Aerodynamic Heating Furnaces (AHTF) in which air or gas is heated to 932-1292°F (500-700°C) without electrical or gas heaters were developed in the former Soviet Union. They were widely used for heat treating aluminum, magnesium and titanium alloys as well as steels.

The AHTF chamber furnace has a centrifugal fan with vanes having a special contour. This fan, operating in a closed system, converts almost all the energy used to turn it into heat. The heat is transferred to the parts by convection. In most plants, aluminum alloys are heat treated in electric resistance furnaces (ERF) or in salt baths. This article deals with a study of the heating conditions for various semi-finished products of aluminum alloys in an AHTF in comparison with an ERF and a potassium-nitrate bath of approximately the same working volume.

Fig. 1. Typical AHTF furnace design

The AHTF Furnace

The invention of the aerodynamic heating process in the former Soviet Union in 1963-1964 resulted in the introduction of a new class of industrial heating device. The principle of aerodynamic heating is a transfer of gas-flow energy generated by a centrifugal fan into heat. The rotor of the centrifugal fan serves as a compressor and as a heat generator. It induces circulation in the furnace atmosphere and generates heat at the same time. The first industrial furnaces were developed by a design team headed by Dr. P. I. Tevis[6] and others.[1-4]

There is no uniform, widely accepted terminology on the subject. For practical applications the most widespread term is “aerodynamic heat-treat furnace (AHTF),” or “aerodynamic loss furnace.” We must emphasize the difference between the meaning of the term “aerodynamic heating” in our case and its usage in the aerodynamics of hypersonic flows. In the latter instance, this term is used to describe the heat generation caused by the friction between a surface of a solid object and high-speed gas flow. This effect is negligible in AHTF.

The AHTF has a number of similar yet different configurations. The differences are generally in the geometry of the heating chamber and in the duct system for both supply and return airflow. Figure 1 shows a typical design.

Basics of Aerodynamic Heating

The generator of the heat in recirculating heating installations is a rotor of the centrifugal-fan type. The source of the thermal energy is various kinds of aerodynamic losses in the driving wheel and to an insignificant degree in the circulating path. Generally, the efficiency of centrifugal fans at working modes ranges from 0.5-0.88%. For some types of isolated wheels – rotors without a case – the static efficiency is reduced down to 0.05-0.15%. In this situation, an overwhelming part of the energy supplied to a rotor is spent in overcoming the flow losses in the rotor.

The mechanism of transformation of these losses into thermal energy can be described as follows. From the mechanics of liquids and gases it is known that energy transforms from one form to another when fluids move in a continuous environment. The change of temperature of a fluid is defined by the amount of heat and mechanical work transferred to the fluid or taken from it.

At adiabatic movement of an ideal gas it is known from gas thermodynamics that the gas is cooled with an increase in speed, and it is warmed when the speed is reduced.

CpTg + 0.5V2 = Const

 

Cp is heat capacity at constant pressure, Tg is absolute temperature of gas and V is the velocity of the gas.

This formula illustrates the interrelation and transformation of kinetic energy (0.5V2) to thermal energy. As the velocity of a flow decreases, its kinetic energy decreases, but it does not disappear and is transferred to heat. The total amount of energy remains constant in conformity with the first law of thermodynamics. On the contrary, a gas being dispersed selects a flow necessary for acceleration, and the increased velocity is accompanied by a decrease in temperature.



Fig. 2. ERF heats air 1.5 times faster than the AHTF

    According to the second law of thermodynamics, the entropy is an increasing function of time in a closed adiabatic system. Its increase shows that an irreversible process or transformation of mechanical energy is going on inside of the system.

    An example of such losses is the loss of kinetic energy due to internal friction. Part of the mechanical energy of a flow turns to heat at the expense of work from the internal forces of friction. This is an irreversible process of transition of mechanical energy to thermal energy. Internal friction (viscosity) in gas and heat conductivity makes up two parts of the same process of molecular transfer.

    For the analysis of work of the fan in a contour, there are various kinds of losses of mechanical energy in the gas flow (friction with the walls and friction inside the flow because of the formation of vortexes, separations, etc.). We will divide the losses into internal (losses in the compressor wheel) and external (losses in a circulating contour).

    An energy balance can be calculated for the circuit of recirculating aerodynamic heating. The rotor of the fan of this furnace works in a closed contour formed by circulating channels and runners in the working chamber. Theoretically, the mechanical energy of the rotor is transferred from the rotor to the air such that each unit volume of gas has an energy level equal to theoretical output pressure, Pt. Actually, the effective or total pressure, Pe, at the output of the rotor will be less than Pt by the amount of internal aerodynamic losses (Pt = Pe + Dhw). The effective pressure is spent to overcome the resistance in the path. This is comprised of external losses, Dhex, and useful work, which is necessary in the process (Dhtec).

    It is necessary to allocate, as the independent part of the loss of kinetic energy, a flow going out from the rotor connected with transformation of dynamic pressure to static due to expansion of a flow on exit from a flowing part – Dhout from losses of Dhex. The other part consist of losses of capacity (work) on friction and local resistance of a path, Dhf: Dhex = Dhout + Dhf. As a result, we receive the equation of energy balance in J/m3: Pt = Dhtec+ Dhw+ Dhout+ Dhf. Though the energy Dhf is spent for formally useful work (moving of air over a contour), it is considered a loss because it is transformed to heat at the end of the process. Thus, the final three components make up the aerodynamic losses. They determine the thermal capacity of the unit.

    In a circulating path, the velocity of the flow typically does not exceed 20 m/s, and the losses due to friction with the walls of the channels are negligible. In the rotor, where the maximum speeds of the flow reach 50 m/s and above, friction with the disks of the wheel and in the channels between blades can be significant and should be taken into account. The main source of thermal energy is aerodynamic losses in the wheel connected with changes in the character of the flow (structure and speeds of a flow in channels between blades and on exit from the wheel). Heating accounts for up to 90-95% of consumed energy of the rotor. And the rest of the supplied energy, minus inevitable and insignificant losses in the shaft bearings, is used for the useful work of moving the air.

    Attempts to use heating “fan” effect for drying of windings of electric motors in the 40s are known. However, an industrial application of this technology for heating has not been found. From the time of its invention by A.A. Sablukov (1835) the fan was used as an air-blowing machine, and all efforts were directed toward the achievement of its maximum efficiency as a blower. The thermal effect of aerodynamic losses can be utilized in industry only with the use of an isolated rotor in a closed contour (or with a small exchange). The key is that the heat generated remains in the unit instead of being thrown out into the atmosphere as is typical for fans in an open network.

 



Fig. 3. Shows temperature drop after metal is loaded

Heating Test

A test to compare the heating ability of the three different systems was conducted. An AHTF, ERF and a salt-bath furnace were compared.

The duration and uniformity of heating in air and potassium nitrate were recorded with 12 thermocouples equally spaced in the working area. The time for heating various semi-finished products to 932-977°F (500-525°C) was determined with thermocouples in the center of the samples of the 2024 alloy (sheets 0.8-10 mm thick, pipes with a wall thickness of 1-4 mm and bars 40-200 mm in diameter) and the AK6 alloy (forgings 40-100 mm thick). The weight of the metal loaded in the furnace and the salt bath was 176-772 pounds (80-350 kg).

It was found that the AHTF produces a more uniform temperature field in the working space than the ERF with the temperature difference in the working space approaching that in the salt bath (Table 1). The air is heated from room temperature to 500°C twice as rapidly in the AHTF (4 hours) as in the ERF (8 hours). The higher heating rate was observed for a furnace of the same size even though the ERF had twice the input power.

Despite a larger drop in temperature in the AHTF in comparison to the ERF when metal weighing 180-350 kg is placed in the furnaces, the AHTF heats the metal to the given temperature 1.5-2 times as rapidly. However, when the load of metal is 80-160 kg, the ERF heats the air to the given temperature 1.5 times as fast as the AHTF (Fig. 2). This is due to the large reserve of heat in the ERF in the mass of the heated brick wall, which also leads to a smaller reduction of the furnace temperature when the metal is loaded into a hot furnace.

The temperature drop of the salt bath when the metal is loaded is minimal because of the large thermal inertia, and the time difference of heating to a given temperature is best as compared to the ERF and AHTF (Fig. 3).

Fig. 4. Shows effect of heating a 353-pound (160-kg) load

Bars and forgings weighing 160-350 kg are heated to 515°C, and sheet and pipe weighing 80-180 kg are heated to 500°C. Both are heated 1.8-2.5 times faster in the AHTF than in the ERF furnace in spite of the more rapid heating of the air in the ERF when the load is small.

The heating time is smallest in the salt bath as compared with the AHTF and ERF (Fig. 4). When bars and forgings weighing 160 kg and 40-200 mm thick were heated to 515°C, the heating time was 5-20 minutes in the salt bath, 40-100 minutes in the AHTF and 60-180 minutes in the ERF. Then the heating rate of the samples in the salt bath declined sharply, and the difference in time for heating to 515°C decreased. The heating time in the AHTF was only 1-2.4 times more and in the ERF 2.5-5 times more than in the salt bath.

The advantage of the AHTF is the uniformity of the temperature field in the working space and the rate of heating the metal in spite of its lower power (one-half) as compared with the ERF. This is explained by the high rate of circulation of the air, the low thermal inertia, the existence of a single thermal zone and the complete absence of heat transfer by radiation. The ERF has three independently controlled thermal zones. Heat transfer is by radiation and convection.

Technological Efficiency

The advantages of the AHTF are uniform and intensive heating. This results in good control of process, with resulting high-quality thermal processing (thermochemical). Also, these units produce a combination of thermal profitability and productivity with simplicity of design reliability. An additional advantage of the AHTF is that these units give good gas tightness of the chamber (in contrast to, for example, electric furnaces), eliminating the need to support a small positive pressure in the working volume. With normal atmospheric air without protective gases, when the tightness of the chamber is not required, the pressure is constantly above atmospheric in the unit. As air is always being let out from the chamber, inflows are excluded, reducing interim cooling of the load and increasing uniformity of the heating. All this simplifies the problem of maintaining the stable air temperature in the workspace.

It is important to emphasize the special requirements for uniformity of a temperature field in the heat treatment of materials with space–time uniformity in the development of structural transformations. As a rule, the discrete control of the temperature on the surface of a material is used in heat-treatment processes. So for a given temperature diagram, only points of control are obtained, but phase transitions occur throughout the entire volume of the material. Temperatures at each interior point are not controlled.

Heat Treatment in AHTFs

The results of research and experience with industrial applications show:
  • The high quality of heat treatment in AHTFs
  • A significant reduction in process cycle times
  • A simplification of procedures
  • A significant decrease in the cost of the process as a whole
  • Improvement and increased stability of the material’s properties
Such effects are realized because of the uniformity of heating, accuracy of regulation and maintenance of a temperature mode. In a number of cases, processing with the AHTF allows one to realize optimum modes unattainable with other methods of heating. For example, the heat treatment of magnesium alloy ML-5 requires heating up to temperature of 788°F (420°C) with an accuracy of ±2°C. This furnace allows one to raise the temperature of the metal up to the top limit of hardening temperatures without risk of burning, which has generated a time reduction by one-half – 16 to 8 hours.

The interest from a heat-treatment-quality standpoint is represented by aluminum and magnesium alloys because they are most sensitive to temperature fluctuations. The ability to obtain quantitative parameters on these alloys guarantees the quality of heat treatment of all other materials.

The heat-treatment quality of deformed aluminum alloys was determined by property comparison of specimens from different semi-finished products (alloys D16, AK6, V65 – Table 2). Properties were compared after complete strengthening heat treatment with heating to the quench temperature with an AHTF and a salt bath. Specimens from D16 were treated with natural age hardening, alloys of AK6 and V65 with artificial aging, and alloys of AK6 and V65 with artificial aging using the AHTF.

Specimens were cut from D16 rolled plates, D16 extruded tube and bar, forged AK6 billets and V95 pins. The results for tensile strength, yield, elongation, shear strength, fracture toughness and electric conductivity of specimens heat treated in an AHTF and a salt bath were obtained (Tables 3, 4, 5). Losses of mechanical properties due to stress corrosion of D16 plates quenched in water after heating with the AHTF are less than after heating in the salt bath (Table 3). During the heating to the quench temperature with an AHTF, the scattering of certain properties was smaller. The depth of diffusion of copper and magnesium into the cladding of alloy D16 specimens with thickness from 0.8-10 mm and tube D16 (30x15 mm) heat treated in the AHTF and in the salt bath were the same.

Investigation of the heating processes in an AHTF for AK6 forgings, which were produced from preliminary deformed semi-finished products, indicated the possibility of a drastic reduction of the heat-treatment cycle times. According to a mechanical-property study of specimens cut from a forging in three directions, sb, s0.2 and d  were practically the same after the heat treatment with a reduced holding time of 100 minutes as compared to the recommended holding time of 150 minutes.

The heat treatment (artificial aging) without preliminary quenching, which needs close control on the temperature, of automobile pistons from heat-resistant alloyed AL0 with the AHTF produced good-quality parts. Optimum processing, such as age hardening at 365°F (185°C) for 8 hours and cooling in air, generated better mechanical and manufacturing properties. The heat-treatment cycle was reduced by 4-6 hours compared with previous technology. Further, positive results were also obtained for large heat-treatment-lot manufacturing conditions – more than 2,000 pistons without special placement.

Age hardening of AK4-1 alloy was also performed with the AHTF. The basic challenge of this process is a high-uniformity heating requirement with a gas temperature variation not more than ±1 to 1.5°C and at the metal ±3°C. Optimum temperature of artificial age hardening for AK4-1 plates is 195±5°C with holding time of 12 hours. For as-quenched, the temperature is 192±2°C and a 24-hour holding time.

The AHTF satisfies the tight requirements that are necessary for heat treatment, and it exceeds special electro-thermal equipment for medium- and low-temperature heating with regard to quality of treatment, productivity and efficiency of process. AHTFs do not concede to the technology parameters of other methods, and sometimes they exceed the most effective heating technologies available.

Conclusions

  • The AHTF is more economical, simpler to manufacture and safer in operation than electric air furnaces or salt baths.
  • The AHTF produces a more uniform temperature in the working space and heats the air from room temperature to the given temperature twice as rapidly as ERFs.
  • Heating of different semi-finished products with sections 40-200 mm thick in loads of 80-350 kg to 500-515°C is 1.8-2.5 times more rapid in the AHTF than in the ERF with similar dimensions in spite of the fact that the ERF has twice the power.
  • The mechanical properties, resistance to corrosion and stress corrosion and electrical conductivity of various semi-finished aluminum-alloy products are the same after heat treatment in the AHTF, ERF and the salt bath.IH


For more information: Dr. Alexey Sverdlin is Professor and Dr. Arnold Ness is Associate Professor in the Department of Industrial and Manufacturing Engineering & Technology, Bradley University, 1501 W. Bradley Ave., Peoria, IL 61625; tel: 309-677-2982; e-mail: sverdlin@bumair.bradley.edu; web: www.bradley.edu

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: electric resistance furnace, aerodynamic heating, entropy, viscosity, heat conductivity, salt bath, convection, age hardening