An environmental control system of an aircraft comprising a supplemental pack , the supplemental pack , comprising: a shaft ; a compressor ; a turbine coupled to the compressor via the shaft; at least one heat exchanger; a first valve ; a second valve ; a third valve ; and a mixing unit ; wherein the compressor receives and compresses a first medium in accordance with power provided by the turbine via the shaft , the first medium being fresh air, wherein the turbine receives and expands a second medium via the first valve to provide the power to the compressor via the shaft , the second medium being cabin discharge air, wherein the at least one heat exchanger is configured to cool the first medium that is compressed and heated and flowing from the compressor , wherein the at least one heat exchanger comprises a first heat exchanger downstream of the turbine on a flow path of the second medium and a second heat exchanger upstream of the turbine on the flow path of the second medium ; wherein to cool the first medium : the heat exchanger utilizes: the exhaust of the turbine , being a cool version of the cabin discharge air ; and part of the first medium via the second valve ; and the heat exchanger , in turn, provides a medium , which is a cool and compressed version of the first medium to the mixing unit via the third valve ; and wherein the second heat exchanger , upstream of the turbine , utilizes a third medium to heat the second medium , which is exhausted from the second heat exchanger as a further medium . a shaft ; a compressor ; a turbine coupled to the compressor via the shaft; at least one heat exchanger; a first valve ; a second valve ; a third valve ; and a mixing unit ; wherein the compressor receives and compresses a first medium in accordance with power provided by the turbine via the shaft , the first medium being fresh air, wherein the turbine receives and expands a second medium via the first valve to provide the power to the compressor via the shaft , the second medium being cabin discharge air, wherein the at least one heat exchanger is configured to cool the first medium that is compressed and heated and flowing from the compressor , wherein the at least one heat exchanger comprises a first heat exchanger downstream of the turbine on a flow path of the second medium and a second heat exchanger upstream of the turbine on the flow path of the second medium ; wherein to cool the first medium : the heat exchanger utilizes: the exhaust of the turbine , being a cool version of the cabin discharge air ; and part of the first medium via the second valve ; and the heat exchanger , in turn, provides a medium , which is a cool and compressed version of the first medium to the mixing unit via the third valve ; and the exhaust of the turbine , being a cool version of the cabin discharge air ; and part of the first medium via the second valve ; and the heat exchanger , in turn, provides a medium , which is a cool and compressed version of the first medium to the mixing unit via the third valve ; and wherein the second heat exchanger , upstream of the turbine , utilizes a third medium to heat the second medium , which is exhausted from the second heat exchanger as a further medium . <EOS>