Abstract: An environment control system which comprises: a cooling turbine drivingly connected to a compressor, means for driving said cooling turbine-compressor combination, and intercooler means, wherein, in a first mode of operation, the working fluid circulates around a closed loop from the compressor to the intercooler to the cooling turbine, thence along a cooling pass to cool equipment or a volume, and back to the compressor inlet, said system including means operable in a second mode of operation to interrupt said loop to cause working fluid to vent to ambient during or after said cooling pass and to introduce further working fluid into said loop.