Abstract: A counter-flow heat exchanger includes helical tube bundles for passage of a primary fluid, the helical tube bundles forming a closed helical flow channel between a core tube and a tubular shell around a common longitudinal axis, with a secondary fluid passing through the closed helical flow channel in countercurrent flow, each tube bundle comprising a plurality of linearly arrayed contiguous helical tubes, the helical tubes each being connected at both ends to a connecting compartment composed of a perforated connecting plate and a cover. The helical tube bundles rest freely on helically arranged brackets which are secured on the core tube and are positioned in different radial planes and extend outwardly to the tubular shell, with straps connecting each bracket in a radial plane. The helical tube bundles may be exposed for cleaning by removing them from the tubular shell together with their connecting compartments, cover plate, core tube, primary inlet and primary outlet.