Injectors work because steam under the same pressure and conditions flows from a contracted nozzle at a much greater velocity than water. The steam cone, or nozzle, regulates the quantity of steam used by the injector. It is both convergent and divergent in order to direct the flow of steam into the combining cone and give it the maximum possible velocity.

The condensation of the steam jet and transfer of its energy to the water takes place in the combining cone which receives the steam and water. In condensing, the steam gives up its velocity to the water, which is then further accelerated by the vacuum in the combining cone caused by the reduction in volume of the steam when condensed by the water. At the inlet end is a jet consisting of a mixture of steam and water, while the outlet end has a jet of hot water flowing at high velocity but very low pressure.

The combining cone is split lengthwise for part of its length and hinged at one end. Should the water supply be interupted the injector stops working as condensation does not occur. Steam when there is no water to condense it can lift the flap and freely escape to atmosphere via the overflow valve until normal water flow is resumed.

The jet passes across a gap after exiting the combining cone and before entering the delivery nozzle. This overflow gap is necessary for disposing of the steam and water before the steam is condensed when starting the injector.

The jet of water is at its maximum velocity as it enters the delivery nozzle, and as it passes through the nozzle and delivery cone its velocity decreases as the bore of the cone increases, so that the pressure will increase until it is sufficient to overcome the boiler pressure and the water can enter the boiler.

When working, the injector would tend draw in air via the overflow pipe, hence the need for a valve on the overflow to prevent this while allowing a free escape for any overflow of water and steam.

To work an injector, water is turned on at the feed cock on the tank, or tender, and then steam is admitted. Any overflow is controlled by using the feed cock to reduce the water flow. To turn an injector off, steam should be shut off first and then the water supply. Water is always turned on first and off last to prevent the injector overheating which would prevent proper condensation of the steam and hence the injector failing to work.

Exhaust injectors are very similar, but have a much larger steam nozzle to allow for the much lower pressure of the steam supply from the engine exhaust. They also have a smaller supplementary steam nozzle (supplied from the boiler) to assist in overcoming boiler pressure. The supplementary steam could also be used to work the injector while the engine was stationary with the regulator closed.

Steam injectors are very efficient wasting very little heat as the steam used is returned to the boiler as hot water.