Cold wall refractory systems are typically backed by steel shells, which are poor at withstanding the thermal stresses that come from reverse heating. API 936 and API TR 980 clearly emphasize thatthe heat-up or dryout process for cold-wall linings must always begin from the hot face- that is, the process-exposed surface.
Heating from thehot faceallows for controlled migration of moisture toward the cold face and eventual venting to the atmosphere. Conversely, heating from the cold face or steel shell side traps moisture between the hot impermeable material and the outer surface, leading to:
Steam buildup
Explosive spalling
Cracks in the material or at the interface with the steel
For these reasons,only the hot facemethod is specified unless the design and dryout plan justify a different method under special engineering review.
Reference:
API Std 936, Section 6.5.2: "Dryout shall be performed from the hot face only unless otherwise specified." API TR 980, Section on "Dryout of Cold Wall Systems": Highlights the mechanisms of vapor movement and the risks of reverse heating.
ASTM C1190(Referenced Method): Supports the use of correct drying schedules and methods.