Since its beginning in the early 1960s, the heat pipe technology has evolved into many different shapes and forms and has been used in numerous applications from computer cooling to spacecraft thermal control. Our Heat Pipe Resources page has the most extensive information on the web about heat pipe operation and limitations.
Standard heat pipes act as thermal superconductors, transmitting heat with minimal temperature drop in both directions. By adding small amounts of Non-Condensable Gas (NCG) and modifying the heat pipe design, it is possible to create many heat pipe variations, such as the following:
- Diode Heat Pipes, which transfer heat in one direction, and shut down when heat is supplied to the nominal condenser
- Variable Conductance Heat Pipes (VCHPs), that can maintain the evaporator temperature during wide swings in the power, and in the heat sink conditions,
- Pressure Controlled Heat Pipes (PCHPs), which can provide precise temperature control. They can also be used to switch the heat flow path at high temperatures, using low temperature components
- Heat Pipe Switches, that passively turn on and off to maintain a component above a set-point temperature
Other advanced heat pipes include high heat flux vapor chambers (> 1,000 W/cm2), as well as heat pipes with new working fluids and envelopes for the temperature range between 150 and 450°C.
Loop Heat Pipes (LHPs) have been developed with a Thermal Control Valve (TCV), allowing the LHP to passively shut down to maintain the evaporator temperature during the 14 day long Lunar night. Heat Pipe Loops offer higher power than heat pipes, while are lower cost than LHPs.
The following pages contain information on our experience with various advanced forms of heat pipes:
High Temperature Monel and Titanium Heat Pipes have been developed that extend the temperature range for water from 150 to 300°C
Intermediate Temperature Heat Pipes have been developed that operate from 450 to 750 K, between water and alkali metal working fluids
High Temperature Heat Pipes use alkali metal working fluids with superalloy envelopes up to 1100°C, and refractory metal envelopes at higher temperatures
ACT’s Vapor Chambers can use direct-bond copper for mounting electronics, and accommodate heat fluxes up to 1000 W/cm2
Variable Conductance Heat Pipes (VCHPs) have a non-condensable gas loading to help maintain the evaporator temperature under changing conditions
Pressure Controlled Heat Pipes are a type of VCHP that allows the temperature to be controlled as tightly as ± 5 milli-Kelvin
Loop Heat Pipes (LHPs) are passive, two-phase heat transfer devices that can carry more power and operate at higher adverse elevations than heat pipes
Heat Pipe Loops, invented at ACT, offer higher performance than heat pipes with much lower costs than Loop Heat Pipes
Heat Pipe Life Tests are necessary to determine compatibility between the envelope, wick, and the working fluid
Using a hybrid wick can increase the allowable evaporator heat flux in Constant Conductance Heat Pipes (CCHPs) by more than a factor of three.