A heat pipe material system includes the envelope material, the wick material, the working fluid, and any braze, solder or weld filler materials used in sealing the heat pipe. Once the operating temperature range of the heat pipe is known, the working fluid is chosen. Next, the heat pipe designer must choose envelope and wick materials that are suitable for the application, and are compatible with the working fluid. Two major results of material incompatibility are corrosion and generation of non-condensable gas (NCG). If the wall or wick material is soluble in the working fluid, mass transfer is likely to occur between the condenser and evaporator, with solid material being deposited in the latter. This will result in either local hot spots or blocking of the pores of the wick. NCG generation is the most common indication of a heat pipe failure. As the NCG accumulates in the heat pipe condenser section, it gradually blocks the heat transfer area, consequently degrading the heat pipe performance.
Another consideration in envelope material selection is that the heat pipe is a pressure vessel. It must resist both vacuum (external pressure) for temperatures below the normal boiling point (100ºC for water), and internal pressure at higher temperatures. Ideally, the heat pipe envelope will have a high yield and tensile strength, and a low creep rate (creep is mostly important for the higher temperature heat pipes, with alkali metal working fluids).
Most working fluids have a number of compatible envelope/wick materials. The heat pipe material is selected based on a trade-off:
- Thermal conductivity
- Strength/mass (high tensile strength allows a thinner wall)
For example, copper is chosen for electronics cooling applications at temperatures below 150 ºC, since it has very high thermal conductivity, and adequate strength. At higher temperatures, copper is not suitable due to the higher vapor pressure, so titanium or Monel envelopes are used instead.
Heat pipes can operate for 15-20 years or more, so fluid/material compatibility is very important. Problems with incompatible fluids/material pairs include:
- Non-Condensable Gas Generation, which blocks a portion of the condenser, and reduces maximum heat pipe power at a given temperature
- Corrosion, which can cause leaks that stop heat pipe operation
- Materials Transport, which can plug the heat pipe wick
Compatibility of working fluids and wall/wick materials is determined by long term Life Tests. In the past, life tests have been conducted on hundreds of different working fluid/envelope/wick combinations. The most commonly used fluid/envelope combinations are discussed below. Other compatible combinations, and suggested operating temperature ranges are given in Compatible Fluids and Materials.
Most Commonly Used Envelope/Fluid Pairs
Most heat pipes are used for either
- Electronics Cooling
- Energy Recovery
- Spacecraft Thermal Control
- High Temperature Heat Pipes
A copper envelope and wick with water working fluid is the most commonly used envelope/wick/fluid system for electronics cooling for temperatures between roughly 25 and 150ºC. Water has the highest Merit number in this temperature range. Copper/methanol is used when the heat pipes must operate at temperatures below 25ºC. Titanium and Monel envelopes allow water to be used for temperatures up to 300ºC (short term) and 280ºC (long term).
The refrigerant R134a is used as the working fluid with copper or steel envelopes in Energy Recovery Heat Pipes Heat Exchangers, since it has no ozone depletion, low global warming potential, reasonable vapor pressure, and is non-flammable. In addition, the HVAC community is comfortable with and experienced with this working fluid.
Spacecraft Thermal Control
Aluminum/ammonia Constant Conductance Heat Pipes and aluminum/stainless/ammonia Loop Heat Pipes (LHPs) are the default combinations used in spacecraft thermal control. Ammonia has the highest merit number in this temperature range For applications where the condenser temperature can fall below ~ -60ºC (ammonia freezes ~ -77ºC), ethane is used in CCHPs, and propylene is used in LHPs.
High Temperature Heat Pipes
Superalloy envelope & wick/alkali metal working fluids (cesium, potassium and sodium) are used for high temperature heat pipes in the range 450ºC to 1100ºC; see Figure 12. At higher temperatures, heat pipes use lithium as the working fluids, and refractory metals such as tungsten or molybdenum as the wick and envelope; see Figure 13. The merit number for the alkali metals are much higher than the merit numbers for working fluids that are suitable at lower temperatures.
More details on fluid/envelope/wick compatibility are given in the following pages:
- Compatible Fluids and Materials, a more detailed discussion of fluids and materials known to be compatible, which also give approximate temperature ranges for each fluid.
- Life Tests, discussing how compatibility is demonstrated
- Process Control Life Tests, for verifying that the heat pipe manufacturing process is under control to produce long-life pipes
- High Temperature Water Life Tests, work by ACT to increase the water upper temperature range from 150 to 300ºC
- Intermediate Temperature Fluids, work by ACT and others on fluids in the temperature range from 250 to 400ºC
- Examples of Incompatible Fluids/Envelope Pairs, demonstrating problems that can occur when exploring potential fluid/envelope pairs