The Heat Pipe: Evaporation, Condensation and a Wick That Pumps

Cut 300 mm of 10 mm copper tube and two 300 mm strips of fine stainless mesh. Roll the mesh into a sleeve that springs out against the inside wall — two layers — so it touches the wall everywhere and leaves the core open. Crimp one end flat in the vise, fold it, and seal it with solder and plumbing flux. Add about 3 ml of distilled water with a syringe: enough to wet the wick, not to fill the tube. **Purge the air:** clamp the tube upright in the vise, open end up, and heat the bottom gently with the torch until steam jets steadily out of the top for half a minute — the steam carries the air out. Wearing gloves and goggles, crimp the open end shut in the vise while it is still steaming, then solder it. Air left inside is the commonest reason a home-made heat pipe fails (Grover's own sodium pipes had to be baked to drive out hydrogen). Make a second tube identical in every way but empty of water — the control.

Tape a temperature probe to the top end of each tube. Stand both tubes upright with their bottom 50 mm in a jug of hot water at about 70 °C — never heat a sealed pipe with a flame again: its pressure climbs steeply above 100 °C. Watch the two top ends. The heat pipe's top rises within seconds to within a few degrees of the water; the empty tube's top barely moves. That is the notebook's comparison, in your hand. Now turn the heat pipe over: hot end at the TOP, dipping the upper end into a shallow tray of hot water, cold end below. The wick must now lift the liquid against gravity. Its performance drops, and if your wick is coarse or poorly seated it stops — the dry-out in the cycle's last line.

**US 2,350,348, 'Heat Transfer Device', Richard S. Gaugler of Dayton, Ohio, assignor to General Motors; filed 21 December 1942, granted 6 June 1944.** A sealed tube with a sintered *"porous capillary structure"* that carries the liquid *"upwardly or in any direction without any forced circulation and without any moving parts"* — proposed to cool the inside of a refrigerator. **US 3,229,759, 'Evaporation-Condensation Heat Transfer Device', G. M. Grover; filed 2 December 1963, granted 18 January 1966.** Grover's specification describes a sodium pipe of 347 stainless steel, 3/4 inch outside and 5/8 inch inside, 12 inches long, with a wick of five layers of 100-mesh 304 screen and 15 g of sodium, run at about 1,100 K; it also sets out the capillary pumping condition the notebook uses. **Honest limits.** A heat pipe works only between the freezing point and the critical point of its fluid, and best in a narrower band. It has a maximum heat load, set by the wick, above which it dries out. It fails silently if it leaks or holds air. And it moves heat; it does not get rid of it — the cold end still needs a fin or a fan.