Applying High-Temperature Heat-Pipe Technology to Precision Heat Treating and Materials Processing

High-temperature heat pipes can be used to build heat-treating and materials-processing furnaces that are capable of extraordinarily precise temperature setpoints and isothermality.

Setpoint accuracy, stability and isothermality of ±0.1°C are common with a single-heated zone using an off-the-shelf temperature controller. Because of the inherent temperature uniformity and stability of heat pipes, they are an integral component in nearly all of the most precise temperature-calibration instruments in the primary calibration laboratories around the world. This technology can also be applied to research, commercial and industrial applications for processes such as annealing, sintering, crystal growing, brazing and controlled diffusion. The following sections will describe in more detail: What is a heat pipe? How do they operate? How are they typically configured? And, finally, what are the available temperature ranges?

What is a Heat Pipe?

When a pan of water is heated on a stovetop, the temperature of the water rises steadily until it begins to boil. At that point, all of the energy or heat is used to change the phase of the water from liquid (water) to vapor (steam). The boiling point of the water is set by the atmospheric pressure surrounding the open pan of water. At sea level, the boiling point of water is 100°C (212°F). The water in the pan will remain at the boiling-point temperature until all of the water is converted to steam. When all of the water has been evaporated, the pan will begin to increase in temperature again (Fig. 2).

Now consider what happens if the water is inside a closed volume and the atmospheric pressure is reduced to zero by temporarily pulling a vacuum on the closed volume and then permanently sealing it. The result is a closed volume with only liquid water and its vapor. The pressure inside the closed volume is set by the vapor pressure of the fluid (Fig. 3), which is water. If heat is added to the water at one end of the closed volume, it will immediately change some of the water to a vapor. This will increase the pressure locally, and the vapor will travel to the cooler, lower-pressure end of the closed volume and condense. If the working fluid is selected properly and the cross-sectional flow area is adequate, this mechanism will keep the closed volume at a nearly isothermal temperature. What was just described is known as a heat pipe, or thermosyphon.

How Do They Operate?

The heat pipe was invented in the 1940s by R.S. Gaugler. It did not become widely accepted and studied, however, until it was independently invented by G.M. Grover in the early 1960s at Los Alamos National Laboratories. The basic concept of the heat pipe is shown in Figure 4. The traditional heat pipe is a long, closed cylinder. As described above, a working fluid is charged into the closed cylinder and the air is evacuated. When heat is applied to one end of the heat pipe, a small amount of working fluid vaporizes. This raises the local vapor pressure and causes the vapor to travel to the lower-pressure, cooler end of the heat pipe, where it condenses. This change in phase does not require a change in temperature. Therefore, the only temperature difference required is the small vapor-pressure difference to drive the vapor from the evaporator to the condenser. By selecting the appropriate working fluid and sizing the cross-sectional area for a low-pressure-drop flow, the temperature difference can be almost negligible, resulting in a very high effective thermal conductivity – a super conductor for thermal- energy transport.

To demonstrate the effective thermal conductivity of a heat pipe versus a solid conductor, a 12-inch-long, ¼-inch-diameter copper rod will be compared to a 12-inch-long, ¼-inch-diameter copper/water heat pipe. The power input to the evaporator end will be set at 25 watts (Fig. 5).

From the conduction equation:

DT = QL/kA, where:

Q = Thermal power (watts) = 25 W
L = Rod length (m); 12 inches = 0.305m
k = Thermal conductivity (W/m-°C); copper = 400 W/m-K
A= Cross-sectional area of rod (m²) = pD²/4 = p*.25²/4 in² or 3.17x10^-5 m
DT = Temperature difference along rod length (°C)

For the solid-copper rod, the temperature difference along the rod is:

DT = (25W * 0.305m)/(400W/m°C * 3.17x10^-5 m²) » 600°C

For the equivalent copper/water heat pipe, a delta T of approximately 3°C is typical. In other words, the effective thermal conductivity of the heat pipe is approximately 80,000 W/m°C, or 200 times more conductive than the copper rod.

DT = (25W * 0.305m)/(80,000W/m°C * 3.17x10^-5 m²) » 3°C

Heat pipes have been designed into numerous thermal-management applications to take advantage of this phenomenon. For instance, copper/water heat pipes have been applied to laptop-computer CPU chip cooling since the mid-1990s, and they are now moving into many other electronics-cooling applications. Aluminum/ammonia heat pipes are used for thermal management on most space satellites. The heat pipes isothermalize the satellite temperatures in extreme hot and cold environments and are used to radiate the waste thermal energy to deep space. And high-temperature heat pipes, typically Inconel/sodium, have been used for high-temperature thermal-to-electric conversion devices such as thermionics, thermoelectrics, Stirling engines and small-scale nuclear reactors. High-temperature heat pipes are also used to isothermalize furnaces.

How are They Typically Configured?

High-temperature heat pipes for heat treating and materials processing are typically manufactured in a cylindrical annular configuration. As shown in Figure 6, the heat pipe is made up of two concentric cylinders and end caps. The enclosed volume between the inner and outer cylinders (orange) contains the heat-pipe working fluid, and the volume inside of the inner tube is the working space for the product being processed. The outer cylinder is generally heated with a single-zone, cylindrical electrical-resistance furnace. The heat pipe can be manufactured with both ends open or with one end closed using hemispherical caps. Thermocouple wells are built into the annular space such that sheathed thermocouples can be inserted and removed as necessary. A thermocouple/thermowell in the heat-pipe vapor space results in extremely accurate and repeatable temperature measurements. Off-the-shelf temperature controllers are used to achieve the setpoint control and stability. Another name for the annular heat pipe is an isothermal furnace liner, or IFL.

Isothermal furnace liners are typically manufactured using Schedule-40 pipe sizes for the inner and outer tubes. The inner diameter of the standard product line spans the range from 1-12 inches. Larger sizes can be accommodated. The length of the IFL is typically 12-60 inches long. However, significantly longer pipes can be manufactured if required. The IFLs can be operated either vertically or horizontally. Two of the most common furnace IFL configurations are shown in Figure 6. Custom features like flanges, extended inner pipe, grounding studs, etc. are easily accommodated.

Annular IFLs provide an isothermal processing zone and are also used to extend the useful furnace length. Re-entrant cavity IFLs are often used in conjunction with freeze-point calibration cells or as black-body calibrators with emissivity ³ 0.95.

What are the Available Temperature Ranges?

The isothermal furnace-liner heat pipes operate across the temperature range of 50-1100°C. However, one heat-pipe working fluid cannot span the entire range. The fluid properties vary too much across this wide temperature range for any one fluid. The four fluids typically used to cover this range are water, cesium, potassium and sodium. The operating range for each fluid is shown below:

• Water – 50 to 250°C (122-482°F)
• Cesium – 300 to 600°C (572-1112°F)
• Potassium – 400 to 1000°C (752-1832°F)
• Sodium – 500 to 1100°C (932-2012°F)

Materials of construction are typically Alloy 400 for water and Alloy 600 for cesium, potassium and sodium. Advanced Cooling Technologies also manufactures a Haynes 230/sodium heat pipe for extended operation near 1100°C (2012°F) because of the significantly higher creep strength.

Summary and Conclusions

Heat pipes and/or isothermal furnace liners can be used to manufacture heat-treating and materials-processing furnaces that are capable of extraordinarily precise temperature setpoints and isothermality. Setpoint accuracy, stability and isothermality of ±0.1°C is common with a single-zone furnace and an off-the-shelf temperature controller. These furnaces are typically 1-12 inches in diameter and can be operated vertically or horizontally. Customized IFL furnaces can be designed for numerous applications, including annealing, sintering, crystal growing, brazing and controlled diffusion. An example of a vertical-orientation isothermal process cavity is shown in Figure 7. IH

For more information: Contact Peter Dussinger, vice president Custom Products, Advanced Cooling Technologies, Inc., 1046 New Holland Avenue, Lancaster, PA 17601; tel: 717-295-6061; fax: 717-295-6064; e-mail: pete.dussinger@1-ACT.com; web: www.1-ACT.com