Operating characteristics and energy efficiency of heat pump dryer for industrial electroplating sludge drying

Conventionally, an electroplating coating factory would handle its process sludge waste from discharge point source until the dewatering stage only. Huge amount of dewatered sludge cake would incur high hidden cost to the production particularly in sludge transportation, storage and disposal. Therefore, further reduction of moisture content in the sludge after dewatering stage is essential in order to diminish the sludge handling cost. Nonetheless, the energy cost for sludge drying must be justifiable and low enough to take the advantage of it. An energy efficient sludge drying technology is indeed greatly in need. Heat pump dryer is known to be energy efficient due to its ability of heat recovery. Therefore, in the present study, a pilot-scale heat pump dryer was developed to evaluate the potential of heat pump dryer in sludge drying. Experiments were conducted to determine the drying characteristics and energy efficiency under drying temperature of 35°C-50°C and air humidity of 10%RH-52%RH. Comparisons were made against samples from conventional hot air drying (35°C-70°C). Results revealed that the drying rate would increase in proportion to wet bulb depression of drying air, where the highest effective diffusivity was recorded at 1.09765 x 10-8 m2/s in heat pump drying at 50°C. Due to the dehumidification function of heat pump dryer, when under same drying temperature, drying air in heat pump dryer would possess higher wet bulb depression as compared to hot air dryer. Therefore, drying rates of heat pump dried samples were higher than hot air dried samples. In term of energy performance, for hot air dryer, higher drying air temperature would consume more energy. The highest power consumption was recorded at 51 kWh in 35°C heat pump drying while the lowest power consumption was recorded at 14 kWh 35°C hot air drying. While lower drying air temperature consumed lesser energy, however, longer drying time was required to dry the same amount of sample. For heat pump dryer, it was found that improper configuration and control strategy could result in high power consumption as compared to hot air dryer. Findings showed that even though a heat pump dryer could possess higher heating COP theoretically, improper configurations of heat pump components (e.g. condenser type, compressor capacity, bypass duct and evaporator) and control strategy (e.g. relative humidity control and air flow rate control) could end up consuming more energy than it supposed to be (37 kWh more than hot air drying). Testing result shows that using condenser to heat the air directly would give different energy consumption profile compare to heat the air through secondary medium such as water. Direct heating would consume lesser energy at the early stage of drying compare to indirect heating. However, once the heating medium of indirect heating mode reaching its temperature limit, the energy consumption will drop and eventually having similar SMER as direct heating mode. Air volume flow rate would play a more significant role in heat pump drying compare to hot air drying. Lower airflow in heat pump drying would increase the moisture extraction rate and thus causing lower relative humidity in the drying air. Lower drying air humidity would have better drying kinetic. In heat pump drying, some air is bypass from going through evaporator to reduce the sensible heat ratio so that more latent heat could be recovered. Finding showed that the drying kinetic would drop significantly when there is no bypass air. This could be due to poor dehumidification of the drying air at the evaporator of the heat pump. Improper strategy in dehumidification control was one of the major issues for heat pump dryer in term of energy efficiency. It was found that when relative humidity of drying air was too low (i.e. when dew point was near to the surface temperature of evaporator) for effective dehumidification, kept running the compressor would decrease energy efficiency as the input energy did not bring any meaningful work. In addition, it was observed that using secondary media (hot water) for heating would limit the temperature that could be achieved by a heat pump and the thermal energy that being stored in the hot water tank was not being used and wasted. Furthermore, proper matching of heat pump size and drying load was playing an important role in the energy efficiency of the heat pump dryer as well. When the heat pump capacity was relatively too large for the drying load, then a huge amount of energy would be wasted due to frequent start/stop of the compressor and operation of auxiliary condenser. However, if the heat pump size was too small, it might not able to achieve the targeted drying air condition. Therefore, for efficient operation of a heat pump dryer, the compressor size could not deviate too much from the drying load, unless the heat pump system had means of capacity control for instance using a variable speed compressor