Things to Consider When Bending or Flattening A Heat Pipe

Just as roadways require curves, overpasses, and bridges to navigate through terrain, cities, and towns, in order to navigate through the space that is densely packed with electrical and mechanical components inside an equipment, heat pipes need to be formed with bends or flattened sections as well.

ENERTRON EXAMPLES OF VARIOUS CUSTOM BENT HEAT PIPE_CHP

In consideration of this design requirement, Enertron recommends several guidelines to ensure for the best thermal management performance possible.

There are four types of heat pipe wick structures that are commercially produced and each wick structure’s bending/flattening requirements varies by type.

  • Groove wick
  • Mesh wick
  • Fiber wick
  • Sintered powder metal wick
Bending

Bending a heat pipe built with Groove or Fiber Wick is not as restricting as bending one with Mesh Wick or Sintered Powder Metal Wick (Sintered Wick hereafter). However, these heat pipe wick structures offer some disadvantages.

For instance, the capillary force of a Groove Wick is poor and unsuitable for operating at an against gravity orientation where the condenser is situated below the evaporator. With heat pipes built with Fiber Wick, you run the risk of blocking the vapor passage; thus jeopardizing thermal performance. Groove and Fiber Wick structures also have a much lower commercial demand than Mesh or Sintered Powder Wick.

In today’s market, most heat pipes sold for electronics cooling applications are with Sintered or Mesh Wick structures. This is because Sintered and Mesh Wicks have a stronger capillary force that allows for them to be operated at an against gravity orientation.

Enertron heat pipes are mostly made of Sintered Wick and our recommended safe bend radius for these Sintered Wick heat pipes is 3 times the pipe diameter.  If necessary, the bend radius can be reduced to 2 times the pipe diameter but we strongly advise against this. A 2x bend radius could lead to a large variation on heat transport capability from pipe to pipe.

In volume production runs, a numerical control pipe bending machine or press formed die is used. In prototyping or small build runs, a groove ring can be machined for bending the heat pipe by hand as the sintered wick heat pipe is fully annealed and soft.

If bending is performed adequately and according to Enertron’s 3x rule, the heat transporting capability will NOT be affected up to a 180 degree bend.

Flattening

A Sintered Wick round heat pipe can be flattened to its entire length or a portion of it. However, there are limitations that must be observed with regards to the flattened thickness to avoid the vapor passage from becoming obstructed or even pinched off by excessive flattening.

ENERTRON VARIOUS HEAT PIPE INCLUDING COPPER, NICKEL PLATED, AND FLATTENED HEAT PIPE

The table below illustrates the flattened width in relation to the flattened thickness. Please be advised that the thinner the flattened thickness is, the lower the heat transport capability will be.

For instance, a 6mm diameter round heat pipe will ahve the same heat transport capability as a 6mm diameter heat pipe that has been flattened to a 3.5mm thickness. Yet, when this 6mm diameter heat pipe is flattened to a thickness of 2.5mm, the heat transport capability is greatly reduced.

In volume productions, the press formed die is used. But in prototyping or small builds, a common shop hydraulic bench press with two parallel surfaces can be used to flatten the heat pipe. When flattening a larger diameter to a thin thickness with a ratio larger than 2:1, the heat pipe should be heated to 150C~200C before flattening. The high inner vapor pressure at the elevated temperature can prevent the flattened surface from caving in.

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