fig1 mounted in cantilever manner out from the side walls 11 12 the roller 27 defines the delivery end 17 of the conveyor and the belts then pass back over a roller 28 disposed below roller 26 and back into the rear of the enclosure and then around the roller 24 as will be described in greater detail below air is evacuated from the enclosure 16 and to restrict ingress of air at the back of the enclosure a further floating roller 29 is mounted to bear on the conveyor above roller 25 and a flexible sealing strip 30 bears on the upper side of roller 29 the sealing strip being mounted on a bracket 31 secured to the box form structure 32 constituting the top 18 of the enclosure the enclosure 16 has a lower back wall 34 terminating in a cross member 35 over which the lower stretch of the conveyor 15 runs an elongated bracket 36 is secured to the cross member 35 at spaced locations to form a narrow slit between the two through which the belts of the conveyor can pass and a sealing strip 37 is secured to the top face of the bracket to bear on the underside of roller 25 as best shown in fig3 the top of the enclosure 16 is formed as a box section extending the.