Systems, methods and devices for optimizing heat transfer within a device or system used to compress and/or expand a gas, such as air, are described herein. In some embodiments, a compressed air device and/or system can include an actuator such as a hydraulic actuator that can be used to compress a gas within a pressure vessel. An actuator can be actuated to move a liquid into a pressure vessel such that the liquid compresses gas within the pressure vessel. In such a compressor/expander device or system, during the compression and/or expansion process, heat can be transferred to the liquid used to compress the air. The compressor/expander device or system can include a liquid purge system that can be used to remove at least a portion of the liquid to which the heat energy has been transferred such that the liquid can be cooled and then recycled within the system.