Capillary Pumped 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

Spacecraft and electronic systems require heat rejection systems which can remove quantities of heat on the order of 10 to 100 times that which present single-phase systems can reject. Another special type of heat pipe has been proposed and extensively tested for this purpose, the capillary pumped loop (CPL) [1][2]. The CPL was first proposed by Stenger [3] at the NASA Lewis Research Center. A basic schematic of the CPL is given in Fig. 1.

Capillary pumped looped heat pipe (CPL).
Figure 1: Capillary pumped looped heat pipe (CPL).

Heat is applied to the evaporator component, which consists of a hollow rod of wick material capped at one end and force fitted into an internally axially-grooved pipe. The heat applied to the outer surface of the evaporator vaporizes the liquid working fluid, which then travels down the length of the axially-grooved vapor channels and into the vapor header. The vapor travels to the condenser, where it is condensed first as a film on the inner wall of the pipe, and then as a liquid slug flow. Before reaching the evaporator, the liquid passes through a subcooler to collapse any remaining vapor bubbles as well as to provide additional subcooling if necessary. The capillary pressure generated in the wick structure continuously pumps the working fluid through the cycle. A two-phase reservoir is provided to control both the working fluid through the cycle and the working temperature of the system, which is similar in function to the variable conductance heat pipe. For multiple evaporator systems, isolators are provided to prevent the depriming of one evaporator from depriming the other evaporators. Despite the wide use of the CPL and LHP in space technology, fundamental confusion still exists regarding their similarities and distinctions in operation, as well as the limitations of these two devices.


  1. Faghri, A., 2012, "Review and Advances in Heat Pipe Science and Technology," Journal of Heat Transfer, 134(12), 123001.
  2. Faghri, A., 1995, Heat Pipe Science and Technology, 1st ed., Taylor & Francis, Washington, D.C.
  3. Stenger, F. J., 1966, "Experimental Feasibility Study of Water-Filled Capillary Pumped Heat Transfer Loops," NASA X-1310, Nasa LeRC Report.