VFT tube leak

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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RNoe
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Re: VFT tube leak

Post by RNoe » Sat Jan 04, 2020 5:34 pm

Half-inch thick flue sheets are standard in full-sized American-built logging steam locomotives thru the 1930s, at least.
Having rebuilt 4 locomotives, including fabricating a new firebox flue sheet for Hillcrest Climax #10, I am more familiar with them than I'd prefer!!

For the new sheet in the Climax we drilled all 140+ of the 2" holes. The 5-1/8" superheater holes were trepanned/bored.
All done on a LARGE radial arm drilling machine. was not a fast operation...

Our smaller boilers can certainly use thinner sheets, and usually do.
But a vertical boiler might benefit from a thicker "bottom" sheets, due to erosion issues during their lifetime.
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Re: VFT tube leak

Post by Oilking » Sat Jan 04, 2020 7:26 pm

The posted image is in reply to Mikes comments related to the effects of fluid films on heat transfer across a tube wall. The image is from Chapter 8- introduction to thermodynamics, Principles of Naval Engineering, NAVPERS 10788-B Image
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Re: VFT tube leak

Post by Oilking » Sat Jan 04, 2020 7:44 pm

I
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I hope this one works better
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Re: VFT tube leak

Post by Lopez Mike » Sun Jan 05, 2020 12:00 am

Interesting diagram. No gradients in the bulk of the fluids? Hmm.

I wonder if they meant for both sides to be actual fluids or was it a generic term which includes gasses? It occurs to me that the 'film' thicknesses and conductivities would be different for gasses and liquids and both film thicknesses would be quite dependent on the velocity. In our applications probably relatively low and maybe thin on the water side and rapid and thicker on the flue gas side. Tending to more laminar in the water due to velocities below the Reynolds number of around 2000. I just don't know how stirred up the water is inside the boiler.

I still think the conductivity of the steel would be much higher than that of the water. The heat flow on the gas side probably depends a lot the velocity. I mean we talk a lot about hoping for turbulent flow in our flues.
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Re: VFT tube leak

Post by fredrosse » Sun Jan 05, 2020 5:14 am

Being the Heat Transfer Specialist for a large corporation that designs steam power plants, I could not resist providing some detail here:

“I wonder if they meant for both sides to be actual fluids or was it a generic term which includes gasses?”
ANS: Liquids and gasses are both fluids, and both liquids and gasses share similar (but not exactly the same) equations governing heat transfer properties.

“It occurs to me that the 'film' thicknesses and conductivities would be different for gasses and liquids and both film thicknesses would be quite dependent on the velocity.”
ANS: That is correct, heat transfer coefficients for fluids flowing over solid surfaces (Tubing generally) is strongly dependent on the fluid velocity, as well as the fluid’s properties, such as viscosity, density, thermal conductivity, etc.

Other parameters also play here, such as tube diameter, but lets not get too detailed here, I can feel the readers (if there are any) falling asleep already.

For a general (and very approximate) quantification, some typical ranges of heat transfer coefficients for fluid conditions, (BTU/hr-ft2-F)
Air or Flue Gas flowing at a few MPH - 2 to 5
Air or Flue Gas flowing at 5 MPH, IE, Inside my boiler firetubes - 6 to 10
Water with natural circulation, moving at less than 1 MPH – 20 to 50
Water flowing in or around heat exchanger tubes, turbulent flow, 15 MPH – 500 – 1500

These approximate values are for single phase fluid conditions, for phase changing conditions such as boiling or condensing water, the heat transfer coefficients go way up, typically 4000 – 10,000

“In our applications probably relatively low and maybe thin on the water side and rapid and thicker on the flue gas side. Tending to more laminar in the water due to velocities below the Reynolds number of around 2000. I just don't know how stirred up the water is inside the boiler.”
ANS: When just warming the boiler, the waterside does have slow natural circulation, with laminar rather than turbulent flow conditions. That is why it took so long to properly warm up big Scotch Marine Boilers, although even with these minimal water flow conditions, the waterside heat transfer coefficient is still far higher than on the flue gas side. Once boiling begins, the water is in a fully turbulent state, with very high heat transfer coefficient.

On the flue gas side, typically the flue gas is close to laminar conditions in firetube boilers. In our watertube boilers flow conditions are typically better, sometimes turbulent.

“I still think the conductivity of the steel would be much higher than that of the water.”
ANS: Very correct, Steel thermal conductivity = 26 Water thermal conductivity = 0.34

“The heat flow on the gas side probably depends a lot the velocity.”
ANS: Very correct. Beyond velocity, smaller tube diameter helps, and flow over the outside of tubes is better than flow inside tubes.

“I mean we talk a lot about hoping for turbulent flow in our flues.”
ANS: With several practical constraints, we do strive for turbulence of flue gas flow conditions. In small firetube boilers we sometimes use “twisted metal tapes” inside the tubes to break up and mix the flue gasses for an increased heat transfer coefficient. These were named, many years ago, “Turbulators”. I use stainless steel coil springs inside my VFT boiler tubes to do the same thing.

The highest boiler flue gas heat transfer coefficients I have seen was with very high flue gas velocity, achieved by using boiler tubes rotating at high velocity inside the boiler casing at about 2000 RPM, giving gas side heat transfer coefficients of several hundred. But that is a special case, not for a steam launch!
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Re: VFT tube leak

Post by fredrosse » Sun Jan 05, 2020 5:29 am

Some figures to give actual numbers to the temperature profiles for typical conditions we find in launch or small locomotive boilers. These numbers are for my VFT 150 PSIG with 1-1/ 4 x 0.095 wall steel tubes, a boiler with 1/16 inch thick waterside scale, vs. a theoretical case with everything perfectly new and clean.

The salient point here is that the clean tube has its metal temperature very close to steam temperature, while waterside scale results in significant increase in metal temperature. The figures are for average conditions, but in areas receiving much more heat flow (radiant heat furnace areas), tube metal temperatures can soar to 500-600F, not good, and possibly having failures with Copper tubes
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Re: VFT tube leak

Post by fredrosse » Sun Jan 05, 2020 5:31 am

Scaled boiler tube
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Re: VFT tube leak

Post by Lopez Mike » Sun Jan 05, 2020 5:48 am

Ah! Now maybe I understand why my VFT pressure rise is so non-linear.

I sort of understood why there was a delay when the water temperature reached 100C. I forget the terms but I know it takes a lot of energy pumped in the water to get past the atmospheric pressure point.

What I didn't understand was why the subsequent rise is so lethargic at first. And stays fairly slow for a relatively long time. And then all hell breaks loose.

My impression is that the takeoff point for the pressure rise rate seems to co-incide with a change in the sounds. First the usual singing like a tea kettle and then lower toned sounds. I'm guessing that the drop in the pitch is because of an increase in the bubble size and thus much increased convection velocities in the water.

I steam up when still on the trailer due to launch ramp congestion and have always had difficulty timing things so that, on one hand, I have enough pressure to back off of the trailer and, on the other hand, don't have to deal with the relief valve roaring away while I'm trying to avoid stray dogs and children while backing down the ramp. Non-linear pressure rise doesn't help!

Something about launch areas inspires people to new heights of idiocy.
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Re: VFT tube leak

Post by Oilking » Sun Jan 05, 2020 5:05 pm

Latent Heat is the term I think you're thinking of.
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Re: VFT tube leak

Post by RNoe » Sun Jan 05, 2020 5:58 pm

A very informative thermodynamic discussion, especially with the graphs.
Thank you all.
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