Abstract: Disclosed herein is a plate fin heat exchanger subassembly. The subassembly includes three plates in intimate contact with each other. The first plate is sandwiched between the second and third plates. Each of the plates has a first at least one fluid opening and a second at least one fluid opening located at the ends of each plate. The first plate has at least one elongate fluid channel extending between the first and second fluid openings. One fluid passageway for a first heat exchanger fluid extends through the first and second fluid openings in each plate and along the elongate channel in the first plate. At least one fin has two thermal transfer surfaces. The first thermal transfer surface is in intimate contact with the second plate. The other fluid passageways for a second heat exchanger fluid are located on each side of the fin.