Blast-pipe - Chimney Proportions

Can anyone advise on draughting proportions for a steam-wagon chimney please?

I've just used a lot of electricity turning some very rough cast-iron into the chimney choke for my 4" steam-wagon, using the scanty information in Martin Evans' book on miniature locomotives, then found I'd got it all wrong...

I am having to design the beast from extremely limited information, and have just revised the choke by the information in Norm Norton's article in the latest ME.... about 2 days too late!

That though is for a 5"g 'Britannia' with simple-expansion cylinders, very short exhaust pipes and a very short chimney (about 3.5" including that in the smoke-box)

Not a compound with 2" bore LP (the guiding diameter I used) and a very long exhaust from the mid-mounted engine. The steam finding the smoke-box will probably be wet fluff about needing to be helped up the spout.

Now, most TE engines have high chimneys often diverging upwards, and helped by natural draught; and it's not clear if they are fitted with a choke above the side-entry exhaust and blast nozzle.

My wagon matches the original photos: the exhaust pipe reaches an elbow under the bottom of the smoke-box, and the chimney is a rather ungainly, parallel stove-pipe. Close to scale, I am using a 20" long piece of rolled steel tube, originally a blackboard roller*, of about 2.7" bore.

The throat re-calculated from Norm's series gives an outlet about 1.2" dia at the maximum taper length (3" I can bore on my lathe. I doubt my Warco '3-in-1' rolls will form a tapered tube that small.

SO......

What is normal practice, possibly on undertype lorries rather than overtypes and traction-engines?

Does anyone have advice, guiding formulae etc., for these, or would using a short (loco style) chimney surmounted by a tall stove-pipe as I have concocted so far, work?

I am not sure of any effect of the sudden transition from choke outlet to pipe, more than twice the diameter.

(I have no existing drawings for this project.)

'

*Blackboard roller. A piece of the "Board" itself, thick rubber sheet on a coarse fabric backing, proved ideal for the canopy roof. I inverted it so the weave shows, and painted it very thinly to maintain the painted-canvas effect I think reasonably so-1908.

My recollection of an aveling full size was that The blast nozzle pointed straight up the tapered chimney, no petty coat. From what you have said there may not be much blast. To make a tapered chimney start well over length, cut, hammer and then weld. when it looks as what you want, cut to length. Drafting was the fall down of many otherwise good engines. If it looks OK and works well, you've cracked it. Good luck, Noel

Thankyou Noel.

I wondered if that was so - having the blast-pipe within a tall, tapered chimney would produce some draw even without the venturi.

My wagon's chimney is not tapered, though. All the photos of the original show just a simple rolled tube with a bit of beading round the top. The exhaust-pipe does clearly enter the bottom of the smoke-box, via an ordinary screwed elbow, but without works drawings (the company collapsed c.1919) or surviving examples, it's not possible to know the internal details.

So I envisaged the best approach is to making a choke rather like that for a locomotive, in the base of the tube.

Edited By Nigel Graham 2 on 14/03/2021 11:31:05

Exhaust blast and it's associated back pressure is a can of worms, unless you have a supercomputer - and lots of time. Compounding will get more out of the steam, but leave less for drawing the fire. All good fun. Noel

As Noel says, draughting can be a can of worms.

Over the years various people have devoted years, sometimes their entire careers, to developing draughting for firetube boilers.

The results have varied widely! The subject is influenced by the steam passages, pressure of the exhaust steam, valve events and the proportions of the blast nozzle and the chimney. What suits a simple engine is not likely to suit a compound, nor a slide valve engine a piston valve engine, without the vagaries of lap, lead and valve travel.

Also what is ideal for one coal on an engine may not suit another coal. Locomotive trials showed that a engine that performed well on hard Welsh Coal did not work so well on soft Yorkshire coal; and vice versa..

What you are aiming to do is to produce a depression in the smokebox to draw, air through the fire, so that it burns well, and the hot gases through the fire tubes. This is usually accomplished by sending a jet of steam into a convergent / divergent chimney.

The size, and position, of the blast nozzle should relate to the sizes and proportions of the chimney. A small nozzle might produce a fierce blast (Sometimes too fierce, making the engine a fire thrower ) but at the expense of creating excess back pressure which reduces the engine's power.

There have been various more complicated forms of blastpipe nozzle, with the object of producing a strong draught but with minimal back pressure, such as the Giesel or the Kylchap. Have never heard of either of these being applied to a Road Engine, rather than a locomotive.

On a road engine the chimney will be longer than that on a locomotive, (But some of the locomotive chimney is hidden inside the smokebox, as will be some of the road engine..)

Since you can't scale nature tghe proportions of a ful scale engine may not work so well whens caled down

A 5 " Full Scale nozzle scaled down to 1/12 size, (under 7/16" ) will have 1/144 th of the cross sectional area, but the air viscosity will remain the same so will not produce the same effects.

A can of worms indeed.

You could spend the rest of your life seeking the perfect nozzle / chimney proportions!

Howard

Locomotives require a shorter chimney due to the speed they travel at - a 40mph+ wind blowing over the top has a lot more pull than 3mph blowing over a traction engine chimney.

If you have kept the blast nozzle a screw on type you can experiment with both length up to the venturi and also the hole size.

Nigel, as Dave Halford says, make an end fitting for the exhaust, such as a BSP elbow, and then you can use BSP plugs with a hole in as the nozzle, and make one about 3/8" to 7/16", and see how it goes, then adjust as required. Open the hole in the plug up a little until it keeps the fire pulling when you ride around. I keep a couple in my tool box so can change them if I want the fire a bit fiercer.

Chris Gunn

Nigel, as Dave Halford says, make an end fitting for the exhaust, such as a BSP elbow, and then you can use BSP plugs with a hole in as the nozzle, and make one about 3/8" to 7/16", and see how it goes, then adjust as required. Open the hole in the plug up a little until it keeps the fire pulling when you ride around. I keep a couple in my tool box so can change them if I want the fire a bit fiercer.

Chris Gunn

Thank you!

For the moment I've diverted to modifying the flywheel, while I work out what to do next about the chimney.

Howard -

Norm Norton does draw on rules developed by several model-engineers from Henry Greenly onwards for designing model locomotives, and shows these proportions (not the same as scale sizes) are not far from full-size practice. Interestingly he notes that Greenly's original cone pattern still applies.

So it was numbers for miniature engines I was after. I know this is one area where Nature can't be scaled!

A tall chimney on a traction-engine creates some natural draught partly offset the engine having no slipstream effect (the wind itself would add to it). Indeed, some owners of the larger-scale miniature engines raise steam by natural draught, induced by an extension tube temporarily plugged into the chimney top.

I don't think the manufacturers of the full-size road steam vehicles bothered over-much about fine details of draughting. I have some reprints of reference-books originally written for the commercial owners of these vehicles, revealing an impressive range of patented boiler designs, but very little about smoke-boxes and blast-pipes. The builders and owners did care about the boilers' efficiencies, but perhaps to a lesser extent than the railway companies. The nearest I found was a generic section of a locomotive-type boiler with the cone of exhaust steam emitted from the blast nozzle reaching the chimney wall at the top of the chimney.

'

Dave -

I do have that in mind, making the assembly adjustable. It will be a long time before there will be an engine putting exhaust steam in the chimney, but for the moment I am largely concentrating on finishing the water and steam side to an operating state.

As it seems road-steam vehicles did not worry too much about such things, perhaps my best bet will be to make a new inner chimney as if for a loco, but this time guided by Mr. Norton's article, and conceal it within the saddle holding the "stove-pipe" that represents the original.

I knew the operating conditions and overall designs are so different between road and rail, so thought I'd better find what is more appropriate for the former.

'

The manufacturers did not help the thing, in full-size. The engine is vertical, between the crew seats, and though the cylinders are lagged they were fed with wet steam by a prominent and prominently un-lagged loop of ordinary iron pipe with a large globe-valve by the (full-size) driver's elbow, as regulator. From the other end of the compound engine the exhaust-pipe appears to have been another length of iron pipe running below the footplate and bunker to the smoke-box perhaps eight feet away.

My replica's boiler barrel diameter is quite a bit less than that of the smoke-box, with an adaptor ring wide enough for bulkhead unions passing the blower pipe and if I fit it, the superheater flow and return pipes, tucked up against the boiler itself, under the cladding. I could run the exhaust the same way, shortening it slightly and reducing condensation, but I will fit a small drain-cock at its lowest point.

(I ordered the boiler from Western Steam, and with non-original super-heater flues to give me the option.)

Chris -

This is odd but your post appeared after I had replied to Dave and Howard, yet is in its right pace in the list! I've noticed this happen elsewhere on this forum.

Anyway, thank you - yes I will make the blast-pipe and nozzle adjustable but I was trying to establish the best way to design the chimney. All the literature seems to concentrate on locomotives, not traction-engines, and I don't know if there are any significant differences other than road steam vehicles having very tall chimneys.

Even reprints of the manuals written for the commercial owners and drivers of the full-size originals do not show much, and I am coming to the conclusion that those engines merely had the blast and blower nozzles pointing up the middle of a long and usually, but not universally, diverging stove-pipe - no petticoat pipe and no choke. The chimney height helped by natural draught. The engine builders often advised that the blower, or "steam jet" as some called it, should be used sparingly, and on some traction-engines the blower valve was by the chimney, not accessible from the man-stand.

However, these things do not scale easily. For example, a full-size railway locomotive can be brought into steam by natural-draught, though slowly and rather messily - but that does not work in miniature. I am trying to establish normal practice for model road-steam vehicles, though, in which the draughting may be proportionally more important than on full-size.

The generally taller chimney saddle on a TE compared to a Loco tends to taper inwards towards the top flange so will act in a similar way to a petticoat. The nozzle being placed at the joint before the chimney tapers outwards, though not all chimneys were tapered. The height of the stack on a TE goes a long way to creating the draught as they moved slowly or worked stationary in the case of belt work or ploughing engines hence the common model practice of using an extension piece to raise steam.

Aha! Thank you Jason!

I have managed to find my copy of Maurice Kelly's The Overtype Steam Wagon, and among its many reproduced works drawings are one or two showing the nozzle in the chimney, at the joint. What they don't show though, is the shape of the saddle in detail - these were GA not component drawings.

The advantage the traction-engine and overtype wagon has over mine is of very short steam paths and so probably better exhaust steam conditions for producing the draw.

I wonder if I've been over-thinking the problem in trying to be sure "wet fluff" will draw up the fire. I may be better keep more or less what I have made so far, but moving the blast nozzle up to the chimney, and trying it. In fact in all the drawings of air-ejectors I have seen, the driving nozzle is at the entrance or the choke-point of the venturi - as is the steam-cone in an injector.

It will still be a long time (it's been far too long already!) before this wagon has an engine but I'm working mainly on the steam-making rather than steam-using for the moment, and once that side is ready, I can experiment with it.

I wouldn't get too worked up about the heat loss from the exhaust pipe. Compared with the heat flux along it you won't lose much. Still a good idea to lag it of course.

My narrow gauge loco has the blower pipe protruding into the very tall chimney with a nozzle pointing straight up on the centre line. When working hard it acts like a tuned pipe and howls. Sorting it out is on the to-do list, but so is a lot of other stuff

Thank you Duncan.

Tuned exhaust eh? A "go-faster" loco!

That's a thought though - lagging the exhaust-pipe.

The cylinders are high above the frame, but the exhaust pipe has to go below the chassis rail top. I'll arrange the lowest point as the elbow into the smoke-box, a good four inches below the chassis top anyway, and fit that with a small drain-cock so the first few puffs don't flood the steel smoke-box.

If I have this right, the blast gets it moving and steam comes out the top,

Then the blast stops suddenly but the motion continues drawing air through the fire tubes and smoke comes out the top.

Surely the perfect length and diameter of pipe depends on the frequency of the puff,

Didn't the old boys put in different lengths side by side to keep it drawing fast and slow?

What we are looking for is essentially the same process as the jet pump used in chemistry labs. In this a jet discharges into a venturi and entrains the surrounding air to reduce pressure locally.

With regard to a loco "howling", in a model, the petticoat pipe and chimney are so short that the resonant frequency is likely to be so high as to be inaudible to the human ear. A 5" long pipe is likely, depending temperature, to have a resonant frequency of about 27 Kz, by my calculations. (memory permitting! ).

Another possibility might be that the frequency of the exhaust from the cylinders coincides with the natural frequency of the boiler tubes? But on a 5" gauge loco it is still likely to be pretty high, given that the speed of sound in cool air is circa 1100 fps, rising to about 1700 fps at the temperatures possible in the gases exiting the fire tubes.

Firing a full size loco, with the firedoor open, the exhaust beats are quite audible in the cab, so the gases passing through the tubes are travelling below the speed of sound.

So how about for adding some confusion?

Howard

Hi Nigel,

In case this is of help, I've found the following guidelines to be reliable to get you in the right ballpark for a single-chimney design for burning coal. The ratios were distilled from proportions published in ME (and other sources) for railway locomotive designs over the years and where the credibility of the author was high and/or the results were plausibly verified. 

I modified the drafting on my 3" Allchin based on these ratios with good success, it will now make steam on local coal that wouldn't burn with the original front-end arrangement. it might be worth a pencil-and-paper exercise to see if you can get these proportions to work within your constraints.

- Blast nozzle diameter (in inches) = 0.005 x grate area (in square inches)
- Choke diameter (in inches) = 0.06 x grate area (in square inches)
- Height of petticoat choke = 1:3 cone (with the tip of the cone below the blast nozzle, such that the cone touches the inside diameter of the nozzle)
- Top of chimney = 1:6 cone (with the tip of the cone below the blast nozzle top as above)

I can send you a simple Excel spreadsheet that calculates the above based on your grate area, if that's of help. PM me if so.

Best regards

Steve

Edited By stephen goodbody on 22/03/2021 19:26:49

Howard -

Intriguing exercise in acoustics, but I am not sure about your sound-speed with respect to temperature. The speed increases with density, so would it not fall in gases that are both very hot, and then slightly rarified as they are drawn through the tubes into the roomy smoke-box?

'

Stephen -

Thank for those formulae! yes, I will take up your kind offer, please!

Equations based on the boiler's dimensions rather than cylinder are likely to be a lot more realistic on my wagon, with its rather low-pressure compound engine and long exhaust pipe.

The problem I might find is trying to apply these to a wide stovepipe nearly 3" diameter and parallel for all of its nearly 21" height. matching the manufacturer's advertising photographs, which are all that survive of the original vehicles. Maybe those designers did not trouble themselves too much with such niceties as proper draughting!

The 1:3 cone is easy enough but the 1:6 cone, which the diagrams all show touches the blast-nozzle rim, might put the blast-nozzle far too low. These geometries seem contradictory unless applied to the very short tube that is a locomotive's chimney, or to determining the chimney's total height and outlet diameter.

'

Some quick sums now, from your formulae...

Grate area: 19 sq ins. (Approx. 5" dia grate - it might be 4.25 but I forget if 5" is the cylindrical firebox's internal or external size. 

'

Blast nozzle: 0.095 ins. diameter. Eh? Never! Have I mis-read your instructions?

If you mean Grate Area X 0.005 gives the nozzle area, then we obtain a diameter of 0.39 ins.

That is close to the sums based on cylinder diameter, admittedly for simple-expansion engines, but it seems normal practice to make the nozzle easily exchangeable with ones of different diameters to "tune" the individual engine.

'

Choke: 1.14 ins .

Seems likely despite the formidable mass of tubes in the tube-plate looking as if demanding something bigger. I set the boiler's overall proportions by scaling ancient photos as best as I could, and from the GA in the original design's patent specification; and I fear it may be a bit under-scale. The tubes though were all by Western Steam's arithmetic, and I trust that.

By comparison, the chimney on my 7.25" g. 0-4-0 loco has a choke of about 1.12" expanding to 1.62" over perhaps 4" height - very roughly as it is not easy to measure.

'

I tried to calculate the heights from those 1:3 and 1:6 rules, but could not see how. It's not just the two diameters above, times 3 and 6, because the two cones cross on the blast-nozzle edge and their apices are somewhere inside the blast pipe. I would have to draw it.

Also, the 1:6 ratio for the chimney height would probably not work directly with my engine's proportions, but might apply to a short inner chimney inside the lower end of the "stovepipe".

I have assembled the choke as made, for now; but can re-bore it to take a new one as an insert, or replace it completely, as it is a cast-iron insert itself, held by a couple of screws inside the fabricated-steel saddle.

Edited By Nigel Graham 2 on 22/03/2021 22:08:29

Hi Nigel,

No problem, I’ll send you the spreadsheet when I’m at my computer tomorrow.

Regarding your post above, yes you should multiply the grate area by 0.005. A starting point of 3/8” diameter for the blast nozzle sounds about right to me, I apologize if my post wasn’t clear. Hopefully the spreadsheet will make the 1:3 and 1:6 blast exit cone distances and diameters clearer as well.

As an aside, I installed a liner inside the chimney of my Allchin in order to reduce the chimney’s internal diameter. That way the chimney looks externally correct but the drafting is much improved because, in addition to the other modifications, the blast always fills the chimney, even when the engine is working hard.

Best regards,

Steve

Nigel G 2

I was only quoting the figures for the speed of sound from Smiths book on "Design of Intake and Exhaust Systems" which deals with tuning Intake and Exhaust systems for ram, which obviously depends upon setting the gas column into resonance. He quoted 1100 fps for ambient and 1700 for exhaust gases on an internal combustion engines, in the days when virtually all I C engines were naturally aspirated.

But nature being fair, what works at one speed, will do the reverse at half the speed. And harmonics creep in to complicate matters.

From limited personal experience, his formulae do appear to hold good.

Howard

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