The intermediate temperature range extends from 450 to 750K (175 to 475 °C). The alkali metals, such as cesium, potassium and sodium, are suitable working fluids at temperatures above this range. In the intermediate temperature range, the alkali metal vapor density is so low that the vapor sonic velocity limits the heat transfer and the heat pipe vapor space becomes too large to be practical for alkali metals.
Water is commonly used at temperatures up to about 425K. Higher temperature water heat pipes can be used with titanium or Monel envelopes at temperatures up to 500K. Their effectiveness starts to drop off after 500K, due to the decrease in the surface tension of water.
In recent development efforts, ACT identified a number of potential intermediate temperature working fluids including Dowtherm, Sulfur/Iodine mixtures, Iodine, Naphthalene, Phenol, Toluene, Mercury and several halides.
Sulfur has a temperature dependent polymerization property at 475K, which increases its liquid viscosity to approximately three orders of magnitude higher than the maximum level for effective heat pipe operation. The addition of 3-10% of iodine reduces the viscosity of sulfur to a level sufficient for effective heat pipe operation. A disadvantage of iodine is its low liquid thermal conductivity.
Another set of potential working fluids is the halide salts of titanium, aluminum, boron, phosphorus and silicon. Russian heat pipe developers have reported good success with titanium tetrachloride (TiCl4), but a thorough examination of related compounds has apparently never been undertaken. ACT believes that related salts such as TiCl2F2 may have better working properties. These working fluids are more polar, which increases the latent heat and the liquid transport factor.
Vapor Pressure and Merit Number are two parameters used to screen potential working fluids. Vapor pressures for some of the potential intermediate temperature working fluids are shown in Figures 1 and 2. Note that the vapor pressure for water is too high and the vapor pressure for cesium is too low in this temperature range, so a vapor pressure between the two extremes is desirable. It can be seen that most of the fluids discussed above have a suitable vapor pressure.
The merit number (liquid transport factor) is a means of ranking heat pipe fluids, with higher merit number more desirable:
where: M = Merit number, W/m2, ρL = Liquid density, kg/m3, σ = Surface tension, n/m, λ = Latent heat, J/kg, μL = Liquid viscosity, Pa
Figures 3 and 4 shows the merit number as a function of temperature for a number of fluids. While water and cesium have good merit numbers, their vapor pressures eliminate them from consideration.
Figure 5 compares the theoretical heat transfer capability (power) for heat pipes with five working fluids: water, iodine, BiCl3, SbBr3 and cesium. Water is the best fluid at the low temperature end and cesium at the upper end. Iodine and SbBr3 offer good performance in the middle. However, Iodine has two potential problems: low liquid thermal conductivity and high corrosiveness.
The discussion above shows that there are many candidate fluids in the intermediate temperature range. While some of the fluids have sufficient physical property data to allow their use, none of the fluids have adequate life test data at appropriate conditions. Previous life test data on fluids in this temperature range is very scanty. Any potential working fluid will require additional life tests before it can be used as a heat pipe or loop heat pipe working fluid.