Gas fired engines

I reckon you do not want to get the heat out of the flame, you want enough space for complete combustion, and then enough heat transfer surface and high gas velocity to get the heat out of the hot flue gas. Taking heat from the flame only increases the volume required for combustion, and if flames actually get inside small tubes they will be extinguished leading to incomplete combustion (and headaches for the driver who will be inhaling CO). You can see why I've not set off down this road in reality, enough half finished jobs.

+1. On full sized oil and gas-fired boilers, the furnace or at least large diameter furnace tube extends the full length of the boiler to allow full combustion without getting flame impingement on the metal surfaces. Then followed by one or two passes through banks of smaller fire tubes to get best heat transfer.

If it comes down to running model locos on gas, building from scratch might be better off, and certainly more efficient, to follow typical modern industrial practice.

For even better efficiency, but with much added complexity in the construction, you could make the rear firebox a wet back like a Scotch marine boiler. But probably the gain would not be worth the complexity of construction on a model.

Edited By Hopper on 20/03/2023 06:27:36

Edited By Hopper on 20/03/2023 06:33:29

Noel,

I think you are doubting my numbers on coal consumption. Happy to share my spreadsheet of IMLEC results. Pm me and we can exchange e mails.

Jasonb & Hopper,

If starting from zip, then I think you are probably right. Usually needs some refractory at the end of the furnace tubes as well. I had conversions or trying to keep a full size outline in mind.

Philip Gardiner,

Good point that draught will almost certainly increase, so more mods needed to blast pipe & chimney. Turbulators might well work, but I think they increase path length rather than Reynolds number. I can also see that an arch increases flame length and improves combustion.

I think we all agree it is quite a problem to get the best out of gas.

Martin

Whilst I see what you're getting at, as the statement sort-of holds for the temperatures in a model boiler of an existing conventional design..

It's not strictly true and ignoring the consequences of that statement only being true within certain conditions (bounds if you will) kind of obfuscates the fundamental thermo-fluids of how you overcome the limitation for people less familiar with the topic at hand.

Most triatomic (Carbon Dioxide, Water, etc.) gasses do have an emissivity profile which results in IR (radiative heat) emission above a given temperature, even though they're not luminous (i.e. don't emit in the visible spectrum).

The rub is that this emission occurs when the gas is held uniformly at higher temperatures, so any boiler design which allows for rapid cooling of the flue-gasses will not allow the radiative heat transfer to predominate and thus effectively wastes that heat transfer opportunity forcing the boiler to rely on conduction and convection.

The result is that a very different boiler design is required, to take the example of the supercritical steam generator design which is typical of modern power-boilers (as opposed to heating boilers) the basic design is always a water-tube or finned heat exchanger.

An example of a typical arrangement is shown in the below figure, and you can see that the design is optimised to give maximal residence time at high temperature for the flue-gasses to emit radiatively, before any convective/conductive heat transfer begins, and the heating of "streams" of working fluid occurs in the order which to maximises overall heat transfer, rather than a convenient co-current or counter-current flow relative to the flue gasses.

Crucially whether it's a water-tube/finned power-boiler or fire-tube heating-boiler (akin to oil-fired marine boilers), the upshot of not having luminous char from a fuel source to help maintain radiative heat-transfer in non-ideal geometries/conditions, is requiring much greater consideration of overall path length, and ensuring the tubes and furnace are sized correctly at each point in that path to achieve the most efficient heat transfer from the flue gas.

Which of course is a completely different way of thinking about boiler design to traditional locomotive designs, which will feel alien to a lot of people who are used to a conventional locomotive "once-through" design...

Although perhaps less so for people who have faithfully reproduced more modern loco boilers which used unequally sized tubes getting larger as they go higher in the boiler for similar reasons, as engineers learned to optimise the combustion of the volatile component of solid fuels.

Edited By Jelly on 20/03/2023 17:20:43

Converting model locomotives to run on propane is really not that difficult all i changed was remove the grate and ash pan and replaced them with the burners fitted in the old ash pan ,i did not change anything in the smokebox . The gas lines were made large enough so you do not get freezing of the pipes , this system is used in the USA they seem to be less regulated than us and they have massive burners in there settups.

I'd guess that Jelly's diagram is for a pulverised coal or oil fired boiler, most stations which burn gas do the burning in a gas turbine and then pass the exhaust of that into a steam boiler (Combined Cycle). Both oil and coal produce lots of radiant heat. Even then if the space full of gas is massive, you can get radiant heat from the gas, BUT in our sizes we do not have thick films of gas and so radiant heat transfer is much reduced. This was described much more clearly in Martin's article in ME, April 2018 et seq

Phillip, your grate, and presumably the firebox, tubes etc are massive by UK 5"g standards, so even if it's not producing as much heat as you get from coal, it's probably still capable of a lot of work. That's not to say it can't be done, in ME 7 Feb 2003, Mr CHJones described his gas fired Rob Roy (3.5"g) and Metro (5"g?), which by his account were successful.

Finally, the model gas turbine chaps bur a huge amount of paraffin in a small space, but they do it at higher pressure. If anyone out there knows how they do it tell us now.

Edited By duncan webster on 20/03/2023 22:23:16

Just found this video of a gas fired loco in the USA.

**LINK**

Yeah it's a generic diagram lifted from my old uni notes, which dealt with boiler design in a sort of fuel-agnostic way.

In practice I have seen both gas-fired power boilers and thermal oxideisers (with heat recovery) which use a comparable construction, usually because they're fuelled by an off-gas or vapour stream which is readily available but would be unsuitable for a gas turbine due to composition/contamination.

The challenge of available space to maintain sufficient film thickness is obviously a problem with scaling which is difficult to overcome when modelling.

Clayton still make steam generators, just need a small one of these unless you have 1:1 models like he does.

Yes the Yanks tend to go on and get stuff done while the rest of us are still discussing why it could not be done and then making red tape to prevent it as much as possible. There is a lot to be said for their "can do" mindset.

Duncan, Gas Turbines - Kurt Schrecklings book on the subject tells how it's done and how to do it ! In small scale. Noel

In the 11/9/1958 edition of ME there is a design for a 5” gauge water tube boiler that it is suggested could work with the ‘tube with slits’ type of burner described above.

Food for thought?

Perhaps best redesign the asbestos lining to the casing though!

Edited By Nick Clarke 3 on 21/03/2023 15:11:57

I've sent a pm to NC3 but no response. Anyone else got a copy of the article referred to above?

Noel, is this the book to which you refer?

yes Duncan it is ! An interesting read. Noel

Oh dear, hand in pocket time again. Fortunately it's my birthday soon.

There seem two threads on one here, and neither are BSP.

One about burner design, the other on tests and certificates. I don't know gas-burner design, but it does seem well covered above.

The matter of insurance seems to have elicited misunderstandings, though. In the UK, club insurance normally relies on following the MELG system, whose requirements for gas-firing cover gas-tanks below 250ml capacity and made specifically for the models. It does not cover the fittings, pipes and burners - though obviously it would be sensible to bubble-test them for leaks while you are at it.

There is no "insurance" reason for not gas-firing locomotives in scales larger than the "garden gauges"; or equivalent road-steam and stationary-engine miniatures.

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Nothing to do with Gas Safe, either. Those cover building installations - and most Gas Safe registrees are trained in mains gas to homes, but less likely also to hold the separate approval for LPG installations, such as serving a house from a big tank. (You can't blame them - few would need it anyway, but despite many overlapping areas the Powers-That-Be decreed accreditation for either type of gas supply needs a separate course, which is very costly for self-employed gas-fitters by the course fees plus the week's lost earnings.)

The objections against gas-firing locomotives of 3.5" and larger gauges, would be technical ones and most are covered by other contributors here. In short:

- The likely fuel consumption may entail a regular gas cylinder and regulator that can only be hidden in a driving-trolley, not a scale tender.

- The cylinder may have to be upright, not on its side.

- Simply raising steam from cold, and maintaining pressure between runs, would use a lot of the gas available.

- The burner would need be designed for the task, especially in a boiler designed for burning coal.

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The MELG scheme covers only specially-constructed LPG tanks, own-made or bought as part of commercially-built locomotives, up to 250ml volume and refilled from standard cartridges such as you'd buy for a blowlamp. It explains that non-refillable cartridges used directly are neither tested nor banned; but does not say anything about the larger, standard, refillable cylinders of the types exchanged in a caravan-site shop.

The Boiler Test Code 2018 (orange print on white covers; the only edition to be used at present) probably assumes only small models with little tanks refilled from blowlamp cartridges. It does not say we cannot use such cartridges directly. It does not say we cannot gas-fire bigger engines from standard, exchangeable, trade-refilled cylinders.

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Finally it is not a matter of "club rules". If your club is affiliated to the MELG (via its own federal body), its Club Rule is simple: test boilers and gas-tanks to the current MELG rules, instructions and certificates. Not to some local idea of dubious provenance or use.

A boiler inspector can decline to test a boiler he or she feels beyond personal knowledge, and the book covers this diplomatic problem; but deviating from the regulations may take the lid off a big tin of wriggly Lumbricidae.

- Weakening or changing the requirements may invalidate the test hence the club's insurance - potentially a very serious matter even if some claim has nothing to do with boilers.

- On the other hand, gold-plating with invented "extras" or specious refusals is likely only to give your club very aggrieved owners unable to run perfectly sound models; and that could harm the club's reputation.

(Specious: Known boiler-test refusal "reasons" have included thicker (not thinner!) firebox plate, a rocking-grate, and feed check-valve locations differing from original drawing!)