<P> The sealed ultra-high - pressure circuit ran at between 1,200 and 1,600 psi (8.27 and 11.03 MPa), depending on the rate of firing . The HP boiler worked at approx 850 psi (5.86 MPa), and the low - pressure boiler at 200 to 250 psi (1.38 to 1.72 MPa). The UHP and HP boilers were of a water tube design, while the LP boiler was a fire tube boiler typical for steam locomotives . The LP cylinders were driven with a mixture of the HP cylinder exhaust and the LP boiler output . Both HP and LP boilers had superheaters . </P> <P> The French PL241P, the German H17 - 206 and the British LMS 6399 Fury all used the Schmidt system, and were of basically similar design . The New York Central HS - 1a and the Canadian 8000 also used the Schmidt system but were a size larger altogether - the 8000 weighed more than twice the Fury . </P> <P> Another way to avoid scaling in the HP boiler is to use steam alone to transfer the heat from the fire; steam cannot of course deposit scale . Saturated steam from an HP steam generator was pumped through HP superheater tubes which lined the firebox . There it was superheated to about 900 ° F (482 ° C) and the pressure raised to 1,700 psi (11.72 MPa). Only a quarter of this was fed to the HP cylinders; the rest was returned to the steam generator where its heat evaporated more water to continue the cycle . </P> <P> The HP cylinder exhaust passed through an LP feed heater, and then the tubes of an LP boiler; this was roughly equivalent to the LP boiler in the Schmidt system, but was heated by HP exhaust steam not combustion gases . Steam was raised in the LP boiler at 225 psi (1.55 MPa), fed to the LP superheater, and then the LP cylinder . The LP exhaust fed the blastpipe in the smokebox . The HP exhaust condensed in the LP boiler heating tubes was pumped back to the HP steam generator . It was a complex system . </P>

Maximum steam pressure in locomotive boiler in bar