Monotube Boiler Success

[Tony]

I have been trying for some years to make a monotube boiler which will produce 80 PSI of dry steam at about 200 C and will control reliably over a wide range of steam demands (see previous posting about 3 months ago).
I think I have now come up with a successful design. The main boiler consists of about 30m of 10mm copper tube and is heated by a standard cast iron ring burner which has been modified to produce about 15Kw from propane. The coil is designed to be kept flooded at all times and is fed into a separator. This consists of a small stainless steel pressure vessel containing a copper tube wound in a spiral. This tube has small holes on the outer sides of it allowing any water to be thrown out onto the walls of the main tube and be drained off at the bottom through a bleed valve, the steam then passes up through the middle and out at the top. It then passes through a superheater coil which is situated in the middle of the main ring burner with a heat shield around it. This is heated by a smaller burner of my own design producing about 4Kw. Both burner valves are controlled by r/c servos driven from a PIC controller. The main burner is controlled using the pressure (P) and the superheater burner using the “steam out” temp (T5). The feed water is supplied by a motor driven pump. The control of the pump seems to be the most critical part of the system and has taken me some time to get the controlling algorithm right. Basically, the controller looks at the temp at the output from the main coil (T3) and the pressure (P) and decides whether it is steam or water at this point. If it is steam it runs the pump for 2 seconds, waits for 2 seconds and then repeats the process. When the system is running properly after initial warm-up there is very little water bleeding off from the separator (this is a manually controlled valve), there is another bleed valve from the main steam output line, this is necessary to prevent the superheater coil from overheating when the main steam valve is shut off. There are still a few problems to sort out. During warm-up the system can overheat and shutdown (T5 > 220, see chart) and then sometimes load up with water an produce wet steam, but after about 5 minutes it seems to be fairly stable. Referring to the chart,
T1 = hot well
T2 = economiser out
T3 = main coil out
T4 = separator out
T5 = superheater out
P = pressure
Pump – shows when pump is running
Main and superheater burners in % opening
The whole boiler set-up is quite light at about 45 lbs (less the propane tank) and another 34lbs for the steam engine.
So, I think it’s about ready to go into a boat!

[Lopez Mike]

If there is one thing that I have learned from being a member of this forum it is that there is an incredibly wide range of notions about how to go about putting together a steam plant. I thought I had reached a high point when I was shown the right and wrong way to use a pipe wrench as a hammer.

I thought Fred's paddle wheeler was pretty high tech. And I feel a little out there by having an aluminum connecting rod!

You need to consult with Wes (87gn@tahoe). His setup is at about the limit of what one can do without any electronics.

Beautiful work. I hope you don't get any water in your computer. I'd hate to try to control your plant manually.

Mike

P.S. Lest you think I'm a complete Luddite, I'm an electronics engineer.

[SailorHarry]

Very impressive. I've always kind of figured it would take a PLC to really get a monotube running up to it's potential. Although my interests in steam run differently I can certainly appreciate what you have accomplished.