperature of 1,920.degree. K. at a power of 2,500 watts, corresponding to 100 watts per square centimeter of wall.

Thus the tubular enclosure and its walls are strictly isothermal over the whole of the length.

This length can be varied in the following manner:

A neutral gas such as argon or neon, under a pressure higher than that prevailing in volume 2a, is caused to flow, by means of the arrangement 9, 10, 11, from the source 8 into the volume 2a. This gas should have such a density that for all practical purposes it does not mix with the vapor present in the volume 2a. It propagates towards the top of the volume 2, so that the upper part of the volume is filled with gas. Accordingly, over the whole of the height portion of the volume 2a, there is no vapor, and accordingly no thermal exchange between the vapor and the wall. The temperature prevailing within the corresponding portion of the volume 2a is extremely low compared to that prevailing in the remaining portion of the volume.

Thus by controlling the quantity of gas admitted within the volume 2a, the height of the volume over which the furnace operates can be adjusted.

FIG. 2 shows a modified embodiment of the furnace of FIG. 1, wherein similar reference numbers designate similar parts. In this embodiment the gas duct is connected at the upper part of the volume 2a, i.e., at the end which is opposite to that were the liquid container is located. This may be particularly useful if the density of the gas and that of the vapor are not very much different, since the separation of gas and vapor by density is facilited.

Of course, the invention is not limited to the embodiments described and shown which were given solely by way of example, in particular in so far as the heating of the liquid, the nature of the gas or even the manner in which the pressure within the volume 2a may be varied, are concerned.

Claims

What is claimed is:

1. An isothermal furnace for heating material placed therein to a uniform temperature, which comprises:

1. a cylindrical, hermetically sealed, double-walled envelope, the inner and outer walls of said envelope defining therebetween an internal volume;

2. a pair of plugs for mating engagement with the upper and lower ends of said cylindrical envelope, said plugs and said cylindrical envelope defining a sealed furnace enclosure in which the material to be heated is placed;

3. a vaporizable liquid in said internal volume and normally lying at the bottom of said cylindrical double-walled envelope;

4. electrically operated heating means, proximate the lower end of said furnace and in thermal contact with said fluid, for applying heat thereto and causing said liquid to vaporize, said vapor rising upwardly in said internal volume, condensing on the walls of said envelope and isothermally transferring heat energy to the material in said furnace enclosure; and

5. means communicating with said internal volume through the outer wall of said cylindrical envelope for introducing, under pressure, a gas which is immiscible with said vapor, said gas displacing at least a portion of said vapor, whereby the amount of said furnace enclosure which is heated may be selectively varied by varying the amount of vapor displaced by said gas.

2. The furnace according to claim 1, further comprising a heat shield substantially surrounding said cylindrical envelope to inhibit thermal losses by radiation and convection therefrom; and said gas introducing means comprises a tank containing a quantity of said gas under pressure, an expansion valve connected to said tank, pipe means interconnecting said expansion valve with said cylindrical envelope, and a valve in said pipe means for controlling the amount of gas supplied to said internal volume.

3. The furnace according to claim 1, wherein said gas is argon.

4. The furnace according to claim 1, wherein said gas is neon.