Air leaking by cooper tubes

[Lopez Mike]

Throw away the copper and weld in steel ones. Using copper tubes is a result of the strange delusion that the thermal conductivity of the tube material matters much. It doesn't.

You can do it now or do it after you fight the copper ones. Sometimes you can pound in ferrules as mentioned but it does obstruct the flow somewhat.

[Mike Rometer]

“(suggests) vertical fire tubes are an inherantley poor design as the excess heat in the top of the tubes is effectively trying to destroy the boiler, not boiling water. The ideal must be where the whole of the tubes are under water, did loco engineers get it right?”

Submerged tubes are better, because differential expansion issues are greatly reduced. The VFT design is good in that it is simple and cheaper to manufacture, and can provide a small amount of steam superheat. However a VFT will always have more thermal expansion of the firetubes, because the upper tube ends will run significantly hotter than the submerged tube part, and the boiler shell, these elements typically running very close to saturation steam temperature. A VFT with a steel shell, and Copper tube has the differential expansion problem exacerbated by the much greater thermal expansion coefficient of Copper compared to steel.

"The VFT design is good in that it is simple and cheaper to manufacture, and can provide a small amount of steam superheat.

That bit I like!

[fredrosse]

Mike, you are absolutely correct that using copper boiler tubes has virtually no heat transfer advantage compared to steel boiler tubes.

In the path of getting the heat of the fire into the boiling water, the metal tube, either copper or steel, is an insignificant resistance to heat transfer. The convection heat transfer coefficient between the hot flue gasses generally dominates the overall resistance to heat flow.

for example, using typical real values, say a boiler operating with 350F steam saturation temperature:

Boiling water temp: 350 F
Steel Tube wall surface exposed to the boiling water 360F
Steel tube wall surface exposed to hot flue gas, 368F
Average hot flue gas temperature, 1200F

The largest resistance to heat transfer is where the largest temperature differential exists, (1200 - 368 in this example), 832F temperature differential.


Copper, having a thermal conductivity 4x that of steel, would have the following temperature profile:

Boiling water temp: 350 F
Copper Tube wall surface exposed to the boiling water 360F
CopperTube wall surface exposed to hot flue gas, 362F
Average hot flue gas temperature, 1200F

The largest resistance to heat transfer is where the largest temperature differential exists, (1200 - 362 in this example), 838F temperature differential.

Here we see the quantified advantage in heat transfer due to copper tubes over steel, ( 838 / 832 ) = 1.007 The Copper tubes bring less than 1 percent advantage over steel.

However the steel is far stronger than copper