Abstract: Relatively small diameter heat pipes having generally horizontally arranged evaporator sections and inclined or vertically oriented condenser sections are provided with a flow separator for conducting working fluid vapor from the evaporator section to the condenser section and liquid working fluid is returned to the evaporator section in the flow passage formed between an outer cylindrical tubular member and an inner cylindrical member forming the flow separator. The flow separator may be formed as a cylindrical tube positioned within the outer tubular envelope such that the ends of the flow separator are open and are spaced from the end closures of the outer tube by a small distance. The hydraulic radius of the inner tube flow path is equal to or greater than the hydraulic radius of the annular passage formed between the tubes.