The purposes of this research were to study the effects of the fraction of evaporator bypass (BP) on heat pump dryer performance and the energy efficiency of two systems of heat pump dryer: the Vary Speed Drive Heat Pump Dryer (VSD-HPD) and the Bypass Working Fluid Heat Pump Dryer (BWF-HPD). For the VSD-HPD, the speed of the compressor varied by using the inverter to control the heating load inside the drying chamber and for the BWF-HPD, the external condenser was used to control the heating load inside the drying chamber. The laboratory scale heat pump dryer was constructed by combining these two systems into one. These experiments used the sliced bananas as the testing materials. The diameter of each sliced banana was 30-35 mm. and the thickness was 5 mm. the initial moisture content of sliced banana was approximately 215-229% dry basis. The experiments were carried out by using the fraction of evaporator bypass as 60%, 70%, and 80% consequently. The working system of this heat pump dryer is the closed loop system which has the constant temperature as 60 °C and the air velocity is 1.27 m/s. This present study is conducted to find out the drying rate (DR) and the specific moisture extraction rate (SMER) of both heat pump dryers. It can be concluded that the BP had slight effect on the performance of the VSD-HPD whereas it mostly affected the performance of the BWF-HPD. The comparison of the average DR and SMER of these two systems of heat pump dryers from all experiments presented that the obtained rate of the VSD-HPD were better than the BWF-HPD. The average SMER of VSD-HPD with 60% BP was 0.55 kg_(water evaporated)/kWh.
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