The vacuum tube geyser pump worked well for about a week, Then, one day, it locked up. The header pipe approached almost 350F, but the vapor condenser remained relatively cool. The system pressure stayed well below one atmosphere, approximately 12 In. Hg vacuum. My theory is that rather than nucleate boiling, the fluid in the header just evaporated, and the header emptied to its ends where a surface of water maintained an equilibrium evaporating just enough into the header to maintain slightly positive pressure. The rest of the system lost enough heat to maintain a vacuum in the vapor condenser. I was able to start the pumping again by drawing a little bit of vacuum or letting a little air in to the system, and then it would pump for the remainder of the day, but would not start pumping the next day.
The solution to this would be to modify the header so that it has active artificial nucleation sites, but the holy grail I am seeking is to use the headers without modification, but I plan to test the nucleation theory by building a straight through 3/4″ I.D. header with a 3/4″ O.D. nucleator.
To build a straight-through header, I will have to build wrap-around sockets for the heat-pipe condensers. I’ll build it with removable high-temperature insulation and install thermal sensors at several points on the header. I’ll start with a 4-socket header, and if it works, I’ll try to pump a 10-tube slave collector. If that doesn’t work, I’ll try building a 10-tube geyser pumping collector with a straight-through, nucleating header.