Available in wide range of diameters & lengths  in  required  voltage & wattage  as per application and generally available from 500 watts to 3000 watts or more. 
Generally used for Quick heating in dryers / ovens /  pharmaceutical, textile, chemical, electroplating plants, ceramic industries where these are used  frequently.  
When procelain  heaters are covered by metal / glass tubes
are used as Immersion heater for water / acids / chemicals.

Generally when a cartridge or porcelain heater is covered with metallic sheath such as Mild Steel, stainless Steel,Titanium,glass or any other material and submerged into liquids are known as “Immersion” heaters.
These are available in various diameters as well as wattage and Voltage.
Photograph of typical “Immersion” heater in stainless Steel sheath with threaded flange at end with replicable cartridge for minimizing downtime.