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

Post by cyberbadger » Sun Jun 27, 2021 1:52 am

Something like this:
baker.png
baker.png (167.72 KiB) Viewed 4807 times
I know if my fireman forgets the feed water source is on I find out when I start hearing the steam main getting carryover and the engine snorts and grumbles through it. Probably some slug flow. :D

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

Post by fredrosse » Tue Jun 29, 2021 6:30 am

Planing to launch an experimental steamer in about 10 days, with a monotube steam generator, oil fired. The coil is 1/2 inch schedule 80 steel pipe, and inlet velocity is about 2 feet per second, subcooled compressed water. Reynolds Number at the inlet is about 25,000 (fully turbulent) and increases significantly as flow proceeds through the coil. With the small engine I currently have in the boat, only about 150,000 BTU/hr (44 kWt) average heat capture is expected. This condition will result in a coil outlet quality of 20+% ,slightly more than 100 PPH (pounds per hour) steam flow, with a coil outlet velocity of slightly more than 70 feet per second.

The expected final condition will double the heat input, and give more than 200 PPH steam flow, with an outlet velocity of about 160 feet per second. These velocities are quite high for a two phase flow, and I will need to keep an eye on the potential for erosion of the steel pipe.

So the conditions within the coil traverse from a very docile 2 feet per second liquid flow inlet, which itself is very turbulent, up to a far more turbulent two phase mist flow at the exit. I have found no experimental data on the flow conditions within the coil (similar to the Baker Plots, which only address horizontal and vertical flow), because the coil centrifugal action will tend to throw the liquid droplets to the outside of the coil radius, leaving a higher fraction of steam on the inner radius of the coil. I hope burnout will not occur, and that turbulent mixing will keep the steel coil from overheating by keeping it wet with liquid water mist.
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Re: What water speed do you work with in monotube boilers?

Post by fredrosse » Tue Jun 29, 2021 2:31 pm

The outlet of the monotube coil is a two phase (steam and liquid water) mixture, and it is necessary to separate the steam and water, sending the steam to the engine, and sending the hot water to heat revovery system. The attached picture shows the internals of the separator, with tangential inlet, allowing centrifugal force to do this function.
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Re: What water speed do you work with in monotube boilers?

Post by barts » Tue Jul 06, 2021 10:03 pm

This is really neat, Fred!

That coil set up would be very nice for a Le Mont boiler as well; the use of the high speed flow could be used w/ a tangential entrance to generate a violet circular motion in the drum, and with a tangential feed at the bottom of the drum to circulation pump one could pick up substantial amounts of dynamic head to help prevent cavitation in the circulating pump.

Is this a hot pressure washer coil by any chance?

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