Abstract: An oven for heating food products on a conveyor includes two chambers. A conveyor belt circulates within each chamber and between the chambers. Each chamber includes a drum oriented with a vertical axis, a motor for rotating the drum about the axis, and a conveyor belt having a length helically surrounding the drum. The conveyor belt has an infeed end and outlet end. A plenum is located within the chamber along an outer extent of the chamber and is curved substantially concentrically with the drum. The plenum forms a substantially closed air pathway having an inlet at one end and an outlet at an opposite end passing air into the first chamber. A fan is arranged to circulate air within the chamber and force air into the plenum inlet, through the plenum and out of the plenum outlet. A heat exchanger is arranged in the plenum to heat the air flowing within the plenum.