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.

THE STEAM NOZZLE. 15 nozzle.* In exhaust injectors said ratio of areas is in- creased to about 16 to 1, on account of tlie lower density of the exhaust steam. These ratios are experimentally de- termined and vary according to the niean pressure of the steam dealt with, being greater for low than for high pressures. For example, if an injector be designed to work with a steam pressure of 160 Ibs. to 200 Ibs. per square inch, the area of the steam inlet nozzle throat may be about 1’96 times the area of tlie delivery nozzle throat; for 120 Ibs. pressure steam the ratio of areas may b& 2’56 to 1, and for 60 Ibs. pressure steam 3’2 to 1. The chief factors which have to be considered in deciding upon the size of steam nozzle for any injector are three. In the first place, sufficient steam must be admitted to the injector to give the requisite velocity to the entering feed water. Secondly, if the feed water is cold it will condense more steam per unit weight in a given time- than if it is hot, but as condensation must be completecl within tlie limits of the combining nozzle, the ratio of steam to water must be kept as low as possible when the feed water is hot. Thirdly, if the injector has to lift its feed water, the amount of steam admitted must not be sufficient to cause choking within the combining nozzle before the feed water is drawn into the appliance. As the greatest wear of the steam nozzle takes place at its throat, the latter is frequently made with a straight or cylindrical portion there (see figs. 5 and 10) for a length equal to from about one-third to the diameter of the throat. It is very usual to divide the total steam inlet ai’ea of an injector into two parts, the one part being of annular and the other of circular cross section. Fig. 13 shows one such arrangement. The nozzle a is known as the “ lifter ” or lifting steam nozzle, and the nozzle 6 as the “ forcer ” or * Formulæ are frequently given for determining the size of steam nozzle for any particular service. These formulæ are interesting, but useless, as tliey are based upon several assumptions as to ratio of steam to water, velocity of entering steam, velocity of entering feed water, etc, which it is impossible to make with any degree of acciiracy. A similar formula has not, tlierefore, been given here. Later tliere will be given an approximate method of determining the sizes of the nozzles of an injector from the results of tests on the latter.