A heat exchanger in the form of a honeycomb with a plurality of rectangular or otherwise polygon in cross-section passages which share common walls with adjacent passages. Two or more flow paths each comprises a plurality of serially connected passages. Each flow path passes through the heat exchanger in a helical pathway, thus through one passage in a first vertical stack of passages, then through a lower passage in an adjacent second vertical stack of passages, then through a lower passage in the first vertical stack, then through a lower passage in the second vertical stack and in this helical manner to the outlet from the heat exchanger. Thus, the flow path comprises alternate passages in each vertical stack, and another flow path comprises the alternate passages in at least one of the vertical stacks not taken up by the first flow path, whereby the flow paths at least partially overlap each other thereby providing both counter-flow and co-flow.
