A number of steam locomotives in the SRPS collection are fitted with a Giesl Ejector. This device is fitted on the boiler smokebox in substitution for a normal chimney. These ejectors represent in the Bo'ness collection the improvements to locomotive boilers which were attempted world wide from the middle of the 20th century, although these ultimately had little effect on the steady elimination of steam traction.
The early locomotives built by George and Robert Stephenson used the steam exhaust from the cylinders, accelerating it through a nozzle in the smokebox on its way to exhaust at the chimney, to create a low pressure in the smokebox. This low pressure drew the gases from the fire through the boiler. This made the boiler fire, to a useful extent, self regulating - the harder a locomotive worked, the greater the amount of exhaust steam and so the greater the vacuum in the smokebox, and thereby the greater the draught through the boiler and the greater amount of air available to mix with the fuel for combustion.
The association between increased velocity and reduced pressure of the fluid in a nozzle is a statement of conservation of energy - the sum of velocity head and pressure head (i.e. the total head) remains the same through a nozzle. The velocity of the jet is increased by the diminishing taper of a converging nozzle, such as a steam locomotive blastpipe, and as velocity head rises so pressure head must fall.
Obtaining the maximum power output from a steam engine depends on maximising the difference between steam admission pressure and steam exhaust pressure at the cylinders. The lower the exhaust steam pressure, the more useful energy is obtained. Unfortunately, the smokebox blastpipe nozzle creates a back pressure which raises the exhaust steam pressure at the cylinders, and so reduces power output.
The dynamics of boiler draughting were well understood by the early 20th century, and it was clear that a balance had to be found between the desired high smokebox vacuum and the unwanted cylinder exhaust back pressure which the exhaust nozzle created. The exhaust nozzle system designed by Dr Adolf Giesl-Gieslingen was effective in increasing boiler draught for a given steam exhaust pressure, and was thereby helpful in enabling steam locomotives to burn low grade fuel. The National Coal Board experimented with it in Britain with some success.
The Giesl Ejector replaces a conventional circular blastpipe steam nozzle by an oblong ejector, consisting of multiple nozzles in line, and an elongated chimney to suit. The effect is to increase the smokebox vacuum. It appears to have been most successful for low steam outputs, such as in NCB service. One was also fitted on a British Railways Class 9F locomotive, no.92250, with the object of enabling more extensive use of the NCB's lower grade product, but with indifferent results.
D. Wardale, The Red Devil and other Tales form the Age of Steam, Inverness 1998, and E.S. Cox, British Railways Standard Steam Locomotives, Shepperton 1966.