power generators operated by the expansion of steam; power generators operated by the expansion of liquids, gases and vapors being a by-product of industrial processes; machines for the recovery of energy from machine exhaust gases and vapors, namely, rotary screw expanders, electrical generators and engines operated by the expansion of such gases and vapors; machines for the recovery of energy, namely, rotary screw expanders, electrical generators and engines operated by the expansion of liq...