a grate assembly for a shell boiler comprising a fire bed supporting means including a first apertured support member and a second support member mounted for mutual relative movement between a first position in which the apertures in the first member are obturated by the second member and a second position in which the apertures are not obturated and drive means for causing relative movement between the first and second positions at a speed to permit passage of ash through the apertures whilst continuing to support the fire bed .