This study proposes an improved design for a typical sludge continuous feeding drying system connected with three air-source heat pumps. The system’s performance was further improved using air-deflectors on the drying chamber’s internal sidewalls, enhancing the heat and mass transfer between the conveyor sludge and circulating airflow. In this study, numerical analysis was performed to elucidate the deflector designs on the airflow field and thermal temperature field distributions in the drying chamber. The specific moisture extraction rate (SMER) value was quantified to evaluate the system’s overall improvement during experiments. With a suitable deflector design, the average percent water content in sludge could be further reduced to 22.2% with drying time of 18.3 h, and the SMER value could be enhanced from 1.38 kg/kWh to as high as 1.83 kg/kWh with an increment of 32.44%. Moreover, to prevent overloading and frequent shutdown of the compressors, an auxiliary cooling subsystem was designed to attain stable operational conditions. By the auxiliary cooling subsystem design, the compressors’ shutdown can be avoided, the temperature difference between airflow inlets and outlet of the drying chamber can be increased, and SMER value can be further increased to a value of 1.94 kg/kWh.
This study proposed an improved design for the typical sludge drying system cooperating with three air-source heat pumps. Performance improvement of the system was conducted by consideration of the internal configuration of the drying chamber and the heat and mass transfer effect of the air in the cabin. Since the heat pump drying system in this study adopts the design of continuous sludge feed, the efficiency of heat and mass transfer between the sludge on the conveyor belt and thet air streams from the condenser in the drying chamber becomes very important. Simulation analysis of the computational fluid dynamics (CFD) software was used to understand the flow field and temperature field distributions in the drying chamber with air deflectors in order to find the appropriate internal configuration design in the drying chamber and system operating parameters. The SMER value (ratio of dehumidification amount and energy consumption) were measured to verify the system improvement effect. The purposed system can enhance the dehumidification SMER value by 32.44%. The simulation temperature differences and the actual measured results are less than 10%, the water content percentage of the discharged dry sludge decrease by 30% after redesign the cabin.
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