Macallan variable blastpipe

The blastpipe is part of the exhaust system of a steam locomotive that discharges exhaust steam from the cylinders into the smokebox beneath the chimney in order to increase the draught through the fire.

The primacy of discovery of the effect of directing the exhaust steam up the chimney as a means of providing draft through the fire is the matter of some controversy, Ahrons (1927) devoting significant attention to this matter. The exhaust from the cylinders on the first steam locomotive – built by Richard Trevithick – was directed up the chimney, and he noted its effect on increasing the draft through the fire at the time. At Wylam, Timothy Hackworth also employed a blastpipe on his earliest locomotives, but it is not clear whether this was an independent discovery or a copy of Trevithick's design. Shortly after Hackworth, George Stephenson also employed the same method, and again it is not clear whether that was an independent discovery or a copy of one of the other engineers. Goldsworthy Gurney was another early exponent, whose claim to primacy was energetically advocated by his daughter Anna Jane.

The locomotives at the time employed either a single flue boiler or a single return flue, with the fire grate at one end of the flue. For boilers of this design the blast of a contracted orifice blastpipe was too strong, and would lift the fire, expelling incompletely burnt fuel up the chimney. It was not until the development of the multitubular boiler that the centrally positioned, contracted orifice blastpipe became standard. The combination of multi-tube boiler and steam blast are often cited as the principal reasons for the high performance of Rocket of 1829 at the Rainhill Trials.

Soon after the power of the steam blast was discovered it became apparent that a smokebox was needed beneath the chimney, to provide a space in which the exhaust gases emerging from the boiler tubes can mix with the steam. This had the added advantage of allowing access to collect the ash drawn through the fire tubes by the draught. The blastpipe, from which steam is emitted, was mounted directly beneath the chimney at the bottom of the smokebox.

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Wikipedia