Monotube Boiler Control

[Tony]

I'm new to this so this is my first post!
I have been working on various designs of monotube boilers over the last few years and find the views on this forum most interesting. My original challenge was to design and build a 3HP double acting steam engine and boiler from scratch to fit in a dinghy. It needed to be light so much of the engine is ally with sealed ballraces throughout. The boiler had to be monotube for safety.
The engine is fine and chugs away quite happily at about 800 RPM. It has some unusual features. The flywheel is driven by a timing belt at 3:1 above crankshaft speed, this enables the use of a flywheel that is only 1/3 of the weight and also the engine can be mounted lower in the hull reducing shaft angle and lowering the C of G. The weight of the engine assembly including condenser and scavenge pump (but not the boiler) is only 34 lbs.
The boiler on the other hand is a different matter! I needed about 80 PSI of moderately superheated steam (about 200 C). It was important that it was limited to <220 to avoid damage to the various O rings in the engine and valve gear. I started off with a monotube boiler and economiser coil in the flue controlled by a PIC and R/C servos and using gas burners from a domestic boiler. After much plying around with the control system it was moderately successful but I was not really satisfied with it so went for a Lamont design with separate superheater coil and burner. The pump was the problem here and I ended up with a much modified central heating pump. Again moderately successful but it was getting very heavy and I was worried about the separator tank which was, after all, a small pressure vessel.
So now I am looking at going back to a single coil again. There is much information on small units for hydroplanes producing amazing quantities of steam from 3/16 dia tube and a very noisy burner, but this is not suitable for a steam boat.
From what I understand, it seems necessary to have a feed flow of at least 1m/sec for removal of vaporisation bubbles forming on the inside of the tube. This leads to very small Dia tubing and a small surface area. I have noticed in the past that if you tap the coils lightly with a small hammer, the pressure rises quite dramatically for a few seconds (until the bubbles reform again?). I am now experimenting with an ultrasonic transducer fitted near the cold end of the coil in an attempt to shake these bubbles loose. Another idea would be to use a very oversized piston type feed pump which could shuffle the water back and forth in the tubing at about 1m/sec velocity.
What do you think?

[fredrosse]

Quite an interesting setup you have put together!

My first steam boat had a 5 square foot monotube boiler, pumping water in at about 200% of the steam flow, to assure the coils were always kept wet. A separator tank bled off the excess water, which flowed back to the feed tank. There was a counterflow heat exchanger so that the return (hot) water would pre-heat the incoming cold high pressure feedwater. This worked well with no problems of surging or other anomolies. There is no reason to fear a pressure vessel if it is designed and built properly. The separator tank does not even have to be a fired pressure vessel if it is kept out of the flue gas area.

[Tony]

I may consider using a similar arrangement you had in your first steamboat. What type of gas burner did you have? And was it force draft?
I like the idea of keeping it simple, I’m fed up with messing about with PICs and complicated control systems. I could use my original economiser coil (80ft of 3/8 copper) as the main one and my Lamont separator. If the coil is maintained fully flooded it should not need to be stainless steel. I may need to have a separate superheater coil between the separator and the engine.

[fredrosse]

The steam scanoe used a high pressure (5-10 PSI) asparating propane burner, available from turkey frying equipment suppliers (I think Bayou Classic). Rated at 55,000 BTU per hour. Worked well, that is the largest burner they sell, in terms of heat output. There is a physically larger burner which they say is 210,000 BTU, but that is a low pressure asparating burner and I could only get about 50,000 BTU out of it, more gas just blows the flame off the burner.

At present (on the Margaret S.) I use domestic house heating gas burners, able to go up to about 100,000 BTU, and very quiet. All of these gas burners use natural draft on the stack, non-electric burners.

The fast steamer I am building (about 14 horsepower) will use a common electric driven oil burner, which uses high pressure atomization of oil, and a forced draft fan for air. That burner will put out about 500,000 BTU per hour. I am going to use a monotube boiler for that steam plant.

Very nice work. How do you make those nice coils? Also, in the separator picture it appears there is a float of some sort?

[Tony]

My reply to your private massage seems to have got stuck in the out box although it says sent on the message, (I'm new to Forums!)
The float switch was suppose to monitor the water level in the separator and run the electric feed pump as necessary but it was not very successful. I think there was too much turbulence and in the end I replaced it with a bleed valve to maintain water level. The pump was then run from an algorithm derived from the gas valve and various temperature sensors. This worked quite well. The burners were from our old central heating boiler, re-jetted for low pressure propane. They were very quiet and controllable but susseptable to wind and not really the right shape; I may try making my own this time.
The coils I did by hand. 10mm Cu tube is quite soft when annealed. I have attached a layout of my last Lamont design.