The drum contained within the cylindrical casing is formed into segments into which are placed removable ( for cleaning) baskets, consisting of vertical plates (to give minimum resistance to flow) The drum slowly rotates, about 4 rev/min, driven via a flexible coupling,gear train, clutch and thrust bearing by one of two electric motors; one mounted on top the other below. As the drum rotates a segment will enter the gas side, here it removes heat from the gas, it continues to rotate until entering the air side where it will give up its heat to the air. The heat transfer is very efficient, however, soot and corrosive deposits quickly build up in the mesh and hence an effective soot blowing method is essential. This normally takes the form of an arm , pivoted at the circumference of the drum with a single nozzle at the other end. This sweeps acros the drum rather like a record arm. One of these arms are fitted top and bottom. Gas leakage to the air side is prevented by the air being at a higher pressure and by fine radial clearance vanes fitted in the drum segments. By passes for both air and gas sides are fitted to prevent fouling with the reduced gas flow and temperature, also during manoeuvring when the possibility of different gas/air flow rates occurring leading to high metal temperatures and possible fires. Failure by uptake fires is not uncommon with this as in most gas/air heater designs.