New Kylchap Chimney & Blastpipe Original Chimney & Blastpipe Kylchap Chimney & Blastpipe Assembly The Original Ashpan

Ashpan Airspaces

A surprise came early in the restoration of No 71000 when a new ashpan was being constructed. The old ashpan was of a through compartment hopper type, but advanced corrosion meant that it was only fit for scrap. No drawings were available at the time so, in order to keep the work moving as planned, team leader Chris Ball fabricated a new ashpan assembly using the dimensions of the original as reference. On completion, the BR drawings had come to hand so Chris checked his new ashpan against them and was amazed to find that the damper door air spaces (through which air passes to the firegate), were too small by 72sq in! The old ashpan was immediately compared and, as it had been the source of the measurements, was also deficient in air space. Quite clearly, the mistake had been made in construction. The new ashpan was quickly and easily corrected.

Certain factors were taken into account when assessing the importance of the deficiency in the damper door air space. The deficiency was all on the front and at the bottom, i.e. nearest the track, which would give it more significance. In 1961 Crewe enginemen had complained that that there was not enough air through the firebed and slots were cut in the sides of the old ashpan in an attempt to rectify this, with a degree of success. However, considered opinion was that this might have been a contributary factor to poor steaming, but not the main cause. Detailed research was put under way in an endeavour to establish if other deep seated flaws were present in the boiler design.

The Chimney & Blastpipe

The Duke's steaming performance was often measured against that of the rebuilt Bulleid 'Merchant Navy' and Peppercorn 'A1/A2' Pacifics, the boilers of which were not so different dimensionally, so it made sense to compare the three types. Also, it seemed worthwhile to look at aspects of other big contemporary British steam engines to put the comparisons in perspective.

This research showed that in theory, No 71000 was at least as good as the other two designs except for the chimney and blastpipe arrangement.The total area of the plain blastpipe nozzles of No 71000, at 25sq in, was less than either the five-nozzle unit of the 'Merchant Navy' at 27.5 sq in, or the twin-cloverleaf pattern of the 'A2' Kylchap blastpipe at 29.5 sq in. Both of the others, because of their different designs, had a surface area of steam that was greater still. Mr Daniels had always considered that not only was the Swindon double chimney design unsuitable for 'The Duke', but it actually caused 'throttling' of the steam at the exhaust ports. Ample evidence is to hand to show that Swindon's principles of draughting, while producing superb results in modifying the 'Castles' and 'Kings', were not necessarily suited to engines other than those of the first-named GWR class. Experiments on LNER Pacifics had proved disastrous.

Comparison of the chimneys produced an equally disturbing result. The total choke area of No 71000's double chimney was less than half that of the 'Merchant Navy' large single chimney. Because the five nozzles of the Bulleid blastpipe were angled outwards it was felt that this might not be typical, but the 'A2' Kylchap choke area was found to be half as big again as that of 'The Duke'. Further checks showed that the 'Britannia' single chimney choke was 5/6 the size of No 71000's total choke area despite the latter's extra cylinder and its potentially fiercer blast resulting from the use of the Caprotti valve gear. Also odd was the fact that the '9F' double chimney would fit straight into position on 'The Duke'. Quite clearly, the double chimney and blastpipe assembly fitted to No 71000 was unusually small. In view of these findings, there was no sense in wasting hard earned funds on duplicating the original blastpipe and chimney assembly. A better proportioned alternative had to be found and the comparative merits of several designs were considered.

Mr Daniels stated that he had asked for a kylchap type chimney to be fitted to 'The Duke' when under construction and he reiterated this preference which was acceded to. As André Chapelon, the inventor of the Kylchap blastpipe, was dead, the only solution was to produce one using existing designs as a foundation. Mr Daniels outlined the important criteria and the starting point was the Kylchap assembly on preserved 'A2' Pacific No 532 Blue Peter.

No 71000 differed from the LNER design Pacifics in that the exhaust ports in the smokebox ran from 'port to starboard' and not 'bow to stern' - something which meant substantial redesign of the blastpipe casting. The characteristics of the new shape did not affect the smoothness of the exhaust in any way. Blastpipe and nozzles were cast by Fletcher and Stewart of Derby. Two attempts were made at producing drawings for the chimney before the desired BR Standard appearance was pronounced acceptable and K. C. Patterns of Peterborough made it. The two pairs of cowls were relatively simple to produce. Despite the design changes, perfect Kylchap proportions were maintained in both blastpipe and chimney - something that was crucial for effective steaming. Following the manufacture of the new Kylchap chimney assembly for 71000, a rumour had persisted that originally, prior to Swindon taking charge of chimney design, something similar to a Merchant Navy assembly had been proposed.

However, a drawing produced from Crewe Works proved differently. The first proposal for the three cylinder Pacific, with rapid exhaust from poppet valves, was no more than a marginal modification of the assembly fitted to the two cylinder Britannia's. Not only was the blastpipe nozzle the same size at 5 3/8 inches diameter, but the chimney flue choke was actually smaller at 15 inches diameter, The Britannia being 15 5/8 inches! The other main differences were in the distance between blastpipe nozzle and chimney flue choke, and chimney taper. The successful Kylchap, fitted to the restored Duke, has two blastpipe nozzles each of 5 1/2 inches diameter and two chimney flue chokes.

Many years later other information came to light from Crewe Works. It would seem that the Works had been frequently asked to deal with a problem of the cylinder relief valves operating unnecessarily but nothing was ever found to be wrong with them or their settings.The fact was that the valves were operating because of a throttled exhaust causing excessive back pressure when the engine was working hard! The steam chest pressure was noted to rise above boiler pressure at these times, indicating excessive compression in the cylinders. It might be logically reasoned that, with relief valves set at 265 lb/sq.in and continually operating, the pressure in the cylinders would have been greater than boiler pressure (250 lb/sq.in) and would have affected steam admission. This would seem to totally support Mr Daniels original theory.

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