Study On The Thermodynamics Of Heat Pump Drying System And The Drying Process Of Typical Materials

Energy consumption depends on several factors such as economy, technology and population growth. It is apparent that drying itself is an energy-intensive process because of the latent heat to be supplied to the material to evaporate the moisture. It is reported that industrial dryers consume on average about 12% of the total energy used in manufacturing processes. In manufacturing processes where drying is required, the cost of drying can approach to 60%—70% of the total cost. Thus, one of the most important challenges of the drying industry is to reduce the cost of energy source for good quality dried products. Heat pump-assisted drying is an energy efficient process because the heat is recoverable. Heat pump assisted-drying provides a controllable drying environment (temperature and humidity) for better products' quality at low energy consumption (cost).This dissertation has conducted a systematic study on the performance of the heat pump drying system, on the energy and exergy analyses of the heat pump unit and the drying cabinet, and on drying mechanism and experiments of heat pump drying of the carrots.The performances of different from heat pump dryer configurations were studied. By comparing the performances of those cycles of the heat pump dryers, it was concluded that the different operating mode of the heat pump dryer depends on the drying conditions. If the air to air heat exchanger is applied to the united cycle system, the efficiency of the heat pump unit will increase, otherwise, will increase the resistance of the process air and the energy consumption of the heat pump drying system. Ideally, the heat pump drying should be carried out at temperatures <60°C and average relative humidity >30%. These are ideal conditions for the drying of many agricultural materials.Based on the first and second laws of thermodynamics and the concepts of energy (enthalpy) and exergy (stream availability), the energy and exergy analyses of the heat pump dryer are investigated. The exergy losses of the heat pump's four main components (the evaporator, the compressor, the condenser and the expansion) and the dryer were studied. The exergy losses due to heat transfer, expansion and compress have been considered. In the design of a system, the exergy method provides the useful information to choose the appropriate component design and operation procedure.The heat pump dryer used in the experiments was assembled from air conditioner parts. Frequency control is a suitable method for an AC electrical motor working at variable speeds. This method changes the discharge rate of the compressor and the fan by varying their rotation speeds to meet the requirement of variable load. The experimental system is suited to the heat pump assisted-drying, the heat pump fluidized bed drying and the combination drying. The heat pump and the dryer were connected by an adjustable air duct system which enables the HPD to be operated as a partially closed cycled or open cycled.The performances of opening air cycle, semi-opening air cycle and closed air cycle configurations were studied. When the heat pump works with the opening air cycle, the heat pump unit works at the steady state and extraction water from the process air is lower than the semi-opening air cycle. When the heat pump works with the semi-opening air cycle, the extraction water from the process air will increase by exhausted air loss. In the closed system, the bypass air rate (BAR) play a significant role inthe system performance. As BAR increases, the amount of air passing the evaporator is reduced and the BAR directly alter the HPD performance. The extraction water from the process was not sensitive to BAR values within the range from 0.4 to 0.7. To obtain the peak SMER, the system should be operated with BAR between 0.4 and 0.6. At BAR greater than 0.6, the peak values seem to be reduced. it was, therefore, concluded that the bypassing air rate (BAR) is effective at the maximum SMER.With the heat pump drying experimental sets, the carrots slices were chosen as the drying materials.