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.
Last edited by
fredrosse on Tue Nov 08, 2022 9:34 pm, edited 2 times in total.