Member postings for Martin Johnson 1

Bob,

Thanks for the clarification on the boiler design. I am going to keep it simple and consider the one without thermic siphon. From a thermal point of view, the siphon has to be a good idea, it transfers more heat to the water by radiant firebox heat, but reduces the heat transfer in the tubes (by lowering the entry temperature). Overall, it will gain more than it loses.

Julian,

I am aware of some of the work you mention on draughting. However, I think this where it is easy to get bogged down in detail. I come at that problem from a jet pump point of view (having been a pump designer in a previous incarnation), and all the various "design guides" are aimed at optimising the efficiency. However, you cannot get much beyond 35% Effy. for absolute optimum geometry, and typical chimneys with restrictions of scale, height etc. will be considerably less. The major factor here is that the blast nozzle area is tuned to the whole engine. I should state here that my interest is in road engines which have a tall chimney, offering much better jet pump geometry.

I agree with your comments on driving style, but again it is a bit of side issue - I know that anything I do will be through a thick fog of statistical scatter.

Martin

Thank you all for your contribution.

Neil,

I agree that a good driver has a huge influence on efficiency. I also think experimental accuracy (or lack of) has a huge influence on just who wins - just tell me again that you can weigh a coal consumption of 2 lbs. to 3 decimal places of accuracy...........

Duncan,

I agree that there are other factors in the safety valve that can ruin it's performance. But if the 'ole aint big enough to start with, then even Gordon Smith's clever designs can't put it right. I ignored other factors to try and keep my survey within bounds. On that subject though, does anybody have trouble getting Torquay Manor and Les Warnett's 9F through the 10% pressure rise test? My correlations show that they both have rather small areas compared to grate size.

Julian,

I agree that the whole subject is fiendishly complex. However, my initial goal is rather more modest - what is the grate loading in a miniature Stephenson engine? I do have a statistical answer beginning to emerge, but the scatter is large. I do have some ideas on narrowing that down a little, but in any case a rough guess is better than no guess.

My reason for wanting to know is to develop a boiler performance calculator. Until one knows that grate loading figure, then you cannot calculate heat input, mass flow, flow velocity, Reynolds numbers, Nusselt numbers or very much else. You will be pleased to know that the boiler calculation module is done and working and has been correlated against some full size experimental results. The superheater calculation module is proving rather "challenging".

Such boiler thermal design work that I have been able to trace in ME and similar is very waffly speculation and of no use in designing anything a bit off the usual.

Thanks to everyone again, I am off to put in some more numbers on the computer..................

Martin

Absolutely brilliant.

Thank you Duncan and Eric.

Martin

Trevor,

That is absolutely brilliant. Thank you very much.

Martin

Hello

I am currently analysing past IMLEC results to try and improve the thermal design methods for small boilers. Please can anyone help with the following dimensions of the Martin Evans “Nigel Gresley” design:

Cyliner bore

Cylinder stroke

Wheel diameter

Grate length

Grate width

Firebox height

Firehole diameter

Smoketube diameter and number

Superheater flue diameter and number

Length of tubes

Working pressure psi

Safety valve bore

Many thanks in advance

Martin

Hello

I am currently analysing past IMLEC results to try and improve the thermal design methods for small boilers. Please can anyone help with the following dimensions of the Martin Evans “Simplex” design:

Cyliner bore

Cylinder stroke

Wheel diameter

Grate length

Grate width

Firebox height

Firehole diameter

Smoketube diameter and number

Superheater flue diameter and number

Length of tubes

Working pressure psi

Safety valve bore

Many thanks in advance

Martin

Hi Andrew,

Yes, the turbulent heat transfer is better due to the thinner boundary layer. Back in the day, there were ridged flue tubes available for that very reason of tripping up the boundary layer; whether or not it would be practical in miniature, I have severe doubts - it never really caught on in full size.

Fully understand you cutting yourself on plastic boundary layer trippers. Wifey used to work in a wind tunnel (very fast military stuff), and BL trippers in miniature are little sticky up razors - used to cut her hands to ribbons.

Also thanks to Duncan Webster who has P.M'd me with some useful leads.

My present task is trying to set up a calculation routine for evaluating superheaters (both smokebox coil and flue type). The numeric complexity is "challenging".

Martin

Duncan,

Was the article by Wallbank concerned with the benefits of superheating or the numbers of the heat transfer into the superheater element? I am working on a calculation method for superheater performance, so am eager to find out what has been done before.

Martin

Just a brief update. I still don't believe in the Ewins boiler factor. Nor, having investigated it's roots, do I believe in the Keiller tube factor.

What I have done is to produce a spreadsheet that calculates from first principles the heat transfer in a boiler.

The firebox segment uses the well known Stefan Boltzman radiant heat transfer equation.

The tube bank section uses accepted correlations of convective heat transfer for the laminar or turbulent flow regimes as appropriate. For a summary of these see this website or degree standard heat transfer texts:

http://web2.clarkson.edu/projects/subramanian/ch302/notes/Convective%20Heat%20Transfer%201.pdf

The whole lot needs a correlation of flue gas properties (from my steam tables) against temperature to provide constants for the equations.

The final key to the problem is knowing the mass flow through the boiler, which has prompted a review of a lot of IMLEC results, amongst others. This seems to be a key aspect that nobody has picked up on before!

I have achieved a good correlation with test results from full size locomotives, but test results from models are as rare as hen's teeth. I am particularly pleased with the agreement of heat transfer Vs. tube length to full size tests.

The program also calculates pressure drop of the flue gas through the boiler (quite easy as part of the heat transfer calcs.). It seems that flow resistance in the tube bank is a relatively small part of the whole - entry and exit losses dominating - so that the basis for Ewins' formulation including tube length as a factor in pressure drop has no basis in fact.

I have confirmed that tube bank flow in "full size" practice is normally turbulent, while in 5" gauge sizes the flow is laminar. Once we get to larger boilers (such as my 4" Burrell TE) it all rather depends.

I am intending to develop the program to look at the whole ashpan to chimney circuit, including blast pipe performance. The reasoning is that any fool can make a good heat exchanger with enough pressure drop. The trick is to design one that does not need lead to a performance loss in the engine by needing an excessively small blast pipe orifice. There is a balance to be struck, but it is not as simple as Ewins suggested.

Hope to write it all up for one of the comics in due course.

Martin

Thanks to all who took the trouble to reply.

The basics of Carnot cycle efficiency limits were drummed into my alcohol fuelled student brain some 40 years ago, but they are of scant help when optimising boiler flue layouts.

Ady1, thanks for the reference to ME 3618 – I shall look it up.

Julian and John, you need to get together and come to an agreement on the Kt factor! It seems to give a figure that will use most of the heat in the flue, but I think it is very empirical.

As I said, I have no problems with the Ee factor, which is all about balancing heat input to steam demand from the cylinders. One term that it does not include is engine speed, presumably because most 3.5 / 5 / 7.25” railways trundle round at approximately the same speed for safety. The other term it really needs is “Grate Loading” – i.e. how many kg/hour/m2 of coal can you put through a grate before you get clinker. I am not convinced about consequences if you take it too low. Grate Loading must be a relevant design parameter that is a function of the chosen fuel – clearly it would not be a limiting factor with gas firing, for example. I have expanded the formula to take account of these parameters for road engine use and it is quite a simple theoretical derivation.

Duncan, I am glad I am not the only one who has problems with Eb (I also agree that superheating is a grossly misunderstood science). Julian, your comments about the rail motor are exactly pertinent to the flaw in the Eb factor – as well as it not being dimensionless. John, I have come to the same conclusion as you that the critical boiler factor is to get the ratio of tube bank area to grate area about right. I can see that a short tube stack might lose some efficiency up the chimney, but can see no other consequence, assuming blast nozzles are adjusted to match the reduced flow resistance.

As a result of all this, I am working on a program to calculate the heat transfer and flow resistance in a bank of tubes from first principles. I am using the summary of heat exchange correlations as reported here:

**LINK**

So far, I have proven that flow in model (and many full size) tube banks will be laminar, and have got the kernel of the spreadsheet working fine. I hope to either prove or disprove some of the assertions about “all the heat transfer taking place in the first n% of length”. Test results in my 1929 Locomotive Engineer’s Pocket book tend to disprove the assertion, and I hope to calibrate the mathematical model against these.

JasonB, I am already in contact with Julia and SuctionHose on TT forum. My alter ego there is tenor. I have a build thread there for the project that is prompting all this at:

**LINK**

or at:

**LINK**

Not surprisingly, this is a stumpy little boiler with a very short tube stack – but how many of what size? And if they are very short, can I use a smokebox superheater coil to soak up some of the waste heat (as Fowlers did) or should I go to flue superheaters? Time and a lot of sums will tell.

I was working on an article about the Fowler design and build which prompted this line of thought. Then when you start to marshal your thoughts, you find they are not very clear in the first place. Hence the plea for any other work. Seems I might now have two articles to write………………

It all keeps the grey matter going.

Martin

Hello all,

Back in the '80s Jim Ewins published some proposals for a set of engine factors to help in designing rail locomotives. I have been looking at how these apply to road locomotives and in the course of that have been doing some research of my own, starting from Jim's articles in Engineering in Miniature in 1985.

Does anybody know of discussions, updates, adherents, opponents or any work that has been done in that vein in the intervening 30 years?

I am fine with the Engine factor Ee. Also accept the tube factor Kt. But from a theoretical viewpoint, I have big problems with the Boiler factor Eb and as a consequence the Overall factor Eo.

Many thanks,

Martin