Purpose of waterwalls tube in boiler

1. To cool and protect the furnace lining.
2. Depend on the type of boiler, waterwall tube may be 10% of boiler's heating surface, yet represent 50% of the total heat absorption.
3. Absorb heat from the furnace to increase the unit's generating capacity.
4. Make the furnace air tight (on pressurized furnaces with tangent welded tubes).