Abstract: A convection steamer apparatus and method having improved heat-transfer characteristics for rapidly and effectively processing food items or the like at atmospheric pressure and at a relatively low constant steam temperature comprising a steam generator and distribution system for delivering steam in a turbulent controlled path to a heating chamber. A diffuser device acts to introduce substantially dry steam into the distribution system and a pressure differential generator device communicates with the chamber for continuously inducing a pressure differential to continuously purge air and/or unused steam from said chamber to maintain turbulent, generally orbital flow of dry steam in heat-transfer relation around the food items to be processed.The invention further contemplates an improved construction for an atmospheric closure for fluid and heat sealing of the heating chamber during the processing of such food items or the like.