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A high-pressure steam locomotive is a steam locomotive with a boiler that operates at pressures well above what would be considered normal for other locomotives. Most locomotives operate with a steam pressure of 200 to 300 psi (1.38 to 2.07 MPa). [1] In the later years of steam, boiler pressures were typically 200 to 250 psi (1.38 to 1.72 MPa).
1833 – Hercules (1829) was modified to use an extra low pressure cylinder, taken from Agrippina, with steam from her high-pressure cylinders. This modification was designed by Dutch engineer Gerhard Moritz Roentgen , [ 11 ] making him the inventor of the naval compound steam engine.
An animation of a simplified triple-expansion engine. High-pressure steam (red) enters from the boiler and passes through the engine, exhausting as low-pressure steam (blue), usually to a condenser. It is a logical extension of the compound engine (described above) to split the expansion into yet more stages to increase efficiency.
Superheated steam was widely used in main line steam locomotives. Saturated steam has three main disadvantages in a steam engine: it contains small droplets of water which have to be periodically drained from the cylinders; being precisely at the boiling point of water for the boiler pressure in use, it inevitably condenses to some extent in the steam pipes and cylinders outside the boiler ...
There is no generation of steam bubbles within the water, because the pressure is above the critical pressure at which steam bubbles can form. It passes below the critical point as it does work in a high-pressure turbine and enters the generator's condenser. This results in slightly less fuel use and therefore less greenhouse gas production ...
Compound engines have either two or three cylinders, in which the steam is expanded in turn. The exhaust of the high-pressure or HP cylinder feeds the low-pressure or LP cylinder. Three cylinder engines also had an intermediate-pressure or IP cylinder, but these were less common than two cylinder engines.
The high pressure necessitated compound expansion; steam being supplied to the two 12-by-26-inch (305 mm × 660 mm) high-pressure inside cylinders and then fed into two larger 20-by-26-inch (508 mm × 660 mm) low-pressure outside cylinders before going to exhaust. High-pressure cylinder diameter was subsequently reduced to 10 in (254 mm).
The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, district heating units, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are needed.