Ceramic Gas burners on Locos

Discussion in 'General Engine Discussion' started by doubletop, Sep 27, 2010.

  1. Sep 27, 2010 #1

    doubletop

    doubletop

    doubletop

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    I'm using a commercial ceramic gas burner on my Rob Roy loco for the first time and I am having trouble getting anything like a good steam pressure.

    http://www.homemodelenginemachinist.com/index.php?topic=9516.0

    [​IMG]


    I've worked out to start with a low flame to get things heated up before slowly opening the gas valve to get more heat. Should I have the fire door open or closed and should I be using the blower to get some air flowing through the system.

    With the fire door open I can have have flames shooting out the door, so I've closed it. I then open up the gas to the point there's probably to much gas and it flashes over and either goes out or I have a moment and shut down, just in case. Sometimes the burners ceramic block glows orange other times I'm just getting a blue flame. Is it the orange glow or blue flame I need.

    [​IMG]


    Any clues on how I should be doing this please?

    Pete
     
  2. Sep 27, 2010 #2

    lordedmond

    lordedmond

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    Pete

    I hate to say it but the burners are not big enough. most people just use open jets with out the ceramic bit, yes use the blower

    you will need enough heat to lift the safety valves with ease and keep them blowing off for at least 10 mins ( for the accumulation test ), note that your boiler is flued for coal not gas fired , ie the flues are to small

    a Rob Roy in steam on the track needs a good fire going to maintain its steam any way


    Sorry I cannot be more help , I have driven but not owned a Rob Roy , I did have a "William" which also have to be fired to the bottom of the door



    Stuart
     
  3. Sep 27, 2010 #3

    steamboatmodel

    steamboatmodel

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    Hi Pete,
    Ceramic burners generate radiant heat when they are glowing orange, and should produce the maximum amount of heat in that condition.
    Regards,
    Gerald.
     
  4. Sep 28, 2010 #4

    doubletop

    doubletop

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    Stuart

    I think you've confirmed my suspicions; this burner just isn't going to do it. I was having these concerns running in my garage, our club track is on the beach in sunny windy Wellington.

    [​IMG]

    So its out with the ceramic burner and re-install the ash-pan and grate. That saves me a bit of time I could have wasted trying in vain to get it to work. The guys in the club can show me how to fire it up.

    Gerald

    I had thought that may be the case it just wasn't consistent enough to rely on it. Any bit of a draft and and the flame was all over the place and at times more outside the firebox than in.

    It's gone

    Thanks guys

    Pete

     
  5. Sep 29, 2010 #5

    steamboatmodel

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    Hi Pete,
    Most of the ones using the Ceramic Gas burners that I know are in Gauge 1 Locos or in Model Boats. I think that the air supply has to be very well controlled and brought in with the burner. There have been a number of them around here (Toronto, Canada) that have gone to gas burners, but I haven't looked too close to there designs as they are too large for my models.
    Regards,
    Gerald.
     
  6. Oct 25, 2010 #6

    doubletop

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    I had given up on the ceramic burner idea and refitted the ash pan and grate when one of the club members gave me a call and put me in touch with a group of Kiwis who are successfully running their locos on propane. One of them another Rob Roy and one a large 5" gauge loco.

    I went round to Dave's place and we got the ceramic burner running and he was pretty happy with what he saw. Eventually we got it up to 80psi. The result of the session was a series of mods I had to do to improve things.

    I've been getting better at getting steam pressure up and keeping the whole lot alive. Yesterday I managed to get a cold engine, with a cold fill, up to 80psi in about 10 minutes. It was outdoors with a bit of a breeze going. So pretty encouraging.

    I'll post the pictures back on the Rob Roy thread when I get a bit of time this week.

    Pete
     
  7. Jan 3, 2011 #7

    doubletop

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    I got a PM from Tups (Cris) for more details on the replacement ceramic burner I have made. This one works and if I don’t forget to open the blower before I stop running it doesn’t go out.

    Some of this is a repeat from the Rob Roy thread but for completeness I’ll do it again here and hopefully by the end it will all make a bit of sense;

    The problem I had been having with running my loco successfully on gas was although I could get the engine up to pressure relatively easily maintaining it was a problem so I ended up going round the track in short runs followed by a recovery of pressure then on again. On the assumption that the more heat you can get into the engine the more steam you make and keep up the pressure. I have been experimenting with burners and learned something from the exercise. Some of it is blindingly obvious but it has helped me understand what is going on, or not as the case may be.

    The basics being.
    · To get more heat you need to burn more gas (I said it was blindingly obvious stuff)
    · The amount of gas you burn is a factor of jet size and jet inlet pressure.
    · You need the right balance of primary and secondary air to ensure complete combustion (its about a ratio 1:24 gas:air for propane (4%:96%)
    I made myself a test rig so I could see what was going on. Regulator on the gas bottle, Input pressure gauge and output pressure gauge after the gas tap

    Gas Control Rig

    [​IMG]




    The assumed problem with the ceramic burner was that it couldn’t get enough secondary air. If it is running on the electric blower its fine, on the steam blower it’s fine. But add more gas and the engine whistles like kettle as it tries to draw air through the holes on the firebox door. If the pressure is too high, and the blower is off, it flashes back with a huge pop or just goes out. If it goes out there's no indication as you can still hear the gas flowing. Not good

    My starting pint was the local guys who had given me a lot of advice based on the success they had with gas firing their loco’s.

    First was to put a baffle on the front of the burner to keep the flames in the firebox for a while rather than shoot straight down the fire tubes. This keeps the heat where you need it and stops you melting the paint on the smokebox. Believe me it works, I ran the loco once with it out and the smokebox paint job looks a bit sad now.

    Stainless Baffle on original burner

    [​IMG]


    Baffle in the firebox, note its at the fire tube end not the fire door end.

    [​IMG]


    (I am convinced this is a major contributor to success)


    The next was make a new burner I started with making a new burner based on some old magazine articles, the intention being that the gaps between the tubes would allow enough secondary air.

    [​IMG]

    This didn’t work too well and made a couple of versions but they just didn’t give the heat and there was lots of un-burnt gas.

    New Burner

    I then turned to resolving the secondary air issue with the ceramic burner I had seen a version of a burner with tubes running through it. I wasn't about to hack my burner about in case I was wrong so set about making a copy

    All the parts alongside the original

    [​IMG]


    New and old

    [​IMG]



    New burner with the lid off

    [​IMG]

    Installed in the loco you can see the baffle in the front

    [​IMG]



    Make sure that the burner is as high up in the firebox as the jet holder will allow e.g hard against the foundation ring.

    The dimensions for mine is 26mm high, 42mm wide 85mm long. This just fits in the firebox allowing for the ends of the stays and a little bit of room for the baffle

    Through the middle are 2 x 12mm tubes for the necessary secondary air . To hold up the ceramic there are 2 x 15mm sleeves around the air tubes and strips of copper soldered on the inside edges of the body.

    The jet holder is 38mm long 12mm dia brass drilled 8mm for the jet assembly, It is cross drilled with 6 x 5.6mm holes set at about 60degrees. The jet is a #15 held in with 6BA screw. The jet holder is mounted as low as it can be in the burner body. I'm sure it can be longer, as your installation would require.

    My original was stainless steel, this one is copper, as that’s what I had. I’m sure brass would be just fine. (either way16-20swg)

    I got my ceramic from Bruce Engineering (Polymodel) in the UK

    http://www.pollymodelengineering.co.uk/sections/bruce-engineering/index.asp

    It cuts really easily; I did the holes for secondary air pipes by using an offcut and twisting it slowly as it worked through the material.

    See these drawings from Sandy C his burner gives a good view of a similar sort of burner albeit smaller and no secondary air holes. However it give you a good idea on sizes. Most of my burners have had some basis on this burner from Sandy and there does seem to be some commonality with parts.

    http://www.homemodelenginemachinist.com/index.php?action=tpmod;dl=item155

    Running with it

    As with any loco a blower is needed to get the burner running otherwise you’ll get lots of flame out of the firebox door. I start at about 5-7psi gas pressure with the blower going, it doesn’t need much of a blower, I use a computer fan in a cut down plastic funnel on a long coper tube so the whole lot doesn't melt. That said with the baffle it not like there's flames shooting out of the loco flue.

    [​IMG]


    Once a bit of steam pressure is available (5-10psi) the steam blower can be opened and the external fan removed. As steam pressure comes up more gas can be applied. I find that 20psi max is good for my burner on this loco. It takes only about 10 mins from cold to full steam pressure (80psi), my boiler takes about 500ml of water.

    OK I understand there’s a but of reluctance to go to gas burners, but when they work they work pretty well, and clean up at the end of the day is just about zero in the firebox area. And if you have a problem with water, as I have been having on my small loco, no grappling for the dump pin. Just shut down, sort the water, if you've got steam pressure and bit of steam blower and re-light.

    Hope that is of use to somebody

    Pete

    PS

    For those of you who missed it, here it is running

    [ame]http://www.youtube.com/watch?v=iugzOVs-73E[/ame]

    Dodgy track at the end caused it to slow and stop
     
    Henry and geoff p like this.
  8. Jan 3, 2011 #8

    tups

    tups

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    Thanks very much Pete, that is exactly the information I needed to get started on my own gas burner, hopefully without my usual progression of buying lots of stuff, finding it doesn't work, buying more stuff, finding it still doesn't work, etc etc ad nauseam. My grate is 67x 55 mm, which gives it nearly the same surface as yours, and the boiler is also 500 mL, so performance should be quite comparable I would guess.
    In any case, firing a blowtorch through the firebox door just isn't cutting it :)

    Again, thanks a bunch, and enjoy your beautiful loco!
    If I manage to actually make something, I'll post onhere.

    Cris





     
  9. Jan 15, 2011 #9

    doubletop

    doubletop

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    I thought I'd add a bit about gas supply.

    The biggest problem I've had with any of the burners I've done, including those for my little engines, has been controlling the gas supply so I know what is going on.

    For my small engines I used camping gas cylinders with the 7/16 screw on connector. Primarily because I had a control valve for these cylinders that came with the original Rob Roy ceramic
    burner.

    [​IMG]


    This didn’t work very well with the loco for two reasons.

    • You have no idea what pressure you are supplying to the burner and you can’t really see what is going on, as the fire door needs to be shut.
    • The gas in these cylinders is 60/40 butane/propane, you really need propane as it provides more heat.

    The light began to shine when one of the club members loaned me his driving trolley that is fitted out for gas running. It has gauges so you know what is going on and carries a 2Kg Primus gas cylinder.
    [​IMG]

    [​IMG]


    The Primus cylinder is ideal as it is a nice size and has a low profile. We get these filled at the local BOC depot and the gas they supply is guaranteed 60/40 Propane/Butane.

    You can see the cylinder has a regulator (orange knob on the right) to control the gas pressure and of course the gauges up front to see what is going on. (the grey cylinder on the left is the accumulator for the vacuum brakes.

    This led me to get my own set up like this for testing. (I use the loaner trolley for the track)

    [​IMG]


    This has a bottle mounted regulator with a POL connector and input (post regulator) and output (jet) pressure gauges. In addition there is a cut off valve and two gas control valves in parallel. The idea of the parallel valves is one is preset for idle or pilot burning and the second for the main control of the gas supply. The main control can be shut right down without the burner going out. If there is a major issue the cut off valve can be used.

    Of course the problem with this is the use of the LPG type bottles with POL (or whatever connectors) they are generally too big and the wrong shape for carrying on a driving trolley.

    The other option is the disposable propane cylinders with the “MAPP” connector. I had one of these for a portable barbecue but had no way of dealing with the regulation issue until I started searching on Ebay and found the Turbotorch STK-R regulator. The trouble was they were expensive (~US$60) and the shipping prohibitive in comparison.

    http://www.thermadyne.com/IM_Uploads/Literature/lit_302_05_56-1252.pdf

    Then I stumbled across this guy in Hawaii who does these and ships worldwide.

    http://ultimatespudgun.com/propane-mapp-regulator-p-220.html

    US$27 and $12 shipping. It regulates 0-40psi which is ideal when I am trying to get 0-20psi for the burner.

    [​IMG]


    This regulator combined with a pressure gauge now gives me a compact solution for gas supply and control.

    [​IMG]


    The regulator itself provides enough control of gas feed but building this into a driving trolley should really also have a cut off valve and two gas control valves in parallel readily available to the driver rather have them fiddle about with the gas cylinder which can be hidden in the trolley.

    There are some issues with these small cylinders

    • They are probably only suitable for smaller locos.
    • They wouldn’t supply enough gas at the right rate for larger locos. They’d cool right down and the pressure would drop off
    • These small cylinders are not refillable and they are more expensive than bottle gas. For example one of these 450gm (1lb) cylinders is NZ$11 whereas a refill of the 2kg Primus bottle is NZ$2.50.
    • Although these regulators fit MAPP bottles I’d suggest don’t even thing about using MAPP gas for your loco.

    Of course my other option is the Primus cylinder but I haven’t found a suitable regulator for one of these and these cylinders don’t seem to be current in NZ. I do have a Primus to POL adaptor but when the cylinder, adaptor and POL regulator are coupled together it’s a really cumbersome. Maybe I’ll get something sorted in this area but for now it looks like I’m going the small cylinder route.

    Regulators side by side

    [​IMG]




    PUBLIC SERVICE NOTICE – You are on your own with this. It’s up to you to make sure you do things safely and within the bounds of your local regulations
     
  10. Jan 15, 2011 #10

    bambuko

    bambuko

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    From my days in G1 I seem to remember dire warnings about using propane!!!
    Only butane was recommended. I will try to find more - but in the meantime I suggest you research it as well.
    It would be a shame to not to learn from the mistakes of other people.

    Chris
     
  11. Jan 15, 2011 #11

    shred

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    Propane can create much higher tank pressures and a non-professionally-built tank (eg: G1) needs much more testing. Using a commercial tank and regulator designed to be used with Propane should avoid that problem.
     
  12. Apr 4, 2011 #12

    tups

    tups

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    Hello Pete,
    Hello forum,

    I pm-ed Pete in december to ask him about the experiences he had with the gas burner he made for his Rob Roy, and he updated this thread with a wealth of details. I've had little time in these past few months, but I finally did succeed in building a gas burner for my OS rocket.
    It doesn't look pretty, but now I know that the geometry works, I could make a second, prettier one. The shapes on the copper on this burner are a little more challenging ... but it glows bright orange in use, there is no unburnt gas (I feed it liquid butane which evaporates in the copper feedpipe) and the loco lifts the safety valve easily when stationary.
    The pressure buildup seems a little slow though, and when running fast (on a rolling road) steam pressure plummets pretty fast. I have not added a baffle yet - maybe the hot gases leave the firebox too fast. I can't wait to see what it does on the track... next weekend we have a running day.

    Some pictures:

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    And the burner in place - space constraints necessitate a bit of a back-asswards mounting, but it works!
    [​IMG]

    I really can't thank you enough for the guidance and advice!

    Cris
     
  13. Apr 4, 2011 #13

    doubletop

    doubletop

    doubletop

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    Cris

    Your burner looks perfect but what you describe is exactly the scenario I had, which then proved to be disappointing on the track. In fact I was out running this weekend and my experiences contribute to my growing understanding and help me answer your question.

    Pressure building seeming a little slow would indicate not enough heat to maintain the steam pressure. The problem is you can't just push in more gas without means of drawing the air through the boiler. So my suggestions, in some sort of order.

    Your loco looks pretty new. It needs to be loose to enable this to work. I was told to run mine on air for a long time to run it in. I ran it for 6 hours plus keeping it well lubricated all the time. Varying speed, reversing etc. It should only need 5-10psi of air pressure to do this, so its not a case of thrashing it.

    Put in the stainless baffle, unless you do, once you get enough heat it will ruin that beautiful paint job around the smoke box.

    If you haven't already got it rig up some means of including a pressure gauge in the gas feed after the regulator, before the jet, so you can see the jet pressure. That way you'll get to learn what works and what doesn't. A 0-60psi gauge will do. I use guages from Norgren http://www.norgren.com8BSP 40mm gauge. They are off the shelf in NZ so wherever you are I'm sure you'll be able to get one.

    Use propane if you can otherwise you'll need more butane to make it work and you may not be able to get enough gas through your burner to make enough heat.

    Use an electric fan on the flue to start with, you'll see I use an old fan from a computer.

    Put in the fan, start the burner on a low pressure (2-3psi) to warm things up a bit. leave the blower, regulator and drain cocks all open to let heat through the system.

    After 2-3 mins shut the blower and regulator and open the gas pressure up to 5-10psi and wait for the loco pressure gauge to rise to about 20-30psi. Crack the loco blower just enough so you can hear it in the flue. Leave it like that for a couple of mins. Boiler pressure should keep rising.

    Remove the electric fan. Increase gas pressure to 15-20psi. Boiler pressure should come up pretty quickly and the safety valve blow off.

    With gas pressure at about 20Ppsi and running on the rollers without a load, and blower shut off it should be possible to run the loco and maintain boiler pressure to the extent that the safety blows off while you are running. If it doesn't try more gas pressure, but make sure its burning all the gas. You'll smell it if it isn't.

    If you shut down the regulator remember to open the blower and reduce gas pressure to about 3-5psi otherwise the burner will go out and be at risk of flash back when you try to relight.

    The gas pressures quoted are what works for me. The jet size plays a big part on the gas pressure required. I have a #15 jet. If you use a smaller jet you'll need more pressure and vice versa as the gas has still got to get to the burner at the right rate.

    I also had an issue with pressure loss when running. I ran it on air, upside down, and used soapy water to trace leaks in glands unions etc. Try that

    My experience this weekend re-enforced all this. My gas bottle was getting empty, which means the pressure dropped off and as I can only get propane/butane mix (LPG) the propane tends to go first leaving the less efficient butane until last. Once the bottle pressure got below 25psi and I couldn't maintain 30psi gas pressure, when running, the loco just packed up and I was back to where I was a few months ago. Its a fine line between right and hopeless.

    Please re-post your question to me on the main thread and I'll copy my reply over as well for all to see.

    Good luck

    Pete

    (just had another look at you pictures, beautiful!!!
     
  14. Apr 4, 2011 #14

    tups

    tups

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    Thanks for the suggestions.

    About the Rocket, I have no merit in its construction, it is a ready-made model by OS Live Steam. In the mean time, I took it to the track, and while it runs very well on its own, I don't think it will keep up steam when it actually has to pull things. The advantage of butane is that you don't need pressure gauges, it is consistently self-regulating - but when the canister cools down too much from the evaporation, it regulates itself too low unfortunately.

    Before switching to propane, I'll try one more thing, which is keeping the canister in a bucket of water to keep it warm. I think, hope, that that will do the trick.

    The weirdness with gas is all these different threads and fttings that don't fit into each other, so switching to a different canister is a major pain in the ass while I figure out what I have to make up now in the way of fittings ...

    And jets. I would love to find a source of jets. It seems impossible here (Belgium). I have a number 16 which I bought in the UK, and that of course leads to more thread compatibility madness.

    Thanks, again!

    Cris
     
  15. Apr 5, 2011 #15

    doubletop

    doubletop

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    I had the same experience at the start with the engine running by itself OK. I even thought of adding radio control as I didn't expect it to get any better. In the end I proved myself wrong.

    I don't know what you mean by self regulating? You do need to be sure you are getting enough gas out of the jet. The bottle pressure will drop off as the gas depletes. I've just taken my cylinder to BOC for a refill. They told me it was 1/4 full. It may be, but there isn't enough getting to the jet to generate enough heat.

    So, I'd suggest that your main problem is lack of heat not allowing the boiler to maintain pressure. At risk of sounding patronising, its a chain of events. The bottle needs to feed the gas at the right rate, the gas has to be the right calorific value, the burner has to convert the gas to heat, the boiler uses the heat to create steam at pressure, the engine converts the steam to mechanical effort. As the effort is expelled the gas needs to provide the replacement energy, preferably at a rate higher than it is being used.

    I know what you mean about standards, It is a nightmare. In the end I found a local source of off the shelf 1/4" flare and 1/8" BSP fittings so its now easy to mix and match. With a 1/8" BSP tap I now make up my own adapters. It is confusing though as 1/8" BSP actually has 3/8" diameter thread. I've got jets from Bruce Engineering in the UK. They use BA threads and fortunately I have BA taps and dies. After a while I did notice a bit of commonality between threads being used and things from different sources started to fit together.

    The biggest problem is the gas canister fitting, but I've now got examples of most so can switch between POL, MAPP, Primus, and disposable cylinders. I have found disposable butane and MAPP type canisters (with Propane) just don't have enough go in them so I am settling on Primus right now. We don't have GAZ in NZ.

    Pete
     
  16. Apr 5, 2011 #16

    tups

    tups

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    Hello Pete,

    I understand pretty well what is involved in getting water to evaporate, and also the direct link between the combustion heat of the gas and the resulting work from the engine - I teach thermodynamics to industrial engineers. But it is good to see others come to the same conclusions independently ;D

    My main problem is that I have difficulty understanding and/or am unable to find good literature on the venturi effect in burners. No-one seems to have researched that properly, and the relation between air holes, their location and size, the jet size, feed pressure and mixing tube length is muddy to say the least - which is why I was so happy to find working starting values for a comparable boiler and burner size in your post. I had to lengthen the mixing tube a bit and provide more holes closer to the end of the jet for it to work better, but now it seems OK - on butane. I'm curious to see what it will do when I increase the pressure and use propane. They have pretty much the same caloric value, it is just that the pressure in a propane canister is much higher than over pure butane.

    If the gas evaporates, the liquid cools down. The boiling point of propane at atmospheric pressure is -44°C, for butane it is 0.5°C. Once the canister gets that cold, no more evaporation takes place. That is why I wrote butane is self-regulating - in the end the amount of evaporating gas is only proportional to the heat flow that is possible from the outside world through the canister wall. This is what limits the amount of gas the burner can burn in the end. The problem should be solved with propane, as this can go much lower in temperature without losing too much pressure, and if the temperature difference with the environment increases, so does the heat transfer to the canister. I found some canisters today with propane-butane mix, which I plan to use when this butane canister is empty.

    Another way around the problem is to turn the butane canister upside down, so it feeds liquid butane to the line to the burner. This then evaporates in the line, unless you're unlucky and it makes it all the way to the jet, in which case the spray will get you bad flare-ups of the burner. On the track I had to leave the canister right-side up on the tender the whole time, and thus performance was paltry.

    In the end I'll buy a small bottle of pure propane and see how I need to change the air hole geometry to cope with much higher feed pressure - but the shop handling that was closed on modays.

    Once more, thanks a lot!

    Cris
     
  17. Apr 5, 2011 #17

    doubletop

    doubletop

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    OK so I'm the one who should be asking the questions ;)

    On the "How much air" question I found somewhere that the gas/air ratio should be around 4%/96% and there should be a similar amount of secondary air as primary air. Primary air being the air introduced in the mixing tube and secondary air at the point of combustion. That's why I added the through tubes to the burner to increase the availability of secondary air .

    How you achieve this seems to be empirical from our point of view. Air vent adjuster on the mixer tube, observation of the flame, combustion smell and in my case a pressure gauge on the jet pressure so there is a means confirming gas feed rate (jet size and pressure being proportional to gas feed and hence the BTU's that can be achieved from that gas.

    I did find this site with a lot of information for propane engineers which helped my understanding. The long link takes you to a series of documents that includes details on troubleshooting systems.

    http://207.200.58.35/Resource%20Library/Forms/AllItems.aspx?RootFolder=http%3a%2f%2f207.200.58.35%2fResource%20Library%2fGas%20Check%2fCETP%206.0%202001%20Gas%20Check%20Cross%20References&FolderCTID=0x01200016E31ECAED4C8E4AAEB16325DB5F5979

    Also take the time to look around the rest of the site lots of stuff there.

    So now my question which you may be able to answer Cris. If we were to look at this question from the "how much heat do I need" point of view we should be able to work back to "how much gas do I need to shove into the system" getting it to burn efficiently then becomes a secondary question. If we can't feed the gas its never going to work.

    I know there will be a whole lot of factors that would need to be fed into any equation but we should be able to make a start.

    We know boiler dry weight and it is copper, we know how much water needed to be heated to 80psi so how much heat does that need? We are now going to vent the pressure at a some rate so how much more heat has to be added to maintain our 80psi. Then we have some losses, how much heat should we allow for that? If we can define the need.

    Maybe a case study for your students? You have the ideal means of putting their theorectical results to a practical test.

    Pete
     
  18. Apr 6, 2011 #18

    tups

    tups

    tups

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    Dear all,

    Well, here is a quick back-of-an-envelope calculation on how much gas you should need in an ideal world, not based on efficiencies, but simply on the amount of steam the engine goes through at a particular speed.

    Let’s assume (for my Rocket) a 14 mm cylinder diameter, and 35 mm stroke, with 120mm driving wheels, and 5,5 bar (80 psi) boiler pressure.
    This leads to a stroke volume of 5390 mm³, four strokes per wheel rotation, and a distance travelled of 377 mm, so in order to travel 377 mm, we use up 21,6 cm³ of 5,5 bar (80 psi) steam when the throttle is fully open and the engine is performing a maximum amount of work.
    If the engine would do that under maximum load while travelling at a fast walking pace (6 km/h), it would do 8.8 strokes per cylinder per second, and thus 17.6 total strokes per second.
    This corresponds to 525 cm³ of steam per second, converted to atmospheric pressure. For this we need to vaporize 0,0234 moles, or 0.42g of water per second.

    Since the heat of evaporation of water is 2.27 MJ/kg, we use 0.95 kJ/s of energy. This needs to be compensated by the burner.
    In order to generate 0.95kJ, we need to burn 0.0203g of propane – per second. This corresponds, in an ideal world, to 73g of propane per hour.

    Obviously, there will be (a lot of) heat loss. On the other hand, the full power of the engine will not be used all the time, and when the throttle is closed more, less steam will be used. This is equally true if a cutoff is used, but the Rocket wasn't that sophisticated ::)

    I wanted to attempt a small calorimetric experiment on the loco tonight along the lines of what Pete suggested, but this was cut short by the lack of a suitable balace to figure out the amount of water in the boiler. I'll see if I can find something useful in the next few days. For now, I generated the following tables, consolidating the drill size to BTU/hr table (which was also a theoretical calculation based on simple assumptions btw ... the numbers follow a second order polynome perfectly) and the jet size data that Pete found links to, into a table that gives grams of propane going through a jet of a particular size:

    [​IMG]

    I did do an experiment where I measured firebox wrapper and stack temperatures versus times and weight of the gas bottle - I had to do this with an analog kitchen scale, so that didn't exactly help accuracy, but for now it's the best I could do.

    [​IMG]

    I have the safety valve blowing off at 80 psi 7 minutes after a cold startup.

    By juggling the butane can and switching it between an upright and upside down position, I can control the burner perfectly. The temperature of the butane can falls to 6,5°C, which is too cold for providing a good gas stream, which is why it needs to be turned upside down for liquid butane to be fed into the line. My troubles at the track sunday were clearly related to temperature issues with the can which had to remain upright on the tender during the entire run. The ceramic element then shows a dull reddish orange glow instead of bright yellow orange.

    Some more worthless data (because of course it depends on the amount of water in the boiler - that is why a good scale would have been so very worthwhile):

    With the loco stationary, I can go up from 3 bar (140°C on the firebox wrapper) to 5.5 bar (155 °c on the firebox wrapper) with the safety blowing off in 37 seconds.

    I can do the same thing with a bit more water in the boiler from 2 bar up to the safety blowing off in 56 seconds.

    I can keep the loco rolling while putting a load on the driving wheels by pushing down on the front end, with the safety blowing off almost continuously.

    And finally, perhaps the most interesting number of tonight: I used a total of 88 grams of butane for exactly one hour with the burner running - minus a few minutes of burning my finger while trying to correct the mixture - that would be a ballpark figure corresponding to 5 psi of butane pressure with a nr 16 jet in the table above.

    I'll try and locate scales that are sufficiently precise to get a better idea of the weight of the loco and the gas canister, and then I can come back with more definitive results on heat loss.

    From what I observed this evening, I am quite convinced that the loco will do well on Sunday, and should be able to pull me around the track without too much trouble with this burner.

    Now if only I could solve my problems with the water level showing in the sight glass being strictly unrelated to what amount of water is present in the boiler ...that would be fantastic.

    Cris
     
  19. Apr 6, 2011 #19

    doubletop

    doubletop

    doubletop

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    Cris

    This is fantastic thanks and helps the understanding of what is going on. The only unqualified aspect is the table I provided, the inclusion of a pressure gauge in the setup would help confirm that the gas pressure was 5psi. I'm going to go and re-read in detail and see if I can reproduce the results and include jet pressure data.

    From your graph below it appears to stabilise at about 220secs. I'd assume that's the system warming up?

    Although I can get my boiler up to 80psi on 5psi gas pressure (#15 jet) it isn't going to go anywhere. Running with 20psi gas pressure the boiler doesn't recover and is soon down to 20psi boiler pressure and fails to negotiate a small rise on the track. With 30psi gas pressure the boiler pressure can be maintained an with nurturing get back to the safeties blowing while running.

    It may have to wait to the weekend its raining at the moment and doing this sort of experiment anywhere but outside is frowned upon.

    Thanks again

    Pete
     
  20. Apr 6, 2011 #20

    tups

    tups

    tups

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    That is actually a good question, I suppose it could be ... water will start condensing everywhere over the water level inside the boiler at this point and flow back cooler, so until the entire boiler is at the normal boiling point of water, the temperature will not rise any further. You also see the residual water in the safety valve begin to sputter and boil at this point, as well as the water in the blowdown line.
    It may also be a mixing effect, where convection from the firebox into the rest of the boiler really starts going. As you perhaps know, the firebox of Rocket is a bit of an oddball design, being outside of the actual boiler. It has two tubes at the top leading the boiling water back to the boiler, and two tubes to the sides of the firebox bringing in colder water from the bottom of the boiler.
    This circulation starting up by the firebox boiling over into the boiler may be the cause of this effect. The inner firebox liner is typically hotter than the flue tubes.

    By the way, I've done all this without a blower or fan - due to the high smokestack the loco drafts very well, and there are no problems at all as long as you keep the firebox door closed.

    The temperature of the smokebox and chimney does not get over 65°C when the burner is running, due to heat soak it heats up to 85°C when the boiler is off. This is not hot enough to damage the paint job, even without stainless steel baffle in the firebox, which on this loco is extremely difficult to fit ... I think I'll try without for now.

    Cris


    Cris

     

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