Abstract: A modular plastic heat exchanger includes at least one thermal exchange element having parallel outer and inner plastic sheets separated by a plurality of parallel ribs extending between said sheets and connected thereto to form a plurality of channels generally rectangular in cross-section with a plastic insulating foam sized to be co-extensive with said outer sheet and secured to the exterior surface of said outer sheet, and a pair of cylindrical manifolds, each having a large central flow tube oriented normal to such channels in said thermal exchange element and fixedly secured to said thermal exchange element on opposite edges thereof, each of said manifolds being operably connected to fluidly communicate said channels to its flow tube at the ends of said channels, whereby the inner sheet can be tightly faced against a cylindrical member to be thermally controlled and temperature controlled fluids can be passed from one manifold to the other to control the temperature of the member.