Abstract
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.
Highlights
2.4 million tons of sludge are being produced in Taiwan annually
To improve the heat and mass transfer between the sludge and circulating airflows inside the drying chamber, deflectors in the drying chamber are designed to guide the circulating airflows to pass through each conveyor belt and increase the detention time of the circulating airflows inside the drying chamber
In the initial design of the drying chamber without the air deflector design inside the drying chamber, most of the supplied airflow from the three air inlets directly flow towards the opposite wall of the chamber, move upwards to its top wall and are sucked into the three return air outlets, as shown in the side view of type I
Summary
2.4 million tons of sludge are being produced in Taiwan annually. With rapid industrial development, the total amount of sludge that needs treatment is increasing subsequently. The high temperature and low humidity circulating air generated by the heat pump system is used to reduce the water content in foods. The dry air temperature, relative humidity, and mushroom weight were monitored under different conditions They observed that a solar-assisted water-source pump system could significantly improve the thermal efficiency. Using the heat pump drying system, the treated sludge’s water content, weight and volume can be reduced with a net increase in energy efficiency and a reduction in carbon dioxide emission. The airflow and temperature fields in a sludge drying chamber of the drying system are discussed to enhance the heat and mass transfer between the sludge and circulating airflow in the drying chamber of a continuous feeding air-source heat pump drying system. The actual performance improvement of the heat pump drying system with a suitable chamber and additional cooling subsystem designs was verified through actual measurements for a long operational period
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