Selecting A Heat Pipe Working Fluid
Evaporation and condensation of the working fluid are what give heat pipes their high effective thermal conductivity, which can be as high as 100,000 W/m K. During heat pipe operation, the working fluid is vaporized in the evaporator and then condensed in the condenser, thus transferring heat.
The first step in selecting a heat pipe working fluid and envelope/wick material is to determine the operating temperature range. For a heat pipe to operate, it must be at saturated conditions, where the heat pipe contains both liquid and vapor. The practical operating temperature range for a copper/water heat pipe is roughly 25° to 150°C.
Learn more about theoretical and practical temperature ranges for heat pipes below.
Heat Pipe Envelope and Fluid Pairings
 Copper envelope with water as the working fluid is common for Electronics Cooling
 Copper or Steel envelopes with Refrigerant R134a fluid is common for Energy Recovery
 Aluminum envelope with Ammonia as the working fluid is common for Spacecraft Thermal Control
 Superalloys: Alkali Metals (Cesium, Potassium, Sodium) for HighTemperature Heat Pipes
As discussed in Compatible Fluids and Materials there are a large number of other compatible envelope/fluid pairs that are used at other temperature ranges, or when additional factors must be considered. For example, copper/methanol is often used for electronics cooling when the heat pipe needs to operate near or below 0°C, the point at which water freezes. Fluid choices in a given temperature range are ranked by the Merit Number.
Heat Pipe Fluid Selection FAQs

Why is water the most commonly used heat pipe fluid, and ammonia used for spacecraft thermal control?

The reason water and ammonia are common working fluids is that they are the best heat pipe working fluids for their respective temperature ranges, determined by comparing the merit number.

At what temperature do heat pipes operate?

Working Fluids Theoretically Operate from the Triple Point to the Critical Point.
Heat pipes are twophase heat transfer devices, therefore, for a heat pipe to operate, a saturated working fluid is required, meaning there will be 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 a 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.
The triple point and critical point for a number of common heat pipe working fluids are shown to the left and in the table below. There are a couple of points to note:
 There are gaps in the cryogenic heat pipe temperature range (below about 100 K), where there is no currently known working fluid.
 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 where a compatible heat pipe envelope is acceptable. For example, while ammonia is a betterworking fluid than methanol, methanol must be chosen when a copper wick and envelope are used.
 The merit number and material compatibility is not the only factor, i.e. for large geothermal thermosyphons, a fluid with a low global warming potential may be selected.
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.)*Scroll right to view table
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
SnCl_{4}
240.15
591.85
33.0
318.7
TiCl_{4}
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
lBr_{3}
370.15
763
97.0
489.9
Sodium
370.98
2507
97.8
2233.9
Lithium
453.64
3503
180.5
3229.9
What are the Practical Temperature Limits for Working Fluids?
In practice, the fluid range is smaller than the theoretical operation from the Triple Point to the Critical Point, since the power that the heat pipe can carry drops off sharply near the freezing and critical temperatures.
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 to the right The peak power occurs at a temperature near 150°C), and drops off at lower and higher temperatures.

What are the Practical Temperature Limits for Working Fluids?

In practice, the fluid range is smaller than the theoretical operation from the Triple Point to the Critical Point, since the power that the heat pipe can carry drops off sharply near the freezing and critical temperatures.
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 to the right The peak power occurs at a temperature near 150°C), and drops off at lower and higher temperatures.

What are the Practical Operating Temperature limits for Water Heat Pipes?

The average practical operating temperature range for water heat pipes is 25150°C (with an average upper limit of 300°C with a titanium or Monel envelope).

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.
 The practical uppertemperature 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.
Titanium or Monel envelopes increase the maximum operating temperature range for water to 300°C. In this case, the uppertemperature 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 the figures below, both the latent heat and the surface tension approach zero near the critical point (373.9°C).
Practical Temperature Limits are discussed in more detail in Compatible Fluids and Materials, where the ‘Working Fluid and Envelope Compatibility Table’ lists practical temperature limits. 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.

Heat Pipe Merit Number

What is the merit number and how is it used in common heat pipe working fluid selection?

Heat pipes fluids are ranked by the Merit Number formula:
where
ρ_{l} Liquid density
σ Surface tension
λ Latent heat
μ_{l} Liquid viscosityHigh liquid density and high latent heat reduce the fluid flow required to transport a given power, while high surface tension increases the pumping capability. A low liquid viscosity reduces the liquid pressure drop for a given power. The Merit number is derived below.
The Merit number as a function of temperature is shown in Figure 1 for a number of typical heat pipe working fluids. From the figure, it is very clear why water is chosen as the heat pipe working fluid whenever possible. Its Merit number is ~10 times higher than everything else except the liquid metals, meaning that it will carry ten times more power (in the proper temperature range) than other working fluids.
Ammonia is chosen for spacecraft heat pipes since it has the highest Merit number (roughly 3 times less than water) in their typical operating temperature range. Methanol is generally the working fluid of choice when ammonia and water are not suitable, since it has the thirdhighest Merit number near ambient conditions.

How do you derive the Merit Number for heat pipe fluids?

The amount of power that a heat pipe can carry is governed by the lowest heat pipe limit at a given temperature. For a given heat pipe, the Merit number ranks the maximum heat pipe power when the heat pipe is capillary limited. (The capillary limit generally controls the power in the midrange, while other limits control at higher and lower temperatures).
The capillary limit is reached when the sum of the liquid, vapor, and gravitational pressure drops is equal to the capillary pumping capability:
The Merit number neglects the vapor and gravitational pressure drops, and assumes that the capillary pumping capability is equal to liquid pressure drop. The equation for the liquid pressure drop in a heat pipe is:
Where
ΔP_{l} Liquid Pressure Drop, assumed equal to the wick pumping capability
L_{Effective }Effective Length
k_{wick} Wick permeability
A_{wick} Wick AreaThe mass flow rate is the heat transfer rate divided by the latent heat:
The wick pumping capability is:
Where r_{c} is the pore radius.
Combining the three equations and solving for Q, the maximum heat transfer when only the liquid pressure drop is considered becomes:
Where the first term consists of heat pipe and wick properties, and the second term is the Merit Number.