The Ultimate Steam Page

As locomotive design progressed, builders realized that the proportions and configurations of the chimney and the exhaust pipe had a signficant effect on how well the exhaust system worked. A major concern was the effect of back-pressure on the performance of the locomotive's cylinders. The locomotive exhaust could be built with a small exhaust nozzle, which caused the exhaust steam to jet up the stack at high velocity, which would produce excellent gas flow through the boiler (draft). However, this small nozzle would impede the flow of the exhaust steam from the cylinders, causing excessive back-pressure. This back-pressure saps power from the locomotive's cylinders, reducing the locomotive's performance. Good draft increased the locomotive's power, but high back-pressure could cancel this out.

This was the chief task of locomotive exhaust designers: how to produce the maximum draft while producing the minimum back-pressure on the cylinders.

Until the 20th century, the physics of gas flow were not understood and the theories and laws which could be used to design exhaust systems did not exist. Consequently, early locomotive exhaust systems were developed through a process of trial-and-error.

This exhaust system, from a locomotive in New Zealand, is similar to that of many 19th Century locomotive exhaust systems. The steam nozzle is at the bottom, and it exhausts through a "peticoat" and finally up through the main chimney (the separate peticoat was a 20th century development). This chimney includes a spark arresting apparatus, which forces the exhaust gases through several turns in order to make sparks and cinders drop out into the bottom of the spark arrestor.

This drawing shows a typical locomotive exhaust system from a U.S. steam locomotive built in the 20th century. The drawing shows a cross-section of the smoke box at the front of a locomotive boiler (the boiler would be to the left of the view). The steam nozzle is at the bottom of the smoke box, exhausting its steam jet up the stack which is at the top. The drawing also illustrates the emperical design formulas which were used to size the components. After building hundreds of exhaust systems, designers decided that the proportions of the components listed above would work best for most locomotives under most conditions.

This drawing shows an early Kylchap exhaust. Chapelon used the exhaust splitter developed by Finish engineer Kylala, which divides the exhaust stream into four parts. The Kylchap draws in gases from more than one level of the smokebox, which Chapelon believed to be an imporant feature in providing an even gas flow through the many tubes of the boiler. Later Kylchap exhausts used two levels of entrainment and two or even three stacks.

The Kylala exhuast splitter was an important part of the Kylchap xhaust system.

In the U.S., several railways developed improved exhaust systems using annular exhaust nozzles and larger stacks. Plan View of Annular Exhaust Nozzle (above)- Sectional Views of Smokebox (left)- Known as a "Waffle Iron" exhaust on the N&W

The Lemaitre Exhaust was developed by Lemaitre, a mechanical engineer from the NORD Belge. The Lemaitre featured 5 nozzles in a circular pattern exhausting up a large diameter stack, with a variable area nozzle exhausting up the center.

This drawing shows the Kylpor exhaust system developed by L. D. Porta from the Kylchap as applied to the 2-10-2s of the Rio Turbio Railway in Argentina.

Finally, this diagram shows the Lempor (Lemaitre-Porta) exhaust developed by Porta and applied to many locomotives. Porta also developed an extensive theory describing the performance and design of these exhaust systems.

One of the most recent new exhaust systems was developed for the Garratt locomotives of the Rhodesian Railways. These were known as "pepperpot" exhausts and were later fitted to many Garratts which were overhauled and restored to service in the early 1980's in the new country of Zimbabwe. This nozzle arrangement was used in combination with a larger chimney, and was developed as alternative to Giesl exhausts experimentally fitted to Garratts in the 1960's. The pepperpot exhaust was preferred because (1) it was locally developed (the Giesl was proprietary and royalties had to be paid for its use) and (2) the Pepperpot was less susceptible to unauthorized tampering which tended to cause problems with the Giesls in normal service.

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