Boiler dimensions/ratios

I'm sure there are some standard ratios for comparing boilers - things like grate area:heating surface perhaps. What are the conventionally useful ones?
Which also leads me on to a thought: I don't think I've often, maybe not ever, seen the capacity of a boiler listed. Seems to me that the volume of water and volume of steam space at a given point - glass full perhaps - might also be an interesting comparison, especially if one were looking at taper boilers against parallel.

Jim C

 

I don't claim to be an expert on boilers but some statistics do appear of importance. The grate area is obviously one such, but so too is the width of the water legs between inner and outer fireboxes. These effect water circulation, as does the tube layout which is usually either a horizontal or vertical diamond pattern. The tube and flue heating surfaces are important, and it's necessary to state whether this area is on the inside or outside of the tubes i.e. fire or water side, likewise the superheater elements. The other ones often mentioned are the A/S ratios (sometimes inverted) where A is the cross-sectional are of the total tube and flue ends and S is the total area of the tubes and flues in contact with the water; and the free gas area: the total tube and flue areas compared with the grate area. The firebox heating surface, surface area of the firebox in contact with the water, is priceless for steam raising as it is the hottest part of the boiler assembly.

I've never seen any figures as to the capacity of the boiler in either water or steam spaces but would agree that these could be fundamental to the ability of a boiler to provide steam over an extended period.

A parallel boiler would usually have a greater steam space than an equivalent tapered type but the latter has the advantage that most of the steam is at the firebox end, so hotter. The narrower front end of the boiler also restricts flow and surge to that end on gradients and braking, so less chance of the firebox crown being uncovered. Belpaire fireboxes also allow a greater volume for steam than a round-topped type. On the other hand, parallel boilers are easier and cheaper to build, as are round top firebox.

Edit: Try A.F. Cook's 'Raising Steam on the LMS' (1999) RCTS ISBN 0 901115 85 1

 

Why not look into Phillipson's book: Steam locomotive design: Data and Formulae
Secondhand available, Camden sold a reprint some time ago.
Kind regards
Jos Koopmans

 

One of my bibles. Thoroughly recommended to anyone who wants to understand the fundamentals of loco design but you must accept that it was written in the 1930's and science and engineering has moved on.

Boiler design is frequently a compromise rather than a collection of ideals. The track gauge effectively limits the width of a narrow firebox as it has to fit between the frames and the need for adequate water space reduces grate width, as well . The need to squeeze a firebox in is also detrimental to good frame design but that's not what we are discussing. The length is limited by the ability of the fireman to throw the coal to the front (assuming hand firing.) Boiler barrel diameter is constrained by the loading gauge at the top and by the need to fit safety valves and a chimney. It is also constrained by the driving wheels. If you want a larger diameter, smaller wheels will give you that. If you want to go larger than about 33 sq ft then you have to go to a wide box. The need for the driver to see ahead also limits firebox and barrel diameter. Just look at a cross section of a Coronation at the firebox and work out how you could get bigger. A wide box needs a combustion chamber which can affect tube length. The front tubeplate position is influenced by the wheelbase of the loco and where you want to put the cylinders. The longer the tube the more heat that you can extract But there is an optimum length to diameter ratio and if the diameter is too large, you start to lose the ability to extract the heat as you get laminar flow and the majority of the gas is not in contact with the tube surface. If you want a high superheat you don't want to have your tube length too long as you don't want to cool your exhaust gases and thus cool your steam but, conversely you don't want your gases going up the chimney at too higher temperature.

Then there's the question of weight. Steam loco design is invariably one of reducing weight to meet the constraints imposed by the Civil engineer; one of the reasons why taper boilers were popular, simply because your big diameter boiler, necessary to burn the coal and allow the gases to escape to the smokebox and chimney, is going to contain an awful lot of water that you don't actually want. Having a lot of water in your boiler may give you a reserve of energy but, for the most part, it isn't a good idea.

 

I don't know if you've come across this one Jim:

http://kesr-mic.org.uk/resources/Locomotive+Boiler+Design.pdf

It's on the K&ESR site with a bunch of other Swindon papers. Probably not exactly what you are after, but there is quite a bit of technical info and various ratios for boiler design from the early Churchward era (1905).

 

IMHO, one of the most interesting books on locomotive boiler design is the RCTS one 'Raising Steam on the LMS' by A F Cook. Full of interesting info on the subject it covers.

 

For narrow gauge locomotives the fire box volume is the most critical parameter. Using the same coal, the flames does not scale with size. The flames take the same time to burn.
The grate area is thus of little importance for fire boxes of less than about 1 m (3 ft) in height.
This is the reason for many narrow gauge engines to have a rear pony truck (or bogie), then you can make a deep and effctive fire box.
Also the articulated Failie, Kitson-Meyer and Garrats allow for very deep fire boxes. (While Mallets and standard Meyers does not.)

 

The depth of the firebox is an interesting one, mainly from point of view of the firehole, and where it should be. Taking things to extremes, a large mainline express loco with a sloping grate has the firehole quite low down, where the fireman can shovel, and a cavernous space above the firehole for combustion, and the water glasses high up where they can be seen, whereas A traction engine has the firehole just below the crown and the water glasses somewhere near the driver/firemans knees, so firing under the door, especially on the move (traction engine) is very difficult, fortunately stopping on the road to fire is not to difficult, then a Sentinel NO! DON'T EVEN SAY IT!

 

Apologies for replying to this very old thread, I came across it via Google when looking for something else.
I'm a bit confused by the first sentence of the last paragraph. Isn't the steam temperature (before the superheater) just dependent on the pressure, and not the type of boiler?

 

I'll draw attention to my first sentence, but the steam and water temperatures will not be constant throughout the boiler as convection currents move both around. For example, the water at the bottom of the boiler will be cooler than that at the top which will have steam bubbles flowing through it, and heat rises in a fluid. The temperature of the water and steam in contact with the inner firebox with the fire inside it will be higher than that at the smokebox tubeplate, which is in contact with the cooler gases on the smokebox side. This why over-long tubes are to be avoided: the heat available at their front end is considerably lower than at the rear.

 

The steam can be more or less wet that is being a mixture of saturated steam and droplets.
Mr Giesel said that max continious steam in kg per hour was (5525* total evaporative area in square meter) divided by(35+(total evaporative area/grate area))
The evaporate area is on the fire side(i e inside pipes and firebox.
steam locomotives around 1900 used 10 kg of steam to make one horsepowerhour and Chapelon around 5kg on a nice day.

 

All this kilogrammes metres stuff and driving wheels in metrics is absolutely meaningless.

 

You could convert it back to Imperial measurements. Tables are available through the good artifices of Dr. Google.

Come to think of it, I could convert the measurements myself too.

 

Inclined to agree. The width of a thumb is about an inch, the length of a foot is about 12" (one foot), and a pace is about 3 feet (a yard). Refined and tolerances reduced to the present day dimensions.

Metric is figures pulled out of the air. How was the distance from the North Pole to the Equator measured many centuries ago when the first person (Admundsen) to get to the North Pole only achieved this circa 1909? Or even as the distance travelled by light in a vacuum as being 1/299,792,458th of a second?

https://en.wikipedia.org/wiki/Metre

 

There is already a thread for people to rant about metric vs imperial - any chance the discussion could disappear there?

https://www.national-preservation.com/threads/metric-vs-imperial.901507/

Tom

 

Please. I had to bite my tongue rather hard as an engineer who has worked in both


Sent from my iPhone using Tapatalk

 

You can disappear this if you like, all I'm saying is a British steam locomotives has 6'8 1/2" driving wheels, and we all know what that means, 16" x 26" cylinders, great. 2020mm, 1800mm 1626.9mm? err.
There's a convention, when dealing with(historical) railway engineering of using imperial units. One wonders what people are attempting to gain by defying that convention.

 

Surely we all know what that means too? It's not difficult and it's been taught as the main system in schools for almost 50 years now.

I'm not one for collecting old books, but I'm fairly sure the oldest railway book I have - from the 50s - is purely in metric.

 

I once got in trouble at my local barbers for asking for 25mm off all round.
(They offered to take an inch off my ears too for being cheeky)