How High can Boiler Output be Forced?

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fredrosse
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How High can Boiler Output be Forced?

Post by fredrosse » Sun Apr 01, 2018 10:45 am

Lionel has sent the following question as a PM, but my poor computer knowledge of how to reply leads me to post here. I hope it is of interest to some of the forum's members.

From Lionel: I have been reading up about the small tethered hydroplanes that they play with in the UK. It is suggested that those guys manage to get 6-7 brake horse power out of a 25 foot length of 1/4 inch tube mono-tube boiler albeit with an exhaust temperature of 600 Deg C plus. For this to be right they would need to achieve something like 3-4 boiler horse power to the square foot of heating surface. Can this really be right? If fuel efficiency is of very little concern, and the only concern is to get the maximum power for the least weight and fuel weight is not important due to the short run time, is 4 boiler HP to the square foot a doable thing?

And the reply: Within industrial boilers, the maximum heat flux per square foot of furnace wall (the most intense area of heat flux) is around 70,000 BTU/hr-ft2, which translates to about 2 boiler horsepower (33,475 BTU/hr) per square foot of heat absorbing surface. As you know, overall boiler surface, for reasonably efficient boilers, is on the order of only 0.2 boiler horsepower per square foot of heating surface, considering furnace as well as convective heating surface.

The upper limits of this parameter is called DNB, (Departure from Nucleate Boiling), where excessive steam vapor generation exposes the tube wall to steam as a gas (with a relatively low heat transfer coefficient) for a long enough time to allow the tube metal temperature to rapidly increase, with subsequent tube failure. For industrial furnace conditions the DNB limit is around 10 - 20 Boiler horsepower per square foot of surface.

The DNB limit is a function of several variables, and with tubes much smaller than typical industrial practice, and steam/water velocities much higher than normal industrial practice, the DNB limits I would expect to be even higher. So, in summary, yes it is possible to force a steam generator to very high outputs per square foot of heating surface, if overall efficiency is not a concern.
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Re: How High can Boiler Output be Forced?

Post by jeff » Sun Apr 01, 2018 1:21 pm

In support of the above, I would argue that nucleate boiling is not relevant in a monotube. It might apply immediate the feedwater encounters the hot tube, but as soon as 1% of the water has evaporated, the volume fraction of steam is already 57% (at 200psig), way beyond a nuclear boiling limit. With 10% evaporation, the steam volume fraction has risen to 94%. The heat transfer inside the tube is effectively forced convection with a steam as the medium. Jeff
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Re: How High can Boiler Output be Forced?

Post by Lopez Mike » Sun Apr 01, 2018 2:22 pm

I would submit that the estimates of the power outputs are just that, estimates. I was a fanatic I.C. powered model airplane addict in my youth. I built a test stand for my most powerful engine which was pushing my control line craft to perhaps 130 m.p.h. The .6 cu.in. (10c.c.) engine was rated at "About 1.5 h.p." by the manufacturer McCoy. I never got that much out of it. There were subsequent engines of this size claimed to have reached 5 h.p. (K & B) but the speeds only jumped 15-20%.

They are getting amazing speeds out of these boats but I wouldn't base any serious calculations on their numbers.

What interests me is the question of whether these craft are boats or planes. They seem more like planes that are taxiing at very high speeds! Just guided in altitude by the surface of the water. I raced inboard hydroplanes (full sized) and once in a while one of us lost pitch control with the obvious resulting excitement.
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Re: How High can Boiler Output be Forced?

Post by fredrosse » Sun Apr 01, 2018 4:25 pm

What I read about British Flash Steam Hydroplanes, they have usually 15cc or 30cc reciprocating engines, running at over 10,000 RPM, with a steam pressure far above anything you might get with a non-supercharged IC engine. Steam generator pressures ranging into a few thousand psi are used in some of these machines, where the steam density is far greater than what 200 psi represents. So on that account the estimates of several horsepower that Lionel mentions are certainly in the ball park.

As far as nucleate boiling not being relevant relevant in a monotube, I can say with certainty that it is indeed relevant, and is a defining upper limit as far as tube burnout is concerned. There can also be several other issues in the heat transfer arena that may lead to tube failures, but within the relatively coarse estimates provided by Lionel, the basic answer to his question holds, yes, that kind of fantastic steam generation rate from these small boilers is possible.
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Re: How High can Boiler Output be Forced?

Post by Lopez Mike » Sun Apr 01, 2018 4:50 pm

They are certainly blurring the line between external and internal combustion engines.

I know very little about critical point steam. At these pressures aren't they near the 3200 p.s.i. that this takes? I'm amazed that they aren't getting to where metallurgy isn't the limiting factor rather than thermodynamics.

What fuel are they using and are they managing to get some sort of forced draft? I suppose an oxy-acetylene flame is next!
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Re: How High can Boiler Output be Forced?

Post by DetroiTug » Sun Apr 01, 2018 5:04 pm

I have a friend of mine who knows quite a bit about nucleate boiling in a monotube as he has taken quite a few of them to failure as a result of nucleate boiling exposing the tubing under hard firing in the 1000 psi range. Steam is made where the temperature and pressure is correct for it to happen, no matter where it is in the coil. His latest Monotubes, start with small tubing and progressively larger tubing as it proceeds up the coil stack, he has better success with that configuration. Nucleate boiling and the prevention of it is the basis for forced circulation generators. Now if we could just find a pump to do that :)

As far as the flash steam hydro hp claims, I tend to believe them, I've read quite a bit from them and they seem pretty well versed in their work.

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Mike, that used to be my hobby too, build em all week and crash em on Saturday. I had a 1/5 scale Citabria on floats that I used to take the wing off and go ripping across the water, probably 40-50 mph but it looked like 200 mph.
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Re: How High can Boiler Output be Forced?

Post by cyberbadger » Sun Apr 01, 2018 10:05 pm

For me those hydroplane steam things are not really what I value in steamboating.

Everything about the whole thing is as contrived as NASCAR to me
1) "vehicles" that can only properly turn in one direction because they are meant to just go in a circle or oval.
2) "vehicles" that are basically so unstable you need to tie them to an anchor so they stay on course
3) Max Passengers: None
4) Manoeuvrability: None
5) Skill Required to Operate : Low
6) Instant Gratification: Way Too high

And to top it off, the Furlongs/Fortnight is just not even in the right ballpark.

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Re: How High can Boiler Output be Forced?

Post by fredrosse » Sun Apr 01, 2018 10:27 pm

Just a clarification of terms,

It is Nucleate Boiling that gives the best heat transfer coefficient, and DNB (Departure from Nucleate Boiling) is the time when trouble begins. This departure leads to the boiling process called "Film Boiling", a film of steam basically insulates the hot tube metal from the bulk of the boiling water in the tube, and burnout rapidly follows.

Racing of tethered steam hydroplanes has a long history, and this sport was founded long before any type of radio control was available for model racing, I believe more than 100 years ago. So the only option for such fast machines is to constrain them to circular running with a cable keeping them from potentially dangerous departure from the proper path.

I believe the burners are generally burning gasoline, with a healthy jet of a vaporizing burner assisting the draft thru the boiling coils. When the boat is running at speed, the progress thru the atmospheric air probably assists the draft. Put your hand out the window of an automobile at 100 MPH, and you will feel plenty of draft pressure available. Add to that the potential for a steam engine exhaust blower, similar to what many of our steamboats have, and there can be copious amounts of draft pressure available.

Have any of the forum members been involved in this competition?
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Re: How High can Boiler Output be Forced?

Post by Lionel Connell » Mon Apr 02, 2018 1:32 am

His latest Monotubes, start with small tubing and progressively larger tubing as it proceeds up the coil stack, he has better success with that configuration.
What is the advantage of starting with a small tube and progressively increasing the tube diameter through the monotube boiler?

I know that the tethered hydroplane guys use a single diameter tube for the entire coil.

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Re: How High can Boiler Output be Forced?

Post by malcolmd » Mon Apr 02, 2018 1:27 pm

As a BTW I have a copy of the old MAP book "Flash Steam for Hydroplanes" and they were certainly running into valve-gear issues with valves running "red hot" due to the temperature of the supplied steam!!! Impressive bits of kit. ..I think all in this book were being fired by Paraffin burners....
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