We need to look to America for the origins of the Diag 94, 94A & 107 Boilers. The Pennsyvania Railroad was at the forefront in development work on Boiler proportions, it started this work in 1905 with the building of two experimental 2-8-0 Class H28, these proved to have excellent boilers with a wide round top firebox. In 1907 the boiler was refined and a wide Belpaire firebox was added, when the new 2-8-0 Class H8 were built . In 1910 a 4-4-2 Class E6 was built, again an experimental engine, the boiler being copied from the Class H8. the boiler was not superheated, but this was soon added, and over the next four years the boiler was subjected to rigorous tests. Another single engine was built by Alco in 1911 and designated Class K29s, it is reported that this too had an excellent boiler with a round top firebox. One can only assume that the Class K29s was tested alongside the experimental Class E6s. However 1914 saw full scale production of the 4-4-2 Class E6s (S being for superheated), which proved to be a great success, and led to the development of the 4-6-0 Class G5s in 1923. The E6s tests and experience gained from other engines was put to good use when designing the 4-6-2 Class K4s. A longer boiler would be required, but the proportions were well established, and a Belpaire firebox was fitted. A combustion chamber was provided extending 36" into the boiler barrel, this ensured that ash and fine coal continued to burn before entering the flues. It is alleged that steaming capacity was increased by 20%, one has to ask the question "in comparison to What???". One Class K4s was built in 1914, and was thoroughly tried and tested before full scale production commenced on the remainder of the Class in 1917.
Gresley ever mindful of developments at home and overseas, took full note of the K4s and its Boiler and used it as a basis for the Class A1 Pacific Boilers.
In Part 7 we looked at the introduction of the Diagram 94 Boiler, as the 180 lbs per sq. ins version, used on the original A1 class, and then was revised to cover the 220 lbs per sq. ins. version used on the A3 Class. Reiterating, that the minor modifications along the way were, the height of the round dome being lowered by one inch, and then the round dome being superseded by the perforated steam collector, more commonly known as the “Banjo Dome”. The round dome was situated to the rear of the parallel front ring of the boiler barrel, the banjo dome was moved back and was placed on the front end of the taper portion of the boiler barrel. The above defined the outward appearance of the Diagram 94 and 94A Boilers. Internally as can be seen from the Dimension tables, three different superheaters were used before, it was finally decided to standardise on the 43 element type for the Class A3 Engines. The unseen difference was in boiler pressure.
Side tracking slightly here, two 2-8-2 Class P1 Freight Engines went into service in 1925 each carrying a Diagram 94 Boiler. I believe it was intended to fit both these boilers with "E" Type superheaters, in the event only the second engine was so fitted. The first engine had the 32 Element Robinson type superheater, and the boiler on the second engine was converted to this layout in 1931. These two engines retained their original Diagram 94 Boilers until 1934. I have been reliably informed that the boiler from the first engine had a very badly corrode parallel boiler ring, which had to be replaced ( the boiler after repair was fitted to Class A1 No. 4481). The Boiler from the second engine was probably beyond repair as it was scrapped. Both engines received refurbished Pacific boilers. It can only be assumed that as the engines had more or less worked solely between Peterborough and London, the hard water supplied along this route must have been a major factor the deterioration of the boilers. Eventually there were no more ex- A1 boilers available, so Diag 94A boilers were used. From about 1942 onwards. The engines were withdrawn in 1945. and their Diagram 94A boilers were taken into the pool and reused on Class A3 Engines.
Another side track insofar as the Boilers for the Class P2 engines introduced in 1933 were to diagram 106. The Tube layout was the same as the Class A3, as was the dimension, tubeplate to tubeplate,(18'-113/4') but a larger firebox with a grate area of 50 sq. ft. was provided, but pressure remained at 220lbs per sq. in.
As you can now see there is a definite trend in the boiler layout which has applied for some 12 years, so that when it came to designing the Boiler for the Class A4, the basic principles were already in place. A first glance the Diagram 107 Boiler appears to be inferior to the Diagram 94A Boiler in having an overall deficit of 72.8 sq. ft. in the total heating surface. However if one looks closely it can be seen that in two areas important gains were made namely in the firebox and superheater elements. In the firebox the tube plate was moved forward into the barrel by 1'-0" to form a combustion chamber, thereby adding 16.2 Sq. ft. to the firebox heating surface. The combustion chamber was omitted from the original design, and there is no doubt that a significant increase in steaming capacity was gained by its introduction on the Diag 107 Boiler. I doubt if the 20% increase to the steaming claimed by the American’s was attained here. Exactly how do you measure an increase in steaming?? With regard to the superheater a theoretical gain here of 42.9 sq. ft. which was due to the provision of longer elements, virtually the full length of the Flue Tubes. It was realised that the ends of the elements nearest the firebox were getting burnt so in 1943 they were cut back by 1'-0"and the thickness of the element walls were increased. This effectively brought down the area of the superheater elements to 706 sq. ft. , equal to the Diagram 94A Boiler.
The loss of 72.6 sq. ft. from the small tubes and 59.1 sq. ft. from the large tubes, due to the shorter distance between the tubeplates does not seem to have had a detrimental effect on the boilers ability to make steam. The combustion chamber ensures that a large proportion of the unburnt small coals are ignited, and useful heat is generated where it is most needed, and not dissipated through the tubes. A much reported comment, of the Ex Great Western Inspector riding on 60033 SEAGULL in the 1948 Locomotive Exchanges, climbing Hemerdon Bank with the boiler pressure rising, was,"You could not do that with one of ours", which is testament enough to the design of the Diag. 107 boiler.
Researched and written by Mel Haigh,
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