Variable Conductance Heat Pipes (VCHPs)
Figure 1 Variable Conductance Heat Pipe (VCHP).
A standard heat pipe is filled with a two-phase working fluid, and a wick to return the condensate from the condenser to the evaporator. In a Variable Conductance Heat Pipe (VCHP) a Non-Condensable Gas (NCG) is added to the heat pipe, in addition to the working fluid. Depending on the operating conditions, the NCG can block all, some, or none of the available condenser length. When the VCHP is operating, the NCG is swept toward the condenser end of the heat pipe by the flow of the working fluid vapor. At high powers, all of the NCG is driven into the reservoir, and the condenser is fully open; see Figure 1 As the power is lowered, the vapor temperature drops slightly. Since the system is saturated, the vapor pressure drops at the same time. This lower pressure allows the NCG to increase in volume, blocking a portion of the condenser. At very low powers, the vapor temperature and pressure are further reduced, the NGC volume expands, and most of the condenser is blocked. This change in active condenser length minimizes the drop in evaporator and associated electronics temperatures over large changes in power and evaporator sink conditions.
Variable Conductance Heat Pipes were discovered by accident at RCA in the early 1960’s, soon after the initial work by Los Alamos on the first heat pipes. At the time, there was no good method for sealing a refractory metal heat pipe, so a lower temperature valve was left on condenser end of the heat pipe. NCG was added to the heat pipe to protect it by providing a blanket of gas between the working fluid and the valve. During operation of this gas charged heat pipe, the principles of VCHPs were discovered. A patent describing VCHPs and Pressure Controlled Heat Pipes was submitted in 1964 by William B. Hall and Fred G. Block (U.S. Patent No. 3,613,773) but did not issue until 1971 (It discussed flattening the thermal decay in isotope powered applications, which was classified in those days).
Many of the benefits of the VCHP are the same as the Standard Heat Pipe, while the only disadvantage over a standard heat pipe is a slight increase in design and fabrication costs. Benefits of VCHPs include:
- High Thermal Conductivity (10,000 to 100,000 W/m K)
- Low Cost
- Shock/Vibration tolerant
- The Non-Condensable Gas allows the VCHP to respond passively to changes in evaporator power and condenser sink conditions, minimizing the changes in the evaporator and electronics temperatures.
The applications for VCHPs include:
- VCHPs for Passively Controlling Temperature
- VCHPs for Over-Temperature Protection
- VCHPs for Variable Thermal Links
- Gas Loaded (Variable Conductance) Heat Pipes for Start-Up from a Frozen State
- VCHP Heat Exchangers
If you’d like more information about our variable conductance heat pipes, please contact an ACT representative today. (717-295-6061 / info@1-ACT.com)