Thermal and Moisture Characteristics of Air from a Galvanized Steel Tubular Solar Air Heater for Agricultural Drying

Abstract Solar air heating has emerged as a promising approach for enhancing the efficiency of agricultural drying processes. In this study, a galvanized steel tubular solar air heater was designed to generate hot, low-humidity air suitable for agricultural drying. The system comprised nine tubes connected in a serpentine flow configuration, with fans installed at the inlet and outlet to control airflow. The effects of heater length (3, 6, and 9 tubes) and airflow velocity (1, 2, and 3 m s⁻¹) on drying air temperature, relative humidity, thermal content, moisture content, and moisture-carrying capacity were experimentally investigated. Increasing the heater length significantly improved the thermal and moisture characteristics, whereas airflow velocity had a comparatively minor effect. Heater length exhibited strong positive correlations with temperature rise, relative humidity reduction, thermal content, and moisture-carrying capacity, whereas airflow velocity showed moderate influence. Peak performance was observed at a heater length of 12 m (6 tubes) with an airflow velocity of 1 m s⁻¹, resulting in an average moisture-carrying capacity of 84.4 g/kg, closely followed by a heater length of 18 m (9 tubes) at the same airflow velocity (83.8 g/kg). Overall, the tubular solar air heater proved capable of producing warm, low-humidity air, offering a simple, energy-efficient, and sustainable solution for agricultural drying. Integrating solar reflectors beneath each tube is expected to further enhance thermal efficiency and provide a practical strategy for optimizing solar-assisted drying systems.