A heat transfer system for refrigeration and/or heating of products includes a housing having an internal chamber, a baffle, a spiral pathway around the baffle, a gas flow pathway and a blower to circulate gas along the gas flow pathway. The baffle separates the chamber into upper and lower portions. The spiral pathway includes an upper spiral pathway within the upper portion, and a lower spiral pathway within the lower portion. The gas flow pathway includes an upper gas flow pathway across the upper spiral pathway and a lower gas flow pathway across the lower spiral pathway in which gas flow in the upper and lower gas flow pathways defines a circulation loop. The width of the upper gas flow pathway and upper spiral pathway are similar, and the width of the lower gas flow pathway and lower spiral pathway are similar.