A bioreactor outlet air conditioning system (10) has a bioreactor vessel (100) and a heat exchanger. The heat exchanger has a fluid flow path (196) from a headspace (108) in the vessel for venting air from the vessel, and a temperature control element (300) in thermal contact with the fluid flow path (196). The fluid flow path (196) may be defined by a disposable portion of the bioreactor vessel (100). A method of controlling evaporation within a bioreactor vessel (100), dependent on the required evaporation rate, adjusts the temperature of the temperature control element (300) to control the rate of evaporation of the liquid media (106) from the vessel (100), and may include monitoring fluids in the fluid flow path (196) to detect at least water content of the fluids exiting the vessel to adjust the temperature and control the rate of evaporation dependent on the detected water content levels.