Abstract: A counterflow heat exchanger having a plurality of heat exchanger elements in a stacked, spaced-apart arrangement forming a plurality of inter-element passages. Each element has fluid passages formed between facing sections of a first plate and a second plate. A first-fluid passage has an outer passage circumscribing an inner passage in fluid communication with the outer passage. An inlet-port traversing passage in fluid communication with the outer passage, an outlet-port traversing passage in fluid communication with the inner passage and a second-fluid traversing passage circumscribed by the inner passage traverse the first and second plates. A first-fluid inlet header includes the inlet-port traversing passage of each element. A first-fluid outlet header includes the outlet-port traversing passage of each element. A second-fluid inlet passage in fluid communication with the plurality of inter-element passages includes the second-fluid traversing passage of each element.