# Working Fluids and Temperature Ranges of Heat Pipes

(Difference between revisions)
 Revision as of 19:55, 12 March 2014 (view source) (Created page with '{{Heatpipe Category}} Image:HPtable2.png [[Image:HPfig21.png|thumb|400px|alt=Operating temperature ranges of various working fluids on a logarithmic temperature …')← Older edit Revision as of 00:23, 13 March 2014 (view source)Newer edit → Line 4: Line 4: - 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 Faghri, A., 1995, Heat Pipe Science and Technology, 1st ed., Taylor & Francis, Washington, D.C. 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. + 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 Faghri, A., 2012, "Review and Advances in Heat Pipe Science and Technology", Journal of Heat Transfer, 134(12), 123001, 1-18.Faghri, A., 1995, Heat Pipe Science and Technology, 1st ed., Taylor & Francis, Washington, D.C. 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 . 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. 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 . 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.

## Revision as of 00:23, 13 March 2014

 Historical Development of Heat Pipes Working Fluids and Temperature Ranges of Heat Pipes
Figure 1: Operating temperature ranges of various working fluids on a logarithmic temperature scale.

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.

## 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, 1-18.
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.