A thermal processing furnace, comprising: a generally bell jar-shaped outer reaction tube having a central axis; and an open-ended inner reaction tube for accommodating a wafer boat holding a plurality of substrates, which inner reaction tube is substantially coaxially disposed within the outer reaction tube, thereby defining a gas passage between an outer wall of the inner reaction tube and an inner wall of the outer reaction tube, wherein at least one of the outer wall of the inner reaction tube and the inner wall of the outer reaction tube is provided with a flow deflector that protrudes radially from the respective wall into the gas passage.