Combined microwave-hot air dryer As shown in Fig. 2, the combined microwave-hot air dryer used the same model of microwave oven and weighing system as described in the microwave-vacuum dryer. Fresh air was forced by a blower (LDL, Model RT 2005, Taiwan) connected to 0.4-kW motor to pass through a 1-kW heater to raise the air temperature to the set point. The hot air then flew across the sample on the two-level tray before exiting the dryer. Some hot air was recycled for energy saving purpose. Steel grilles at the inlet and outlet of the oven were used to prevent microwave leakage.