Blast Pipe Orifice

[mott]

Hi, guys. I picked up a spreadsheet on calculating the size of the blast pipe nozzle from the Alan Stepney site and ran the 14XX through it. It came out with a figure of 0.35" which seems quite a large hole. My model, as supplied, has an orifice of 0.1562"(5/32"). Interesting that there should be such a big difference and I don't know quite what to make of it. Has any current 14XX runners made any changes to the nozzle size. When using the spreadsheet I used cylinder diam of 1.25", stroke of 2.25", 6" wheel diam.and 90lbs boiler pressure.
www.alanstepney.info/blast.xls.Mott

[greasemonkey]

HI All
A smaller blast pipe orifice will pull the fire harder and make the model easier to steam under light loads. If you work the engine hard you may find that it draws the fire to much and the larger daimeter may then be the better bet.
Andy

[dscott]

Dear Blastpipers,

On the full sized engines there was fitted a jumper blast nozzle. This increaced the size as the power went up, a bit like a supercharger or turbocharger, very difficult in our sizes but could be experimented with.

David Scott. 517 tank designer at the moment.

[Phil Sutton]

The "jumper " blast pipe opens whe the loco is working heavily in full gear and privides an additional passage to the exhaust steam,thus increasing the nozzle area.This I think reduces the "pull"on the fire and so tends to prevent it from being lifted through the tubes,saving coal(and the poor fireman ;D)

Phil