My *gas fired* boiler

[joanlluch]

I hesitated a lot to create this thread, because of reasons that do not have anything to do with model engineering, or steam technology. So, I just want to show my current approach for my boiler, and leave the place open to any comments or suggestions that could eventually help me to make improvements on it.

In case somebody has the temptation to post a *non-constructive* criticism or a sarcastic comment against me, my approach, or my build please refrain from doing so. I should not be in the need to make a mention about this, but unfortunately such a very deplorable behaviour has happened yet again very recently, which never add anything good to the forums.

I very much appreciate this forum for all the knowledgeable members that regularly post here, who are definitely helpful. So all constructive criticism is welcome because the making of my boiler has yet to begin, and it is now the right time to make any design corrections.

I have chosen to open separate threads for questions rather than posting them in my own build thread, because this will ultimately keep the build thread cleaner, simpler, easier to follow, and to the point.

So this topic is about the boiler:

These are my 3D plans of (several parts) of my boiler.

BoilerAssembly-Tubs3 by joan lluch, on Flickr


BoilerAssembly-Tubs2 by joan lluch, on Flickr


BoilerAssembly-Tubs by joan lluch, on Flickr


I chose a Round Top boiler over the Alfred Belpaire type because it's easier to make and because virtually all full scale boilers used in my country had the top round design, so that shape feels more natural to me. I understand that the Alfred Bellpaire type boiler has some steaming advantages, though.

These are the main characteristics of my boiler:

- The regulator is a standard full-port ball valve.

- Other valves are commercially available needle regulating valves. They are for feeding the smokebox blower, and a donkey pump. I am not putting injectors. Valves are not scale because I do not see the point in doing so, and I have a small disability in my right hand that only went worse over the years, and I worry about not being able to handle too small valves in the future. So please, no discussions about the size of the valve handles.

- The boiler is gas fired.

- The firebox incorporates water pipes that are right in the path of the gas burners.

- Water pipes should draw and convey water from the bottom of the boiler to the top of it, by natural convection.

- Smoke pipes are bigger diameter than usual, and shorter than usual for this size of boiler, particularly they are 20mm diameter. This is an attempt to help natural -or semi natural- flow of combustion gases towards the smokebox. Forced draught has less sense in a gas fired boiler because of the different behaviour of gas burners with respect to a bed of coal. Ideally, the boiler should be able to run with no or very small forced draught from the smokebox. That's the goal. Any inefficiency derived from this approach should be compensated (or balanced) by adding turbulators in the pipes. This adjustment can be done at any time after the boiler is made by inserting said turbulators from the smokebox side.

- Two radiant, coaxial, superheaters are installed. Efficiency of them is increased by insulating the return tube inside them, so the already hot-dry steam is not cooled back down as it travels out of the boiler through the inner tube of the superheater. 

- The regulator intake pipe is located in the longitudinal middle of the boiler, and as high as possible. I will have a small cavity at that location -like a small dome- which keeps the intake pipe as high as possible. Steam can enter to this cavity through two relatively big holes which are a bit separated apart in an attempt to minimise priming as both holes should not be flooded with water at the same time.

- About the height of the firebox crown, I attempted to get a compromise between it being as low as possible with as much as possible smoke pipes area. So the roof of the firebox is somewhere higher than the middle line of the boiler, but still (I hope) allowing enough steam space and evaporative water surface in normal conditions.

- There's no ash pan grate. Instead the firebox is completely surrounded by a 10mm thick layer of water.

- Structural stays for the boiler barrel and firebox are not yet drawn, so they are missing from the drawings above. 

Any feedback would be appreciated. Specially if there's something remarkably wrong that I can correct before making it. Thanks.

[steam4ian]

joan.

I suggest you correspond with our USA cousins regarding gas fired boilers as it is relatively common over there.

From my little experience of full size stationary boilers I suggest that the loco firebox is not the best shape for gas firing. Gas tends to have a very long flame with much less radiant heat which will not suit a firebox designed for solid fuel. At model sizes much of the heat input to the boiler occurs in the firebox principally through radiant heat off the fire bed.

I suggest you also look at having two large flues each with a gas burner much as used in model boat practice. At the end of the extent of the flame you can either have smaller tubes or Galloway (cross) tubes.
If you stick with the locomotive type boiler you will need to have a combustion chamber on front of the firebox to get ample flame path.

I think you have too many circulation tubes which may cool the flame before it has reached maximum temperature and so reduce the radiant heat transfer.

Rule of thumb (to be contended by others) keep the firebox crown no higher than 5/8ths of the boiler diameter.

[joanlluch]

Hi Ian,

I have a total space of 49.6mm from the top of the firebox to the inner wall of the boiler barrel above it. The barrel at this point has a inner diameter of 168.3mm, so if I understand your rule of thumb correctly, this would give (168.3-49.6)/168.3 = 0.705 , which is higher than your suggested 5/8 = 0.625. So my boiler has possibly a slightly too high firebox. Still the picture on the Modelworks Britannia looks worse than that, so I assume that I still have something acceptable in this regard.

I have investigated what they use In the USA, and I found that their boilers are basically standard ones which grate is replaced by a number of gas burners. These burners do not have a very large flame, as they resemble small kitchen burners. These boilers do not usually fit water tubes either. They work, of course, but I wonder whether this arrangement produces any powerful boiler at all. I have yet to see a USA locomotive being driven at the speed they sometimes do in the UK. Gas burners used in the US will certainly heat the boiler and make some steam, but how sustainable is that I do not know. It comes to me that these burners may fall short of the power required to fully steam a locomotive at high speed and load. I may be totally wrong of course.

I see your point about the two large flue tubes with a burner each. This is used in small 45mm gauge locos with great success. But then, how efficient is that?. I can imagine that temperature of combustion gases at the smokebox is much higher than on coal fired boilers, thus hurting efficiency. In such small locos the amount of butane/propane required to produce enough steam is not important, because consumption will be relatively small after all. Again, I may be wrong on that assumption.

So, I was searching for something in the middle.

You may have a point about the length of the flame and the excessive number of water tubes. In an earlier design, I had less water tubes (this can still be recovered) and a firebox that somewhat entered the cylindrical zone of the boiler, thus giving a much longer path for the flames before entering the smoke tubes. But at some time I rejected that design because the area available for the tubes was reduced. Without diverging too much from the current design, I think I have some space in the cabin to place the burners in a further back position, which should provide more space for the flame to develop. I have tested the burner in two ways. I have placed it at one end of a 50mm diameter, 600mm long tube and found that the flame sustains itself through the tube. I have also tried to leave it in an open space and distort the flame by placing objects (such as pipes) in the middle of it. It looks to me that the hotter area of the flame just changes its position or length, but not its temperature. The max temperature of the flame is theoretically related only to the amount of combustion air you allow to enter, provided the combustion is always complete, and it looks like so in practice as well. I also think that if you manage to lower the temperature of the flame, that's because you are removing heat from it, which is the ultimate purpose of it, so maybe doing so is not that bad after all. Provided in all cases that combustion is complete and you don't end distorting the flame that much that part of the combustible is not burnt.

In any case, your points are worth to consider and I shall make some testing arrangements.

 
 

[joanlluch]

Hi Adam.

Please, this was amply discussed before, and everybody knows that it is not in my interest to use coal. I don't want to go again this route as it goes nowhere. Please understand that people needs, goals and interests may be different. I wonder what would you say to the ones that built and drive a battery powered electric loco?. I don't think they don't enjoy themselves, quite the contrary. Also, I do not live in your Country, not even in a English speaking one, so please don't assume anything, because you probably just don't have a clue. If you can't understand such simple facts, please ignore me for once. I do that for you unless you directly reply to me. Your post was not asked for, as it is mostly to express your personal choice, and as a corollary to dismiss my interests. Please stay at helping me with any technical challenges of my boiler design if you can.

About your comment about the the boiler lacking air entrance at the bottom of the firebox, this is an intended feature. All air will enter through tubes that surround the burners. The type of burners I use are Sievert type. Part of the combustion air enters in them and mix to the gas, and there's a secondary entrance of air around the burner. If I left the bottom of the boiler open, then the temperature in the firebox will drop below what is required. Some gas fired locos in the US suffer from this defect, they ultimately have to close that air intake with a metal flange that just allows a small quantity of air entering there. In coal fired locos, it is the bed of coal which prevents too much air entering there. Combustion air intake is controlled by the blast pipe. In gas burners combustion air intake is controlled by the design of the gas nozzle and other design aspects of the burner.

[4fbuilder]

Hello Joan,

I have to say your boiler is an interesting concept. A couple of pointers that may be of some use, firstly there were a couple of articles in the Model Engineer in the seventies by a German model engineer who built a gas fired German pacific also I think he also wrote a series of articles in the American Live Steam magazine.

I would also draw your attention to an article in the Model Engineer by the late Jim Ewins on boiler performance, albeit coal fired, also he always maintained 2/3 rds of the boilers heat transfer was achieved in the firebox with only a 1/3rd by way of the tubes, interesting.

Having spent a lifetime involved in heat transfer by all manner of fuels I can safely say with gas firing overall boiler efficiency is generally below that of oil and certainly solid fuel, principly as someone else has pointed out, due to the reduction in the radiated heat component.

If you look back over the years in the Model Engineer and the IMLEC competion you can see the overall thermal efficiency of the locomotives is pretty poor but it may give you some idea of the demands placed on the boiler whilst operating under load in reasonably controlled conditions.

Regards,

Bob

[joanlluch]

Hi Bob,

Thanks for your input.

Unfortunately, I have no easy access to old ME magazines, so anything that you could find and would share with me would be very appreciated.

I am fully aware that efficiency for a gas fired boiler will be lower than a coal fired one. In fact I have already implied so in one of my previous posts. As you point out this is basically because it is more difficult to use the heat from a gas flame. Making the boiler work is alone a challenge that I face, so making it work reasonably well is yet more challenging. But It's an interesting one to me.

I have found many resources on the subject on the internet, but nothing really conclusive for 5" locomotives. I think that most designs simply won't work. Americans use propane for their locos, they make gas fired 7.5 gauge locos, but I always thought that all of them are underpowered. You'll never see one of them running at speed or carrying a lot of load. Since Americans drive their cars ridiculously slow, I suppose they do not expect anything different out of their locos. So as you point out, the key must be in looking at what the Germans have done, although I wish I could read German.... On the other side, there are plenty of tubular boiler designs for 45mm gauge locos, which seem to work just fine.

About the energy requirements of a steam loco, I created some time ago a complete spreadsheet covering it. The results are indeed very surprising as a huge amount of power is required at the burner to just feed the cylinders with enough steam for a particular speed and cut-off. The required heat input is in the order of tens of kilowatts to provide steam at full speed and long cut-off. And this is before even considering any boiler efficiency.

So thanks again for your reply and as said any additional pointers or ideas are of course very appreciated.

[steam4ian]

Joan.

I don't know where you got your information about American locos and operating speeds from but you are not correct.

USA steam locos were the biggest in the world and on appropriate routes regularly travelled at 100mph (160kph). Their models are much the same. Cylinder size and boiler pressure set the power and they certainly had the biggest cylinders. The 7.5"G locos I have seen in the USA were quite large and powerful and capable of pulling significant loads.

Gas firing in needed on the West Coast on account of the fire danger because may of their lines run through forests. Coal apparently is not readily available there but can be obtained on the East Coast.

As I said look to American gas burning experience; there will be plenty of people willing to tell you what to do (in usual American fashion).

Have you considered preheating your combustion air and vaporising your gas so the flame is more intense?

As for 45mm G loco many burner designs have a stainless steel mesh on the flame path to radiate heat to the flue surface.

Regards
Ian

[jma1009]

hi joan,

i think your nicholson type cross/themic syphon tubes in the firebox are excellent. i would fit 4 not 2. i wouldnt fit them myself to a silver soldered copper boiler, but yours will be of stainless.

i do not see any need for a water space under the firebox, and will preclude you burning coal if you fancy this change later on. it will also i suspect cause welding up the firebox rather difficult. i have never seen this design detail before. heat travels upwards, so your below burner firebox water space will achieve very little. so far as i am aware there is no fullsize prototype with this feature to the firebox.

i am not so sure why propane would behave differently when compared to oil firing. it is not something i know anything about or have any experience of. i cannot myself see why propane would not create as much radiant heat in the firebox as oil or coal. it would be nice to think your loco could be converted to run on coal at some stage, especially if you received some decent Welsh stuff!

cheers,
julian

[joanlluch]

Oct 12, 2015 23:12:25 GMT steam4ian said:

Joan.

I don't know where you got your information about American locos and operating speeds from but you are not correct.

USA steam locos were the biggest in the world and on appropriate routes regularly travelled at 100mph (160kph). Their models are much the same. Cylinder size and boiler pressure set the power and they certainly had the biggest cylinders. The 7.5"G locos I have seen in the USA were quite large and powerful and capable of pulling significant loads.

Gas firing in needed on the West Coast on account of the fire danger because may of their lines run through forests. Coal apparently is not readily available there but can be obtained on the East Coast.

As I said look to American gas burning experience; there will be plenty of people willing to tell you what to do (in usual American fashion).

Have you considered preheating your combustion air and vaporising your gas so the flame is more intense?

As for 45mm G loco many burner designs have a stainless steel mesh on the flame path to radiate heat to the flue surface.

Regards
Ian

Hi Ian,

Sorry, maybe my post was misleading. I have refereed all the time to 7.5 gauge locos, not full size ones. I can not put your post in doubt because I don't know, but all vids I have seen show guys sit on top of their locos like having a slow walk with them. One thing is engine power and another one is whether the boiler will sustain prolonged high steam demand that is required when pulling load at speed. You don't need a lot of steam to get power at low speed. But you need a lot more to sustain it at hight speed. I can be wrong about the Americans but videos made by the British often show their locos a lot more free running even at considerable speeds and loads. 

The forums I have been looking at are "the home machinist". Any other recommendation?

I will take butane gas from a 2.5 Kg 'campingaz' bottle, so the gas already gets out in gaseous form at a pressure around 2 to 4 bar depending on ambient/bottle temperature. One concern is that temperature (thus pressure) of the bottle will go down if consumption is high. If that happens I may place the bottle into contact with the tender water so it helps to keep the bottle warm. I expect that butane freezing is less of an issue in my country than in northern Scotland, to say.

Having a stainless mesh is a good idea to get some flame heat converted into radiant heat, but I wonder whether this may cause too much drop on the draught in the boiler. My understanding is that if you have the flame directly hitting one of the walls of the boiler, or the water tubes, most of the energy in the flame will be transferred. The ultimate goal is to achieve a relatively low temperature at the chimney, provided we use the less excess combustion air as possible. Much excess combustion air is counter productive because you are diluting/lowering the temperature of the flame, which is used to just heat the air. The output temperature for a given excess air and fuel input is an indirect measure of the fuel energy that is actually transferred to the water, regardless of whether the transfer was radiant, or convective.
 

[jma1009]

and no injectors! a working reliable injector with stainless cones and body would be IMHO the pinnacle of achievement in miniature! i know your water supply temperature in Spain is problematic but it is something i'd like to tackle. double skinned tender tank with insulation from sunlight effect?

donkey pumps are very good and sound good too but can be a bit of a pain to get to work ok. as Bob Youlden suggested elsewhere, you need to avoid condensation on the steam side, and keep the pump and the steam supply hot and the pump well lubricated.

dont worry too much about our American cousins, they run in full gear and have odd ideas. they need re-colonising and taught a lesson!

cheers,
julian

[ejparrott]

Ok, someone needs to go and look up train mountain.

Underpowered? Get your head out of your self made clouds. They run gas fired loco's because there is now a ban on solid fuel. They haul huge lengths of trains for a considerable distance. Some of them run at considerable speed. You need to get your facts right before making proclamations of our fellow model engineers.

[joanlluch]

Oct 13, 2015 0:03:42 GMT jma1009 said:

and no injectors! a working reliable injector with stainless cones and body would be IMHO the pinnacle of achievement in miniature! i know your water supply temperature in Spain is problematic but it is something i'd like to tackle. double skinned tender tank with insulation from sunlight effect?

donkey pumps are very good and sound good too but can be a bit of a pain to get to work ok. as Bob Youlden suggested elsewhere, you need to avoid condensation on the steam side, and keep the pump and the steam supply hot and the pump well lubricated.

dont worry too much about our American cousins, they run in full gear and have odd ideas. they need re-colonising and taught a lesson!

cheers,
julian

Hi Julian, After reading your recent thread on injectors I am open to propose some testing if you like. Actually, with the current CNC techniques and modern cutting materials, it would be very possible to make an injector entirely in Stainless Steel. 

My choice of using donkey pumps is merely based on what I have seen in my country. The general believing is that injectors are difficult to work out and people just avoid them. Most of the time they are not even considered. This causes that very few model engineers -if any- know anything about injectors. I suppose that's some sort of closed loop, such as this: "Injectors are believed to not work" ->so-> "nobody fits injectors" ->so-> "nobody knows how to make them work ->so-> "injectors are believed to not work".  In full size, donkey pumps -or any kind of steam powered mechanical pump- were fit as standard, so the presence of them in a miniature loco in my country does not totally distort the look of it when compared with full size ones that operated here.

The problem with hot water for miniature locos is not just that the tender gets hot, but the tank where it's stored at the clubs. Tanks are usually exposed to the ambient/sun at an elevated level and are usually painted in black to resemble old full size ones. Water is keep there for days and it inevitably heats sometimes to skin burning temperatures (above 45 C) due to sun radiation. That's just 10 C above peak temperature in Summer, so that figure in the tank is reached very quick. This is not bad for locos because they are fed with pre-heated water (to say), and it's believed to be a welcome effect. In fact, I listened once a conversation about water tanks being preferably placed where they will receive more sun radiation. I'm sure that all this mix of circumstances contributed to the believing by local model engineers that injectors doesn't work. I think that they are right when referring to my country.

An interesting experiment that I would like to conduct at some time is to bring a British made mini loco -and its driver- to my club in the peak temperature days of Summer. That's the easiest way to determine whether all the hate of injectors that exist in my country is justified. I have seen a loco bought in the UK by a local person to get its injectors removed/replaced by a donkey pump. But I can't know whether this is to be attributed alone to the lack of experience of said person with injectors.

[joanlluch]

Oct 13, 2015 8:30:08 GMT ejparrott said:

Ok, someone needs to go and look up train mountain.

Underpowered? Get your head out of your self made clouds. They run gas fired loco's because there is now a ban on solid fuel. They haul huge lengths of trains for a considerable distance. Some of them run at considerable speed. You need to get your facts right before making proclamations of our fellow model engineers.

Hi Ed, That's excellent news. This indeed means that gas fired boilers can actually keep with the steam demand that is required in some circumstances as well as coal fired ones. So after all this turns to be a matter of getting the correct boiler design that works as intended. 

[jackrae]

Energy-wise you might be better looking at propane burning rather than butane unless availability is an issue.

[joanlluch]

Oct 13, 2015 12:46:34 GMT jackrae said:

Energy-wise you might be better looking at propane burning rather than butane unless availability is an issue.

You are right about propane advantages versus butane. I originally wanted to use propane, but it is only available in relativelly big bottles in Catalonia or Spain and tranfering it to small containers is apparently forbidden. Bottled propane is mostly intended for services companies or industrial uses in these areas.

On the other hand, I know propane is a lot more used in cold weather countries because it does not freeze until extremely low temperatures. It requires more pressure in the bottle, so mixtures of propane/butane are used as well to reduce that requirement.

In France, they have a sort of 5kg Propane bottles, which are readily availabe and easy to buy, but they are still too big for being carried on a tender.

I have even investigated the possibility to fill containers from petrol stations having automotive-gas, or even with natural gas at home. But apparently nothing of this is possible or allowed.

So the only remaining option around my area is campingaz type butane bottles. Replacement/refilling is relativelly cheap and that's what I will attempt to use.

[jackrae]

My experiences with my two domestic woodburning stoves suggests that the one with a water back boiler runs much cooler than that with insulated walls which suggests the 'wet' wall causes the fire to run much cooler and hence the stove is more prone to produce flue blockage problems.

If you could arrange for the boiler wall that the burner points towards has a thin sheet of white ceramic fitted, it would glow white hot and act as a radiant surface to heat the other firebox walls. Since combustion temperature is totally reliant upon air aspirated by the injected gas stream it is essential that you generate as hot a firebox as possible.

Maybe something like this :-
www.alibaba.com/product-detail/Made-in-china-infrared-honeycomb-ceramic_60198200880.html?spm=a2700.7724857.35.17.2riCcD&s=p

[joanlluch]

Hi Jack,

What you stated about your stove with a water back boiler is totally consistent with reality. The same observation explains why stainless steel boilers work just the same as cooper boilers specially considering that heat transfer figure for copper is 25 times that of stainless. It's simply because inner surfaces of stainless firebox walls are hotter than copper ones.

The ceramic piece you pointed out is actually a part of a ceramic burner. This is the working link:

www.alibaba.com/product-detail/Made-in-china-infrared-honeycomb-ceramic_60198200880.html

These kind of plates wont't work directly as burners because they don't generate enough power for a loco. It would be very easier to simply replace the grate of a standard boiler by one of these ceramic gas burner. But unfortunately this won't work.

I understand what you meant by using them just to hit the flame of a conventional gas burner (sievert type). That's possibly a very sensible idea and something I definitely will consider. Doing it properly possibly implies a major redesign of the firebox. One difficulty is how to replace the ceramic part when it's worn out. More food for thought. :-). So thanks for that.

[jma1009]

hi joan,

would you not re-consider a coal fired boiler?

Don Gordon used stainless mesh to create maximum radiant heat on his incredible gas fired boat boilers.

you could order a tonne of steam coal from wales and it would last you quite a few years!

cheers,
julian

[jma1009]

at your club house in lovely Spain you will have a mains water supply for the clubhouse to make tea and coffee etc and work the toilets.

this mains supply will feed the outside water tanks for loco use to which you refer. these tanks heat up as you have noted in the Spanish climate.

however, if your club fitted standpipes from the mains water supply for direct supply to tenders and tanks, then i see no reason why injectors should not work. the water supply should then be cool enough.

cheers,
julian

[steam4ian]

Joan

The data copied to the Superheater thread strongly supports the contention that most of the heat transfer is in the firebox. Considering the relative surface areas of the total tubes vs the firebox radiant heat has to be firebox walls the major heat transfer mechanism. this fact is supported by the design of power station type boilers down to package boiler size.

If you are gas firing why are you going down the conventional boiler route?

You seem well capable of lateral thinking so here is an idea.

Have you considered a Franco Crosti type boiler? This would allow a long flue or flues for the gas burner along the main barrel then fire tubes in the lower barrel. Because the loco is gas fired only a simple ejector would be needed to draw the flame though the boiler. No firebox would be needed and the boiler could be fabricated out of simple tubular sections.

Have you looked into how much gas you will need to burn and what size burner is required? If the loco is developing 1kW and the boiler/engine is 5% efficient then you will need a 20 kW burner as a minimum.

Ian