INDUCTION HEAT TREATMENT OF BULK DIELECTRIC MATERIALS

E. D. ZAITSEV, A. I. SLOSMAN, V. V. KHARCHENKO

The difficulties of heat treatment of finely dispersed and caking materials are well known. One method of treating such substances is their processing in a boiling bed of inert particles [1]. However, for high-temperature treatment of substances that require special purity, collection of decomposition products, or purification of the fluidizing agent, this method is relatively ineffective.

For the heat treatment of finely dispersed and caking substances, it is possible to use a high-frequency electric field. When bulk substances are treated in the electric field of a high-frequency capacitor, heat is continuously released within the volume of the dielectric, caused by conduction currents and dielectric losses. However, the intensity of heating and drying under the action of an electric field on a dielectric is low if the dielectric losses are small and the conduction currents are weak. The efficiency of installations for heating dielectrics by high-frequency currents does not exceed 40–50 percent [2]. Such installations are temperamental in operation, interfere with radio communication, and require good ventilation of the capacitor when drying substances with high moisture content.

S. G. Romanovskii applied induction drying of wood by currents of industrial frequency, which in this case proved effective [3].

We investigated the heating and drying of bulk dielectrics by an induction method. The application of the induction method became possible as a result of introducing metallic particles into the material being treated. The inert metallic particles introduced into the bulk dielectric are continuously operating heat sources uniformly distributed in the dense layer of the material. Heat is released in the inert metallic particles as a result of eddy currents induced in them by the electromagnetic field of the inductor.

The heat-treatment process by this method can be carried out continuously; the residence time of the material in the treatment zone is easily regulated; high productivity is achieved with small apparatus dimensions; uniform temperature treatment of the material is obtained; and it is possible to dry finely dispersed materials without dust entrainment, as well as materials with high moisture content.

The intensification of the heat-treatment process of bulk dielectrics occurs as a result of substantial heat release in the metallic—

particles, close contact with the material being treated, and a developed heat-exchange surface.

In addition, intense heating of the moist material surrounding the particles leads to gas flows in the layer, which loosen and mix the finely dispersed loose dielectric.

If metallic particles are used, for which the magnetic permeability \(\mu \gg 1\), they are set in motion as a result of the interaction of the particles with the electromagnetic field of the inductor, as a result of which the mixture is mixed and thereby uniform thermal treatment is achieved.

It should be noted that moisture transfer is affected by a thermodynamic force represented by the product of the gradient of the magnetic-field induction \(\nabla B\) and the magnetization vector \(\pi_M\) [4].

I. Experiments at a frequency of 2500 Hz

The setup consists of a PV 50—2500 generator, measuring and control instruments, an inductor 180 mm in diameter made of copper tubing 14 mm in diameter and having 18 turns, and a quartz apparatus in the form of a truncated cone (larger diameter — 130 mm, smaller diameter — 100 mm, height — 250 mm). The material being treated is mixed with metallic particles (balls of ShKh15 steel with diameters of 3.15 mm and 7.93 mm). The concentration of metallic particles was varied from \(1/3\) to \(2/3\) by weight, calculated on the dry mixture. The power consumed in the inductor was varied in the experiments from 0.5 to 6 kW.

Into the apparatus was loaded 1 kg of mixture with various concentrations of balls and various moisture contents of the material, and the apparatus was placed in the inductor. To investigate the kinetics of heating and drying, typical dielectrics were used: white corundum and quartz sand, whose granulometric composition is given in Table 1.

Table 1

The study of heating of metallic balls 3.15 mm in diameter showed that a layer 100 mm high is heated relatively uniformly; the greatest temperature variation was observed along the vessel and amounted to \(\pm 50^\circ\text{C}\) at an average ball temperature of \(700^\circ\text{C}\). The maximum temperature fluctuations in the cross section in the middle of the apparatus are \(\pm 26^\circ\text{C}\). The nonuniformity of heating can be explained by convective heat exchange on the surface of the apparatus. Heating of a dry and moist loose dielectric (corundum and quartz sand) does not occur without the addition of metallic particles.

The greatest temperature difference over the volume when heating a mixture of corundum and balls 3.15 mm in diameter (at a concentration of \(1/2\) by weight) is \(\pm 25^\circ\text{C}\) at an average mixture temperature of \(500^\circ\text{C}\), and 1 kg of mixture is heated within 10 minutes to the above temperature at a power consumed in the inductor of 4 kW.

Visually, motion of the balls in the apparatus was observed; they appeared to boil slowly (move about), rising above the bottom of the apparatus.

after energy is supplied from the high-frequency generator to the inductor; after the supply of energy is stopped, the balls fall and lie motionless in an even layer; upon reaching a temperature of 700°C the balls cease moving in the inductor field and arrange themselves in chains along the lines of force.

The mixture of balls and dry corundum is also in motion and is mixed.

When drying a mixture of dielectric and balls, at first slow mixing is observed (heating to 95–99°C), and then intensive mixing, which continues until the end of drying.

Fig. 1. Drying and temperature curves for corundum (solid lines are drying curves, dashed lines are temperature curves):
1 — power consumed in the inductor 3 kW, concentration of metallic particles 50 percent, diameter of metallic particles 3.15 mm;
2 — power 3 kW, concentration 70 percent, diameter 3.15 mm;
3 — power 1 kW, concentration 70 percent, diameter 3.15 mm;
4 — heating without addition of metallic particles.

Next, the kinetics of drying was studied by taking samples of the material being dried every minute, followed by drying them in a drying cabinet at a temperature of 250°C to constant weight.

During drying, the temperature was measured with a chromel–copel thermocouple, whose emf was recorded by a PP-63 type potentiometer. (Experiments showed that the chromel–copel thermocouple is not heated in the field of an inductor supplied with current of frequency 2500 Hz.)

Figures 1, 2, and 3 show the temperature and drying curves for quartz sand and corundum without an additive and with the addition of metallic particles. Heating and drying of corundum proceed faster, which is evidently due to the fact that corundum particles are much smaller than sand particles; therefore they are mixed better by the gas flows and metallic particles.

It is evident from the figures that, for these experiments, the optimal concentration of metallic particles is \(1/2\) by weight, calculated with respect to the dry mixture.

Increasing the power consumed by the inductor reduces the heating time, so that the optimal energy characteristics ...

the operating conditions of the induction dryer will be nominal for the given high-frequency generator.

Fig. 2. Drying and temperature curves for quartz sand (solid lines are drying curves; dashed lines are temperature curves): 1 — power consumed in the inductor 3 kW, concentration of metal particles 70 percent; 2 — power — 2 kW, concentration — 70 percent; 3 — power — 4 kW, concentration — 50 percent; 4 — power — 2 kW, concentration — 50 percent; 5 — power — 4 kW, concentration — 50 percent; 6 — heating without the addition of metal particles; curves 1–4 — diameter of metal particles 3.15 mm; curve 5 — diameter of metal particles 7.93 mm.

Fig. 3. Drying and temperature curves (solid lines are drying curves; dashed lines are temperature curves): 1 — corundum, 50 percent metal particles, power consumed in the inductor 0.5 kW; 2 — sand, 30 percent metal particles, power 3 kW; 3 — heating of corundum and sand without the addition of metal particles.

Increasing the particle diameter (which must be selected from the conditions of optimal heat generation and maximum heat-exchange surface of the metal particles) from 3.15 mm to 7.93 mm (curve 5,

Fig. 2) does not reduce heat generation, but the uniformity of heating and drying of the material deteriorates owing to the cessation of mixing and the reduction of the heat-exchange surface.

II. Experiments at a Frequency of 200,000 Hz

The installation consists of an AZ-43 tube generator, control and monitoring instruments, an inductor 65 mm in diameter consisting of four turns made of 6 mm tubing, and a porcelain crucible 60 mm in diameter into which quartz sand was loaded; the dimensions of the latter are given in Table 1, in a mixture with balls of ShKh15 steel having diameters of 3.15 mm and 2 mm. The concentration of metallic particles in both series of experiments was \(1/2\), calculated on the dry mixture. The weight of the mixture loaded into the crucible was 200 g. The temperature of the mixture was measured with an KhK thermocouple, which was placed in the crucible after the current supply to the inductor had been switched off.

The results of the experiments are summarized in Table 2.

Table 2

Thus, the experiments showed that in the thermal treatment of loose dielectric materials, especially finely dispersed and caking materials, the use of induction heating may be promising.

REFERENCES

1. L. S. Akselrod, G. B. Panter. On the operating regime of an apparatus for drying and calcining ferrous-metal hydroxide in a pseudo-fluidized layer of inert material. Chemical and Petroleum Engineering, 1968, No. 4, p. 14.

2. A. V. Netushil et al. High-Frequency Heating in an Electric Field. Moscow, 1961.

3. S. G. Romanovskii. Induction Drying by Industrial-Frequency Currents. Kiev: Gostekhizdat of the Ukrainian SSR, 1963.

4. A. V. Lykov. Theory of Drying. Energiya Publishing House, 1968, pp. 531–534.