Evacuated Tube Steam Generator



Evacuated tubes are normally used gather heat for solar DHW systems. Using them to generate steam is unconventional and could be dangerous, because evacuated tubes produce high temperature and high pressures. They are able to do this because very little heat is lost through the vacuum. When evacuated tubes are used to boil water in an enclosed environment the pressures that could produced could be hazardous it the precautions are not taken. 

As you can see from the illustrations felt is being used to hold water against the inner surface of the glass tube that gets hot when exposed to sunlight. Felt conducts water like a blotter, however even blotters have their limits so water at the bottom of a tube is not automatically drawn to the top of the tube by capillary action alone. This is why a water injection system is needed. The distribution of water inside the tube is influenced by the presence of felt, but gravity is also an important factor. Water is injected through a tiny tube and allowed to trickle down from the top. The quantity of water that would be injected would be controlled by a valve that would be controlled with a temperature probe just below the water entry point. At temps above 300*F water would be allowed to flow. This is a delicate feedback control system. Could use some help with this one. 



I am planning on using this kind of water distribution T on the top part of the heater hose wrapped with felt. This should provide a simple method of introducing water above the polyester felt. As the water trickles down through the felt it should cool the surrounding area and indicate that the felt has been saturated and  shut the water inlet valve... 

All of these things are a bit speculative on my part. I wish I could do more, but with limited funds progress is slow. 



The following video is used to further introduce the concept, but an automated injection system is not used in this video.


A more conventional evacuated tube assembly would go like this:

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