a system includes a substantially sealed , substantially airtight cabinet sized for housing a vertical array of heat - producing units , the cabinet having an exterior shell and the system including an interior divider wall disposed inside the cabinet , the shell and divider wall providing an equipment chamber adapted to support the array such that the array cooperates with the shell and divider wall in use to define a first plenum , the first plenum having a first inlet defined by the divider wall for receiving a flow of cooling gas and having a first outlet defined by a plurality of openings through the array whereby the first plenum communicates with the openings in use to exhaust substantially all of the flow of cooling fluid through the openings and hence through the array , wherein the divider wall is configured to allow the first inlet to admit the gas to the first plenum in a substantially horizontal direction .