the pipes has an outside diameter of 22 mm and the total length of the cooling pipes is 100 meters the rate of flow of cooling water through the pipes is 60 liters per minute air is fed to the gas supply pipes 18 at a pressure of 69 u00d7 10 3 n m 2 in a cycle time of 60 seconds for the toughening of successive glass sheets the gas supply pipes are operated for 40 seconds and are switched off for 20 seconds the time of immersion of each glass sheet in the fluidised bed is 8 seconds occurring during the middle of the 20 second period when the gas supply to the pipes 18 is switched off when toughening glass sheets of thickness 2 3 mm and overall size 1 5 m u00d7 0 66 m with the glass at an average temperature of 650 u00b0 c when it enters the fluidised bed 55 kilowatts have to be removed from the bed in order to maintain the bed temperature at 85 u00b0 c when the glass sheets are being toughened in succession one every 60 seconds this rate of cooling is achieved with a rise in temperature of the cooling water from 9 u00b0 c at the inlet to 22 u00b0 c at the outlet the rate of heat exchange with the particulate material of the fluidised bed is governed by regulation of the rate of feed of cooling water through the cooling pipes around the tank side walls and attached to the lid as well as by regulation.