Chimney Design

[chiptim]

I'm fabricating the chimney for my loco from 3 separate pieces. The main shaft is being turned from mild steel round and is slightly tapered. I can turn the outside taper easy enough but would prefer to bore out parallel.

Does a parallel sided chimney draw the fire as well as a tapered chimney or is the external taper purely ascetic?

Regards Tim

[Blue Peter]

Railways weren't around when ascetics started their self denial. The locomotive designer J.G. Robinson, however, appreciated the aesthetics of a good looking chimney as he said the wrong looking chimney was like a man without his hat on. I understand the chimney doesn't actually draw the fire, it's the exhaust blast in the Stephenson front end that pulls the hot gases through the horizontal flues and tubes. The blower is used to keep the fire pulling when stationary so the tapered chimney is probably purely aesthetical.

Regard
Egon Peter

[chrisb]

A taper chimney can be of great advantage in full size steam as advocated by L D Porta with the "Lempor" blast pipe set up. How much of an advantage this would be on a miniature I could not say but there are those who have dabbled with the idea.

There is some info here - home.ca.inter.net/~mguy/index.htm

Chris...

[maunsell]

Some years ago Don Young did an article on the experiments at Swindon by S O Ell on smokebox design. About that time (1968 ish) I had completed one of the first Simplex locomotives having tried to keep pace with Martin Evans description. It did not have the steaming capability that I expected so I redesigned the smokebox including the chimney internal tapers to be in line with the Don Young article. The internal tapers in the chimney are rather like an injector cone and in my view important.
The other factor was to increase the depth of the ashpan under the grate so as to give adequate air flow to the fire.
All these alterations dramatically improved the locomotive performance, so yes the external shape is according to taste, but the internal profile is very much an analytical study.

[drjohn]

Hi maunsell.

Yes, the Martin Evans' original Simplex design did leave a lot to be desired.

Never bothered to change the chimney as the venturi at the bottom, if the blast nozzle and blower are properly aligned, is adequate, and the aesthetics are fine to my eye for such a loco - in fact, on my latest Simplex, the draw from the blast nozzle pulls holes in my charcoal fire

The fire was constipated if you used Martin's ashpan design. On my first Simplex, I just removed the whole front of the ashpan to let some air in, and everything was hunky-dory. Here, on my own track with a concrete bed, I don't even bother with an ashpan.

DJ

[GeorgeRay]

Hi in answer to the original question. I think it is true to say that the outside form of the chimney often doesn't reflect the inside form. In earlier designs a tapered outside had a tapered inside. A parrallel outside did not necessarily mean a parallel inside but often did. Laterly asthetics came into it and the outside shape was designed to look right and the inside was tapered. Remembering that on the more modern designs with short chimneys there was a lot inside the smoke box and this was tapered once past the initial curved petticoat.

[maunsell]

Locomotive chimneys actually had a chimney liner inside suspended on four long round bars threaded at the top with nuts and locknuts to attach them to the top of the smokebox. We aligned them up with a bar across the top of the chimney and a string line with a weight at the bottom which lined up with the centre of the blast pipe. This alignment was always looked upon as critical to the smokebox performance and therefore steaming.
As I said earlier the chimney liner internal design, blast pipe height, and depth of chimney liner into the smokebox was a rather involved study.
In practice for our purposes I have always relied on the old third-third and third principle for the height of the blast pipe, gap between the blast pipe and petticoat pipe to the top of the smokebox.
The air flow into the grate is also important and the American philosophy was to provide a 20% of the grate area as an opening in the ashpan while in the UK 15% was looked upon as being adequate.
No doubt today with computer modelling there would be a much more scientific approach than the perhaps empirical thoughts of yesterday.
The outside of the chimney might mildly be to the designers taste, but of course there would not want to be a surfeit of cast iron in the chimney production or increased cost and locomotive weight.

[steam4ian]

G'day Tim

I modified my little loco by make in new upper chimney.

I have 15 sq" of grate with a 22mm diameter chimney throat. The chimney was about 7" long with no taper, truth is it was a piece of 1" copper water pipe. I could see the chimney was choking the blast; easy if the loco is priming.

The 1:3 to 1: 6 rule has been discussed here before (or was it another group?). The top of my 1" pipe gave much, much less that 1:6. I built a new tapered section opening out to about 32mm at the top for about 12 deg taper.

The outcome is that to my assessment the loco steamed much better and showed up the next round of shot comings. I didn't alter the blast pipe from 1/4" for 1-1/4" cylinders.

My assessment is apply the 1:3 to 1:6 rule and taper the upper part of the stack at about 8-12 deg. I think my stack has a littel too much flair.

Regards
Ian

[drjohn]

My goodness, for an aborigine standing on his head, that was awffy erudite Ian - I didn't understand it, I I suspect many others didn't as well.

Regards

DJ

[Gary L]

I found this (lengthy) discussion helpful, especially the contributions by Jos Koopmans:
www.chaski.com/homemachinist/viewtopic.php?t=82682&postdays=0&postorder=asc&start=0
In particular the S Ell ratios (Swindon, c.1953) allow you to dimension everything with confidence except the blastpipe nozzle. (Ell's formula produces a nozzle that is correct for full size but too big for miniature locos, according to Koopmans.) Fortunately the nozzle is relatively easy to experiment with.

Hope this helps

Gary

[steam4ian]

G'day DrJ

Was that a compliment? ;D ;D

I must admit my message was somewhat telegraphic.

You will note that I made a couple of contributions to the discussion to which Gary gave the link.

One correction I sought to provide to that discussion was that the 1:3 to 1:6 rule originated in steam antiquity and not by Greenly. There is some practical basis for this relationship between chimney throat and chimney top heights from the blast pipe orifice.

Later research does not refute the old rules of thumb, just gives a more precise outcome. Using the rule of thumb will give a working result but it may not be the best.

The logic in the outward taper in the chimney is to minimise the drag on the gases caused by the chimney walls and to allow the combined blast and flue gas to slow down as they exit the chimney. This maximises transfer of energy from the blast to the flue gas.

Why is this important? Maximising blast nozzle area lowers back pressure hence more cylinder power. Optimising flue gas flow gives the best firing. Note I did not say maximising as too much draft can lift a fire as burners of charcoal may have observed. What we want is the minimum back pressure for the best draft through the fire.

From my reading of full size performance, getting the blast right has transformed many locos from pigs to pullers

Regards
Ian

[alanstepney]

Jos Koopmans is THE expert on draughting, and is probably the only person to have been awarded a Doctorate this century for work on steam engines.

He has written one article that is on my website, plus others, and is very helpful should anyone wish to contact him with specific problems.

[steam4ian]

G'day Alan.

I agree with you about Jos on both counts. My few dealings with him have been most cordial and very helpful.

Regards
Ian

[JDEng]

Mar 7, 2010 22:41:18 GMT alanstepney said:

Jos Koopmans is THE expert on draughting, and is probably the only person to have been awarded a Doctorate this century for work on steam engines.

He has written one article that is on my website, plus others, and is very helpful should anyone wish to contact him with specific problems.

"The Fire Burns Much Better......" by J J G Koopmans.

I got a copy from Camden Miniatures, I don't know whether they still do it.

[chiptim]

Wow, just looked at this thread having been away for the weekend. Loads of useful info! Thanks guys.
Now how do I bore an internal taper on my lathe...
Tim

[steam4ian]

G'day Tim

Regarding boring, use the compound (top) slide with the angle set at the desided taper.

Alternative, make a liner; that's often what the big boys had.

My tapered chimney was a combination of a bored base which included the collar over the stub chimney and a rolled flare. The latter I made from a piece of 32mm (1.5") copper pipe. I cut a V notch up the pipe and annealed it. I then used Jubilee Clamps* to pull the notch closed and silver soldered the seem. A bit of bashing over a morse taper, a brass trim ring on the top, some high temperature black paint and the guys at the club were impressed. (*Jubilee Clamps = hose clamps)

BTW, I was reading an old book by Goss which described his research demonstrating the effectiveness of the tapered chimney over a parallel sided stack. One other bit of logic, the taper acts like the diffusing cone on the delivery side of an injector or ejector transfering velocity to pressure.

Regards
Ian

[chiptim]

Thanks Ian.

I'll post some photos when done. Also of the boiler as it's nearing completion.

Regards Tim.

[robmort]

I, for one, would still be interested in the photos you mentioned.

For making a flared petticoat pipe, and also for chimney bases, it puzzles me why people here don't use spinning techniques more often; it's easy and quick and saves a lot of machining.

This is how I did one such; I used copper but steel could also be used.

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