Mono-tube boiler website

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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TahoeSteam
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Re: Mono-tube boiler website

Post by TahoeSteam » Sun Nov 06, 2022 4:43 am

Elver1 wrote:
Sat Nov 05, 2022 2:32 pm
Isn't the biggest draw of a monotube boiler the fact that conventionally it contains only a small amount of superheated water at any particular moment, and so the damage due to a rupture is less likely to be catastrophic? Would a water level/steam separator drum remove that advantage?
There is that, but there's also the element of making steam VERY quickly from cold, and doing so in a very lightweight and compact package.

It takes me 5 minutes to get steam up and the engine running from cold. Almost half of that is just for preheating the kerosene vapourizing burner. Mind you thats with a completely mechanical system. A system that includes electric pumps and such like a Doble can get steam up in a minute.

To me that's a much bigger draw than the 'safety' factor. They all contain a fluid in a state that if released and impinged upon flesh in the appropriate manner could maime or kill. An automobile can do the same I'm told.
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Re: Mono-tube boiler website

Post by Elver1 » Sun Nov 06, 2022 2:40 pm

Thank you Pete.

How much pressure would there be in the separator? And what temperature would the water in there be at?

Sorry, I do know that would depend on the specifics of the system, just trying to imagine it... wouldn't the pressure be the same as the working pressure of the delivery to the engine, and the temperature of the water be higher than 212 degrees? Probably somewhere close to the saturated steam delivery temperature at that pressure? Otherwise wouldn't it act as a condenser?

Thanks!
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Re: Mono-tube boiler website

Post by TahoeSteam » Sun Nov 06, 2022 8:36 pm

Elver,

The separator would be at the same pressure as operating pressure of the boiler, with the associated saturated steam temperature. It could potentially act as a condenser if not adequately insulated.

Any potential condensation would be expelled with the occasional emptying of the separator, or flashed back into steam as it traveled though the superheater (if so equipped).

Hope that is helpful.

There are other systems that use flow motors, quartz rods, and such to carefully meter the feed water into the coil to prevent unnecessary excessive flow. In turn negating the need for a separator and heating excessive feed water that won't be used to turn the wheel.
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Re: Mono-tube boiler website

Post by Elver1 » Sun Nov 06, 2022 10:44 pm

Yes it is, thank you, TahoeSteam.
I guess if it was just a steam separator it would get emptied by hand as needed, but bart mentioned earlier using it as a water level regulator, so I'm guessing that wouldn't be emptied normally.

Does the steam separator on your boat act as a regulator, or just a steam separator that gets emptied once in awhile?

Thanks!

ps I have a small unknown mfr. Westinghouse style twin that seems similar to an old brochure I have from a company called Acme Marine.
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Re: Mono-tube boiler website

Post by TahoeSteam » Tue Nov 08, 2022 6:36 pm

Mine doesn't have a steam separator. There is a de-superheater coil that adds feed water to keep the steam temp within a reasonable amount of superheat.

Bart, could you elaborate on what your meant in your earlier post?

Do you have any photos of your engine? There are lots of folks on here who can sometimes identify an engine from a photo.
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Re: Mono-tube boiler website

Post by fredrosse » Tue Nov 08, 2022 7:37 pm

Been running a monotube steamboat this past summer, with reasonable success. It takes about 1 minute for the steam pressure gauge to move off the peg from cold, then at 2 minutes I have 150 PSI steam. The uniflow engine allows the steam to be turned onto the cold engine and it starts immediately, with big exhaust ports to avoid water damage in the cylinder. The boat makes 7 MPH according to the GPS with 150 PSI steam, and the next steps will increase steam pressure to 250 PSI, with a bigger 3 cylinder engine, bigger oil nozzle, and a bigger boat. More information within this forum under "Members' Websites & Boats", {Monotube Javelin}, and "Technical - Engines and Boilers", {What water velocity ... for Monotube Boilers}.

A brief (and approximate) description of the monotube steam system for my Javelin sailboat conversion. Please note that this is a basic description of the cycle with several design features left with no details, to avoid over complicating within this thread. Controls, interlocks, and safety features are also not detailed here.

This is just a description of the basics.

Steam flow is 100 pounds per hour, (PPH) this is the design basis.

1. Feedwater pump (electric driven) delivers 270 PPH cold feed (lake water) to the letdown counterflow heat exchanger.

2. Feedwater is heated to about 250F in this letdown heat exchanger.

3. The heated feedwater then enters generating coils (oil fired) where 100 PPH of the water is turned into steam, 365F, and this two phase mixture (about 35% steam 65% liquid water) exits the oil fired coil.

4. A large fraction of liquid water provides protection of the coil from overheating, which is often the downfall of monotube steam generators. This is because the heat transfer coefficient of boiling water far far exceeds the rather low heat transfer coefficient of superheated steam, by a factor of almost 100:1 in this case. The liquid water keeps the coil metal temperature close to the boiling temperature, peak tube metal temperature is around 370F. If superheated steam is generated in the coils, tube metal temperature could soar to over 1200F, not good!!!

5. The two-phase mixture then enters the separator vessel, saturated steam flows upward and enters the main steam pipe, flows thru the engine and produces power, then exhausts to atmosphere.

6. The separated water falls to the bottom of the separator, where it flows thru the "letdown" outlet. This 365F hot water then flows thru the counterflow heat exchanger, and heats the incoming pumped feedwater. The letdown flow is cooled to about 100F, and discharged overboard. The letdown flow is manually controlled with Swagelok needle valves.

7. A thermodynamic steam trap is also connected to the separator vessel, at a slightly higher elevation than the letdown connection. This assures that the separator vessel will not become flooded if letdown flow is not enough.

The steam/water mixture enters the separator at about 100 MPH, and that level of turbulence can significantly impede proper separation of the steam and water. This requires separator internals that can properly quell the violent turbulence here, keep water out of the steam outlet pipe, withstand the pressure spikes, resist the significant forces involved, and assure a reasonable maintenance of a water level in the bottom of the separator vessel for the letdown and trap connections.

Additional features are required to provide a safe and reliable system, and proper engineering is required for several aspects of the design. Therefore, it is recommended that attempts to cobble together a similar system not be attempted unless proper detailed engineering, as well as operating procedures, are provided.
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Re: Mono-tube boiler website

Post by fredrosse » Tue Nov 08, 2022 7:45 pm

350cc uniflow steam engine that powered my house, with exhaust steam heating my house, for several years in the 1980's. Now with a temporary home in the small steamer.
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Re: Mono-tube boiler website

Post by fredrosse » Tue Nov 08, 2022 7:46 pm

Drawing of separator vessel internals
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Re: Mono-tube boiler website

Post by fredrosse » Tue Nov 08, 2022 7:50 pm

Separator Vessel being fabricated, berore final seam welding.
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Re: Mono-tube boiler website

Post by fredrosse » Tue Nov 08, 2022 7:58 pm

Monotube Javelin, first trials. Boat makes 7 MPH with the 350cc engine, holding 150 PSI steam pressure with oil burner continuously firing. If steam to engine is throttled, steam pressure immediately rises, with a pressure switch shutoff of feedwater pump and burner. Then pressure drops to about 125 PSI and pump/burner restarts, cycling as required for any desired cruising speed.
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