Monotube Boiler Design – Radical Thinking

A special section just for steam engines and boilers, as without these you may as well fit a sail.
dhic001
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Re: Monotube Boiler Design – Radical Thinking

Post by dhic001 » Sat Oct 07, 2017 6:19 am

I get busy for a few days and you guys go mad. No Mike, the load doesn't change dramatically, the load on the propeller only changes if you are lifting it in and out of the water. Sure you are altering the speed, but that doesn't alter the load. Once the stroke is set on the pump to match the consumption, the load doesn't alter. If you speed up or slow down, the feed pump does the same.

I guess the important question here is how many of you have actually been involved in running simple monotubes, or have actually run them in a boat?

Daniel
Lopez Mike wrote:Ahh. I hate to break the news but these launches are always changing the load. Speeding up and slowing down and heading into a breeze. You name it.
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Re: Monotube Boiler Design – Radical Thinking

Post by DetroiTug » Sat Oct 07, 2017 8:28 pm

Under higher load and the same RPM more steam is being packed in the cylinder. More work costs something.

As the load the changes the steam consumption can change as does the amount of water needed to make the steam. I've tried it several times on my boat with a needle valve bypass on the feedpump feeding the VFT, can get it set pretty close and then speed up or slow down and the bypass has to be adjusted to it again.

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Re: Monotube Boiler Design – Radical Thinking

Post by RGSP » Sun Oct 08, 2017 1:58 pm

dhic001 wrote:I get busy for a few days and you guys go mad. No Mike, the load doesn't change dramatically, the load on the propeller only changes if you are lifting it in and out of the water. Sure you are altering the speed, but that doesn't alter the load. Once the stroke is set on the pump to match the consumption, the load doesn't alter. If you speed up or slow down, the feed pump does the same.

I guess the important question here is how many of you have actually been involved in running simple monotubes, or have actually run them in a boat?

Daniel
Lopez Mike wrote:Ahh. I hate to break the news but these launches are always changing the load. Speeding up and slowing down and heading into a breeze. You name it.
I (at least partially) agree with Mike on this one. Try running an electric boat with an ammeter in the motor circuit: it's quite remarkable how much the load does change as the boat accelerates, turns, or hits rough water. With my light electric canoe, the difference in power required between going upwind and downwind is significant, though it might not be for a heavier boat.
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Re: Monotube Boiler Design – Radical Thinking

Post by DetroiTug » Sun Oct 08, 2017 2:22 pm

With the Monotube, I think the actual fine tune regulation is in the monocoil itself. The feedpump is set to a coarse adjustment on feedwater requirements. If there is more water in the coil, it simply produces more steam, if there is less water in the coil, it produces less, but there is always some water in the coil as it is being fed with each stroke of the engine, which keeps it from going completely dry.

The Serpollet car operated with no steam throttle at all. To increase speed the fire was increased to increase steam production. And that system used a variable volume feedpump. I think, as the fire increased so did the feedpump output, and the inverse, when the demand was decreased and the fire lowered so did the amount of water delivered to the coil to keep from flooding the coil. That system reportedly worked very well, but complicated.

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Re: Monotube Boiler Design – Radical Thinking

Post by Oilking » Sun Oct 08, 2017 4:42 pm

Maybe just adding to what's been said, but here goes:
The Knox class DE that I was a BT on had the following fuel power characteristics:
RPM-------------- 122----------------- 244
Shaft HP--------- 4750---------------- 35000
FO in 100#/hr--- 44------------------- 244

I won't dispute that a mono tube with a properly adjusted engine driven feed pump can auto regulate the feed flow, but the "load" on the system does change.

Dave
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Re: Monotube Boiler Design – Radical Thinking

Post by fredrosse » Mon Oct 09, 2017 3:01 am

"I guess the important question here is how many of you have actually been involved in running simple monotubes, or have actually run them in a boat? "

Yes, my first steamboat had a monotube boiler, and worked satisfactorily. The only operational problem I had was when the feed pump intake filter became dirty, increasing the pressure drop leading to the feed pump suction.

Eventually this led to enough restriction that the feedwater flow was reduced. The system was designed to allow flow reduction to about 50% of feedwater pump capacity, and still provide 100% steam flow, so all was well for some period of time.

Further reduction in feed flow resulted in superheat increasing, and for a period of time engine performance was better and better as steam temperature climbed. When steam temperature was approaching 900F, the steam flow was reduced such that engine performance decreased, not enough steam flow. At this point the insulation was burning up on the main steam pipe.

I finally realized what was going on, and changed the filter element, normal performance was restored.

Very little storage capacity in this monotube. If the engine inlet was suddenly shut, the safety valve (175 psi setpoint) would quickly lift, then re-seat, then lift again ....etc. This would happen about 10 times per second, sounded like a machine gun.
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Monotube 5000PSI Design Pressure
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Re: Monotube Boiler Design – Radical Thinking

Post by cyberbadger » Mon Oct 09, 2017 12:57 pm

I have played with several drive ratios on Nyitra and the more I have loaded up the Toledo the more dramatically it responds to changing engine loads. 4:1 was the most I tried and a few strands of seaweed can change it from making headway to loosing ground! This summer I have been running 2:1 and happy with that ratio.

If you want to run a monotube boiler, go for it. Just know that very likely the monotube will be the difficult bit to get right.

They weren't dumb back in the day, they tried everything they could that was available to them.

They didn't have microcontrollers, this is another option for controlling a monotube...

-CB
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Re: Monotube Boiler Design – Radical Thinking

Post by barts » Mon Oct 09, 2017 4:28 pm

One style of monotube that hasn't been discussed is the 'flash overfeed' design. This uses a dynamic steam trap about 2/3 - 3/4 of the way through the coil, and the feed water pump is adjusted so that the trap is always discharging at least some water back into the hot well or better yet condenser. This keeps steam temperatures from getting too high w/o fancy control systems. I've talked to people who have run solid fuel monotubes using such a system; this apparently worked quite well. Note that such a system could be used to deliver only mildly superheated steam to the engine if the trap is towards the end of the coil.

As to the load varying during steaming, this happens all the time. The boat runs easier downwind, the engine lugs in a tight turn, etc.


- Bart
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Bart Smaalders http://smaalders.net/barts Menlo Park, CA
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Re: Monotube Boiler Design – Radical Thinking

Post by fredrosse » Mon Oct 09, 2017 7:45 pm

"...... and the feed water pump is adjusted so that the trap is always discharging at least some water back into the hot well or better yet condenser. "

I have built systems similar to this, in my case the feedwater pump gives 200+% of the design basis steam flow rate, so fully 100+% of the design basis steam flow is liquid water directed back to the hotwell. Much wasted energy going back to the hotwell? Not really, I have used a counterflow heat exchanger, with this return hot water flowing thru the heat exchanger tubes, and cooler feedwater from the feed pump on the shell side. This recovers the great majority of residual heat in the return flow, and puts it right into the incoming boiler feedweater, and thus reduces the heat required from fuel.

Another relevant feature here is that a steam trap is not required. It is a character of a simple orifice that the flow of saturated steam is about 10% of the flow for saturated water. This property will let plenty of saturated water flowing back to the hotwell, but only allows a small fraction of the flow back to the hotwell if it is steam. This makes a self regulating device, with no moving parts.

This setup can be used to automatically maintain a water level in a conventional boiler too. My steam driven Domestic Heat-Power Module had just such a water level control system. I designed it for my Hypro feedwater pump, a two cylinder pump which routinely gave trouble when one of the pump cylinders would stop pumping. Even if this occurred, the steam plant would continue to function, providing 100% steam flow with only half the feedwater flow being pumped. All automatic, all with no moving parts for reliable level control.

This type of design can be configured to function similar to a Lamont Boiler, keeping all the steam generator tubes in service, always with liquid water present, thus no overheating of the tubes, no difficult control problems, and without the Lamont circulating pump. In the traditional Lamont boiler setup, there is a circulating pump that needs to take suction from the steam/water drum, and push a large mass flow of water into the generating coils to keep them flooded with saturated water. The duty of the Lamont circulating pump is difficult as it must draw saturated water, which tends to flash and thus destroy pump suction. There are other difficulties with the Lamont circulating pump design, only needing to supply a few PSI differential pressure, but working inside the boiler pressure vessel at a much much higher pressure.
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Re: Monotube Boiler Design – Radical Thinking

Post by Steam Captain » Sun May 31, 2020 8:21 pm

I recently read a thread on the steam car forum, which had a conversation going about using the same pump as a circulation pump as well as the feed pump. The idea sounds interesting.

https://steamautomobile.com:8443/ForuM/ ... #msg-27437

I don't fully understand the valve described there, but I suspect it serves a similar purpose as the 3-way valve I have in mind. Let's say the pump is adjusted to keep track of the average steam consumption or higher to keep the monotube wet. In order to keep the water level within designed limits, one could avoid the readjustment of the pump alltogether and use a 3-way valve.

In case of the water level reaching the high water mark, a water level control would actuate a 3-way valve to shut off the water feed and link the pump to the water inside the steam dryer flask. (If the monotube design HAS a water-holding vessel) So, the liquid water inside the flask gets recirculated until the water level reaches a low water mark, which in turn actuates the 3-way valve to return to the original position, linking the monotube with the feed water.

The advantages I see are: Even in case of excess water inside the flask, the liquid water being removed from the flask won't exit the pressurized system. It simply gets recirculated. This means the energy being invested in pumping that amount of water into the pressurized system won't get lost. The same with the thermal energy already inserted into the water. The recirculation pumping doesn't swallow as much energy as the feed pumping, as the former just needs to overcome the friction losses inside the monotube and not the pressure differential between the hotwell and the pressure vessel on top of that.
As an example: A pump could be adjusted to deliver 4 times the average needed water. It would not consume 4 times the energy. It would always only consume the energy used for the water needed for the engine. All the excess water delivery would just need to overcome the friction losses inside the monotube. So, if a pump works at 4 times the need for the engine, it would only work at 1/4th of the time against the full pressure. The 3-way valve would be switched to recirculation mode the other 3/4ths of the time. A pump delivering 4 times the needed average is just exaggerated to bring the point home.

The drawback in comparison to the aforementioned method of using a small orifice is the need for a sensor to read the water level and use the signal to actuate the 3-way valve. Other than that, it definitely isn't a complex system to plumb. And this method of boiler water level control should work with all kinds of boilers having a water level.

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