The bottom two-thirds of the kettle is covered by a metal jacket; shallow ones may be fully-jacketed.
There is a sealed air space left between the outside of the kettle, and the jacket, into which steam is pumped, which does even heating and cooking of the food in the kettle. Steam, though, never touches the food.
On top, there is a hinged lid.
There is a faucet at the bottom to allow liquid to come out of its spout. Some Steam-Jacketed Kettles can also be tilted, and have a spout on them for this.
They will all have safety valves to let off excess steam pressure.
Tilt ones can hold up to 80 gallons, stationary ones from 10 to 150 gallons. Small ones, 12 gallons or less, can be mounted on tables or counters.
Deeper ones are used for dishes such as beans, gravies, lentils, sauces, soups, pasta, puddings, and pie fillings, and can be used to make large quantities of scrambled eggs.
Shallow ones are used for moist cooking of meat, and stews.
Full-training in their use for safety and maximum effectiveness is required. They require special cleaning procedures as well.
It is best to use distilled water as the water for the steam source, as tap water can cause mineral deposits on the heating coils, lowering their effectiveness.