Rosebud grates

[Roger]

Jan 10, 2017 6:41:48 GMT cnc3d49 said:

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

Hi Geoff,
That all makes sense to be except for the last one. In my opinion the velocity through different size holes is not only going to be different, but there will be more air going through the larger ones. Taken to the limit, tiny holes and huge holes on the same grid pitch are going to pass very different amounts of air, and that will surely make a big difference.

I think the key observation is that most very small coal pieces drop onto the surface where they are roasted and burnt instead of being lost. More than that, larger pieces that are being broken down as they burn don't prematurely end up in the grate while they could still burn a little more. The 'fluidised bed' does seem to me to be something in the imagination rather than in reality.

[robmort]

Gingerneer and Geoff,

thanks for the info, but do you have no problems cleaning out the fixed grate (e.g. clinker or unburnt coal) after steaming?

[cnc3d49]

Roger

Thanks for you response.

The velocity is solely dependant on pressure difference, no pressure difference, no flow, e.g. turn the blower off, fire dies down. The mass flow per unit time will increase with increased total hole area, subject to the flow coefficient, boundary layer, surface finish, hole profile etc., but I'm not clear as to how increased mass flow in certain parts of the firebox would help?

GeoffV

[Roger]

Jan 10, 2017 10:08:44 GMT cnc3d49 said:

Roger

Thanks for you response.

The velocity is solely dependant on pressure difference, no pressure difference, no flow, e.g. turn the blower off, fire dies down. The mass flow per unit time will increase with increased total hole area, subject to the flow coefficient, boundary layer, surface finish, hole profile etc., but I'm not clear as to how increased mass flow in certain parts of the firebox would help?

GeoffV

Hi Geoff,
That's what I was suggesting, that the flow is subject to those other conditions and not solely on pressure differential. That's a side issue though, and not really important in this context. It's a distraction really and one for the pedants to debate if they are so inclined.

The amount of air feeding the fire is of more interest though. If you look at the state of the fire over the whole grate, is it absolutely even or is there more energy being created away from the edges? My guess is that the temperature away from the edges is higher because the coal there is surrounded be more coal that's burning brightly. At the edges, at least one side is relatively cold. I'd suggest that the temperature of the grate is hotter away from the edges.

Would it be fair to say that grates burn away in the centre and not so much at the edges?
If this is the case, I'd ask the question as to what you could do to make the temperature across the fire more even? I'm making the assumption that this is desirable because you presumably want to maximise the use of the small area that you have. (You could also argue that the cold edges of the fire are where the water is that you want to heat, so that would be a good place for the fire to be hot.)

One suggestion is to change the amount of air available close to the wall compared to away from the wall.  Generally, when you blow on a fire you get more intense combustion, so I'd suggest that larger holes might cause the fire to be hotter where holes are bigger.

One point that I don't think has been mentioned, is how close a fit the grate is to the firebox wall. Traditional grates could arguably have an edge clearance which is the same as that between the bars of the grate. However, if you're going to restrict the flow with a smaller percentage area, I'd suggest the the fit needs to be in proportion to that reduced area. In fact, if you want the grate to control the fire, it ought to be a really good fit and for the holes to decide what air goes where.

I would suggest that a poorly fitting Rosebud grate might give a totally different result to a well fitting one with exactly the same hole arrangement.

[uuu]

One of the Pumphouse regulars was burning out his old grates whenever the ash built up underneath to locally block the flow of air. He's impressed with the steaming of the rosebud. Time will tell if ash build up is reduced.

Wilf

PS it slopes.

[jma1009]

I am not convinced of the advantages of rosebud grates yet, where in the UK we have pretty cood club supplies of decent coal. What this supply will be like in a further 10 years is very uncertain, however I have enough Welsh Steam Coal in one of my sheds to last me out.

So far as the UK currently is concerned there are good supplies of decent steam coal or alternatively anthracite. The anthracite will continue to be available probably long after the current very limited extraction of Welsh Steam Coal ends.

Most UK miniature locos have very poor grates and ashpans. The best way to optimise these features is to make your own grate using the optimum features and details, and carefully study how the ashpan can be improved upon. Allied to this is improved smokebox draughting, and quite a number of published designs fall well short of the optimum requirements.

Cheers,
Julian

[steamcoal]

Julian.

For those unfamiliar your referring to the following:

Welsh Steam Coal with the emphasis on the "Dry" steam coal in your description as opposed to the Welsh "Anthracite" steam coal. There are a number of antipodean souls not familiar with the "Dry" variety and it all gets lumped into one,( pun intended )

We have a local member here in New Zealand who has acquired a new loco to him (NZR Fa class )and its grate is 1/8th air gap. He is just going through the exercise to develop a grate and drafting setup to allow this New Zealand engine which used previously the AusChar to now burn Welsh Anthracite beans. From my point of view its an interesting exercise and he has built a new grate with 5mm spacing. Steamed up well but did not run well on the track. Our suspicions is that the exhaust blast is not sufficient and being under minimal load things are not quite working as designed. Smaller blast nozzle is the next step and then to load up the engine.

The fact remains that the coal has not changed, well for the last million years so it must be something else.

Hayden

[Jim]

I thought a couple of photos might be of interest:

The first photo is of my rosebud grate showing the layout of the holes to give a total of 10% of the grate area which is the basic starting point with these types of grates.



The second photo shows the much used grate from a NSW C38 class locomotive. The 38's are of almost identical proportions to the Britannia and obviously performed similar tasks over their lifetime.



Comparing the two, you can see that Les has opened up the holes around the sides of his grate to increase the air flow in those areas. This is a handy feature of the design in that holes can be enlarged or plugged and re-drilled to a smaller diameter or closed off completely if needs be.

The grates are of 3/8" plate and the holes counter bored on the underside to allow ash and small particle to drop through into the ash pan. The grate simply slides out to the side for removal.

I don't doubt that my grate will need modifying once regular steaming starts.

Jim

[cnc3d49]

Roger

I understand your desire to raise the fire temperature around the edge of the grate, however, if these grates work better by reducing the amount of air flow to achieve the optimum stoichiometric ratio, then increasing the flow around the edge of the grate would surely be counter productive, and actually cool the fire? If one fits the grate with a 1mm gap, this equates to about 5% pass area or one third of the total pass area, so a close fit would seem to be very important.

GeoffV

[Roger]

Jan 11, 2017 5:23:44 GMT cnc3d49 said:

Roger

I understand your desire to raise the fire temperature around the edge of the grate, however, if these grates work better by reducing the amount of air flow to achieve the optimum stoichiometric ratio, then increasing the flow around the edge of the grate would surely be counter productive, and actually cool the fire? If one fits the grate with a 1mm gap, this equates to about 5% pass area or one third of the total pass area, so a close fit would seem to be very important.

GeoffV

Hi Geoff,
What I'm really suggesting is that I would have thought that the fire should burn equally brightly across the grate. If it doesn't, then it can't be burning at the same rate across the grate and so the optimum ratio may not be present across the grate. The centre of the grate is presumably the hottest place because it's surrounded by the most fire on all sides and it's away from the cooling influence of the wall. The velocity of air in an open pipe falls to zero at the walls where the boundary layer is, so I'd suggest that there will always be slightly more flow in the middle even in an evenly restricted pipe ie the grate. These factors might lead to there being a benefit to allowing a little more air to reach those places that don't get quite as much as the centre and have the effect of making the fire evenly bright across the grate.

I think a 1mm gap is on the large side, I'll be aiming for a little less than that. You could argue that larger holes in the outer part of the grate would encourage the air to pass through the grate rather than around it, although the effect it likely to be small.

I'm not suggesting that this is a huge effect, but the restriction of air flow in this type of grate does mean the situation is unlike conventional grates, and there might be benefits.

On a long narrow grate like the one on my locomotive, I doubt if there's any point in having different size holes, but on a more square design it might help.

[gingerneer]

robmort i don't have problems cleaning out after wards. There it is main fine almost dust/course sand like ash with some bit of partburnt coal. Its mostly on the grate, there is not much ash in the grate (compared to a bar grate) The Canterbury Lamb's grate is a good fit but can be drop straight down with the ashpan. Ayesha's does not drop out. I find its easier to use my shop hoover to suck the remains off the grate. I do get lazy and clean the tubes from the firebox end to save opening the smokebox door, holding the hoover over the Chimney to suck out all the ash/soot. I can get the grate out but have to drop the rear trailing axle out and undo 3 screws holding the ash pan and grate in. Only a couple of mintues work but only done when/if it needs a deep clean. Last season when i was visting tracks i would run her twice, and had no problems making the fire second run on the remains for the first fire. Maybe running the rake around to dislodge the ash. I do find that you have to get the charcoal burning well and bright before adding the coal, and at that a little and offten, especilly with the wide firebox on Ayehsa. Other wise the new fire can sulk.
How close are you to Steaming A3?

[robmort]

Gingerneer,

thanks, very useful information.
I need to steam my A3 in the next few weeks for its first live steam test.
The original plan showed a rocking grate, so I hope the new one will be practical to use as the grate cannot be easily removed.
I've used larger holes around the outside than the middle.

[Jim]

That seems to be the general pattern Rob. Those I've spoken to who use these grates have done much the same as you based by observing the fire and deciding which areas could do with more air. As you can see in the second photo I've posted Les did much the same as you and increased to hole diameters in a band round the sides of his grate.

Jim

[jma1009]

Hi Roger,

It is perfectly possible to calculate the amount of air required for a coal fire to burn properly, though depending on the type of coal there are 2 sources of air and different proportions are required of each for different coals

1. Primary air through the grate

2. Secondary air (or top air) through the firehole door

Any decent fireman will try and achieve as thin a fire as possible on plain level grates without 'holes' forming in the firebed. A lump of coal, when alight, ought to burn the same whether it is at the side of the grate or in the middle.

Sloping and part sloping grates require a different technique to fire, also varying with the coal used.

Cheers,
Julian

[Roger]

Jan 12, 2017 11:11:51 GMT jma1009 said:

Hi Roger,

It is perfectly possible to calculate the amount of air required for a coal fire to burn properly, though depending on the type of coal there are 2 sources of air and different proportions are required of each for different coals

1. Primary air through the grate

2. Secondary air (or top air) through the firehole door

Any decent fireman will try and achieve as thin a fire as possible on plain level grates without 'holes' forming in the firebed. A lump of coal, when alight, ought to burn the same whether it is at the side of the grate or in the middle.

Sloping and part sloping grates require a different technique to fire, also varying with the coal used.

Cheers,
Julian

Hi Julian,
I don't think a lump of coal would burn the same if it's at the edge of the fire compared to the middle for two reasons. Firstly, one side of the coal isn't being heated by neighbouring coal when it's at the edge, it's being cooled. If it's cooler, I can't see how it can burn as quickly. Secondly, the amount of air at the side of the firebox is likely to be less than in the middle. The air is attached to the wall of the firebox so the velocity there is zero. The velocity increases as you get further away from the wall. These effects are not insignificant so I think it's wrong to assume they make no difference. I don't know how scale affects these things, so it might be that on a large locomotive it's irrelevant, but it might play a big part on a 2-1/2" gauge locomotive.

[suctionhose]

Thought I posted on this thread before - must have abstained!

Rosebud grates have sprung up all of a sudden in local engines (in the last 5 years) with improved fuel consumption reported - with some opposing consequences as well.

Reading 1910 blacksmithing textbooks, there is a specified fuel / air ratio for combustion. No surprise there when you think of fuel air compression for I/C engines.

For blacksmiths, you wanted to avoid oxidation - to much air - or carburisation - too much fuel so there was a known depth of fire under the metal being heated in a forge fired with coke that was optimal (5").

Our first locomotive (40 yrs ago) was purchased from a blacksmith who had built the thing without reference to any model engineering literature at all. (He was surprised to find other people built them too)

The grate was made with tapered bars (to assist passing of ash) with spaces less than 1mm. In hindsight the builder clearly understood fuel / air relationship.

In those days, we did burn Lignite (brown coal briquettes were available at the local hardware store). However, the fines in the fire cascaded from the chimney like Versuvius and burnt anyone that rode the train!

We had to open up the spaces to let the fines drop in spite of the sacrifice in yield from the fuel (we didn't care. we weren't running a railway company. we were having fun!)

Lessons learn't over the 40 years following:

Miniature engines' grates are sized around dealing with ash so you can run for a satisfactory length of time. (Fuel specific)
Blast pipes are sized on creation of sufficient draught to maintain steam when running continuously.
Most likely, the preceding points lead to excessive air for ideal combustion but result in satisfactory running.

Rosebuds:
The adoption of Rosebuds is a paradigm shift. You want to control the fuel air mixture at the grate. (local experience suggests 15 to 20% open area).
The blast pipe is excluded from this other than the best vacuum with the least back pressure is a prerequisite of satisfactory breathing at the cylinders. (this is the best starting point regardless of grates or anything else for that matter)
Rosebuds will promote more complete combustion with corresponding reduction in fuel consumption. A stronger draught than for bars is usually necessary and more blower when stationery is required. You still need to deal with the ash. I would advocate a rocking section in the grate or some means of fire cleaning on the run.

Mostly in Oz we have used the dream fuel "Char". Char is so simple to use that anyone brought up on it knows absolutely nothing about firing engines! As it happens, I have a broader experience having run engines on everything from carpet scraps to welsh coal (Hayden's). Even in full size road engines.

Roger's thoughts on using various hole sizes to distribute the air evenly is a valid one. Only experimentation will tell. He's right about the burning in the middle of a bar grate.

From a performance point of view, model loco's are severely debilitated by constriction in the exhaust so always apply best practice in the smokebox before anything else. (My father experimented with a home made manometer to optimise our blast / chimney arrangements) This is the most critical aspect of overall locomotive performance. Everything else follows (proper timing and mechanicals is a given).

The other option to control fuel / air mixture is with a damper. Most models don't have them. Mine does. I use it. Full size operators report fuel savings if the damper is used too.

Go forth and experiment! So much variation is possible with different fuels and running conditions that the final arrangement is quite individual. It can only be found by experiment and tuning to suit the local conditions.

In models you need to deal with the ash. You need to maximise draught with minimum back pressure. Combine those two and you can have whatever grate you want!

[jma1009]

I agree with Ross generally, though I have no experience of rosebud grates or using 'char'.

His comments on smokebox draughting and low back pressure with a smokebox draughting optimum arrangement I totally agree with plus good valve gear and setting of same.

I think a study of Jos Koopmans' book and writings, and Don Ashton's books and writings on valvegears deserve more attention particulary in the UK.

Cheers,
Julian

[gingerneer]

Well said Ross, I have ordered a digital manometer (yes one can be made, but were do you fit it on a 3 1/2 Tich?). So i will be experimenting, i did get carried away by talking about a fluidized bed. I will dig out the notes from when i made both grates and put the exact dimensions up.

Will

[steamcoal]

Ross.

I see my name mentioned down amongst the discarded carpet scraps.:-(

We are beating our heads to death here against a brick wall (and the guy in Brisbane too) with the apparent delerious debate about the merits of Welsh coal as a fuel for miniature locomotive which is held by the Australians.

Its now been 3 years since I bought the first container to New Zealand and there is plenty in Brisbane and Melbourne and still after trial shipments over the last two years, my attendance at two national conventions yacking to the bods and plenty of opportunities for self trial they still hark back to stinky black coal and what ever else burns (Carpet scraps of all things you say ??) and then they tell us that Welsh Coal is total crap, doesn't work and is useless. I am absolutly astounded.

Here in New Zealand we got over the grate issues ages ago, we are 100 miles down the track mate and not looking back. There is everything from Juliets to Staffords using it, Phantoms, Limas, Britannias, Maisies, Bantam Cock and Hunslets in all gauges and low and behold they even have Rosebud grates some of 'em.

I cannot see for the life of me what the huge issue is, apart from the fact that someone will drive 2000 miles to buy Ipswitch coal at $400 a tonne and still complain its smoky and gutless when they get home.

Its something I cannot fathom really, the best fuel in the world is now on your doorstep, even if it is not as good a grade as Julian secret stash in the back shed but hell it all goes well.

Rather than being out there enjoying the live steaming, between fire bans, everyone is busy scrounging for fuel. Sure Welsh may be slightly more expensive and come from the hallowed valley's of Wales but its there, packed, bagged and ready to use. Its even got twice the BTU's for your buck so thats a 50% saving for a start.

Its disappointing to keep hearing this debate going on, and on. Is it the cost, have the firing skill been lost using char.... heaven knows? All I know and I am sure every Engilish and Welsh model engineer will confirm is that Welsh coal is not rubbish. I still fill my tender looking for that rubbish Welsh coal, not found it yet.

Tell me if I am wrong Ross, if I am you can sweep me back under the carpet!

Hayden


p.s I have not given up yet.

[steamcoal]

Whats better to burn Welsh coal  than in  a...... Hunslet.

Nuff Said.