Operating Temperature Range

Heat pipes are two-phase heat transfer devices.  For a heat pipe to operate, a saturated working fluid is required, with both liquid and vapor in the heat pipe.  The working fluid latent heat is transferred by vaporizing the liquid in the evaporator, and condensing the vapor back to liquid in the condenser.  Theoretically, the heat pipe will operate at temperature just above the triple point (the unique temperature and pressure where the working fluid can be in liquid, vapor, and solid form), to just below the critical point (vapor and liquid have the same properties).  As discussed below, there are other constraints that shrink the practical temperature range.

Hard limits on working fluid temperatures (Triple Point – Critical Point).  Freezing point is used for the halides, cesium, and lithium, since the triple point is unavailable

The triple point and critical point for a number of common heat pipe working fluids are shown in Figure 4 and Table 1.  Two points should be noted.  First, there are gaps in the cryogenic heat pipe temperature range (below about 100 K), where there is no currently known working fluid.

Second, there are many potential working fluids at a given temperature, for temperatures above 200 K.  The fluid selected is normally the fluid with the highest Merit Number when a compatible heat pipe envelope are acceptable.  For example, while ammonia is a better working fluid than methanol, methanol must be chosen when a copper wick and envelope are used.  For large geothermal thermosyphons a fluid with a low global warming potential may be selected.

Table 1.  Selected Heat Pipe Working Fluids, Triple Point and Critical Point.  Freezing point is used for the halides, cesium, and lithium, since the triple point is unavailable.

Fluid

Triple Point Temp., K

Critical Point, K

Triple Point Temp., °C

Critical Point, °C

Helium

5.20

-268.0

Hydrogen

13.95

33.15

-259.2

-240.0

Neon

24.56

44.49

-248.6

-228.7

Oxygen

54.33

154.58

-218.8

-118.6

Nitrogen

63.14

126.19

-210.0

-147.0

Propylene

87.8

365.57

-185.4

92.4

Ethane

91

305.33

-182.2

32.2

Pentane

143.46

469.7

-129.7

196.6

R134a

169.85

374.1

-103.3

101.0

Methanol

175.5

512.6

-97.7

239.5

Toluene

178.15

591.75

-95.0

318.6

Acetone

178.5

508.1

-94.7

235.0

Ammonia

194.95

405.4

-78.2

132.3

Carbon Dioxide

216.58

304.1

-56.6

31.0

SnCl4

240.15

591.85

-33.0

318.7

TiCl4

243

638

-30.2

364.9

Water

273.16

647.10

0.0

373.9

Cesium

301.6

2045

28.5

1771.9

Napthalene

353.5

748.4

80.4

475.3

Potassium

336.35

2239

63.2

1965.9

AlBr3

370.15

763

97.0

489.9

Sodium

370.98

2507

97.8

2233.9

Lithium

453.64

3503

180.5

3229.9

In practice, the fluid range is smaller, at both the low and high end of the temperature range.   For example, a water heat pipe will carry some power between the water triple point (0.01°C) and the critical point (373.9°C).  Maximum power calculations for a typical water heat pipe are shown in Figure 5.  The peak power occurs at a temperature near 150°C), and drops off at lower and higher temperatures.  Practically, most water heat pipes are designed to operate above ~25°C).  At lower temperatures, the vapor pressure decreases, as well as the vapor density, so the vapor velocity for a given amount of power increases.  At temperatures below about 25°C, the viscous and sonic limits become important, limiting the heat pipe power.

Figure 5.  Heat pipe performance (add a link to the ACT heat pipe calculator) typically peaks somewhere in the middle of the temperature range between the triple point and the critical point.  (need to get a better figure – this one is somewhat blurry)

Figure 5. Heat pipe performance typically peaks somewhere in the middle of the temperature range between the triple point and the critical point.

 

The practical upper temperature limit for copper/water heat pipes is roughly 150°C, and is set by the maximum allowable stresses in the copper envelope; see Figure 6.  At 150°C, the saturated water vapor pressure is 69 psia (477 kPa).  Since copper is relatively soft, the required diameter at wall thickness above 150°C C becomes impractical.

Saturated Water Vapor Pressure as a Function of Temperature.

Figure 6. Saturated Water Vapor Pressure as a Function of Temperature.

 

Titanium or Monel envelopes increase the maximum operating temperature range for water to 300°C.  In this case, the upper temperature limit is set by the fluid properties.  As with any saturated fluid, the saturated vapor and liquid properties become more and more similar as the critical point is approached.   A good heat pipe working fluid has a large latent heat and a large surface tension.  As shown in Figure 7 and Figure 8, both the latent heat and the surface tension approach zero near the critical point (373.9°C).

Water Surface Tension as a Function of Temperature.

Figure 7. Water Surface Tension as a Function of Temperature.

 

Water Latent Heat as a Function of Temperature.

Figure 8. Water Latent Heat as a Function of Temperature.

Practical temperature limits are listed in Table 2.  Note that the upper temperature range for some of these fluids is set by the fact that a superior fluid can be used at higher temperatures.  This is discussed in more detail in Compatible Fluids and Materials.

Operating Min Temp., °C

Operating Max Temp., °C

Working Fluid

Envelope Materials

Comments

-271

-269

Helium

Stainless Steel, Titanium

-258

-243

Hydrogen

Stainless Steel

-246

-234

Neon

Stainless Steel

-214

-160

Oxygen

Aluminum, Stainless Steel

-203

-170

Nitrogen

Aluminum, Stainless Steel

-170

0

Ethane

Aluminum, Stainless Steel

CCHPs below Ammonia Freezing point

-150

40

Propylene

Aluminum, Stainless Steel, Nickel

LHPs below Ammonia Freezing point

-100

120

Pentane

Aluminum, Stainless Steel

-80

50

R134a

Stainless Steel

Used in Energy Recovery

-65

100

Ammonia

Aluminum, Steel, Stainless Steel, Nickel

Copper, titanium are not compatible

-60

~ 100  to 25

Methanol

Copper, Stainless Steel

Gas observed with Ni at 125°C, Cu at 140°C.  Aluminum and titanium are not compatible

-50

~ 100

Acetone

Aluminum, Stainless Steel

Decomposes at higher temperatures

-50

280

Toluene

Al 140°C, Steel, Stainless Steel, Titanium, Cu-NI

Gas generation at higher temperatures (ACT life test)

20

280, short term to 300

Water

Copper, Monel, Nickel, Titanium

Short term operation to 300°C.  Aluminum, steels, stainless steels and nickel are not compatible

100

350

Naphthalene

Al, Steel, Stainless Steel, Titanium, Cu-Ni

380°C for short term.  Freezes at 80°C

200

300, short term to 350

Dowtherm A/Therminol VP

Al, Steel, Stainless Steel, Titanium

Gas generation increases with temperature.  Incompatible with Copper and Cu-Ni

200

400

AlBr3

Hastelloys

Aluminum is not compatible.  Freezes at 100°C

400

600

Cesium

Stainless Steel, Inconel, Haynes, Titanium

Upper limit set by where K is the better working fluid.  Monel, Copper, and Copper-Nickel are not compatible

500

700

Potassium

Stainless Steel, Inconel, Haynes

Upper limit set where Na is the better fluid.  Monel and Copper are not compatible

500

800

NaK

Stainless Steel, Inconel, Haynes

Upper limit set where Na is the better working fluid.   Monel and Copper are not compatible

600

1100

Sodium

Stainless Steel, Inconel, Haynes

Upper limit set by Haynes 230 creep strength

1100

1825

Lithium

Tungsten, Niobium. Molybdenum, TZM

Lithium not compatible with superalloys.  Refractory metals react with air

 

Return to Working Fluids…

Return to Heat Pipe Materials, Working Fluids, and Compatibility…