The solar drying process is interrupted during the cloudy or rainy days and also at night. Thus, a combination of mixed mode dryer using solar as main input and biomass burner as auxiliary source of thermal energy will compensate for the absence of the solar radiation. This study describes a detailed design procedure of a thermal back up unit which is flexible to supply hot flue to dry solid waste or to supply warm dry air to dry food and fish. The unit consists of biomass burner and gas to gas heat exchanger topping the burner. The design calculations are based on requirements to dry 2.5 kg of palm fiber (EFB). A mathematical model was developed to calculate the geometries of the heat exchanger. The model is based on the energy balance, convection heat transfer correlations and suitable initial assumptions. The material selection criterion is presented since it is crucial for the success of the design. The fabricated unit is tested experimentally and the measurement results are in good agreement with the basic design assumptions © 2011 Asian Network for Scientific Information.