The Steam Injector
A theoretical and practical treatise on the design and operation of injectors and on the flow of fluids through and the design of nozzles.

14 THE STEAM INJECTOR. TABLE 1.—Table of Ratios of Areas ok Nozzle Mouth and Throat for Different Ratios of Initial and Final Steam Pressures. Valnes of Vi Vz 1-732 4 8 10 20 50 70 100 Am 1 1-35 2-07 2-436 3-966 7’98 11-55 13-8 It is generally held that the best form of the divergent part of a nozzle is slightly concave and not conical, as usually made, but owing to the expense of manufacture, such form is not commercially practicable. All injector steam nozzles are made of circular cross section. This form is probably the most efficient, as it will not cause any eddy motion in the jet, and it is the most economical to manufacture; but provided a nozzle is well rounded at all parts, the losses due to eddy motion should be negligible. O o To provide an injector steam nozzle of theoretically accurate proportions for any service, it would be necessary to have a fixed steam pressure, so as to obtain a uniform density of steam at the nozzle throat and a fixed countei'-pressure or pressure at the exit end of the nozzle. Such conditions do not obtain in practice. The nozzle must therefore be designed to give a maximum efficiency at mean steam and counter-pressures. It may be best to have one form of nozzle for high-pressure and anotlier for low-pressure- service. The counter-pressure will vary only within very narrow limits. If a steam nozzle is reasonably correct in design and is working within the limits of pressure for which it was designed, its efficiency as a means of converting pressure energy into kinetic energy may be as high as 95 per cent. The total steam inlet area of an injector adapted to work with live steam of any usual boiler pressure varies from about 1'8 to 3 times the area of the throat of the delivery