Abstract: A heat exchanger of the kind having a group of upright open ended tubes which are of rectangular cross section. The tubes are in parallel and laterally spaced relationship so that the space between any two adjacent tubes is in direct communication with at least one other space of the same kind. The interconnected spaces form at least part of a steam chamber and spacer bars are provided at one or more sides of the tube group to form a lateral boundary of that steam chamber. Each bar extends parallel to the tubes and is located within the space between the longitudinal outer edges of a respective two tubes. Similar spacer bars are also preferably used to close-off the spaces at the terminal end portions of the adjacent tubes and consequently those bars extend transverse to the axis of each tube. A continuous evaporator may incorporate two such heat exchangers in laterally spaced relationship with a downcomer chamber between them and that chamber is divided into cells by transverse baffles.