Boiler calculations- steam output

Hello,

Can someone point me to a formula for calculating max steam output from a boiler please?

I am drafting a boiler design but want to match it to the cylinders, to make sure the boiler could keep up.

Regards

Jon

There are no simple, single formulae, but you need the boiler to feed the cylinders at its optimum rate, not flat-out.

Whole text-books were written for the trade and university engineering-courses in this area!

You need work back from theoretical steam consumption in the cylinders at maximum power - not at all easy to assess accurately due to largely-unquantifiable losses. The power depends on swept volume and cut-off, hence mean effective cylinder pressure, and speed; even before you try to consider steam lost in the clearance volumes and heat losses. In full-size practice this often entailed using a so-called "Diagram Factor", a multiplier of <1 of the theoretical Indicated Horse-Power, largely derived from analysing tests on actual, similar engines.

So that's how much water to boil....

It is relatively easy from physics and heat-engine design texts, to calculate the amount of heat to evaporate a given weight of water at the required pressure in a given time from fuel of known mean calorific value; but you also need calculate the boiler for optimum combustion, heat transfer, superheat (if used), etc.

And a boiler is not very thermally efficient, so one rated purely on calorific value, specific heat, temperature and pressure sums is not likely to keep up with the engine.

There are practical calculations in the model-engineering literature for e.g., grate-area / tube proportions; but greatly simplified from the Nature whom LBSC used to say is non-scaleable. They work, but most are based on miniature locomotive performance; and if you look at the IMLEC reports, the winning efficiency is the least worst value!

Possibly the easiest way it to try to compare existing designs; but the basic rule is that you need supply much more heat than expected to give the engine the steam it demands..

Nigel,

Thank you for the thougher reply.

I worked out what space I had to play with scaled downfrom the full size engine, and then worked out the inners. Starting from the boiler, 4" dia tube 13swg x 11.75" long. Split for 4.5/8" along its length and opened out to make the firebox wrapper. This makes the firebox 2" deeper below the bottom of the main tube, with no extension pieces to solder on. So overall height of boiler can be anything upto 6". Grate area scales down to 12sq in. Making the outer firebox 4" x 4.5/8" with a 1/4" water space and using 13swg again for the inner wrapper of the firebox I've got the grate area to 11.718sq in.

The tubes are worked out as 3/8" x 20swg @7.3/8" length. Inside dia tube area needs to be between 1/5th and 1/6th of grate area, so this gives a figure of between 6 and 7 tubes. There are model examples of 16SWG tube been used for the main barrel at this dia. So it may be very possible to decrease this, thus making it easier to solder up, and requiring less heat, whilst also gaining a little extra water sspace.As you can see the boiler is by no means large.

It looks like there's not enough tubes, but the calculations I've done say this is correct.

Also Making it an easier loco to build is the need for only three forming plates, one for front tube plate, one for inner firebox, and one for backhead. The throatplate been able to use the latter too. No extension pieces for the firebox.

The firebox top stays will have some metal removed to allow water circulation across the top of the firebox.

Biggest issue I see is the absence of a dome and priming. However been so small I was wanting to work out the steam output and use that to design the cylinder block, which itself is only small cylinders for a 5" engine. I believe these may have to be increased in dia from scale size, to increase the swept volume.

The boiler would be for a NER Y7. Using Joy valve gear, and twin cylinder block between the frames. Wheels are 3.7/8" dia.

If any of this information is at all helpful for any possible calculation.

Regards

Jon

Jon,

Your main issue will be where will you run it (if you intend to.)

If a local club, will the resident boiler testers except your design? If not, you cannot run on their track. It might be better in this case to present your outside dimensions to them as they may be able to suggest an existing design that fits the bill. Opening out the barrel seems to be out current of favour.

Can you say where this idea that opening the barrel is not in favour comes from?

Many existing designs do this and all have 'grandfather rights' so while any boiler inspector is perfectly within their rights to refuse to test any boiler - giving this as a reason is probably not valid.

Other factors may of course apply as well, justifying their decision.

Where on earth has that come from? Hundreds of not thousands of boilers made this way, not heard of any failing at this feature as long as properly made.

I'd find an existing boiler design of about the right size (Henry Wood's Emma Victoria) comes to mind, then base your cylinder size on his allowing for different wheel diameter.

Exactly, being a new design grandfather rights would not apply.

The redesigned Minnie boiler with 4 hollow big stays per side with the rest as 1/8" rivets silver soldered instead of all threaded stays and Comsol. Both are valid, but where the new design is published is not known to me.

There isn't a singular formula for that.

Because of the sheer number of variables it has to be modelled by a system of equations, and solved for a range of values of one (or more) variables, to give a "region" of acceptable solutions.

I think I still have my course notes and worked problems from when I did a "Boiler Thermal Hydraulics" module as part of my masters... If you're interested I can try to locate them for you.

I might even have some pre-configured excel spreadsheets or [shudders] FORTRAN scripts to match the approaches taken in the course notes too.

Regardless of other things I would strongly encourage you to use a validated calculation method for sizing the pressure relief valves based on the worst-case scenario.

Edited By Jelly on 15/10/2022 23:22:09

If you really want you do it from first principles see Martin Johnson articles in ME. No doubt someone will provide issue numbers. Beware you need quite a bit more steam than swept volume would suggest.

And don't forget , it doesn't end there.

It is essential to get the draught/blower system correctly set up too.

Thanks for the warmup, Duncan.

My xl spreadsheet which was the subject of articles starting in ME 4584 is available for the asking and is still being developed asI get the time. PM me ifmyou would like a copy.

Try and get your boiler tester onside sooner rather than later, but if you design to the Ozzie code for strength, there should not be too many horrors awaiting.

I would imagine the foundations of my program are similar to Jelly's lecture notes, but mine was written for models.

Martin

Jon,

It is the internal cross section area of the tube you should be using, not the internal surface area.

3/8" x 20swg has an internal cross sectional area of 0.0721ssq in.

At 1/6th grate area (1.935 sq in) this would give you 27 tubes.

At 1/5th Grate area (2.3436 sq in) this would give you 32 tubes.

Hope this helps.

Sandy.