Gas fired engines

[duncan webster 1Participant @duncanwebster1]

So use a standard gas bottle. The only bespoke thing is then the burner assembly, which is very small volume. I really can't see why a gas fired loco is different in principle to a camping stove, so why are people saying they are banned? Is there any actual evidence of this? The most dangerous thing with a gas bottle is if they are involved in a fire and overheat. This causes them to burst causing a BLEVE (boiling liquid expanding vapour explosion). Quite spectacular, but difficult to see the relevance to operating model steam as long as you keep the gas tank away from the source of heat.

[BazyleParticipant @bazyle]

A new one on me – Marty burners thanks for that.

I wonder if making the slots at an angle to create a vortex would help.

[noel shelleyParticipant @noelshelley55608]

Following on from Duncans post !

Since gas (butane & it's mixes) is readily available off the shelf in tin cans one can reasonably assume that it's storage and transport is considered safe ? The vapour pressure of butane is quite low and below freezing is zero, ask any one who has tried to use a butane room heater on a very cold day ! As has been said a 5" loco needs a lot of heat so tin cans would not last long. Gaz do/did heavy steel bottles as small as 0.5Kg and also a 1.5Kg. The thread is a standard metric hydraulic/gas size and for high pressure piping ( to 4000psi) hydraulic hose with a perforated outer case. BSP O ring fittings. All the above are commercially available, but since the regulator fits on the bottle there need be NO high pressures involved, depending on the burner type the redoced pressure will be low. Soapy water will show leaks and you will be outside. Baring idiots trying for a DARWIN award, I fail to see where the problem is ?

In frosty weather I opened a damaged Gaz canister and poured out the gas – an oilly liquid, into a glass jar ! It was still there the following day – it was to cold to boil off ! Noel.

Having looked at the Marty Burners, The burner unit in a domestic gas boiler cut down to suit the fire box in question would seem perfect, give or take tuning. I have already put one aside with this idea in mind. Noel

Edited By noel shelley on 18/03/2023 10:33:48

Edited By noel shelley on 18/03/2023 10:42:21

[duncan webster 1Participant @duncanwebster1]

The firebox on the loco I'd like to gas fire is 5.875"*2.625" grate, 15.5 sq.in. If I assume 25 lbs/sq.ft/hr (which is I think a reasonable estimate of the coal actually burnt in 5"g loco working hard) this is 34,800 BTU/hr. Marty burners are 8000 BTU/hr, so I'd need 3. The site linked to above (which is excellent) recommends the burners be 1" from a wall, and 2" centres, but the 2" can be reduced if you intermesh the flames. This means I can actually get 3 in my firebox. Why haven't I done it instead of pontificating? Well the regulator is seized, and so wants sorting, and too many other unfinished projects, and I'm hoping someone else will do it and show us the way (probably the real reason). Phillip's grate is massive compared to mine, nearly 3 times as big, and the firebox is probably a lot taller, mine is 4.375 grate to crown

Fitting things which are essentially round shapes into a rectangular framework is possibly not the best, Noel's suggestion of a longitudinal tube with transverse slits might well work better. I'm off to ring my tame plumber to get him to recover a central heating burner when he next does a boiler swap.

The next problem is the gas bottle cooling due to gas evaporation. The model boat guys get over this by feeding liquid gas from the tank to a vapouriser above the flame, but in my view you might as well burn paraffin if you've got a vapouriser, much easier to handle than gas.

I've collected loads of info about gas burners from old copies of ME if anyone is interested send me a pm

Edited By duncan webster on 18/03/2023 12:14:24

[JellyParticipant @jelly]

Posted by duncan webster on 18/03/2023 09:19:14:

So use a standard gas bottle. The only bespoke thing is then the burner assembly, which is very small volume. I really can't see why a gas fired loco is different in principle to a camping stove, so why are people saying they are banned? Is there any actual evidence of this?

I am mystified too, I even went to look at the primary legislation and it's clear that all model trains/traction engines, would be out of scope of the gas regulations and treated the same as other boilers.

There's some complications with steam-plant type model boilers, but nothing that can't be worked around with basic awareness.

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One wonders if it like the situation with welded and non-copper boilers, where they're already permitted but actually finding someone within the model engineering community who would test one under those rules is hard causing them to be viewed with suspicion and generate misconceptions.

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As JasonB points out you can already do pretty much whatever you want as long as it's safe, if you're willing to submit to a true independent inspection against the relevant regulations, (and crucially pay that inspector).

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To Hoppers about Aus, in the UK we have some of the most non-restrictive [whilst still existent] safety regulations in the world, but it does put the onus on you to prove that whatever you decided to do is in fact safe, which is often a complication as it takes resources and documentation.

(The UK approach should be contrasted with only being allowed to do prescribed things which the government has decided will be safe).

[Martin Johnson 1Participant @martinjohnson1]

"Fitting things which are essentially round shapes into a rectangular framework is possibly not the best, Noel's suggestion of a longitudinal tube with transverse slits might well work better. I'm off to ring my tame plumber to get him to recover a central heating burner when he next does a boiler swap."

Thinking back to LBSC's designs for parafin firing, after a venturi it was a longitudinal tube with transverse slits. Substitute gas for vaporised parafjn and thè job is done.

BTW, Duncan I think you are a bit light with 25 lbs/sqft/hr for 5" gauge. 40 would be a better design value based on my analysis of IMLECS.

Martin

[JasonBModerator @jasonb]

So would the boiler need a redesign to get the best out of gas firing? There won't be much in the way of radiant heat going to the firebox sides and different flue tubes may be more efficient but until someone does the design and gets it approved that will be another stumbling block

[Michael GilliganParticipant @michaelgilligan61133]

Forgive the intrusion please …

This, which crosses the Flintshire countryside kinda puts things in perspective

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Not sure what pressure it runs at … but that’s a lot of gas !

MichaelG.

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Edit: __ and maybe coming soon:

https://www.dailypost.co.uk/news/north-wales-news/energy-boss-how-firm-refill-21573018

Edited By Michael Gilligan on 18/03/2023 14:02:19

[Harry WilkesParticipant @harrywilkes58467]

Fellow club member ran his two Folwer ploughing engines on gas he had no problem with his engines or gas bottle getting tested or insured. The only drawback was running the engines on the rally field they struggled however when he ran them static they performed OK The small sized cylinder was stored in the riding trolley.

[duncan webster 1Participant @duncanwebster1]

Posted by Martin Johnson 1 on 18/03/2023 12:

​​​​​​……..

BTW, Duncan I think you are a bit light with 25 lbs/sqft/hr for 5" gauge. 40 would be a better design value based on my analysis of IMLECS.

Martin

I meant burnt usefully, not ejected unburnt down the tubes or into the ash pan.

[phillip gardinerParticipant @phillipgardiner]

In Australia or at least in NSW all of the fitting must be approved by the AGA and you must have a blowback valve also a thermocouple in the instalation.

[Martin Johnson 1Participant @martinjohnson1]

Duncan, I know you will know this but for the benefit of others who might be looking at gas firing in the larger scales there are a few obstacles to overcome:

1. You don't get the full benefit of the "calorific value". That is because CV is based on cooling all the products to ambient, which would condense out the water vapour and reclaim the latent heat. Gases like propane and butane have roughly 3 times more hydrogen by weight than "coal", so that loss of CV becomes a lot more important. That is why we all have condensing boilers at home. (Actually I have a heat pump, but thats another story)
2. Burning a nice blue flame is efficient combustion, but it doesn't radiate so the firebox surfaces are not working as they would with nice bright flames and a yellow hot bed of carbon. That is quite a loss in heat transfer.
3. Tubes in coal burning models are actually sized to prevent blockage with ash etc. (No I don't believe Jim Ewins or Keiller when it comes to boiler tubes!) So for an optimum gas fired boiler you would really want smaller tubes and more of them.
4. Coal is very forgiving of air control, whereas with gas or oil you need more active control of the air. I think a bank of Marty burners are likely to be running with significant excess air drawn up between the burners, which is dropping the combustion gas temperature and making for poor heat transfer again. Those of us with silver hair remember the fun and games the Festiniog had when changing to oil firing. So some experimentation with dampers and ashpans is likely to be the order of the day.

That is not to say it won't work and can't be done, but as Harry Wilkes' post shows, it can give performance problems. So I would not want to set too low a design target knowing some of the problems that lay ahead. 40 lbs/sqft/hour is pretty much mid range of IMLECS that I have analysed for 5" gauge engines.

Martin

[duncan webster 1Participant @duncanwebster1]

I said it wasn't going to be easy! If using the Marty burners I'd be tempted to have a plate with 3 big holes to restrict secondary air so it has to pass through the flame, but I think the central heating boiler approach is more promising. Is it worth trying a thin stainless divider down the fire tubes to increase the Reynolds number? This has been reported as beneficial when people have taken superheater elements out, just down the big flues, but with gas and so no cinders it might help in small tubes. I'd still have elements in the fkylues of course, but again the lack of radiant heat might affect superheat as well. You're beginning to see why I haven't started, I anticipate problems, and I've got enough projects on. For a start I'd be happy if it just pulled me and perhaps one passenger, I think getting the high output you can get with coal is a long term project.

[noel shelleyParticipant @noelshelley55608]

For give me gentlemen IF I have misunderstood BUT 40lb/sqft/hour from IMLEC ? Even big 5" locos were recording under 2Lb for 30 mins, so given a grate of 20Sq/in 25 or 30 I would fall for but 40? Getting the heat out of the flame would require a fire box crown with a load of 1/8"(3mm) copper rivets, say 1/2"long to be soldered in to draw the heat into the copper and possibly shorter fire box legs, even may be a dry back. It's one thing to convert and make the best of the job but if one was starting from scratch one would NOT use a design for coal firing. My thoughts would be half the heat would be drawn by the "crown sheet" a little from the fire box and the rest via small diameter firetubes and thought given to superheat. The crown sheet, think chip shop gas frier, or old fashioned cast iron boiler ! The boiler could even be easier to make, a thick crown sheet and smaller flat surfaces. Yes Jason ! The bottom of the fire box would have a little ventilation, but most would be top air.  This thread has enough ideas to make a superb gas boiler between us ! Noel

Edited By noel shelley on 19/03/2023 19:58:47

[JasonBModerator @jasonb]

Not sure you would want a "firebox" of any sort if starting from scratch. Why not three large flues with cross tubes down the length of the boiler and a big burner for each. Much like a big version of the horizontal boilers we se used in model boats, this sort of thing at 3, 6 and 9 o'clock. Burners where the firebox would have been and a dummy backhead for the fittings.

[phillip gardinerParticipant @phillipgardiner]

I have fitted an arch in the firebox and also fitted turbulators in the tubes to slow the flow and get the maximum heat and find that you need the blower turned up to help draw the flame so you get a nice blue flame and a complete burn it is a bit of a learning curve ,but you do not have any ash plus you wont have to clean the tubes constantly ,i also run a separate carriage dedicated for the gas bottle only.

[duncan webster 1Participant @duncanwebster1]

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.

[HopperParticipant @hopper]

Posted by JasonB on 19/03/2023 20:11:29:

Not sure you would want a "firebox" of any sort if starting from scratch. Why not three large flues with cross tubes down the length of the boiler and a big burner for each. Much like a big version of the horizontal boilers we se used in model boats, this sort of thing at 3, 6 and 9 o'clock. Burners where the firebox would have been and a dummy backhead for the fittings.

+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

[Martin Johnson 1Participant @martinjohnson1]

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

[JellyParticipant @jelly]

Posted by Martin Johnson 1 on 19/03/2023 16:50:10:

Burning a nice blue flame is efficient combustion, but it doesn't radiate so the firebox surfaces are not working as they would with nice bright flames and a yellow hot bed of carbon. That is quite a loss in heat transfer.

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

[phillip gardinerParticipant @phillipgardiner]

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.

[duncan webster 1Participant @duncanwebster1]

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

[Stuart Smith 5Participant @stuartsmith5]

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

**LINK**

[JellyParticipant @jelly]

Posted by duncan webster on 20/03/2023 22:21:26:

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.

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.

[JasonBModerator @jasonb]

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

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