Deep Firebox Performance vs Shallow Fireboxes

[L-105]

The advantages of a deep firebox, in terms of increased evaporative capacity relative to a shallow firebox, are known and I have seen it mentioned before in various topics and websites, including the 5AT Advanced Steam Locomotive website where two statements got my attention:

 

"The deep firebox of a 4-6-0 has, size for size, a higher evaporative capacity than a shallow firebox boiler"

 

"The reason for condemning the 4-6-0 is its supposedly inadequate boiler capacity. Even in the past this was not necessarily true, for, as Chapelon has pointed out, size for size a boiler with a deep firebox, as generally used on 4-6-0's, has greater capacity than a shallow firebox boiler"

 

Its interesting that two of the largest deep firebox configurations ever applied to a locomotive, the PRR Q2 and C&O H-8, are regarded as being among the most powerful and having the highest evaporative capacities for any steam locomotive class, despite there being other classes of locomotives that existed with shallow firebox configurations which had more grate area and with similar or slightly more direct heating surface. 

 

While the grate area and direct heating surface are typically listed in the stats of a steam locomotive class, the firebox depth isn’t, even though it seems to be one of the important factors in determining the evaporative capacity of a boiler. I know there are a large amount of variables, but for a boiler of given fuel, convection section, pressure and smoke-box configuration, is there an approximate equivalency factor to get an estimation on just how much advantage a deep firebox has over a shallow firebox in terms of evaporative capacity and vice versa? How much larger would a shallow, over-the-drivers, style firebox configuration need to be in order to match the performance of a deep firebox of given size?   

[Paul Milenkovic]

The other confounding factor is the use of a "combustion chamber" (extension of the firebox between the brick arch and the front plate of the tubes).

Alfred Bruce liked combustion chambers.  He states that total firebox volume, not grate area, is the limiting factor in firebox performance.

David Wardale dismisses combustion chambers, but then he wonders how the Pennsy M1 boiler had such an unusually high "grate limit" (rate of combustion per area of grate where the drop off in combustion efficiency owing to stuff just blowing through the tubes and up the stack makes further increase in combustion rate of no use to further power).  According to Bruce, the M1 has an enormous combustion chamber compared to other locomotives.

When it comes down to it, Wardale's data on his "unique application of the gas producer combustion system (GPSC) to a stoker-fired locomotive" show that the GPSC is not a "magic bullet" to completely eliminate stack losses at high combustion rate.  He expresses great frustration with the combination of the GPSC, especially the high expectations for it with the powdery coal in China and what he complains was a lack of the enthusiasm, at least in comparison with the shop workers that he enjoyed in South Africa.  If you look at the combustion rate graphs for the M1, could a proper combustion chamber give much of the performance gain of all of the firebox surgery needed to deepen it for the GPCS.  Like with the superheating/compounding controversy, should both be used?

The combustion chamber is not without problems, namely it is thin-walled just like the proper firebox and adds to the number of inspection and repair-intensive staybolts.  The problems of the GPSC -- read Wardale's whinging about them.

Then again, bring-on-the-staybolts-if-you-have-good-water-treatment, but Wardale whinges about water-treatment woes in South Africa (and those darned SAR chemists giving him weak-tea anti-foam).

So part of the deep-firebox is Wardale's point-of-view.  With that, I turn things over to . . . Overmod!

[Juniatha]

 

[Paul Milenkovic]

Ah, water treatment!

Seems so simple, but Wardale was frustrated by the trouble with it, mainly the ineffectiveness of the anti-foam to allow the high dissolved solids the water treatment plan called for.

[M636C]

Because reality is always less believable than fiction, there was a single locmotive type that was fitted with both shallow and deep fireboxes and also with both narrow and wide grates.

This was the Queensland Railways 3'6" gauge class B13. The B stood for three coupled axles (they had no single drivers, so A stood for two coupled axles) and 13 was the cylinder diameter in inches. This was not a large locomotive, a 4-6-0 with 39 inch driving wheels weighing 22 tons as built and 27 tons in final form.

As first built it had a very low set boiler with a shallow firebox with a 10.5 square foot grate set above the trailing coupled axle. Later versions had a longer coupled wheelbase, placing the trailing coupled axle behind the firebox, which allowed a lower grate location. The boiler pitch was raised slightly providing a deeper firebox.

At this time, there was some change in the engineers responsible for locomotives. William H.Nisbet, who had been the chief engineer of the Westinghouse Brake Company in England joined the QR as CME in 1899. The QR had changed to locally mined coal about this time which was of poorer quality than the previously imported coal and this had caused some concern. Nisbet designed a new boiler with a Wooten style firebox which required a higher boiler pitch. This had a grate area nearly twice the size of the original at 18.75 square feet.

However, before these boilers could be installed, in 1901, Nisbet's brother, who had replaced him at Westinghouse, died, and Nisbet resigned and returned to his old job in England.

Nisbet was replaced by George Nutt, from the London and South Western Railway, who disagreed with Nisbet's views. So only 25 of the B13s were fitted with the wide firebox boilers, and Nutt designed a new narrow firebox boiler, retaining the higher boiler pitch of the Nisbet boiler with an 11.25 square feet grate area, but with a larger heating surface.

Nutt carried out comparative trials of all three boiler types in 1903 and the Nutt narrow but deep firebox proved to be most economical regarding coal consumption.

The deeper firebox of the final design provided not only a larger direct radiant heating surface through the deeper firebox, but also a larger firebox volume that allowed more complete combustion. Had the wide firebox boiler been fitted with a combustion chamber, it may have been more economical, but it is possible that the larger grate areamay not have been justified in such a small locomotive.

Much later, in the 1950s, the Western Australian Government Railways obtained some light 4-8-2s (W class) for secondary lines that were fitted with a large grate and also a large combustion chamber which provided adequate volume for complete combustion of poor quality coal and these proved much more economical than earlier designs. There is a technical paper covering these in the Journal of the Institution of Locomotive Engineers.

Peter

[daveklepper]

Thanks.  Very interesting indeed