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[Lopez Mike]

That may be what the designer was thinking about.

It does occur to me that about a zillion ordinary VFT boilers have been operating for a coon's age with the area above the water level running at these elevated temperatures. Of course, few of them are force fired though some serious rumbling comes out my unit when the blower is at full snort.

As far as I know, none of the locomotive superheaters were made of anything but relatively ordinary steel although that doesn't say much as these power plants were notorious for their short service lives compared to marine practice.

[fredrosse]

Yes, with a VFT, or any other boiler for that matter, the hot flue gas that is exiting the boiler should have had its temperature reduced significantly within the boiler before entering the steam sections of the pressure vessel.

However on a VFT, the only thing that is potentially threatened with overheat is a relatively small boiler tube, and it is assured that the hot gasses have already pased thru most of the tube while water cooled, hence assuring energy reduction of the flue gas.

On a watertube boiler, there may be instances where a fierce fire gets thru much of the tube bank, allowing higher temperatures of the stack gas. Add to that the fact that the potentially overheated part is the main steam drum (failure would possibly be catastropic), rather than the potential collapse of a simple firetube. Probably more risky with the watertube of this type.

[DetroiTug]

The Stanley folks with their VFT's, they shoot the exhaust with an infrared thermometer, if it's too high they add more economizer or water wall. "Water wall" refers to a wrap of copper tubing around the circumference of the boiler top to bottom. I think they said they try to keep it under 400°F. They also add baffling to slow the gases down. My Tug runs less than 300°F on the outside of the funnel. Long tubes and the turbulators are getting about all of the heat from the combustion.

-Ron

[fredrosse]

With gas firing, radiant heat transfer is poor, and without turbulators a single test firetube had 1100F firetube outlet temp. This tube had atmospheric boiling water on the outside, and was carrying 10 inches water level, 8 inches steam space. Putting in a turbulator, outlet temp dropped to about 300F, a safe temperature, and far better boiler efficiency.

[Lopez Mike]

Even with my scrap wood fired VFT, I noticed an immediate change when I added turbulators.

I had always thought of the area of the tubes above the water level as some sort of low grade superheater. I guess I hadn't thought it through about how much lower the gas temps would be by the time they got that for up.