Compatible Fluids and Materials

Since heat pipes were rediscovered by George Grover in 1963, extensive life tests have been conducted to determine compatible envelope/fluid pairs, and a large number have been found.  Some of these life tests have been conducted for decades.  As discussed in Most Commonly Used Envelope/Fluid Pairs, most heat pipes are fabricated for electronics cooling, and are either copper/water or copper/methanol.  Most spacecraft heat pipes are aluminum/ammonia, most heat pipes for HVAC applications are copper/R134a or steel/R134a, and most high temperature heat pipes are superalloy/alkali metal.  These envelope/fluid pairs cover the vast majority of heat pipes used today.

This webpage discusses the compatibility for these envelope/fluid pairs, and other pairs used in special circumstances.  Table 3 lists most envelope/fluid pairs used today, as well as some envelopes that are known to be incompatible.  Research is still ongoing in the temperature range from 150 to 400°C, see High Temperature Water Life Tests and Intermediate Temperature Fluids Life Tests for more details.

An estimated upper and lower fluid temperature range is also shown.  In most cases, the lower limit is set by the sonic limit, while the upper limit is set by the maximum vapor pressure that can be contained with a reasonable envelope wall thickness.  Heat pipes can be built that operate at lower temperatures with a large diameter to maximize the sonic limit, theoretically down to the triple point).  Heat pipes can also be built that operate at higher temperatures, theoretically up to the critical point, but will require a thicker envelope to withstand the vapor pressure, and will have a reduced capillary limit.

The practical upper temperature limit for copper/water heat pipes is set by the vapor pressure at around 150°C; Monel or titanium are used at higher temperatures.   It is very important to note that Table 3 lists the generic type of material, such as Monel or Superalloy.  In many cases, only some alloys are known to be compatible, while others have not been tested.  Only some stainless steels are suitable for cryogenic heat pipes, since other steels become very brittle at low temperature.  For additional help in selecting fluids and materials, please email one of our capable engineers at info@1-ACT.com or call us at (717)-295-6061.

Table 3.  Working Fluid and Envelope Compatibility, with Practical Temperature Limits.

The upper temperature limits for cesium, potassium, and NaK are set by ranking the properties of suitable alkali metals at a given temperature.  For example, cesium is not normally used at higher temperatures than 600º, since potassium is a superior working fluid.   This can be seen graphically in Figure 14, which compares heat pipe power versus temperature for identical heat pipes using either cesium or potassium as the working fluid.  On the left side of the graph, the maximum heat pipe power is set by the sonic limit (the roughly parabolic part of the curve), while on the right side of the graph, the maximum power is set by the capillary limit (the roughly flat part of the curve).  At lower temperatures, more power can be carried with cesium, since it has a higher vapor density (and higher sonic limit) at any given temperature.  Once the temperature is increased above roughly 500°C, the potassium heat pipe carries more power (for this particular design).  This is the reason that Table 3 states that cesium is not normally used above 600°C.

Figure 14. Sonic and Wicking Limits for Cesium and Potassium Heat Pipes. For these specific designs, the sonic limit controls the power below 400ºC for cesium, and below 500ºC for potassium.

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