Cold Fires

There are a few people out there who are expert at light & bright but you have to be very accurate with the blade to avoid holes and you are firing continually. However from the boilersmiths point of view (ask Dave Mitchell, who's probably repaired more Bulleid fireboxes than we've had hot dinners) a well filled back end is far kinder to the firebox. So basically for most of us lesser mortals, and those with respect for the locomotive and owner, keep to the bigger back end method.

 

Steady on a bit please! The comment about "showing respect for locos" is perhaps going too far, especially in response to a valid point raised by someone you've never met or worked with.

I think the biggest problem with Bulleids is that they're overkill on preserved lines, including mine in some circumstances, and people end up running around with next to no fire in an attempt to keep them quiet. That's no good to anyone, as it's going to pull into holes leaving stations, etc, and will cause the problems you refer to.

That is not the same thing as firing "light and bright" when the engine is working. As for there not being many that can do it, well we've got a shed full of people that have to be able to, as with some of our engines there's no other option.

 

Edward I think we may be talking about different interpretations of light & bright? From personal experience a Bulleid Light Pacific boiler is quite controllable with a reasonable back end built up, what generally puts them out of control is too much fire near the syphons. Are we misunderstanding each other with regard 'light' and 'thin'? Once the back end is in there you only need fire 'lightly' to keep it there but don't let it go thin. If you have a thin fire in the back of a Bulleid you have to feed it continuously and you are going to get mightily hot doing so. A bigger back tends to keep the cab cooler. On heritage lines we generally do fire the light pacifics 'lighter' than you would on the main line but not as 'light' (thin) as has been suggested at the start of this thread. I have yet to meet an ex Southern main line fireman who fires a Bulleid with a thin back, yes there may have been some that did but as I say I have yet to meet one.

With regard your comments about other locos, yes some can be fired lightly with no problem, but I was talking solely Bulleid Light Pacifics. Firing thinly in the back corners on a wide 'box, not so many people are good at that and I still maintain that isn't good for a Bulleid 'box. No disrespect to anyone was intended in my comments, I'm sure you have have a good set of crews at Grosmont.

 

I suspect that we have fallen into the trap of trying to describe a practical skill on the internet! I also suspect that this thread was started by someone newish to the job, who had been confused by a particular theory.

By firing on a thin fire, I was suggesting a layer of 8 - 10 inches thick all over the grate. Bear in mind at the Moors, we'll get that incandescent, and keep it that way for some time; if you can keep up with it, that's the most efficient way of firing - you fire as it burns away. That was my late colleague's theory. We also get the "cold air through firedoors" myth from time to time as well. There's definitely an issue if you're not burning too much, but if you've a white fire, generating serious amounts of volatile gasses, then it's not true. We had an Inspector who used to hold peoples hands in front of the firedoor going up Goathland bank who came out with that one... "Is that cold...?" he asked!

I get the feeling that we're descrbing methods that work on our particular lines, which offer rather different challenges.

My personal method on WC's? A good 10 minutes shovelling in Grosmont platform - back corners filled as far as poss, then sit down and enjoy the scenery for the next 18 miles.

 

I think you may be right there Edward.

 

I've heard a lot of Western Enginemen talk about this. I think there was perhaps a mainline technique used on western engines. If so I suspect it depended on the vibration of the locomotive to spread the coal out over the grate.

I don't think it's accepted practice for 3850 running on the WSR. ( :-$ I have not yet met two firemen who agree on technique anyway)

 

Clag bombs for the video recorders, (PSOV I think). That's not fire -chucking.

 

What utter nonsense. You don't see haycock fires these days, because they're really not required for preserved lines. We don't do the work that needs a fire like that. I don't know what your experience of firing Western engines is, twr12, but my 27 years of firing and driving them has clearly given me a somewhat different view to you, hence my first sentence.

 

Fire bombs! utter rubbish!

Were you on the trip then??

This was ordinary Daw Mill coal - at each firing of this there is an appreciable amount of smoke due to the nature of the coal.
If we still had proper Welsh steam coal available - the coal for which GW locos were designed - there would not be such displays of smoke.

 

I was not. If you were on the footplate that day and you say it did not happen, then I will apologise and say I got it wrong. However, I have spoken to many support crew members and I am well aware of the practise of throwing in plastic bottles of old oil at selected locations to "improve" the clag for photographic purposes and that's what that looked like to me.

I have fired Welsh and every other type of coal that has arrived at the SVR in the past 27 years and I am well aware of their characteristics and what they do to exhaust plumes, but thanks for the briefing anyway.

 

Think about it logically, if you have at thin fire, it will be hotter than a thicker fire due to higher oxygen levels to combust. I've heard talk that the 'cold air' term doesn't really mean cold air but an un-even temperature distribution due to the higher levels of combustible gases. This means that the plates are subjected to constant cooling and warming creating repeatable stresses. This is reduced with a thicker fire as it’s a more constant flow of combustible gases. Bulleid boxes especially don't like repeatable stresses because of its welded nature and even though the welds should have been normalized there’s no real guarantee to their physical state. From my understanding this is why it’s not good for the firebox anyway.

 

If you try it, rather than think about it, on a piece of railway that will let you, you can get a thick fire just as hot as a thin one, remembering that most of you heat comes from volatile gasses combusted above the firebed.

In my experience, "cold air" is what people who do not properly understand how a firebox works/ have never seen an engine working in anger/ have only experience on small industrial engines with no brick arches, mean by secondary air, ie air admitted through the firedoors. They do not recognise that the correct proportion of this is necessary to maximise the firebox temperature.

Far more damage to boilers is done on preserved railways by irregular use of locos, meaning that they heat up - cool down far more often than on a mainline shed.

 

I'd recommend all of you to the Handbook for Railway Steam Locomotive Enginemen which Nat Pres have discounted at the moment (AFAIK)
The Section on Combustion starting on page 23 explains all the basics for the novice and even mentions the differences between high and low volatile coals and gives a glimpse into the different techniques used.

Personally I'd always listen to fireman who have main line experience and try to learn as much as you can from them about the art of firing. Few of us preserved railway newboys have any chance of coming close to their knowledge and most of us will waste a lot of coal whist trying! #-o

 

If you think about it logically the temperature of the fire is not dependent on whether it is thick or thin. The firebed and components of combustion are theoretically at their hottest when the stoichiometric ratio is unity. If it has too much air then the excess air cools the fire bed and flue gas. If we have too little air, then combustion is incomplete leading to a cooler firebed and flue gas. So, for a thick or thin fire, the temperature depends on the amount of air that it is getting and that is dependent on the amount of volatiles and the mix of primary and secondary air. The good (black) book says that you should fire to the smoke and that a light smoke gives optimum combustion. This isn't strictly correct as, if you have complete combustion, you get no smoke. The trouble is, with no smoke, you don't know whether you have got it spot on or you are putting in excess air and cooling the fire so the light smoke option is the best, but not most perfect, way of controlling things.

The rate of combustion is dependent on the amount of air that is passing through the firebox. To a large extent, this is dependent on the blast up the chimney. We can only control this air by adjusting dampers and firehole door and, in an ideal world, we provide it with the right quantity of coal to burn. A thinner fire will offer less resistance to the air flow through it. A thicker one will offer more resistance so for a given draught will allow less air through. The stronger draught will also tend to pull more air through the firehole door as it is easier than pulling it through the firebed. The important thing with any firing is to have a firebed that is burning evenly all over and that is usually one that is fairly even. However, there are too many variable in the equation to say it is as simple as that. The air entering the ashpan will take the easiest route through the firebed and this is usually the thinnest but the distance it has to travel also has an effect. It's all about resistance and flow. If you think about the firebox as being a room with doors in (the ashpan doors) a dividing wall (the brick arch) and a mass of people (the firebed) in the room and the tubes as being the exit doors and you (an air molecule) trying to get through that room from the way in to the way out, you will look for any gaps in the crowd (holes in the fire!) and also the shortest route that you can take to get to where you are going and take the easiest route to achieve your target.

If we were able to automate our loco boiler, the control system would be fairly simple. We would monitor the boiler pressure. If the boiler pressure started to reduce, we would increase the fuel feed to the combustion chamber. We would also increase the air feed in direct proportion to maintain the stoichiometric ratio to maintain the boiler pressure at the required operational level. It's dead easy with oil or gas fired boilers, a bit harder with coal fired ones because the fuel content is variable. Optimum combustion would require the fitting of a CO and oxygen analyser to monitor the flue gas and vary the air input to keep it at no CO and minimum oxygen level. On a steam loco, all this, and more, is done by the fireman using his brain and skills.

 

What you say is true, but only to a point. It is important to remember that what is required for mainline running is often very different to that on heritage lines. The best idea is to listen to everyone and develop your techniques to suit the job at hand.

 

I fully agree. I should have said experienced firemen. Those that were out on the mainline had to do their turns on the branches and the station pilots as well though. We have an excellent ex SR fireman on the WSR, I won't mention his name for fear of embarrasing him. I always pay attention to what he has to say and like all really knowledgeable people he has time for the trainee's and is able to explain things clearly.

 

just as a sort of small point ive never actually heard of a cold firebecause to be honest they are a bit hot rather than cold to be honest.

 

Yeah but "hot" is a relative term here as is "cold". While no fire is cold those fires that do not make enough heat to raise the required amount of steam are said to be "cold" as are fires that are making too much CO.

 

If this were possible, you'd have to remember to allow for the additional 20% oxygen that travels right through the firebox and boiler due to the speed at which the air is drawn through. You can't burn 100% of the oxygen, as you'd not get enough air through and therefore end up with reduced temperatures again! #-o

 

So my understanding is the amount of air, and hence oxygen has to exceed the stoichiometric coefficient?