Working Fluids and Temperature Ranges of Heat Pipes

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Historical Development of Heat Pipes Operation Principles of Heat Pipes Types of Heat Pipes Working Fluids and Temperature Ranges of Heat Pipes Capillary Wick Designs and Structures in Heat Pipes Heat Transfer Limitations of Heat Pipes Heat pipe Start Up Heat Pipe Characteristics Heat Pipe Analysis and Simulation Heat Pipe Applications

Each heat pipe application has a particular temperature range in which the heat pipe needs to operate. Therefore, the design of the heat pipe must account for the intended temperature range by specifying the proper working fluid. Table 1 ^[1][2] lists some of the commonly used and proposed working fluids, their melting and boiling points at atmospheric pressure, and their useful ranges. As a rule of thumb, the useful range extends from the point where the saturation pressure is greater than 0.1 atm and less than 20 atm. Below 0.1 atm, the vapor pressure limit may be approached. Above 20 atm, the container thickness must increase to the point where the heat pipe becomes limited by the thermal resistance through the container.

Longevity of a heat pipe can be assured by selecting a container, a wick and welding materials that are compatible with one another and with the working fluid of interest. Performance can be degraded and failures can occur in the container wall if any of the parts (including the working fluid) are not compatible. For instance, the parts can react chemically or set up a galvanic cell within the heat pipe. Additionally, the container material may be soluble in the working fluid or may catalyze the decomposition of the working fluid at the expected operating temperature. A compilation of the most up-to-date information concerning the compatibility of metals with working fluids for heat pipes is given in Table 2 ^[2]. Figure 1 presents various working fluid boiling points and classifies them into four categories: cryogenic, low, intermediate and high temperature ranges. The working-fluid inventory of a heat pipe is the sum of the masses of the vapor and liquid phases, assuming the wick is full of liquid. This criterion is slightly over the optimum requirement because the meniscus recedes into the evaporator wick during normal operation. However, this situation is more advantageous than underfilling the heat pipe, which may significantly reduce the maximum heat transfer. With extreme overfill, however, any excess fluid might collect as liquid in the condenser section and increase the thermal resistance, thereby decreasing the heat transport capability of the heat pipe.

Table 1 Working fluids and temperature ranges of heat pipes
Working Fluid	Melting Point, K at 1 atm	Boiling Point, K at 1 atm	Useful Range, K
Helium	1.0	4.21	2-4
Hydrogen	13.8	20.38	14-31
Neon	24.4	27.09	27-37
Nitrogen	63.1	77.35	70-103
Argon	83.9	87.29	84-116
Oxygen	54.7	90.18	73-119
Methane	90.6	111.4	91-150
Krypton	115.8	119.7	116-160
Ethane	89.9	184.6	150-240
Freon 22	113.1	232.2	193-297
Ammonia	195.5	239.9	213-373
Freon 21	138.1	282.0	233-360
Freon 11	162.1	296.8	233-393
Pentane	143.1	309.2	253-393
Freon 113	236.5	320.8	263-373
Acetone	180.0	329.4	273-393
Methanol	175.1	337.8	283-403
Flutec PP2	223.1	349.1	283-433
Ethanol	158.7	351.5	273-403
Heptane	182.5	371.5	273-423
Water	273.1	373.1	303-550
Toluene	178.1	383.7	323-473
Flutec PP9	203.1	433.1	273-498
Naphthalene	353.4	490	408-623
Dowtherm	285.1	527.0	423-668
Mercury	234.2	630.1	523-923
Sulphur	385.9	717.8	530-947
Cesium	301.6	943.0	723-1173
Rubidium	312.7	959.2	800-1275
Potassium	336.4	1032	773-1273
Sodium	371.0	1151	873-1473
Lithium	453.7	1615	1273-2073
Calcium	1112	1762	1400-2100
Lead	600.6	2013	1670-2200
Indium	429.7	2353	2000-3000
Silver	1234	2485	2073-2573

Heat pipe fabrication, processing, and testing involve several detailed procedures which are recommended to be strictly followed in order to achieve the highest quality possible. Faghri ^[2] provided a detailed procedure for the fabrication, processing, and testing of low, moderate, and high temperatures.

Contents 1 Cryogenic Temperature Range 2 Low Temperature Range 3 Intermediate Temperature Range 4 High Temperature Range 5 References

Cryogenic Temperature Range

Cryogenic heat pipes operate between 4 to 200 K. Typical working fluids include helium, argon, oxygen, and krypton. The amount of heat that can be transferred for cryogenic heat pipes is quite low due to the small heats of vaporization, high viscosities, and small surface tensions of the working fluids.

Low Temperature Range

The low temperature range is from 200 to 550 K. Most heat pipe applications fall within this range. Commonly used fluids are ammonia, acetone, the Freon compounds, and water. Water, which is perhaps the most widely used working fluid, has good thermophysical properties such as large heat of vaporization and surface tension, and has the added benefit of being safe to use during handling.

Intermediate Temperature Range

The working fluids in the medium temperature range, 450 to 750 K, are mercury and sulphur. Compounds such as Thermex or Dowtherm-A (diphenyl/diphenyl oxide eutectics) are also employed in this range. Mercury has extremely attractive properties inherent in a liquid metal such as its high thermal conductivity. However, problems with wetting the wick and wall present difficulties in using mercury in capillary heat pipes. The toxicity of mercury is also a significant problem. A review of intermediate temperature fluid life tests experiments was reported by Anderson et al. ^[3].

High Temperature Range

Sodium, lithium, cesium, silver and a sodium-potassium compound (NaK) are often used in the high temperature range (750 K and above). The heat transport rates for liquid-metal heat pipes are generally much higher than those in the other temperature ranges because the surface tension coefficients, latent heats of vaporization, and thermal conductivities of liquid metals are very high. Faghri ^[2] presents thermophysical property data for most heat pipe working fluids and container materials along with polynomial temperature-property relations for the working fluids.

References

1. ↑ Faghri, A., 2012, "Review and Advances in Heat Pipe Science and Technology," Journal of Heat Transfer, 134(12), 123001. http://dx.doi.org/10.1115/1.4007407
2. ↑ ^{2.0 2.1 2.2 2.3} Faghri, A., 1995, Heat Pipe Science and Technology, 1st ed., Taylor & Francis, Washington, D.C.
3. ↑ Anderson, W. G., Bonner, R. W., Dussinger, P. M., Hartenstine, J. R., Sarraf, D. B., and Locci, I. E., 2007, "Intermediate temperature fluids life tests - Experiments," Collection of Technical Papers - 5th International Energy Conversion Engineering Conference, 2, 926-941.