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Cogen system

Posted by mekennedy1313 
Cogen system
June 11, 2020 09:07PM
Hello, inveterate tinkerer in NE Ontario I am hoping to develop a cogen plant for winter heat. As cars use monotube boilers for weight and safety and I have no desire to host a potential bomb a la a regular boiler this seemed a good place to seek advice from those more experienced than I. The basic elements I'm looking at are monotube boiler, 2kw (electric) steam engine/generator, water pump, condensers, fuel auto feed. In order of importance for completion are the (monotube boiler, water pump, condensors) set that will provide heat. Then add the 2kw steam engine/generator and finally the fuel autofeed. As I have access to massive quantities of forestry slash free for the taking I am looking at beginning with a monotube boiler similar to;


This has an inner and outer tube wall with a 3rd helix in the middle. Thoughts at this time are to feed water starting at the top of the outer water wall counter current to heat, up the middle helix where it should become wet steam. Here it would go out to a Waterman style feed pump control and back to the inner coil wall for superheating (looking to attain 150PSI initially but will go up a bit if all works well.) Based on the dimensions given and looking at the photo's it looks like 0.5"OD tubing, a tubing wall height of 19" and mid coil helix spacing of 1/2 the tubing OD or 0.25" I calculate a total tubing length of 265' using L = (H^2 + C^2)^(0.5) where H = tube OD for the inner and outer coils and 1.5 OD for the mid coil and L is the length of 1 turn. #turns = system height (19"winking smiley/0.5 for inner and outer coils and 0.75" for the middle coil. Is it me or looking at coil lengths for other monotubes online is 265' a bit long? If long where did I go wrong?

As a simplification of water feed controls I am thinking of using this system;


Though fairly simple I have a couple questions. 1) do I need a wet steam separation chamber as in that diagram or would it be easier to set up as in the patent application.


2) No steam pump for this kind of application seems to be in production. The Phorum archives seem to indicate at 1 point that a model aircraft engine could be modified to act as such a pump for a steam powered hydrofoil. That would I think be too small for this application. Here is where I rely on your greater experience. What would be a simple pump diy suitable for this project without a great deal of machining? I am a wood worker and lack metalworking lathe etc. though when not isolated by covid19 have access to my High Schools metalworking shop and do have experience in an automotive parts supplier with lathes, mills etc. Also, if there is an easier water feed control system, I am desirous of system long term durability and ease of maintenance, please advise.

Finally for condensors I am looking at recycled automotive radiators suitably pressure tested. I say finally as this is where I hope to get before this coming winter heating season. If I can get further, great.

For the steam engine, again looking for a diy mod, such as the bash spring steel valve in a uniflow design;


that requires relatively little machining. Have seen weed whacker and oil less compressor conversions. Efficiency not a big deal as the heat recovery will be a large element of the overall efficiency. Water lubrication as in the Cyclone engine would be preferred over oil lubrication.

I'm working on an fuel wood autoloader design and have some idea's on that but it is the last element I will work on.

Well thar she be, any help, suggestions, commentary welcome.
Re: Cogen system
June 11, 2020 10:03PM
This is a "spillover" system quite similar to what Bill Besler installed in his commercial boilers. Coincidentally, Tom Kimmel wrote me yesterday about his book projects and asked about Besler spillovers and when they were invented. After a few hours research my best answer was that they were developed by Babcock and Wilcox for the SteaMotive powerplant back in the Thirties. This was a drumless, high performance boiler meant to be used locomotives and other applications. As it turns out, the spillover was installed to carry contaminants out of the boiler before they could cake onto the tube interiors and thus begin tube burnout.

The number thrown out is typically "ten percent", meaning that you feed ten percent more water into the boiler than you withdraw as steam with that ten percent going through the spillover tube. If you add too much water, the boiler floods. Add too little, the separator goes dry. You know, just like any other boiler. You'll know the water level has dropped below the separator because the pressure will drop drastically when low density steam rather than high density water flows out of the spillover tube. (Note, you need a restriction in the spillover tube so as to maintain back pressure in the system).

Anyhow, this begs the question, "How are you going to keep the feed coming in at ten percent greater flow than the outgoing steam?" This isn't too bad if your steam demand is absolutely stable and you can adjust manually but it is more demanding if flow varies. I would suggest a reasonably large separator so as to minimize the rate of change in the separator and then add some kind of level control mechanism such as the old SACA standby sparkplug controller.

As for a feedpump, you used to be able to buy cheap pressure washer pumps on Ebay.


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