Abstract: A heat exchanger having a number of sheets retained in spaced relationship with one another by projections outstanding from the sheet surfaces. Some at least of the sheets are provided with return flanges and the arrangement is such that return flanges of a sheet lie contiguous with an adjacent sheet so as to define therewith a fluid flow passage. The flanges of the next adjacent sheet are angled with respect to those of the first to define a second passage which extends in a different direction. Two sets of passages are formed thereby and the sheets constitute heat transfer means between adjacent passages. Each set of passages forms a fluid flow conduit. The pack is formed without cementing or welding the sheets together.