steam jacket

from The Century Dictionary.

• noun An inclosure adapted for receiving steam, and applying the heat of the steam to a kettle, tank, steam-cylinder, etc., surrounded by such inclosure.
• To apply a steam-jacket to or surround with one; furnish with hollow walls within which hot dry steam may be circulated to supply heat: used of engine-cylinders, feed-water heaters, cookers, sterilizers, and other devices where heat is to be maintained against condensation, radiation, or other losses.