New VFT boiler for Frøya

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fredrosse
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Re: New VFT boiler for Frøya

Post by fredrosse » Tue Feb 07, 2017 2:39 am

I have checked some numbers, using data from my VFT boiler, which has 48 tubes, steel, 1.25 inch outside diameter x 0.095 wall, 18 inches long. (31.75mm outside diameter x 2.41mm wall, 457mm long), with total heat transfer surface area of 20 square ft (1.85 M^2). About 75% of the tube length is submerged in boiling water, 25% is above the water.

Considering only the heat transfer in the submerged portion of the tubes, If the tubes were changed from carbon steel to commercial copper, heat transfer in the submerged tubes would go up from 62075 BTU per hour, to 62112 BTU per hour (18.193 kW to 18.204 kW), this is very very small increase.

The concept that heat conduction in the axial direction will allow copper to bring extra heat to the boiling water (and thus increase boiler output) is correct, however this will also result in only a very small increase. One reason for this is that the hot flue gas has already been taken down to a relatively low temperature when the gas reaches the water/steam interface. There is not much temperature difference between the hot gas and the boiling water. Temperature difference forces heat transfer, and most of the hot gas energy has already been removed from the hot gas at this point.

Another major factor here is the tube heat transfer surface area (metal) in the axial direction, which is quite small, being only 0.11 square feet of conducting surface, this is about 1/2 of 1% of the nominal boiler heat transfer surface. Area drives heat transfer, and this metal conducting area is so small that the benefit is very very small.
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Re: New VFT boiler for Frøya

Post by cyberbadger » Tue Feb 07, 2017 4:10 am

fredrosse wrote:Another major factor here is the tube heat transfer surface area (metal) in the axial direction, which is quite small, being only 0.11 square feet of conducting surface, this is about 1/2 of 1% of the nominal boiler heat transfer surface. Area drives heat transfer, and this metal conducting area is so small that the benefit is very very small.
This jogs my memory going back and forth with my boiler maker on a design. That's why I choose a larger diameter firebox because it contributes so much to the square footage of heating surface. Loosely inspired by some drawings of large Manning VFTs for factories. Actually Manning is referenced on my P2. (ASME boiler birth certificate)
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http://www.holyokemass.com/pichampden/p141.html

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Re: New VFT boiler for Frøya

Post by DetroiTug » Tue Feb 07, 2017 3:02 pm

The copper vs steel subject rears it's head again and I still question the consensus.

Quote: "One reason for this is that the hot flue gas has already been taken down to a relatively low temperature when the gas reaches the water/steam interface. There is not much temperature difference between the hot gas and the boiling water."

Isn't this a snapshot of only one point in a steam generators heated surface? Other areas where the flame is impinged on the surface as it is with tubesheet and lower boiler flues, the temperature difference is going to be much greater in the thousand degree range. The way I understand it, thermal conductivity or heat transfer coefficient either matters or it doesn't. Sure, whichever flue material will heat to the same temperature, the problem with shooting at the enemy is, the enemy shoots back. The cooler water on the boiler side in the lower portions is cooling the material. The BTU transfer over time, the temperatures one side vs the other and according to the levels of thermal conductivity between the two, there must be more of a difference than a few percent throughout.

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Re: New VFT boiler for Frøya

Post by Ethelred » Wed Feb 08, 2017 3:04 pm

I read somewhere that there's actually little difference in performance between copper and steel tubes as it's masked by the boundary effect. Improving the circulation is more effective as it scours the boundary layer. This is certainly with true in water-tube boilers, though I don't know how you'd improve circulation in a VFT. Perhaps using the make up water as a jet would work.
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Re: New VFT boiler for Frøya

Post by fredrosse » Wed Feb 08, 2017 4:04 pm

"The copper vs steel subject rears it's head again and I still question the consensus. "

I can say with great certainty there is virtually no difference in heat transfer if Copper vs. Steel tubes are used in a steam boiler. I have worked as a professional engineer in the heat transfer, fluid mechanics, and thermodynamics arena for nearly 45 years, and, at least in these realms, I know the real technology.

A very reasonable analogy can often make this more clear. Let us say we are having a traveling race, having three separate segments, for 9 miles total distance.

The first 3 miles of the race is done with brisk walking, on foot only,
The second segment is using any motor vehicle you like, and is also 3 miles distant.
The third segment for the remainder of the race, again three miles, and you use a good horse.

In this analogy, the first segment of the race, on foot, represents the travel of heat from the flue gas to the surface of the tubes, and this process is by far the greatest resistance to travel, at 3 MPH, taking about 60 minutes time.

The second segment of the race, with any motor vehicle, say a Ford Pinto, or a new Corvette. This represents the travel of heat through the tube metal, Steel tubes (Ford Pinto) or Copper Tubes (Corvette). This process, takes 3 minutes for the Ford, at 60 MPH, and only 1 minute with the Corvette at 180 MPH.

The third segment of the race, with a thoroughbred horse, represents the travel of heat from the tube surface into the boiling water, a traveling method far faster than walking, but much slower than the automobile, say 30 MPH taking 6 minutes to complete.

So in this travel race, the entire three segment trip with the Ford Pinto takes 69 minutes, and with the Corvette 67 minutes. While the Corvette is wildly faster than the Ford Pinto, the overall trip time is virtually no different. Yes, the Copper tubes do make better heat transfer, but not in any way significantly.
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Re: New VFT boiler for Frøya

Post by fredrosse » Wed Feb 08, 2017 4:15 pm

"Isn't this a snapshot of only one point in a steam generators heated surface? Other areas where the flame is impinged on the surface as it is with tubesheet and lower boiler flues, the temperature difference is going to be much greater in the thousand degree range. "

That is correct Ron, however the issue I was addressing in the previous post was at the tube path just above the steam/water interface in a VFT boiler. It had been suggested that Copper tubes at this location would bring significant quantities of heat down into the boiling water. At that particular location, the hot flue gas has already lost most of its available temperature energy, tnd thus does not have much drive to transferring heat to the boiling water.
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Re: New VFT boiler for Frøya

Post by fredrosse » Wed Feb 08, 2017 4:30 pm

"I read somewhere that there's actually little difference in performance between copper and steel tubes as it's masked by the boundary effect. Improving the circulation is more effective as it scours the boundary layer. This is certainly with true in water-tube boilers, though I don't know how you'd improve circulation in a VFT. Perhaps using the make up water as a jet would work."

When you write about improving circulation, you are talking about the metal to boiling water heat transfer, and the analogy presented about the 9 mile race applies here. What you would be doing is improving the speed of the race horse, while the walking part of the race remains unchanged. Using this analogy, the circulation increase (horse speed) can go up, say 10%, and the overall trip time only improves by about 0.6 minutes, again an insignificant improvement in the total trip time of over 65 minutes.

The only significant area where performance improvement efforts can be effective is with respect to the heat transfer coefficient between the hot flue gas and the tube metal surface. Any other improvements due to increased tube metal thermal conductivity, or increased boiling water side heat transfer, provide only trifling improvements.

There are other reasons why promoting good circulation within the steam/water space of the boiler is a very good practice, however improving this boiling water heat transfer coefficient is not one of them.
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Re: New VFT boiler for Frøya

Post by Ethelred » Wed Feb 08, 2017 10:16 pm

It's fascinating, thanks for the explanation Fred.
So is it the case that to improve the heat transfer then the more tubes the better. The late Simpson Strickland boilers were wide and not very deep, presumably because more short-tubes are more effective than a similar surface area made up of a smaller number of long-tubes.
Several people have found that a diameter of 1.25 inches is optimum. Is that generally the case in industrial boilers?
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Re: New VFT boiler for Frøya

Post by froya66 » Wed Feb 08, 2017 11:02 pm

Hi Guys

Thanks for all your comments regarding steel versus copper. I might get so interested in this question that I produce a small test rig, in order to actually measuring the difference - specifically pointing against the VFT boiler.

I shall give you a full report - if not my laziness kills it before it is born.

Best regards
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Re: New VFT boiler for Frøya

Post by fredrosse » Thu Feb 09, 2017 1:31 am

"Several people have found that a diameter of 1.25 inches is optimum. Is that generally the case in industrial boilers?"

For solid fuel firing of small steamboats, 1-1/4 or 1-1/2 inch tubes in a VFT are a good choice. The 1-1/4 inch tubes allow getting more heat transfer surface area within the limited space for the boiler, but will need more frequent cleaning. For clean fuel firing (such as propane, or smokeless oil firing), smaller tubes would be the choice, as was used for the Stanley steam cars, which used 1/2 inch tube outside diameter in their VFT boilers. With the small diameter tubes you could get much more heat transfer surface area into a small boiler.

For a watertube steamboat boiler, tubes should generally be somewhat smaller, 1/2 inch to 1 inch outside diameter is a good choice. With the hot flue gas on the tube outside diameter, heat transfer is much better, in the distance race previously described, the walking man is now replaced by a jogger, and the 60 minute walking portion of the trip changes to perhaps 30 minutes of jogging.

Large industrial or power plant boilers, some of which are thirty stories high, generally use boiler tubes of larger size, typically ranging from 1-1/2 inch to 3 inch diameter.
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