Loop Heat Pipe

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Types of Heat Pipes
  1. Two-Phase Closed Thermosyphon
  1. Capillary-Driven Heat Pipe
  1. Annular Heat Pipe
  1. Vapor Chamber
  1. Rotating Heat Pipe
  1. Gas-Loaded Heat Pipe
  1. Loop Heat Pipe
  1. Capillary Pumped Loop Heat Pipe
  1. Pulsating Heat Pipe
  1. Monogroove Heat Pipe
  1. Micro and Miniature Heat Pipes
  1. Inverted Meniscus Heat Pipe
  1. Nonconventional Heat Pipes

The loop heat pipe (LHP) [1][2][3] was invented in Russia in 1971 by Gerasimov and Maydanik [4]. The LHP is based on the same physical processes (evaporation and condensation) as those of conventional heat pipes. The LHP in its simplest form (Fig. 1) consists of a capillary pump (or evaporator), a compensation chamber (or reservoir), a condenser, and liquid and vapor lines. The wicks are only present in the evaporator and compensation chamber. The high capillary force is created in the evaporator due to fine-pored wicks (primary wicks) such as sintered nickel, titanium and copper powder with an effective pore radius of 0.7-15μm and a porosity of 55-75%. The compensation chamber is an important component in the LHP and is often an integral part of the evaporator. The purpose of the compensation chamber is to accommodate excess liquid in an LHP during normal operation. A secondary wick (usually made of larger pores) physically connects the evaporator to the compensation chamber in order to supply the primary wick with liquid, particularly when the compensation chamber is below the evaporator, or when the LHP is operating in microgravity conditions. The motion of vapor and liquid flow in the primary wick proceeds mainly in a radial direction. The evaporator meniscus is inverted down toward the wall being heated. Both the liquid and vapor lines are made of small diameter tubing with no wicks. LHPs can be made flexible and bendable. LHPs provide heat removal over long distances without sensitivity to gravity.

Loop heat pipe (LHP).
Figure 1: Loop heat pipe (LHP).

Several factors make the LHP an attractive option for spacecraft cooling over conventional heat pipes. Since the wick structure is only in the evaporator, the rest of the container walls can be smooth, which reduces pressure drops in the vapor and liquid flows. The pressure drops throughout the system are also reduced because the vapor and liquid flow are co-current, as opposed to the counter-current flow in conventional heat pipes. For these reasons, the LHP is a more effective thermal bus, and the heat source and sink can be separated by a longer distance than with conventional heat pipes.


  1. Faghri, A., 2012, "Review and Advances in Heat Pipe Science and Technology," Journal of Heat Transfer, 134(12), 123001. http://dx.doi.org/10.1115/1.4007407
  2. Faghri, A., 1995, Heat Pipe Science and Technology, 1st ed., Taylor & Francis, Washington, D.C.
  3. Maydanik, Y. F., 2005, "Loop Heat Pipes," Applied Thermal Engineering, 25(5-6), 635-657. http://dx.doi.org/10.1016/j.applthermaleng.2004.07.010
  4. Maydanik, Y. F., Ferchtater, Y. G., and Goncharov, K. A., 1991, "Capillary Pump Loop for the systems of Thermal Regulation of Spacecraft," 4th European Symposium on Space Environment and Control Systems, ESA SP-324. Florence, Italy.