the like in operation inert gas which may be preheated is pumped into the machine the heater panels 18 and 40 are energized the fans 26 are started and the conveyor 12 moves printed circuit boards along the tunnel the inert gas is forced down through the apertured panels 18 and back up the gas channels 36 as indicated by the arrows the forced convection assisting in ensuring even heating across the conveyor 12 each fan 26 is driven by a fan shaft 52 which is disposed vertically and passes through respective apertures 54 in the roof 22 of housing 15 each shaft 52 is supported on two spaced bearings 56 the lower of which is adjacent the roof 22 of housing 15 a cylindrical housing 58 receives the fan shaft 52 concentrically therethrough and sealably encompasses the bearings 56 which are located at opposite ends of the housing 58 the upper end of each fan shaft 52 is provided with a pulley 60 which is connected by means of a drive belt 62 to a pulley 64 mounted on the drive shaft of an electric motor 66 mounted remotely with respect to the machine 10 a source of inert gas such as nitrogen is connected to the housings 56 via pipes 68 and pumps 70 to supply the housings 56 with inert gas at a pressure greater than that inside the housing 15 the cylindrical housing 58 thus acts as a gas seal preventing air from leaking through the bearings and.