Loop Heat Pipes


Loop Heat Pipes (LHPs) made by ACT can transport larger amounts of heat over longer distances than heat pipes. Loop Heat Pipes (LHPs) are often used in conjunction with Constant Conductance Heat Pipes (CCHPs) and Variable Conductance Heat Pipes (VCHPs) to transfer waste heat from spacecraft payloads to radiators. Ammonia is the most commonly used working fluid for temperatures between -40 and 70°C. Propylene and ethane are used for lower temperatures. ACT has also developed titanium/water Loop Heat Pipes (LHPs) for temperatures between 70 and 250°C.

Unlike some of our competitors, all of our Loop Heat Pipe parts, including the primary and secondary wicks, are quality made here in the United States at ACT under strict AS9100 quality controls. When you contact us about Loop Heat Pipes (LHPs), you’ll be speaking to the people that engineer and manufacture them.

Typical Applications of Loop Heat Pipes (LHPs)spacecraft loop heat pipes

  • Payload thermal management
  • Heat transport
  • Radiator panel enhancement
  • Avionics cooling
  • Aircraft anti-icing

Loop Heat Pipes (LHPs) Manufacturing

Throughout the Loop Heat Pipes (LHPs manufacturing process, parts are continually checked to verify that they meet the performance requirements. For example, the LHP primary wick is characterized to determine its porosity, pore radius, permeability, and thermal conductivity.

The Loop Heat Pipes (LHPs) secondary wick performs the critical function of hydraulically linking the primary wick and the reservoir. ACT has the facilities and processes to fully characterize the performances of Loop Heat Pipe (LHP) secondary wicks at various power and body force conditions. ACT also has a Loop Heat Pipe (LHP) evaporator/reservoir characterization apparatus for quick and accurate assessment of the performance of an LHP evaporator/reservoir assembly, prior to installing it into the overall transport line and condenser assembly.

Download: ACT Loop Heat Pipes Brochure

Download: ACT Loop Heat Pipes Brochure

Loop Heat Pipes (LHP) Testing

These tests typically include

  • Start-up
  • Transient power
  • High power
  • Shut-down
  • Un-balanced condenser heat removal (in case of multiple condensers)


It is sometimes desirable for the LHP to stop transferring heat to maintain the temperature of the payload and minimize the magnitude of the temperature swing through an orbit. The figure below demonstrates the ability to shut down a LHP almost immediately and with a very small heater power. With a small heat input of approximately 10 Watts to the reservoir, the condenser temperature dropped quickly to match the environment, while the evaporator stays nearly constant in temperature.

LHP Shutdown Demo

Loop Heat Pipes: How They Work



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