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Post by Jim on Mar 18, 2016 21:42:21 GMT
I was just thinking about grates, with a "conventional" grate most of the combustion seems to happen in a small area in the centre. If this occurred in a rosebud grate it would be easy to open up the holes in the cooler areas to get an even temperature fire over the whole grate Hi Andy, The owner of this loco did exactly that, he opened up the holes in the cooler areas to get a more even burn as can be seen in the photo I took of his grate.  I'm working off a base of 10% airflow with Boadicea's grate and will open the holes where needed conversely it would be quite easy to block off any unwanted holes. The other advantage with the Rosebud grate is that it just slides out from under the fire box making maintenance easy. Jim |
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steam4ian
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Post by steam4ian on Mar 19, 2016 0:28:48 GMT
From what I can see, and my 5"G loco had a rosebud grate, the grate acts to control the airflow and not the fire thickness. IMHO most model grates have too much air opening compared to the 12"to 1' locos.
Drivers with rosebud grates report being able to run a much thinner fire. Certainly from my observations less unburnt fuel finished up in the ash pan from falling through the grate. Half of my ash with the bar grate is unburnt fuel.
Off for a day's run.
Ian
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Post by cnc3d49 on Mar 19, 2016 7:02:06 GMT
Springcrocus
Regarding your valid concerns over CO, I have completed some very broad brush, I hesitate to say calculations, more mathematical doodlings, using Jim Ewins test data as a basis. Employing his mass flow and fuel consumption measurements and estimating a typical bar grate to have 50-65% pass area and assuming good quality fuel, my sums suggest that a bar grate flows about three time more air than is required for the optimum stoichiometric ratio. Whether or not the figure is 2 times or 4 times more air than necessary, there will be considerable unnecessary cooling of the fire due excess N2. On this basis there is probably little to fear from CO, however, only measuring the exhaust gas will tell for sure, as you correctly suggest.
GeoffV
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Post by Roger on Mar 19, 2016 7:41:47 GMT
Springcrocus Regarding your valid concerns over CO, I have completed some very broad brush, I hesitate to say calculations, more mathematical doodlings, using Jim Ewins test data as a basis. Employing his mass flow and fuel consumption measurements and estimating a typical bar grate to have 50-65% pass area and assuming good quality fuel, my sums suggest that a bar grate flows about three time more air than is required for the optimum stoichiometric ratio. Whether or not the figure is 2 times or 4 times more air than necessary, there will be considerable unnecessary cooling of the fire due excess N2. On this basis there is probably little to fear from CO, however, only measuring the exhaust gas will tell for sure, as you correctly suggest. GeoffV Does anyone know a tame Gas Fitter with the right kit, who could be persuaded (tricked if necessary) into doing this? |
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jma1009
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Post by jma1009 on Mar 19, 2016 12:29:44 GMT
I have had a quick check of Jim Ewins' tests but cannot find anything of help, other than in miniature 5"g locos the gas velocity is approximately 1/12th of fullsize.
The BR 'black book' has some interesting comments. It is necessary to supply about 20% more air than is theoretically necessary to complete combustion in a locomotive firebox. 1 lb of coal requires 12 lb of air for combustion of which 9 lb is nitrogen.
Coal contains 'volatile (gaseous) matter', and 'fixed (solid) carbon'. Both burn in a different way.
Air is supplied as 'primary air' through the grate and firebed. 'secondary (top) air' is supplied via the firehole. Different types of coal require different amounts of air via the firebed and firehole.
If carbon monoxide is produced due to insuffient air, it produces 70% less heat than if carbon dioxide is produced.
Incomplete combustion is best gauged by noting the colour of the smoke from the chimney.
Some of the BR Rugby Test reports contain quite detailed analysis of air required for combustion. There is also the well known case of BR pacific 'Duke of Gloucester having inadequate primary air supply due to the dampers being too small.
Unless some kind of gas producer effect is being produced on a rosebud grate, the coal ought to burn in precisely the same way as on a bar type grate.
The amount of air drawn into the firebox via the grate and firehole is primarily a matter of smokebox draughting. There are obviously other factors which affect the draughting elsewhere such as any resistence due to poor ashpan openings to allow air in and ash out, and depth of fire and grate design.
Welsh steam coal and anthracite have a high amount of fixed carbon compared to volatile matter, and need more primary air through the grate.
I do not know why rosebud grates obviously work. To my mind they defy logic and science. They would appear to be unsuitable for anthracite, yet there is plenty of evidence from other forum members that this is not the case.
I dont ever recall having unburnt coal in an ashpan. I have always used thinner bars with wider gaps than usually seen on drawings.
Cheers,
Julian
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Post by fostergp6nhp on Mar 19, 2016 19:08:54 GMT
From the large quantity of small coals lifted under large regulator opening I would suggest a larger blast nozzle bore might be in order.
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Post by Jim on Mar 19, 2016 20:39:06 GMT
I think it's important to keep in mind that grates, be they rosebud or bar are designed to burn particular types of fuel which is why rosebud grates were used extensively here in full sized practice on the New South Wales Government Railways (NSWGR). An interesting article on the origins of the rosebud grate can be found on page 10 of the Taunton Model Engineers Magazine 'The Oily Rag' the link is here: www.tauntonme.org.uk/oilyrag/images/autumn2013web.pdfJim |
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Post by miketaylor on Mar 19, 2016 20:42:09 GMT
Alright,
I don't know much about boilers and I've never fired a loco (will get there one day) however it seems to me that the mode of operation of the rosebud grate approximates to some extent to that of a fluidised bed rather than a traditional bar grate and needs to be considered int hat light.
Perhaps there is a real life boiler man in the audience who could contribute on this theme??
Mike
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smallbrother
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Post by smallbrother on Mar 20, 2016 14:43:52 GMT
At the 2015 7.25 AGM, a bloke was driving his loco with a Rosebud and it was very impressive. He said it didn't matter too much what size or quality of coal went in, it all burnt very well.
I think he said he was going to put something in print soon. Hope he does.
Pete.
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jackrae
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Post by jackrae on Mar 21, 2016 13:58:18 GMT
Assuming smokebox vacuum is the similar for both grate types then the velocity of the primary air in the rosebud grate will be higher than in a fire-bar grate. It's not illogical to presume that "blast" air rather than "slow" air will produce a hotter and more complete combustion.
Not long to go before we see the classification or ranking question of "what type of grate have you" cropping up in efficiency competitions.
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Post by andyhigham on Mar 21, 2016 16:01:10 GMT
I have just ordered a 100mm x 100mm lump of 10mm steel plate, to make a rosebud grate for Chattox.
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penybontshed
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Exiled to sunny South Wales
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Post by penybontshed on Mar 25, 2016 19:10:17 GMT
Just to add to this debate. I have been trialling a Rosebud on my Polly 5. Initial steaming seemed to indicate inadequate air flow with a surface area of around 10%. More holes will be made to bring it closer to 15%. Easier to add holes than lose them!
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Post by andyhigham on Mar 25, 2016 19:48:41 GMT
Something that crossed my mind, feel free to tell me I am talking $hit.
If there is too much air space in the grate (either type) some air could be pulled through the fire without being used in combustion, this fast free moving air and products of combustion will then rush down the tubes and up the chimney.
Slower air speed would be preferable, the slower the air/combustion products are moving, the longer they are in contact with the boiler surfaces and the more heat will be removed to the water.
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Post by Jim on Mar 25, 2016 19:54:12 GMT
I haven't really looked into it but is that the reason for having swizzle sticks in the fire tube in some cases, to slow the flow of gasses?
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uuu
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Post by uuu on Mar 25, 2016 21:23:53 GMT
To speed the flow I think.
Wilf
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steam4ian
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Post by steam4ian on Mar 25, 2016 22:41:39 GMT
I haven't really looked into it but is that the reason for having swizzle sticks in the fire tube in some cases, to slow the flow of gasses? Jim The swizzle sticks cause both turbulent flow and a longer path. Both factors improve heat transfer. It is a good idea to put swizzle sticks in super heater flues is the superheaters are out. Peny.... try some more secondary air before drilling out the plate. I doubt you tube area is much over 10% of grate area, probably about 12%. Ian |
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jma1009
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Post by jma1009 on Mar 25, 2016 23:40:11 GMT
Andyhigham raises some very interesting points!
My old fashioned approach is that the coal sits on thin firebars with wide air gaps. The ash is free to fall to the ashpan, and there is little restriction on incoming primary air. The amount of primary air sucked in through the grate ultimately depends on the smokebox draughting. This can be fine tuned to quite a degree to give an optimum steaming rate to match the loco and the coal used.
However, the Rosebud grate rather turns all this on it's head!
I dont know the answer to this one. I will ask Jos Koopmans what his view is and post on here in due course. Ross might also have a view.
Cheers,
Julian
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Post by cnc3d49 on Mar 26, 2016 7:48:37 GMT
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Post by keith6233 on Mar 26, 2016 12:05:11 GMT
H1 GeoffV
I made a grate 152x194mm with 285 4mm holes on a 10mm grid out of 10mm m/s c/bored with a 8mm center drill.Yesterday which was very busy the engine was in steam for 5 hours and at the end of the day the ashpan was virtually empty. The smoke box was quiet full but non of this affected the steaming,the reason probably is the blast is a bit fierce and the spark arrestor is very efficient .The only thing i altered is driving style with more use of the blower,but overall i think it is a great improvement because it stops the ash pan from being choked and after approx 20 miles running i am very happy .
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jackrae
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Post by jackrae on Mar 26, 2016 13:30:33 GMT
Which suggest that the upblast of primary air is sufficient to fluidise the fire bed (so improving combustion) and preventing ash from migrating down into the ashpan.
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