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.

INTRODUCTION. 3 force is obtained for carrying or injecting water into a boiler under a pressure equal to or even higher than that of the steam used. If we consider the relationship betwæn water and steam from a thermal standpoint by comparing the heat energy represented by 1 Ib. of steam with that represented by 1 Ib. of water at the same temperature and pressure as the steam, we at once appreciate the advantag© of the steam over the water as a mechanical force. It is a por- tion of the superior force or excess of heat energy of the steam, represented by its latent heat, which is utilised in the injector for forcing water into the water space of the boiler in which the steam was generated. To illastrate the foregoing statements, we will consider a jet of dry saturated steam issuing from a boiler at 10 Ibs. gauge pressure, which mingles with, and is ulti- mately condensed by, a water je-t under atmospheric pres- sure and a head of 2 ft., the ratio by weight of the mixture being 1 Ib. of steam to 9 Ibs. of water.* At the point where the steam and water unite a vacuum exists (due to the condensation of the steam), and we will assume this to be 20 in. meroury. The theoretical velocity of dry saturated steam at 10 Ibs. gauge pressure issuing into a vacuum of 20 in. mercury (5 Ibs. pressure per square inch absolute) is about 2,330 ft. per second, and the theoretical velocity of water under atmospheric pressure and a head of 2 ft. into the same vacuum is about 40 ft. per second. The resultant velocity of the combined jet (condensed steam and water) per unit mass (1 Ib.) will therefore be 269 ft. per second. f A water jet discharging into a region at an absolute pressure of 5 Ibs. per square inch with a velocity of 269 ft. per second would be * In the rough calculations liere given, the effect of the temperature of the water upon the results lias beeii ignored, so as to make the figures as simple as po^sible. One pound pressure per square inch has been taken as equal to a water head of 2'3 ft. The valnes have been calculated by formulæ given later. t For: Momentum of steam jet + momentum of water jet = momentum of combined jet 1 X 2330 + 9 X 40 = 2690; i.e., 10 X velocity = 2690; therefore velocity = 269 ft. per second.