Here in the U.K.There is an article in the Feb 2012 issue (out now) of "Engineering in Miniature" which begins to explain how complete control can be applied to Monotube boilers. (the article is to be continued in an unknown future issue/s) This could trigger a seachange in how this type of boiler is handled.
While you may indeed benefit by popping out and getting yourself a degree in systems control on the way to the newsagent, this is not just an exercise in theory, as he has extensively tested all the "gubbins" over the last couple of years, and has got the control nailed such that it will happily run a simple Stuart D10 via radio control. He can even connect it to one of those newfangled "computermabobs" and do data logging!
Anyone with half an interest in anything steam related should take a gander, then throw their boiler certificates out the window and get the soldering iron out
Rog.
Last edited by rogercrier on Sun Jan 22, 2012 9:00 pm, edited 1 time in total.
Personally, I'd much prefer a forced circulation boiler such as a Lamont to a precision controlled monotube. Yes, one needs a pump capable of withstanding boiler temp and pressure - but no boiler control system at all, just the usual sight gage and a float in the hotwell.
I have a monotube in my launch.. I plan on some day building my own launch, and it will either have a steam-powered circulating pump on a Lamont or some other displacement-type watertube. I'd much rather be steaming than developing the "tune" with mechanical and/or electronic controls.
If any of my all-mechanical controls should fail, I risk damage to the boiler as well as the engine and am dead in the water. It would happen VERY quickly even while going "only" 9mph.
Controls, whether mechanical or electronic, are not infallible.
Monotube Boiler control, automatic with no moving parts.
Arrange the feedwater pump to supply about 150% of the steam generation rate. Steam/water mixture exits the monotube, and enters a separator tank, steam leaves the top, going to engine or superheater. Excess water leaves the bottom, and is drained back to the hotwell, flow regulated by a simple needle valve. Counterflow heat exchanger, with water from separator traveling in one direction, cold feedwater traveling in opposite direction. This recovers much of the energy that was used to heat the water in the monotube boiler, and cools down the hot water that is returning to the feed tank. Regular sight glass on the separator tank allows monitoring of water level. I also place a thermodynamic steam trap just above maximum separator level, this will come into play and prevent water carryover to the engine if the return water line is throttled shut too much and water builds up in the separator tank. This scheme has been used on monotube railroad steam heating units in the last century, and keeps all the monotube from overheating. Generally difficult with hand firing, but fine with oil/gas firing that can be held fairly constant.
This "no moving parts" monotube boiler control setup requires a couple of percent extra fire, about 2% more than would be required for an ordinary drum boiler.
I'm the author of the above article in "Engineering in Miniature" and its a pleasure to see that it has reopened the topic about monotube control.
Without reproducing the 7000 word article on this forum, I would like to give a bit of the background of the project.
I had built the Stuart D10 from castings and was looking around for a boiler just at the time new European legislation came in requiring boilers including "model" ones to have certified design documents and the "CE" mark applied. Well, this drove a lot of the one-man (and women) types of model boiler makers out of the business with a result that there were long lead times to obtain one - plus all the insurance testing and certification.
I had heard of "flash" boilers and was told that they were uncontrollable and could only be applied to the tethered high speed hydro boats (>120mph) or the straight runners.
In my professional career I was a power station control systems engineer used to the 4 million lb/hr variety of boiler, so that when somebody told me that monotubes were impossible to control it was like a red flag to a bull!
I chose a computer based (not PC or laptop) control system simply because nobody knew how to control the boiler and therefore I required a very flexible control system that could be amended at will.
After two years I mounted the system in a rather basic model boat hull of 72lb displacement and with a 5 inch prop and gradually refined the system. The vertical boiler currently comprises 50ft of brake pipe giving a copper to water surface area of 240 sq.inches. I use camping gas cylinders inverted to supply liquid fuel to an evaporator wound around the burner which prevents the gas cylinder from freezing even in the winter. I am currently achieving 600RPM (continuous) with the engine.
There is no steam regulating valve betwixt boiler and engine with the steam coming out of the bottom(!) of the boiler straight to the engine. Speed is regulated by the amount of water being electrically pumped into the boiler. There is no separation chamber just the one continuous tube between input and output.
From the transmitter I only control the rudder (naturally), the reversing linkage on the engine and the cc/min of feed flow that I want to go in the boiler. Since I’m not “on board” the control system has to look after everything.
Please read the article for a more in depth understanding of the system
Ian
p.s. No I haven’t got a degree in control theory – just tenacious, that’s all.
I am currently designing a monotube boiler that I think will be capable of meeting automotive and marine needs. I have been reading everything I can get my hands on, joined several forums, and examined the Doble, White, Scott-Necomb and other designs closely.
Having large thermal histeriesus is needed to keep the system stable, as rapid changes in firing rates will throw a monotube into imbalance quickly, and the use of a normalizer is mandatory. My current design is a varriable pressure/varriable firing rate design to get the best fuel economy possible with out compromising performance. Conversly my engine design also works on a varriable pressure/cut off priciple.
I'll keep everyone updated as I proceed with my trial and error tests. Maybe one day I'll have a boat that can out race Wesley's
I've watched your videos "the walkaround" the steam plant on shore and the exciting "Foul weather on Fallen Leaf Lake" - I can understand your need for a very robust and reliable system!
Do you use a steam/water separator on the monotube boiler and control the level by feed pump by-pass and have you a schematic diagram?
The main difference with my control system compared with the “traditional” ones that control steam temperature with the pumps and pressure with the heat input, is that I don’t bother controlling the pressure, since both heat input and feed flow affect it.
I use electric feed pump(s) to maintain a desired feed flow rate independent of boiler pressure – if I want 50cc/min of flow into the boiler then that’s what the control system will maintain whether the boiler is firing or not.
The next control loop is the steam temperature, which is measured by a thermocouple in the boiler steam outlet manifold, and is compared with the desired temperature usually a couple of degC above steam saturation temperature. The controller then operates a servo controlled gas valve to modulate the heat input.
By having a pressure independent feed flow, very stable control is maintained throughout the power range – this is what makes this system so different to that used over the last one hundred years.
To increase the boat speed, I increase the desired feed flow setting and the more water going into the boiler the more steam comes out and the engine speeds up (and vice-versa).
My system is designed with an engine driven pump in mind with the electric pumps providing a start-up and standby service. The system would start on electrics, but, as the engine driven pump takes over the flow, the electric pump would slow down (eventually to a stop) and then start pumping again when the engine driven pump slows down.
Hi Boatbum,
I’m not sure of your location, but if you have access to the “Engineering in Miniature” magazine (Tee Publishing,UK) then my article is in the February/March 2012 issues and includes photographs and schematic diagrams of my control system based on the AE-35 control unit.
I'm on the other side of the pond from you. Sounds like your system is well thought out and I would love to see it in operation. Is there any chance you could scan the article you mentioned and email it to me? I'm planning on using electric feed pumps with mechanical back ups, solenoid valves, thermocuples and solid state controls to maintain desired output of superheated steam.
