Loop Heat Pipes
Loop heat pipes (LHPs) are passive, two-phase heat transport devices. The technology was invented in the former Soviet Union in the 1980s for spacecraft thermal control. The figures below illustrate the operating principles of a typical loop heat pipe.
How Do Loop Heat Pipes Work?
Here’s an overview of how a loop heat pipe works. Heat enters the evaporator and vaporizes the working fluid at the wick outer surface. The vapor flows down a system of grooves and headers in the evaporator and the vapor line toward the condenser, where it condenses as heat is removed by the cold plate (or radiator). The two-phase reservoir (or compensation chamber) at the end of the evaporator is designed to operate at a slightly lower temperature than the evaporator (and the condenser). The lower saturation pressure in the reservoir draws the condensate through the condenser and liquid return line. The fluid then flows into a central pipe where it feeds the wick. A secondary wick hydraulically links the reservoir and the primary wick.
LHPs are made self-priming by carefully controlling the volumes of the reservoir, condenser and vapor and liquid lines so that liquid is always available to the wick. The reservoir volume and fluid charge are set so that there is always fluid in the reservoir even if the condenser and vapor and liquid lines are completely filled.
The Importance of Pore Size in Loop Heat Pipe Design
In general, small pore size and the resultant large capillary pumping capability are very desirable in a wick. Unfortunately, the capillary pumping capability of a wick is inversely proportional to its permeability (a measurement of the pressure drop during flow). The designer must balance the wick pumping capability against the wick permeability when designing a heat pipe or loop heat pipe.
The LHP has flexible fluid and vapor lines
In a heat pipe, the wick extends along the entire length so there are long lengths (and large pressure drops) for the liquid flow in the wick. In contrast, there is only a short flow path for liquid inside a LHP wick. Consequently, LHPs can have much finer pore sizes and higher pumping capability. This allows for heat transport across long distances, against large adverse elevations or accelerations and through flexible liquid and vapor lines. Flexible liquid and vapor lines allow the use of LHPs with deployable radiators and for vibration isolation between the evaporator and condenser.
ACT manufactures LHP wicks in-house. To date, ACT has manufactured Nickel, Stainless Steel, Titanium and Monel wicks with effective pore radii as small as 0.85µm and porosity as great as 75%.
Fluids Used With Loop Heat Pipes
Most current LHPs use ammonia as the working fluid and operate at temperatures between -40 and 70°C. Propylene and ethane have been used in LHPs operating at lower temperatures. A number of applications are emerging that will require operation at temperatures between 70 and 250°C. Water is a good working fluid in this temperature range. However, most current LHPs are fabricated with aluminum and stainless steel parts, neither of which is compatible with water.
A recent spacecraft radiator trade study found that radiators with titanium/water heat pipes or LHPs had the highest specific power in the temperature range from 20 to 275°C. In addition, titanium LHPs would increase the specific power by roughly 1/3 when compared with titanium heat pipes.
We Now Use Titanium to Manufacture Heat Pipes
In the past several years, ACT has been developing high temperature titanium/water LHPs. Titanium has a number of advantages over other materials:
- Titanium has high strength and low density, ideal for low mass radiators.
- Titanium has a coefficient of thermal expansion that better matches the carbon-carbon fins than stainless steel.
- Titanium is compatible with a large number of fluids, including ammonia, water and the alkali metals.