A heat exchanger comprises a plurality of first heat exchange tubes extending through the exchanger, and through a plurality of laterally extending heat exchanger chambers, each chamber having at least one entry from a first chamber and at least one exit to a second axially adjacent chamber, and each chamber having a plurality of transverse interconnecting zones, each of which is defined by at least two of said tubes, and at least one first zone has an entry to said first chamber and at least one second zone, different from said first zone, has an exit to said second chamber. Also included is a vessel for mixing or distributing streams of a first fluid passing axially from an upstream to a downstream location, which comprises transverse baffles across said vessel in at least two successive rows, which rows of baffles define an open transverse chamber, the baffles in successive rows having a different spatial distribution across the vessel. Preferably the apparatus has the heat exchanger with the distributor downstream of it, and the whole apparatus is a compact reformer.