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Routing steam engine exhaust up boiler chimney.

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Hi all,

I'm hoping I can get a bit of advice on the size of exhaust pipe to route exhaust back to my boiler and give adequate draught for coal firing.

The current position is I have a 1/4 x 40 fitting on the boiler (a 5" dia vertical fire tube design) which accepts a 5/32 exhaust pipe from my engines and also the same size pipe part way up the chimney - the exit hole in the pipe has a soldered fitting on so the outlet is about 2mm dia. At the moment that seems fine for the largest of my engines (a Stuart no 4) but yesterday I tried the same set up with the boiler powering a Stuart no1 and it clearly chokes the engine. It affected the running of the engine to the point it slowed considerably and ran with a 'lumpy' action, so that it became necessary to allow the engine to vent into atmosphere.

I'd like to run the Stuart no1 again shortly as my boiler easily copes with it so I'm looking at increasing the dia of the exhaust but still want to maintain adequate draught to run on coal, can anyone please suggest the size of pipework and the size of the final outlet in the chimney?

Any help greatly appreciated, thanks  :)

Hi Gas Mantle,

May I suggest first confirming that the engine will run ok with that 5/32 exhaust, use a similar length, or a bit longer than the one fitted to the boiler.

Then I would concentrate on that fitting at the pipe end.  If you can make up one that is screwed on instead of soldering you can more easily experiment with the nozzle size.

It is that nozzle which helps the flue gases out the stack to create the necessary draft, so itís diameter and exact location are quite important to how well it helps the boiler, and at the same time the hole diameter must not restrict the engine too much.

The exhaust tube cannot be too large for the engine, (apart from appearance), but if the tube or the nozzle at the end is too small, that will choke the engine.  I must admit that I aim for 1/4 inch exhaust tubing on my engines, but we have discussed that one before.  The smaller diameter is easier to make neat.


john mills:
the no1 has 1/2"exhaust pipe  and the no4 3/8"  the exhaust nozzel will need to be closer to these sizes
The 5/32 would be fine for a steam blower.Look at the arrangement on some of the say 5"gauge locomotives.
It will depend on how much load is on the engine ,how high the pressure is in the cylinder.the blast nozzle might be more like 5/16 "dia.

Folks......could we go a few steps backwards

We are I assume talking 'OD tube' and not 'NB pipe' If I am correct, formed 90 degree elbows and 180 degree copper tube fittings in both short and long radius, are available to suit 1/4" OD x 0.014" wall K&S telescopic brass tube, whereas they will be difficult or not impossible  :facepalm: to find the same fittings to suit 5/32 " OD tube

Due to an apparent steam back pressure scenario, I have increased & revised the exhaust design to incorporate the same 1/4" OD brass tubing to overcome my issue

I chose an external chimney and with the lower temperatures used soft solder. If the exhaust is destined to be internal or within the chimney, naturally the higher flue gases may require silver soldering of the tube & fittings

As MJM suggests, a temporary screwed fitting on the exit point of the exhaust would allow trialing of different orifice sizes etc


Just for an idea of size this is the blast nozzle on my 2" traction engine 1.375" bore x 2" stroke, the outlet is 5/32" bore and the pipe serving it is 7/16" OD, base of chimney 1.275" ID and the boiler is 4.75" so similar to yours. The traction engine is designed to do work.

The nozzel can be unscrewed to alter size if needed and the small pipe at the size is the blower.

Having an external exhaust pipe as shown above will do nothing to help draw the fire and you would need to watch that a long vertical stack like that did not cause condensation as the exhaust steam condenses.


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