As discussed in Fluid/Envelope/Wick Compatibility there are a large number of fluid/envelope pairs that are compatible, with some life tests conducted for decades. There is no longer any question that such envelope/fluid pairs as aluminum/ammonia, copper/water, copper/methanol, and superalloy/alkali metals are compatible when fabricated and processed properly. Most Heat Pipe Life Tests today are conducted as a Quality Control measure to confirm that the heat pipe fabrication processes are under control. For example, samples of each of ACT’s extrusions for aluminum/ammonia Constant Conductance Heat Pipes (CCHPs) are on life test at elevated temperature, to demonstrate the long term life required in heat pipes for satellites.
One area for active research on compatibility is the intermediate temperature range, from roughly 250 C to 400ºC. As discussed in Intermediate Temperature Fluids Life Tests ACT has been examining different envelope/fluid pairs for the past decade, and demonstrated several new compatible fluid pairs. Other pairs were found to be incompatible.
Possible problems with incompatible fluid/envelope pairs include:
- Non-Condensable Gas Generation(most Common)
- Materials Transport
This webpage provides examples of these problems. Please remember that these problems were observed during research on new envelope/fluid pairs, or when life tests are conducted at higher temperatures than previous tests, and are not representative of standard envelope/fluid pairs.
Non-Condensable Gas Generation
The most common symptom of incompatibility is Non-Condensable Gas (NCG) generation in the heat pipe. During operation, the NCG collects at the end of the condenser, reducing its effective length. In turn, this reduces the power that the heat pipe can carry at a given temperature. NCG generation is generally caused by impurities in the working fluid or contaminants on the wick or wall.
Gas generation can also be caused in organic working fluids when the temperature is too high. Eutectic Diphenyl/Diphenyl Oxide is an intermediate temperature fluid that is sold under the trade names of Dowtherm A and Therminol VP-1. Therminol VP-1 heat pipes were life tested at three different operating temperatures, 350, 400, and 450°C, and monitored for gas generation.
As shown in Figure 1, the life test pipes operating at 450°C showed a significant increase in ΔT after only 90 hours, most likely caused by excess non-condensable gas. These pipes were taken off life test and purged, then put back on life test. Non-condensable gas generation continued at a high rate. The 450 °C pipes were taken off life test after 300 hours, sectioned, and analyzed. As shown in Figure 2, a portion of the Therminol charred during the life test.
The gas generation rate is dependent on the operation temperature. Figure 1 shows that the gas generation rate was much lower when the heat pipe operation temperature was reduced to 400°C. No gas generation was observed in the 350°C life tests after 1,000 hours of operation.
Corrosion occurs when the working fluid is not compatible with the envelope or wick (NCG can also be generated, as discussed above). Corrosion products can block a portion of the wick, reducing the heat pipe maximum power. In more extreme cases, a leak can develop, with the heat pipe ceasing operation.
ACT has conducted a series of life tests in the 250 – 400°C with superalloy envelopes and halide working fluids. While several of the halides have been shown to be compatible with superalloys, gallium trichloride is not compatible. The GaCl3/superalloy pipes all leaked at the pinch-off weld after roughly one week of operation at 360°C (633K); see Figure 3.
Material transport of the wick/envelope can occur when the working fluid has a high solubility for one of the wick/envelope components. During heat pipe operation, working fluid is vaporized in the evaporator, and then condensed in the condenser, transferring heat. The working fluid vapor is very pure with no impurities, so the working fluid in the condenser is also very pure. If the working fluid has a high solubility for one or more of the components in the wick or envelope, these components will dissolve in the working fluid, and be carried along to the evaporator. When the working fluid vaporizes, the dissolved components are left behind.
A beautiful example of material transport with an incompatible envelope/fluid pair is shown in Figure 4. ACT (Passive Thermal Management for a Fuel Cell Reforming Process) examined a Variable Conductance Heat Pipe (VCHP) Heat Exchanger with cesium as the working fluid. Cold sea-water was a potential coolant for the heat pipes (with a large ΔT between the heat pipe interior and the water). Monel and copper-nickel are known to resist sea water corrosion, so they were potential heat pipe envelope materials.
Since there was no known life test data between cesium and Monel or copper-nickel, heat pipes were fabricated with Monel 400 and 70/30 copper/nickel envelopes, and life tested at 475°C for 1000 hours with cesium. The heat pipes were then sectioned and examined; see Figure 4. The copper grains in the evaporator show that copper was transported in the cesium from the condenser to the evaporator, hence the Monel and copper/nickel is not compatible with cesium.