A closed-loop vacuum conveyor controlling the pressure in a plenum. Pressure sensors in the plenum send pressure signals to a processor that controls the speed of a blower motor sucking air from the plenum and through openings in a foraminous conveyor belt against which conveyed aluminum cans are held by air pressure. The processor controls the blower speed with a variable-frequency drive to adjust the pressure in the plenum to accommodate a varying product density or the shape. size. or weight of conveyed products on the outer surface of the conveyor belt. The plenum may be divided into separate chambers by partitions or adjustable dampers. The pressure in the chambers can be controlled by adjusting each chamber's blower speed or the opening of its dampers.