A kettle is a hemispherical vessel with a double wall or “jacket” that holds steam. It can be open or closed on top. Steam kettles were first built in the Nineteenth Century out of copper and were held together with rivets but most kettles today are stainless steel and are welded. Kettles are designed to cook food products and perform other processes that require heat. They are like double boilers but on an industrial scale. The steam jacket is designed to take pressure. The more steam pressure in the jacket, the more heating abilty the kettle has.
One of the leading producers of steam jacketed kettles is GPE, formerly known as Groen. GPE’s standard size kettles range from 40 gallons to 1000 gallons. (smaller sizes are available for restaurant duty). Standard jacket pressures range from 40 psi to 125 psi. Other leading kettle manufacturers are Hamilton Kettles and Lee Industries and they offer other sizes and jacket pressures. Other brands are Legion, Hubbard, Pardo and Toronto Coppersmithing
Many kettles are agitated. Many have scraped surface agitation to keep the contents from burning against the hot wall of the steam jacket. Some have a counter rotating agitator just inside the scraped surface agitator. These are called double motion kettles. Generally, a more viscous product requires double motion agitation while more liquid products can be processed in a single motion kettle. Some kettles have a third agitatator in the middle, usually shaped like an inverted Christmas tree. These are triple motion kettles. The third agitator is often high shear.
Most kettles are heated by an external steam source, a boiler or steam generator. Others have a built in source of heat to create the steam. This can be powered either natural gas or electricity. These are called self contained kettles. They tend to be smaller and are often used in restaurants and food preparation facilities.
Most larger kettles have valves at the bottom to empty the contents. However, some large kettles and most smaller kettles have no bottom valve but are designed to be tilted to empty.
Some kettles are closed on top and are built to withstand internal pressure or vacuum. Liquid in a vacuum kettle evaporates at a lower temperature than liquid at atmospheric pressure.