What water speed do you work with in monotube boilers?

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What water speed do you work with in monotube boilers?

Post by Steam Captain » Wed Jan 30, 2019 1:00 pm

Hallo,

The subject already brings it to the point - I read the recommended water speed should be at least around 1m/s / ~3ft/s. I tried to find monotube boiler stats, but few come up, that mention bot tube diameter and steam production or engine consumption.

I would like to hear what water speeds people with launch-sized monotube boilers work them with.
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Re: What water speed do you work with in monotube boilers?

Post by barts » Thu Jan 31, 2019 3:13 am

Required water speed depends on tube diameter - you must have turbulent flow, which means a Reynolds number (dimensionless) of at least 4000 in a tube. This helps prevent overheating of the tube at very high firing rates. Note that this requires that there's water flowing through the tubes whenever the fire is on; for this reason Lamont (and other pumped circulation boilers) are popular as they're much easier to control. The engineering and controls required for a successful monotube are quite complex; the only folks I know who've built running boats w/ monotube boilers are engineers or professors. The Herreshoff flash overfeed boiler system is pretty useful (overfeed a monotube boiler and use a dynamic steam trap 2/3 of the way through the tubes; adjust feed pump continuously to maintain flow out the trap), and worked in the 19th century and is about the only means practical if you wish to not use automated controls for the fire.

If you can find a copy for a reasonable sum (or borrow one from a library) , Steam Generators by RUDORFF, Dagobert W. is well worth reading, as is Experimental Flash Steam although that's less technical. The 1955 edition of Steam by Babcock and Wilcox is readily available and very detailed technically.

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Re: What water speed do you work with in monotube boilers?

Post by Steam Captain » Thu Jan 31, 2019 11:46 am

I see I used a word ambiguously: I wanted to pose this question for all boilers labeled as LaMont/flash/once-or-more-through/forced-circulation/monotube as long as they don't possess any drums.

These books seem promising. As a student, I can get my hands on fairily rare books :idea:

Yes, some means to have a recirculation system seems to be a good idea. I think forum member Fredrosse built and successfully runs a once-through monotube, but it seems these boilers are best controlled by the burner output. I will burn wood, so it may be a challenge. But I am getting off the railroad track here.

By the way. I am sure there are some people, who would like this link: There is a site, where you can automatically let your boiler pipes get calculated for pressure drop and it even spits out intel like reynolds number or more easily readable - if the water flow inside a tube would be laminar or turbulent.

Here is the link and I hope it will stay up:

http://www.pressure-drop.com/Online-Calculator/

So, as far as I understand - there is less concern about the speed of the water, than the turbulence of its flow.
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Re: What water speed do you work with in monotube boilers?

Post by fredrosse » Fri Feb 01, 2019 12:06 am

Having practiced as a fluid systems engineer for over 50 years, I can state that laminar flow virtually does not exist in water/steam systems. The only places where laminar flow is of interest in my experience is with high viscosity lubricating oils under gravity flow, and the cooling flow for electronic components with natural circulation around components of very small dimensions. Steam and water flows are, almost without exception, fully turbulent.
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Re: What water speed do you work with in monotube boilers?

Post by fredrosse » Fri Feb 01, 2019 12:55 am

Image

Flow regimes for two phase liquid & gas flows, considerably more complicated than single phase flows. When water begins to have steam in the lines, then velocities increase rapidly, and usually a homogeneous mixture model is the acceptable character.

Look up "Baker Plot" to learn more about this.
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Re: What water speed do you work with in monotube boilers?

Post by Steam Captain » Sun Feb 03, 2019 5:04 pm

I suspected the slug flow, annular flow and dispersed bubble flow, but in no way would I've thought it might be that complex.The scientist in me wants to calculate all the conditions of the medium throughout the system. But before I sink into theoretical spheres all too deeply, I'll stick with a basic calculation of speed and pressure drop for pipe sizing.
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