Operating characteristics of sludge heat pump dryer and influencing factors of performance: A field experiment in an electroplating factory

Heat pump drying technology has broad application prospects in sludge drying due to its clean, low-carbon, and high-efficiency features. A field experiment in an electroplating factory was carried out to reveal the variations and distribution of the circulating air in the sludge heat pump dryer (SHPD). Subsequently, the influence of crucial operating parameters on the performance of SHPD was investigated based on the simple variable method. Field data showed that the condensed water generated at the air precooling dehumidifier (APCD) accounted for approximately 20% of the total. The bottom and middle conveyor belts were the major locations for sludge drying, and the top conveyor sludge was mainly in the preheating stage. Increasing the supply air temperature and circulating fan frequency can obtain sludge with lower moisture content. The increased circulation fan frequency raised the specific moisture extraction rate (SMER) from 1.98 kgw/kWh to 2.07 kgw/kWh, while the supply air temperature was reversed. Moreover, extending the sludge residence time in the drying chamber reduced the water removal, which indicated that the adverse effect of increasing the sludge thickness was greater than the beneficial effect of extending drying time. This full-size SHPD field experiment provided reference and support for the application.