The plate exchanger comprises a plurality of flat parallel compartments formed between adjacent plates which are maintained in spaced relation by means of rings each having a T-shaped cross-section. The side arms of the T are welded to opposite edges of orifices formed in adjacent plates and the outwardly directed stem of the T is disposed between the plates. The interior of the array of spacer rings forms a header for the low-pressure fluid which flows through the compartments and a cylindrical tube extends through all the spacer rings in order to serve as an internal support.