Steam ejectors act as pumps without moving parts to create a predefined vacuum. Steam Jet ejectors are work horses and it is more reliable and requires less attention and maintenance. Steam ejectors are simple, achieving very low vacuum levels, operate unattended, and offer exceptionally long service life. A Steam ejector operates by the conversion of steam's pressure energy (any compressible gas or a liquid can provide this motive force) into a comparatively higher velocity, which entrains and accelerates the gasses on the suction side of the device to create a vacuum. The structural design of the device defines its performance. This principle can be combined in a single stage device or multiple-stage devices linked together to achieve specific vacuum performance objectives.

In instances where suction gases, or materials entrained in the suction gases, might freeze inside an ejector, an external heating jacket is often employed. High vacuum steam jet ejectors, such as V and W stages which operate below 2 torr (the partial pressure of water ice at 32o F), will have a external steam heating jacket running from the end of the suction head to the beginning of the bore section of the throat. Low pressure steam at 10-15 psig is often used to keep the ejector wall temperatures at > 40o F, thus preventing ice build up inside the throat. (Ice build up restricts the flow through the throat. This can lead to erratic or poor high vacuum ejector performance.) External steam tracing coils may also be used.

In the plastics industry it is common to use a full hot oil jacket on an ejector, covering the suction connection, the suction head and the entire throat all the way to the discharge connection. Entrained polymers often freeze at several hundreds of degrees, so the ejector walls have to be kept at temperatures exceeding this freezing point to prevent build-up.

Sulfur pit ejectors, entrain sweep air and sulfur laden vapors at slightly below atmospheric pressure. To keep elemental sulfur from freezing inside the ejector, a full steam jacket with 50 psig (or more) steam, is frequently used to keep the ejector walls at about 300o F.

Conversely, in situations where ejectors have to handle short duration, high temperature gas loads (e.g. rocket engine testing), steel ejectors have been equipped with external water cooling jackets.This obviates the need for expensive high temperature alloys in the construction of the jets.