TWO ZONE, SINGLE CAPACITY (-2) (One hot zone & one cold zone) The products under test travel via a carrier basket, alternating between the hot and cold zones. TWO ZONE, DOUBLE CAPACITY (-2D) (Two hot zones & one cold zone) In the two zone, double capacity system, the cold zone is always in use, which contributes to the efficiency of the system. THREE ZONE, SINGLE CAPACITY (-3) (One hot zone, one ambient zone & one cold zone) In the horizontal three zone, the products travel via a carrier basket between the cold and ambient zones, and the hot zone travels to envelop the carrier basket.