A heat exchanger transfers heat between primary and secondary fluid coolants. A housing has a longitudinal central axis lying within a transverse housing plane. A bottom wall with a concave inner surface faces upward and extends along the central axis. A plurality of arcuately helical lower sector fins are spaced apart along the central axis on the bottom wall inner surface. A cover has a longitudinal central axis lying within a transverse cover plane. A top wall with a concave inner surface faces downward and extends along the central axis. A plurality of arcuately helical upper sector fins are spaced apart along the central axis on the top wall inner surface.A copper tube is assembled into the housing in a direction transverse to the housing axis with the tube closely adjacent the lower sector fins. First and second sealing means seal the tube ends to the housing. The cover is assembled into the housing in a direction transverse to the housing axis with the upper sector fins closely adjacent the tube. The upper sector fins, the top wall, the lower sector fins, the bottom wall, and the tube outer surface define a helical passageway around the tube outer surface. Primary coolant nozzles will direct the primary coolant through the tube. Secondary coolant nozzles and secondary coolant conduits will direct the secondary coolant through the helical passageway.The housing, the primary nozzles, the secondary nozzles, and the secondary conduits, are molded as a single unitary piece from a polymeric material consisting of either thermoplastic or thermoset resins. The cover is also molded as a single, unitary piece from a polymeric material.