Draughting arrangements for Bulleid Pacifics including the Giesl ejector

That's an interesting idea Jos. Are there more slides to that presentation (and would you be happy to share them with me)? I had planned analysis of a desing with more nozzles than 5, but we ran out of time. If we run the project again I may explore that...

 

That is ok with me, please let me know your em address, this site does not allow .ppt extentions. As for the others, my main points of critics are that the total orifice area could be larger, I proposed 7 orifices instead of 5, secondly
the distance from orifices to chimney entrance was too large and thirdly the chimney exit was too large. If the exit velocity is too low it cannot cope with the underpressure that exists on top of the smokebox. As for the orifice to
entry distance, the entrainment during the long travel is already the total needed, it renders the chimney useless. The MN frontend could be vastly improved by just altering the dimensions while retaining the Bulleid-Lemaitre concept!
Kind regards
Jos

 

I didn't intend to convey that I thought the Giesl design is in any way inherently likely to render a loco claggier, merely that 92's mainline runs, 30-odd years ago, did, let's just say, tend to be a bit too ready to accede to the requests of certain amongst the photographic fraternity. There's a mass of published photographic evidence from back then.

 

I believe that we have to look at more than the exhaust system. You can obtain the advantages of improved steaming and improved power output due to reduced back pressure but the effect of drawing even more primary air through the grate is not being considered.

 

Are you suggesting something like the GPCS?

 

Sure 242A1 will answer in due course but i think the suggestion will be along the lines of a whole steam circuit approach, but not necessarily GPCS.

 

I would think there is a calculation you can carry out.
If you start with the required steam flow you can calculate the required combustion air flow through the boiler.
Then given the resistance of the boiler, grate, ashpan etc. you can determine the required smokebox vacuum to produce this flow.
Allowing for some losses of steam (leakage, injectors etc.) you can check whether the exhaust system will adequately steam the boiler, and what the back pressure on the cylinders will be.
I think this is what 242A1 is probably getting at, but I'm sure they will reply is it is something else.

 

Could the clag with 34092 be down to the awful Russian coal that was about at the time. It wasn’t the only engine to suffer from black clag in the early 90s.
It certainly didn’t happen on the Southern in steam days. Coal probably came from various sources but I know Nine Elms was supplied from Betshanger in Kent

 

One thing I forgot to mention is that a locomotive can sometimes steam well at high outputs, but the pressure falls away when running at reduced power , so it's necessary to look at this too. A Merchant Navy should be a good starting point though!

 

Like certain BR Standards...

 

One of the reasons for that was a too large distance between orifice(s) and chimney throat. Both Bulleid and LMS Pacifics suffered from that.
Kind regards
Jos

 

So is this project intending to rebuild the loco back to its original condition or is it intended to be yet another "modified" Bulleid Pacific? The scheme is starting to sound a bit confused to me.

Peter

 

0.1 seconds and 0.1 mm. In a locomotive firebox working at high evaporation rates and without significant secondary air 0.1 seconds was the time taken for a coal particle to pass through the firebox. This was sufficient time to completely burn a particle of 0.1 mm or less in diameter. A 10mm diameter particle would need 1000 seconds to be completely burned. The grate limit is reached when 50% of the fuel fired exits the chimney as unburnt losses and 10mm is the order of the particle size ejected through the entrainment process. In addition to work carried out by the railways the NCB had the Coal Research Establishment and work was carried out to establish the relationship between particle size and the gas velocity required to lift these particles off the firebed. Steady flow was assumed and the situation in a steam locomotive is not steady and so particles of at least 10mm in size tended to be lifted from the fire.

You can of course deliberately design the exhaust system with a low front end limit in order to prevent a locomotive from being worked inefficiently. The combustion rate is limited or restricted by the draughting.

However an exhaust system should function across the wide range of power outputs demanded of the locomotive that it is fitted to, it should automatically link the supply of steam to the changing demand. If a system fails to do this then it is not a suitable design.

Carry over is not the only source of fuel loss, up to 10% of the fuel fired can end up in the ashpan and this wastage passes by unnoticed by the casual observer and also by quite a number of the not so casual.

The water vapour which rises into the air above cooling towers can be confused as smoke and regrettably images used in the media have aided this confusion. You might well be working your locomotive in accordance with best responsible practice, with a clean plume of vapour rising into the air and a light grey haze at the chimney top however across the fields, away over the road, there is a chance that you will be observed. And you will not be seen as being at work on a heritage railway, you will not be seen as keeping history alive. You will be seen as an irresponsible polluter with all that that entails.

We cannot assume that we will continue to be looked upon in a favourable manner.

What we can do, and indeed should have done, is modify our locomotives to reduce fuel consumption by adopting reversible changes. Exhaust systems are an obvious item but any component prone to leakage should be replaced with a redesigned version. If it isn't particularly visible why worry about it? There is a cost but there are savings to be made. We should not think of ourselves as a special case, something so precious that it should not be expected to change but ought to be accommodated without question. We need to maintain support and be persuasive in our arguments. There is so called common knowledge out there which we need to correct but we need deeds as well as words, something material to display to help give weight to what we feel the need to say. And in these days of 'peculiar shouting' we need our word and actions to be very well prepared,

 

GPCS would help to mitigate the above problem, although you can get into trouble if you add too much steam, as the reaction between coal and steam is endothermic.

In terms of consuming coal, injecting steam below the grate is equivalent to injecting pure oxygen.

In normal combustion C + O2 > CO2
In the GPCS reaction C + H2O > CO + H2

An increase in secondary air is then required to burn the carbon monoxide and Hydrogen.

Although as Torgormaig has suggested, do you really want to go there?

 

If you look at the whole reaction, the difference between a conventional combustion of carbon with a stoichiometric amount of air; and a GPCS combustion with a stoichiometric amount of air is basically that for every mol of carbon burnt, you waste a mol of water as superheated steam (entering the boiler cold and emerging from the chimney at flue gas temperature) with all the attendant energy loss of the latent heat of evaporation of the water etc.

(Overall, the conventional reaction is:

C+O2 --> CO2

and the GPCS reaction, in two stages, is:

C + O2 + H2O(l) --> CO2 + H2O (g)

So whatever the advantages in preventing clinker etc, the GPCS reaction represents a net decrease in efficiency. It seems to me the sort of thing you wouldn't choose to do, unless other conditions - such as very poor coal - made it sufficiently desirable to offset the loss of thermal efficiency.

Tom

 

Maybe wrong here but i think thattheres a bit more to it, isnt the thing that gets coal to burn in the first place - its not pure carbon - the amount of hydro carbons present. In normal combustion the breakdown/ combustion of these is happening in the fire bed and the high temperatures in the fire bed causes the clinker. In gpcs the deep firebed/lack of oxygen liberates this with only partial combustion, the rest of the process taking place above the firebed in the presence of 'secondary air' .
The thing that always worries me about gpcs is the rather unpleasant smoke when the loco is at rest, and the potential for this to 'pop' back to life when work recommences...

 

That all makes sense, as far as it goes, but don't you get back some of the energy from the hot steam as heat in the gases going into the tubes? Insofar as there is loss of energy, couldn't you alleviate that by using saturated steam from before the superheater?

Edit: Maybe we should move some of this to a new thread: Steam locomotive thermodynamics.

 

As I understand it, it's exhaust steam that's added to the grate, so there will be an increase in temperature between the input to the grate and the flue gas, but it doesn't include the latent heat. So, yes, there will be a small loss of efficiency, but it's not that bad. The draughting would need to be adjusted to cope with less steam passing up the blastpipe, and more passing via the boiler tubes.

 

The plan is to return the loco back to it's pre-rebuilding condition (air-smoothed casing, chain-drive Bulleid valve gear, balanced (post 1954) crank axle), but we may explore improvements to the draughting and implement some changes that were made to the original Light Pacifics after 1959.

 

So I'm not the only one who thinks that if you are producing a new build, it should include all the warts, as well? The only mods should be to cater for things that are no longer reasonably practical or possible.