Back in the early 1990's they were marketed by B+W Hudson division, and also by Foster Wheeler. At that time, these were verylarge primary air heaters for coal fired power plants.
I recall circa 1992 one mfr was scheduled to give a paper on his company's heat pipes at a conference in Wash DC. 2 days prior to the conference, their largest application for heat pipes suffered a catastrophic failure , at the same site the paprr was to discuss.
The failure was: the heat pipes in the hotter zone ( gas temp = 800 F) become overpressured ,and the end caps burst off. They were shooting around the power plant like mortar shells. The US supplier, whio had purchased the heat pipe fluid from a japanes concner, was not able to translate teh japanese warning that the fluid may not be used above 600F, since it will dissaociate into a compound that will have more moles of vapor than the orignal product. At 800 F gas temp , the heat pipes were overpressurized to greater than 3200 psig, and the end caps failed.
The emergency repair required the heat pipe air heater to be replaced with a conventional tubular air heater on an expedited basis and a $28 million USD write down.
The author of the proposed paper showed up at the conference 2 days after the accident, but he suddenly changed the topic of the paper to a completely other topic, so we were unable to ask embarassing questions.
So, the ASME changed the code to require these heat pipe tube to have overpressure protection, whcih basically killed the market for that device.