Boiler dimensions/ratios

That's because you didn't ask for 25.4mm off all round...

Richard.

 

The Sanzin (Giesel) formula is quite usefull for feetinch locomotives as well.
Evaporative area of a V2 is 225 square meter and grate 3.8.
The formula gives a usefull hourly steam rate of 13.3 ton that is 29300 lbs.
BR test report says 31000 lbs making 1990 indicated horsepower.
The Rugby test was on rollers with washed coal fresh from fridge and two young firemen

 

I've always been a bit puzzled by water circulation in boilers. It is occasionally mentioned in the discussion of boiler design, and power station boilers have circulation pumps, but after all, when water gets hot enough it boils - doesn't it?
I decided to try to do some research on the internet to find out what is going on, and came to the conclusion that it's not that simple.
In a boiler, the formation of steam bubbles only occurs on nucleation sites due to the roughness of the heated surfaces, and there seems to be a limit to which steam bubbles can break away from the surfaces.
In contrast superheated water (liquid water above the boiling temperature) can and does exist, and I'm coming to the conclusion that the convection of superheated water contributes a lot to the heat transfer within a boiler. Steam bubbles and convection of superheated water appear to work together. Steam bubbles rising to the surface help the convection, and the water movement helps to wash the bubbles from the surfaces.
Some water tube boilers also have "downcomers" which allow denser water (i.e. without bubbles) to return to the bottom of the boiler. Therefore, when superheated water reaches the steam surface, it loses its superheat by evaporation, and then returns to the bottom.
The steam bubbles themselves are also quite interesting. For very small bubbles (less than a millimeter diameter or so) the surface tension of the water means that the steam inside the bubble is at a higher pressure than the surrounding water, and therefore, for the bubble to be in thermal equilibrium with the surrounding water, the water must be superheated. It also follows that if you have steam bubbles of different sizes in water, all at the same temperature, the larger bubbles will tend to grow and the smaller ones will collapse. This idea also probably explains why boiling doesn't occur spontaneously.
I was wondering if there is any evidence that anyone had experimented with downcomers in a conventional locomotive type boiler. In a tapered boiler there would be space at the wide end between the tube bank and the barrel for pipes or passages. Around the firebox the stays would get in the way, but there might be a bit of space in the waterspace in the rear corners at the backhead end. although obviously you wouldn't want to block flow around the firebox.
I've read Churchwards paper on water circulation (wider firebox legs etc.) but there is no mention of downcomers. A lot of research into boiling also seems to have occurred in the nuclear industry in the 1950's (and ongoing) which is obviously after much steam locomotive development came to an end.

 

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