thermosiphon tubes

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lostintime
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thermosiphon tubes

Post by lostintime » Tue Jan 16, 2018 3:13 am

[*] any opinions on thermosiphon tubes around the tubes of vft? I didn't find anything in the search function, but I'm wondering about running slightly larger tubes inside of the shell with "ports" on the top and bottom to allow the steam bubbles to "pump" water up the full length of the tubes irregardless of the water level (within reason)
Basically a loose tube around the actual fire tube, held in place with a couple bent flanges
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Re: thermosiphon tubes

Post by Lionel Connell » Tue Jan 16, 2018 4:45 am

I think that you may run into trouble if the water level in your boiler falls below the top of your siphon tubes. If this happens the water in the siphons will stop flowing and you may end up with over heated tubes. I think that you would need to make the top of your siphons a point below your minimum water level, which I am guessing removes or reduces the effect that you are hoping top achieve. It may be better to look at a VFT design where the tubes are always fully submerged.
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Re: thermosiphon tubes

Post by cyberbadger » Tue Jan 16, 2018 1:09 pm

You want it connected directly to the steam space.

It will naturally make it's own water and deal with that itself. (Some like to drain for winter layup) This is the siphon part. Don't overthink it, It's really to protect the gauge from direct steam touching it.

You want 1 valve between it the boiler. This is to isolate it from the boiler should the gauge fail in any way under service.

Get a valve here that is rated for steam service at your steam pressure. Small is fine, you probably will not use valve. Some like the little gas valves because the handles are short. If you have a little brass stop cock, that's more or less the most desirable.

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Re: thermosiphon tubes

Post by TahoeSteam » Tue Jan 16, 2018 4:20 pm

Cyberbadger, I believe you've misunderstood what the original post was referring to. My understanding from the description is a tube that would surround each individual fire tube, only having an inlet at the bottom and an outlet at the top, and would in effect force water to be in contact with the entire length of the tube. This would make for maximum steam production for the given length of a tube. In theory it may work, but in practice one would have to be using incredibly clean water and never have a chance of corrosion or scale on the tubes... I think the suggestion of a submerged tube firetube, like the Simpson Strickland "Kingdon" design would be the simplest option providing a similar effect.
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Re: thermosiphon tubes

Post by cyberbadger » Tue Jan 16, 2018 11:29 pm

Sorry. All I read was thermo siphon and I thought it was about the gauge siphon.

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Re: thermosiphon tubes

Post by lostintime » Wed Jan 17, 2018 4:15 am

TahoeSteam wrote:Cyberbadger, I believe you've misunderstood what the original post was referring to. My understanding from the description is a tube that would surround each individual fire tube, only having an inlet at the bottom and an outlet at the top, and would in effect force water to be in contact with the entire length of the tube. This would make for maximum steam production for the given length of a tube. In theory it may work, but in practice one would have to be using incredibly clean water and never have a chance of corrosion or scale on the tubes... I think the suggestion of a submerged tube firetube, like the Simpson Strickland "Kingdon" design would be the simplest option providing a similar effect.
Precisely my thoughts, though I neglected to think about scale/fouling. I was thinking of this as opposed to fully submerged on the thought line that the pumping action would scour the tubes of forming bubbles (essentially break the cohesion of the steam on the tubes) to get a higher effective wetted area.
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Re: thermosiphon tubes

Post by lostintime » Wed Jan 17, 2018 4:49 am

Here is a pic of the general thought (albeit from a NH3 system). It works remarkably well in that service, water however... perhaps with very little make up water.
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Re: thermosiphon tubes

Post by dampfspieler » Wed Jan 17, 2018 8:56 am

Hi,

i think it will work properly if the outer tubes are completely in water. The flow of the water is more directed than without the outer tubes. You have also two heating areas of water - the fire tubes and the outer (directing) tubes.

A similar flow-directed design you can find in field-tube Boilers.

That directed flow is very effectively i have found with the BULLERJAN hot air oven of a friend. He has it in his very wide shop and needs only a short time to reach pleasant temperatures for working.

Image
linked from bullerjan.com

Best Dietrich
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Re: thermosiphon tubes

Post by fredrosse » Wed Jan 17, 2018 2:56 pm

"You have also two heating areas of water - the fire tubes and the outer (directing) tubes."

The "outer (directing) tubes" (sleeves surrounding the firetubes), would not produce any extra heating areas of the boiler, they would just extend the vertical effective length of the firetubes which are in contact with boiling water.

Depending on the sizing of the sleeves, and the sizing of the water entrance/exit flow areas at the bottom and top of the sleeves, there are several possibilities that could occur.

1. If the flow areas are too large, then the sleeves could possibly just cause no increased boiling water height along the firetubes, hence no benefit.

2. If the flow areas are too small, then the sleeve dimensions could possibly starve the firetube of sufficient flow, and could even cause decreased boiling water height along the firetubes, hence not only no benefit, but a loss of function with lower steam generation.

The purpose of the proposed sleeves is to carry boiling water circulation higher in the VFT boiler, however if the sleeves carry boiling water circulation higher, then there is a higher tendency of water priming in the outlet steam, another factor that would require proper sizing of flow paths.

Sizing this configuration is required, and in my view the expense and complication here would not be prudent. Far easier and less expensive to just make the firetubes longer to get the submerged length you want.
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Re: thermosiphon tubes

Post by lostintime » Thu Jan 18, 2018 1:45 am

Thank you, that's why I asked. In the unit in the picture (taken through the vapor outlet) it has a very large separator drum above it, followed by a second vapor acumulator to prevent priming. Odd thing is that I've experimented running it fully flooded (i can raise the liquid level 3 ft above the tubes before it spills to the secondary seperator) and keeping the the level at the bottom of the outlet piping. I've repeatedly gotten consistent increases of 35% (measurments made in btu's) in vapor production when run at the lower end. I'm close to being stumped by it. (Data gathered by pumping fluid through the "firetubes" measuring incoming and outgoing fluid temps via rtd's, and flow rates with certified mass flow meters, while maintaining the same suction pressure in the vessel while it is filled with ammonia). I do believe there is something more occurring than just the increased static area. I fall back again to thinking it is the forming vapor "bubbles" sticking to the pipes and reducing their area when in flooded mode.
(Edit) that is a very fine looking stove.
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