Re: Types of stacks and why chosen

> Why did straight stacks replace the old diamond
> stacks?

Smokeboxes were lengthened and the spark arresting apparatus was put there instead of the stack. The diamond contained a spark screen, so it was no longer needed.

> Why was a Cabbage or Rushton stack better at
> dealing with sparks from wood than a diamond
> stack?

Aside from some internal differences in baffling and gas flow, one of the big differences was in the size of the hopper to contain spend embers. Larger designs held more sparks, and needed less frequent emptying. They could also have a larger surface area, meaning that the screen could be finer and still pass a sufficient quantity of exhaust gas.

> What about Radley-Hunter stacks?
> The huge ballon stacks.

See above. The really large stacks were used more on wood-fired engines, because wood tends to produce more sparks than coal. More sparks means more need for ember storage.

> Finally, why are some straight, or shotgun stacks
> curved slightly or fitted with a cap at the top?

A straight pipe is easier to manufacture. A tapered nozzle affects how the gas flows through it, which has a major effect on the fire and how well the engine steams.

If you've ever held a loose piece of stove pipe, you know how flimsy it can be. A rolled edge or a cap makes the top a lot more rigid. It's also decorative, which was the main purpose once cast stacks became the norm.

Kenneth Rickman
Researching and modeling the Danville & Western Ry.
[southern-railway.railfan.net]