On a steam locomotive, how does the boiler feed water injector work, when the only motive steam available is that from the boiler itself, which is at the same pressure to which the injector must raise BFW?  And how show on a Mollier diagram?

I understood that, using steam ejector reasoning, that a high pressure steam, expanded isentropically, could then mix with low pressure water, before the mixture is allowed to slow down in velocity in a divergent section before discharging at an intermediate pressure, this IP always being less than the HP.

From Audels "Power Plant Engineers Guide (1945)

"An injector forces water into a boiler because the kinetic energy if a jet of steam is much greater than that of a jet of water escaping from the boiler under the same conditions."

It goes on to say:

"In operation, steam from the boiler, entering the steam nozzle, passes through it, through the space between the steam nozzle and the combining tube, and thence out through the overflow. This produces a vacuum which draws in the water through the water inlet. The incoming cold water condenses the steam in traversing the combining tube, and the water jet, thus formed is driven at first through the overflow, but as the velocity of the water jet increases, sufficient momentum is obtained to overcome the boiler pressure, with the result that the water enters the delivery tube and passes the main check valve into the boiler."

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