Chill-and-blow - thermal-shock cleaning campaign on recovery-boiler superheaters

Chill-and-blow is the periodic thermal-shock cleaning campaign performed on kraft recovery-boiler superheaters when in-service cleaning is no longer sufficient. The boiler load is rapidly reduced; the superheater tubes cool quickly; the temperature differential between the cooled tubes and the consolidated deposit cracks the deposit; intense sootblowing then dislodges the cracked deposit.

Why it matters operationally

A chill-and-blow campaign typically interrupts the boiler at full load for several hours and may require a brief mill production cutback. Mills target intervals of 12-18 months between chill-and-blow events; each additional week of run time defers a chill event and improves the mill's bottom line.

Continuous cleaning to extend the interval

Sonic horns and infrasonic cleaners installed on the superheater extend the chill-and-blow interval substantially by preventing deposits from consolidating to the point where chill-and-blow is required. This is the headline operating-cost argument for acoustic-cleaning installation on recovery boilers.

Distinguishing from water wash

Water wash is the more aggressive offline cleaning during a full boiler shutdown, where high-pressure water removes baked-on deposits that even chill-and-blow could not address.

Procedure and operating logic

A chill-and-blow campaign deliberately changes the thermal state of the deposit. The boiler is taken down or heavily reduced, the affected surfaces cool, and differential contraction cracks deposits that were too bonded for normal sootblowing. Operators then use intensive sootblower operation, air lances or controlled cleaning passes to remove loosened material before the unit returns to load.

The method is common in recovery boilers because sodium-rich deposits can become thick, insulating and mechanically stable during long runs. A fully developed deposit may bridge between superheater tubes, block gas lanes, increase draft loss and disturb steam temperature control. Cooling weakens the bond between the deposit and the tube without introducing water into a hot smelt environment.

Risks and planning

The campaign must be planned around smelt-water explosion risk, boiler chemistry, refractory condition, tube-metal temperatures and access. If the unit is cooled too quickly or unevenly, thermal stress can damage pressure parts. If loose deposits are not removed from hoppers and ledges, they can fall later and damage tubes or plug ash-handling equipment.

Mills usually treat chill-and-blow as a recovery action, not routine cleaning. It costs production time, disrupts steam balance and can expose hidden tube wastage. The decision is driven by draft margin, superheater temperature control, inspection findings, sootblower ineffectiveness and run-length targets.

Relationship to acoustic cleaning

Sonic horns reduce the need for chill-and-blow by keeping deposits mobile while they are still thin. They are not a replacement once deposits have become massive and bonded. The practical value is run-length extension: fewer emergency reductions, fewer manual cleaning interventions and more freedom to schedule the next chill-and-blow inside a planned outage window.

Field checks

A chill-and-blow is planned around deposit condition, furnace inventory, smelt spout status, draft control and safe access. The boiler is cooled in a controlled way so deposits contract and release, then air movement and cleaning methods are used to remove loosened material. The timing is chosen carefully because cooling too fast can stress tubes and refractory, while cooling too little leaves deposits bonded to the heat-transfer surface.

The work carries stored-energy hazards. Deposits can fall from height, smelt or char can remain hot, and confined-space entry cannot begin until gas testing, isolation and ventilation are complete. Plants use pre-job inspection points, tube leak checks and deposit mapping to decide whether a full outage is needed or whether online cleaning can continue. Acoustic cleaners help delay chill-and-blow by keeping early carry-over and ash deposits from consolidating, but once a boiler needs this procedure the acoustic system becomes only one part of the recovery plan.