Cycloidal reactors are the basis of all sulphur reaction chambers (reactors), developed by HP since the 1970s. A cycloidal reactor is one in which Sulphur is burnt with air entering the reactor chamber tangentially, as a swirling flow.

Oxy-Reactors have been designed for gases up to 18 vol% SO₂ generation in one pass, with the lowest NO_X levels guaranteed by the optimised air supply. One of the features of the reactor is the complete reaction of S to SO₂ and the homogeneous temperature distribution at the outlet of the reactor.