In this study, an air source heat pump drying system with an external condenser circuit connected in series was developed. The system was designed to enable the external condenser to be activated serially with the internal one via the solenoid valves controlled by the digital thermostat when the drying temperature of the cabinet reaches the desired level. Temperature control of the drying chamber was achieved by activating the external condenser along with the internal condenser. Exergy analysis was conducted for the process of drying banana slices via a closed-loop air source heat pump dryer (HPD) at various drying temperatures. Exergy efficiencies and losses for the HPD system were calculated. As a result, the exergy efficiency of the dryer was calculated to be between 75.93% and 80.95%, while the exergy efficiencies of the system and heat pump were found to range from 7 to 13.07%. Moreover, the expansion valve was found to have the highest exergy efficiency with 93.32%. The highest exergy losses were also found in the compressor and condenser with 0.557 kW and 0.366 kW, respectively.