An environmental control system for conditioning air in a vehicle compartment, such as an aircraft cabin. Cool expanded higher-pressure air from a turbine is mixed with recirculated warmer, lower pressure air from the compartment. The air is mixed and recirculated by a jet pump which includes an inlet section, an outlet section, a diffuser section and a mixing section. The mixing section includes a plurality of separate mixing chambers. The diffuser section includes diffusers for receiving mixed gases from each of the mixing chambers. The outlet section receives the mixed gases which are then conveyed to the compartment. The inlet section includes for each of the mixing chambers, a primary nozzle for introducing the higher-pressure air into a respective mixing chamber, and a secondary nozzle for introducing the lower pressure air to a respective mixing chamber.