From "The Ultimate
Steam Pages," I find that a blast nozzle of half the
area pulls 70% of the draught on the fire, given the same amount
of back pressure. This would make sense:
Area = pi R².
Circumference = 2 pi R
Take a circle with an area of 2 pi, then.
R² is 2,
R is 1.414.
The half area, pi, means
R² is 1
R is 1. This R is about 70% of the R of the bigger circle.
Would this suggest, then, that the draught is pulled by the
circumference of the nozzle, while the back pressure is
determined by the area?
Bring this all down to the Mogul, in real terms.
The original blast nozzle of the Mogul was a straight bore,
19/64" diameter. We could not get any steam at all out of
her boiler on her first few runs. We had to keep the blower open
all the time to get any draught action on the fire. Seen in
section, it looked like this:
Note the very thin wall at
the threads. I was still able to back-bore it a tiny bit, with an
11/32" drill, without breaking into the threads.. That gave
it a total constriction at the top end of an entire 3/64",
only 46 thou. Still, the fire now roars when the engine was
pulling with the blower closed.
Most important on getting this constriction right is that the
length of the constriction must be less than its' diameter.
They say the stack diameter should be equal to, or up to 10% less than, the diameter of one cylinder, for a two-cylinder engine. The old Mogul had a 1½" stack, and 2" cylinders. Is she getting choked here, too? This will be upped to a 1-5/8" stack on the new smokebox. The trouble now is getting the cone right.
Given a larger stack diameter, this means the nozzle point
must be 9" below the top of the stack, or getting pretty
close to being INSIDE the smokebox tee.
Big Boy had this problem, too. On the prototype, twin stacks
needed to be so large in diameter that the exhaust nozzle would
have had to be 8" below the rail height in order to pull a
draught. The UP solution: multiple blast nozzles.
By spacing the nozzles out to the edge of that 1:6 cone, they got the same draught effects as having the nozzle low enough. Plus, the 1:3 cone starts higher up - at the level of the nozzles.
One more change: Add a second petticoat pipe to the inside of the smokebox, to allow the 1:3 cone to be held back. This raises the bottom of the stack more, and pulls a more even draught over the tubes.
But how do you make a multi-port exhaust nozzle small enough
to fit in a 1/8 scale locomotive? I took a piece of brass hex
stock, and put a small block under one jaw of the 3-jaw. Drilled
a 1/8" hole, turned the hex, and drilled again - 6 times.
The back end of the nozzle was then drilled out as it should be - to a good size diameter. (It doesn't matter what it is, as long as it is larger than the exhaust passages and all 6 holes are in it without constriction.) The thickness of the top is a shade under 1/8". I did not make any effort to square up the bottom of this bore - (I may regret this,) - it is 60º, just like the tip of the drill bit.
1/8 diameter = 1/16 radius. Pi R² gives 0.01227 square
inches, by 6 = 0.0736 square inch nozzle area. I am within 5% of
the old nozzle area, (0.0692,) but the perimiter is 2½ times
greater. I will tell you in the spring if this has worked out.