Double chimney

A double chimney (or double stack, double smokestack in American English) is a form of chimney for a steam locomotive, where the conventional single opening is duplicated, together with the blastpipe beneath it. Although the internal openings form two circles, the outside appearance usually forms a single elongated oval.

The classic exhaust design for a steam locomotive began with Hackworth's invention of the blastpipe, placed centrally within a tall chimney. Victorian developments reduced the chimney's height, such that natural draught was no longer significant. The standard design was then a circular drumhead smokebox, with a single blastpipe nozzle leading into a chimney with a flared petticoat pipe beneath it. From the work of theorists such as W.F.M. Goss of Purdue University, and later S.O. Ell of Swindon, guidelines were developed at each locomotive works, describing how these were to be proportioned.

It was recognised both that a particular diameter of chimney and blastpipe would be needed for the steam-raising capacity of each boiler, and also that the conical taper from blastpipe to chimney could not be made too steep. As boilers became more powerful, not only did the chimney diameter need to become greater, but also the minimum height for the chimney was becoming longer – just as the increasing size of boilers restricted the clearance height available within the loading gauge. A chimney height of at least 24 inches was considered the minimum workable. By the 1930s, it was increasingly difficult to provide such a height and other solutions were sought.

A solution to this limit was to adopt a double chimney. This allowed adequate cross-section area for airflow, whilst reducing the diameter of each and thus the minimum height needed for an acceptably gentle taper.

A simultaneous development was the Kylchap blastpipe, combining the Kylälä spreader by Finnish engineer Kyösti Kylälä, and a further flue choke tube added by the French engineer André Chapelon. This split the blastpipe area into four smaller nozzles, and the vertical draught induction across three stacked venturis. Although the total blastpipe area remained constant, their perimeter, and thus the area for mixing with the exhaust gases, was doubled. The additional petticoats also improved the effectiveness of the blast in inducing a draught.

Although there is no reason why one approach, either the double chimney or the Kylchap blastpipe, depends on the other, interest in both was generally simultaneous and so both were often installed together.

The first 50 of the Ivatt class 4MT 2-6-0 were built with double chimneys. These performed poorly however, and were noted as poor steamers. Work on the static test plant at Rugby discovered that there was both no advantage to the double chimney and also that it had been poorly designed initially. When revised with a single chimney and improved gas flow in the smokebox, their steaming rate was raised from 9,000 lb/hour with a double chimney to 17,000 lb/hour with a single chimney, even though this was still below the theoretical limit, restricted by firegrate size, of 19,000 lb/hour.

A minor disadvantage could be a 'softer' exhaust blast for the purpose of lifting the external smoke clear of the driver's vision. When the LNER A3 class were fitted with double chimneys in the late 1950s, they suffered problems with smoke obscuring the view from the cab. The solution to this was to fit small Witte-type smoke deflectors of the German pattern.