Abstract: A heat exchanger block includes a series of rows of tubes, the end rows of which are joined to form walls, and two further rows of tubes at the opposite sides of the series, also formed as walls. The tubes of the further rows are shorter than the other tubes and, at one side, are spaced downwardly from the top and, at the other side, are spaced upwardly from the bottom. A pair of lateral openings at the opposite sides and ends of the block results. The blocks may be joined to provide composites or they may be separate although joined at one pair of their lateral openings. A pressure vessel may enclose the blocks to relieve them of rupturing pressure.