The injector (Fig. 168) is an apparatus for forcing water against pressure by the direct action of steam on the water. It is universally used on locomotive and sometimes on stationary boilers. Steam is led from the boiler through a pipe, which terminates in a nozzle surrounded by a cone. This cone-shaped pipe is connected with the water tank or well where the water is stored. When steam is turned on, so as to pass into the injector, it rushes from the nozzle and thereby creates a partial vacuum in the cone. Since this pressure in the cone is now less than the atmospheric pressure in the water well, the water is forced up to the cone. As the steam meets this water it condenses, but not before its force has imparted enough of its velocity to the water to give the latter sufficient momentum to force down the valve that prevents the steam and water of the boiler from escaping. An injector does not work well if the feed water is too hot, as in that case the steam does not condense quickly.

An ejector is similar in form and operation to an injector, but is used to lift water without forcing it against pressure.

An inspirator is a double-jet injector; one jet lifts the water, and the other forces it into the boiler.