One embodiment is directed to an oven for heating fibers. The oven comprises a supply structure disposed within the oven between first and second ends of the oven. The supply structure comprises a plurality of plenums stacked one above each other with gaps therebetween. The plenums are in fluid communication with a heating system. At least one plenum comprises at least one side wall comprising a plurality of passages formed therein, said at least one plenum configured to direct at least a portion of the heated gas into an interior of the oven from the plurality of passages. Each of the plurality of passages formed in said at least one plenum has a respective tapered cross-sectional shape.