Working Fluids and Temperature Ranges of Heat Pipes

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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 <ref name="FR2012">Faghri, A., 2012, "Review and Advances in Heat Pipe Science and Technology", Journal of Heat Transfer, 134(12), 123001, 1-18.</ref><ref name="Faghri1995">Faghri, A., 1995, Heat Pipe Science and Technology, 1st ed., Taylor & Francis, Washington, D.C.</ref> 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.
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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 <ref name="FR2012">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</ref><ref name="Faghri1995">Faghri, A., 1995, Heat Pipe Science and Technology, 1st ed., Taylor & Francis, Washington, D.C.</ref> 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 <ref name="Faghri1995"></ref>. 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 <ref name="Faghri1995"></ref>. 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.

Current revision as of 00:27, 13 March 2014

 Related Topics Catalog
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

Operating temperature ranges of various working fluids on a logarithmic temperature scale..
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.

Contents

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.