Rosebud grates

[steam4ian]

Mar 25, 2016 23:40:11 GMT jma1009 said:

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

G'day Julian

It certainly would be great to get Jos's thoughts on the matter..

Experientially my colleagues have found that increasing free air area of bar grates exacerbated the problems experienced with clinkering of the char that became available to us. Some quite experienced drivers got into real difficulties with passenger hauling. Some of us found that regular raking through the fire helped when using this fuel on bar grates. The problem with raking through is that is allows lots of smaller coal particles to get into the ash pan with the ash.

Alternately the rosebud grate seemed not to create a clinkering problem with this char.

It is now reported that rosebud grates are great (pun intended) for burning the anthracite that is being imported (thanks Hayden). I am yet to experience direct evidence of this

In models there is a disparity between the air spaces in bar grates, the coal lump size and the fire bed thickness. A typical fire bed might be around 1" (25mm) for 5"G, maybe 1.5"-2" for 7.25" G. A full size loco could not run with such a thin fire. The point I am making is that in model form our fires are not deep enough to regulate and even out the air through the fire. The rosebud grate seems to be able to do this. It has also been found to allow running with thinner fires.

I wait.
Ian

[steamcoal]

Ian.

No worries. Down in Cobden for convention and engines running on anthracite, Juliets and other 5" locos. Some for first time with good reports and low consumption than both Char products.

A variety of grates and setups.

I am happy.

Hayden

[Jim]

Do you have a rosebud grate Hayden?

Jim

[steamcoal]

Hi Ian.

No not yet but with the information on this site and the issues of model engineer I will make one for my two engines. I have plenty of material and just need to drill a few holes and experiment. Railmotor is easy and Hunslet is very straigthforward also.

I saw a couple of grates at Cobden and the ash and clinker from a 5" LNER loco which had accumulated a vast covering of BM Char clinker.

Actually drove my first engine fired on BM Char so that was a bit of a thrill ! One was Warrick Allison's Aussie class something and Alans Juliet. We changed the Juliet on anthracite and ok. The grate is very fine in the airgap. Went well. Agree char is a non-event but just not the same. Almost anyone can run 'em. Just have to fill them up and go.

Caught up with Ernie Riding and John Monte so have a better understanding of the situation and future requirements. Think they are worried the supply might dry up. You will not be the only ones worried if it ever does, Poms , Aussies and Kiwis all together!

Will do a bit of working out for bigger volumes into Adelaide in the next week or so. There are a couple of scenarios for costing and freight so will do a bit of working out.

Off back to Chch now so catch up.

Better book for Newcastle I guess.

Hayden

[cnc3d49]

Gentlemen

Having gained a few months of experience with the Rosebud grate design, several of us had noticed a reduction in coal consumption. Fortunately for us, one of our group decided to try and quantify this saving by running a series of controlled tests using his 5"gauge 0-6-0 Pansy. As was anticipated this is not an easy task, but with persistence and careful measuring of coal burnt he established that, on his engine, the saving is about 28%. I've not conducted any controlled tests, but after three sessions of passenger hauling, each lasting 5 hours, I find his test results quite believable.

Regarding initial thoughts about there being a reduction in clinker build up, I've not found this to be true, and have accumulated significant clinkering on occasions which I put down to the varying quality of fuel.

GeoffV

[Roger]

Jun 19, 2016 8:31:50 GMT @paul1979 said:

Hi All,

With my interest in imlec I was interested in the apparent coal consumption reduction thro a rose bud grate.
I tried one in my 7f, 150 3/16" holes in a 7"x 3" grate. The result was awful on uk coal, never got above 25psi. Back to conventional firebars.

Cheers

Paul

Surely this is all about proportions? I'd be surprised if one size fits all. I would imagine you'd need to experiment a bit to get the results that others seem to achieve?

[miketaylor]

I dipped my toe in this pool early on and have done a bit of reading since.

The general opinion definitely seems to be that these grates function as a fluidised bed which makes them a very different animal to the bar grates.

They are going to need a blast up through those holes to make the fire bed work properly. If the holes area is too great it will weaken the blast effect without giving the open grate effect of the normal fire.

I notice from earlier posters who have described succesful rosebud grates that they are looking at around 10-12% of the grate as hole area.

Paul's grate is about 20%. Perhaps this is the reason for lack of success.

Fluidised bed technology was developed, in part, as a means to burn anything combustible efficiently. This seems to be borne out by the reports of success with a variety of coal type fuels in this thread. Char to anthracite.

The main problem which might be encountered would be if the fuel contains incombustable matter which won't turn to ash (rocks). Many industrial fluidised beds support the fire on closely spaced jets of air whilst leaving voids between these jets for incombustable stuff to drop through. Obviously this won't happen with a rosebud grate, therefore fuel has to be reasonably suitable in this regard.

I would think another possible problem will be that almost all the ash will end up in the smokebox with little or none in the grate. Raking the fire will stir it up but as long as the air is jetting up through the rosebud, the ash won't drop down.

Maybe a loco designed from scratch for rosebud grate will need a larger smokebox??

Mike

[Jim]

Mike I can't speak from personal experience on this as my Britannia is yet to have its first steam up but from talking to others who have rosebud grates in very similar locomotives such as the NSWGR C38s ash in the smoke box doesn't seem to be any more or less of a problem than with standard grates.

Jim

[miketaylor]

Hi Jim,

I think I must have had Keith's post from back in March (page 2)in mind:

"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"

It certainly seemed logical. The ash has to go somewhere and it would seem much more difficult for it to find its way down through the rosebud than through a bar grate. It is also possible that a rosebud working efficiently will burn more of a given fuel and therefore produce less ash overall. Of course that will depend on fuel characteristics as well.

There is also a comment somewhere (maybe the Taunton article which you referenced) about needing too make more use of the blower with a rosebud; that also seems logical since the rosebud form would be more dependent on a forced airflow throguh the small holes than would be the case with a bargrate.

Mike

[Jim]

Not a problem Mike. I must say I'm just itching to set a fire in Boadicea to among other thing see how my rosebud grate functions. Those I know who use them have done some tweeking of the holes to ensure an even air flow to eliminate hot spots.
Like a lot of things associated with steam adjustments are needed before thing start working as we want them to.

Jim

[cnc3d49]

Paul1979

Paul
I've just done some sums and it appears you have about 23% pass area, assuming the peripheral gap is 0.040", and much more if the gap is larger as you suggest it might be. My observations are, the gap around the grate must be small not only to control the air flow and hence the velocity of the air jets, but also to prevent a stream of cold air between the fire and the walls of the fire box.
I do hope you find the enthusiasm to make another grate with a total pass area around 15% total, 4mm holes on a 10mm grid and a 1mm gap seems about right.
GeoffV

[ruston165]

I had a go on John Cottams Merchant at the pacific rally at nottingham, and couldnt stop it making pressure, he has a rosebud fitted and if i remember correct he said he had 514 holes in his.

A chap in my club us experimenting with 1 in his brit, and its pretty succesful, a normal sunday with standard grate he would use 2 tenders of coal, hes using less than 1 now, with no ash in the ashpan and some unburnt in the firebox at the end.

We also have 1 in a 7 1/4 13XX thats proving good aswell.


Lee

[steamcoal]

Lee.
Obviously they are burning Welsh Anthracite? Beans or small nut size? Does it make a difference?

[ruston165]

We are buring anthracite small nuts that we get from Fergusons. Its good stuff. Best weve had in a long while. Plenty heat and not much ash.

Lee

[robmort]

Now that I've decided to make one of these for my 2.5" gauge A3, I'm wondering about some things that don't seem to have been discussed, namely:

1) a single plate is typically used with no rocking section to drop any residue. Is this rocking section not necessary? And can any clinker be easily cleaned out e.g. through the firehole?

2) a thick plate (e.g. 5mm) is typically used but, if stainless steel, maybe a 2mm plate will work equally well?

Rob

[Jim]

Jan 8, 2017 20:57:56 GMT robmort said:

Now that I've decided to make one of these for my 2.5" gauge A3, I'm wondering about some things that don't seem to have been discussed, namely:

1) a single plate is typically used with no rocking section to drop any residue. Is this rocking section not necessary? And can any clinker be easily cleaned out e.g. through the firehole?

2) a thick plate (e.g. 5mm) is typically used but, if stainless steel, maybe a 2mm plate will work equally well?

Rob

Rob I can't comment on the use of stainless steel so will leave that to others. On the issue of clinker, the owner operators of rosebud grates that I spoken to and observed seem to have no problems. The ash drops through the grate as dust and any unburnt fuel drops off the grate as it is pushed out to the side of the fire box.
I anticipate some adjustments needed when I steam up the Britannia due to a change in fuel as the char previously used is no longer being supplied or of poor quality. But then the rosebud grates were designed to burn the poor quality Rosedbud coal in the US.

Jim

[4fbuilder]

Good morning all,

I think Jim has touched on an essential point, insofar as the original rosebud grates were designed to burn the Rosebud lignite used in the USA, a fuel that would have benefited from leaving in the ground for another million years, but some of the poor quality fuels available may benefit from the use of a rosebud type grate, whereas bituminous and anthracite type coals generally should always burn on conventional grates. I feel much of the poor combustion and heat transfer will be attributed to the low calorific value of the selected fuel, but also the grate, ashpan, and smokebox arrangements will all be major contributors to the overall boiler performance.

Bob

[Roger]

I'd be really interested to hear from everyone who uses these, with the Locomotive's gauge, grate dimensions, number and layout of holes etc so that a picture can be built of typical proportions. I know there are a few in this thread, but the more the merrier.
I'm particularly interested in what people have found when using different (mostly larger) hole sizes around the edges. Presumably in a narrow firebox, you would only increase the hole sizes at the front and back?

[gingerneer]

Have run with 2 smaller gauge locos for almost 2 seasons, i am very pleased with the results of the Rosebud grate but i have more questions now than answers. As i under stand it, you need the depth of material in the grate for the tapered hole, to funnel the air (to fluidise burn the coal on the grate ensuring a more complete combustion of the coal) in the same way a blast nozzle does with the exhaust steam. On both of mine i used 5mm steel plate because that was to hand. I did some digging around and even the 16mm coal fired locos were using grates with holes no smaller than a 1/8" or 3mm dia. I have used 3.4mm and 3.9mm dia holes. As for a pattern of holes or using different size of holes or both i have no idea. The fuel, what ever sort of coal, or bio mass/charcoal will each have a optimum setting. Both the fire boxes on my locos are square, so i have no experience of a long narrow grate. Both grates are horizontal, while i think that i would be possible to use a rosebud grate at a very shallow angle. The angle of Speedy's grate is too steep, i think as the fire will want to move/slide down to the lower end leaving a uneven fire thickness. The same with a dog leg grate. Then there is the question of secondary air how much to admit and were? A brick arch or stainless steel arch will help with the combustion of the gases and disturbution of heat around the inner firebox. Wardale pointed out (in the book The Red Devil and other tales from the steam age) the more you tune/ refine the engine the more temperamental to the type of coal and firing technique, when the Gas Producer System Combustion (next step on from Rosebud Grate) worked it worked very well, but it did have its off days, and it would just burn any old coal. Its still a imperfect science, more so with the miniatures than full size. What it really needs is a test rig/s to be able to test the variables 1 change at a time. Until then its back to the old empirical ways.

The grate forms only 1 part of the whole engine, I belive that a loco should be designed around the front end/exhaust system as this is what limits/determines the locos performance. The valve gear, boiler design and grate are then designed and tuned to the limits of the exhaust not the other way round.

[cnc3d49]

Gentlemen
I've see no evidence that the Rosebud grate performs like a fluidised bed, for a start the velocities are far to low and there were no signs of the coal lumps moving in the video I published on youtube, furthermore sloping grates perform quite satisfactorily. From Jim Ewins test data, the firebox vacuum reached a maximum of 0.3 in/swg and was typically 0.15 in/swg or 0.768 psft, not psi, which is in the order of 15mph, barely enough to blow your hat off, never mind lift lumps of coal.
I suspect the reasons for such good results in miniature locomotives is simply, better combustion as rice grain sizes of coal get burnt rather than falling into the ash pan, and much less unnecessary cooling air passing through the fire. Regarding the shape of the holes, the taper is to try to prevent holes from blocking, it will not increase the air velocity which is dictated by the pressure drop across the grate. With this in mind it is difficult to see a good argument to mix the hole sizes in different places around the grate.

GeoffV