Abstract: An apparatus is provided for controlling gas temperature during compression or expansion. The apparatus comprises a chamber for containing gas, a piston for changing the volume of gas in the chamber, a plurality of atomisers for spraying liquid into the chamber and means for delivering liquid to the atomisers. Each atomiser comprises a spray aperture and means defining a flow path for imparting rotary motion to the flow of liquid about the axis of the aperture so that on leaving the aperture the liquid divides into a conical spray. Spray apertures are positioned adjacent one another and the axes of adjacent spray apertures are oriented such that their respective sprays intersect at a position proximate at least one of the respective adjacent spray apertures.