Steam jet compressors use the energy of a high pressure vapour to compress low pressure vapours to a medium pressure level. Normally such steam jet compressors are operated by steam and mostly serve to compress water vapours. Basically, however, steam jet compressors can be operated with most normal vapours to compress a variety of different vapours. The motive nozzle converts the energy of the motive steam into velocity. The motive jet produced draws in the vapours to be compressed, mixes with them thereby accelerating them.