Thermal and moisture characteristics of air from a galvanized steel tubular solar air heater for potential drying applications

Solar air heating is a promising approach for improving agricultural drying efficiency; however, few studies have comprehensively characterized the psychrometric behavior of drying air in tubular solar air heaters. This study presents an experimental investigation of a galvanized steel tubular solar air heater designed to enhance the thermal and moisture-related properties of drying air under real outdoor conditions. The novelty of this work lies in the integrated evaluation of the thermal and full psychrometric performance, including temperature, relative humidity, enthalpy, and moisture-carrying capacity, together with a systematic assessment of the effects of heater length and airflow velocity. The system was tested using three heater lengths (3, 6, and 9 tubes corresponding to 6–18 m) and airflow velocities of 1–3 m/s. The results showed that the heater length was the dominant factor affecting the performance, whereas the airflow velocity had a secondary but statistically significant influence. The maximum outlet air temperature reached 45.8 °C, and the highest percentage temperature increase was 41.3%. The relative humidity reduction reached 57.1%. The moisture-carrying capacity of the air increased significantly with heater length, reaching a maximum value of 84.4 g/kg at 12 m and 1 m/s airflow velocity, closely followed by a heater length of 18 m under the same airflow conditions (83.8 g/kg). Overall, the proposed system improved the thermodynamic state of the drying air by simultaneously increasing the temperature and reducing the relative humidity, demonstrating its suitability for solar-assisted agricultural drying in arid and semi-arid regions. The integration of solar reflectors beneath each tube is suggested as a potential improvement for further enhancing the thermal efficiency. These findings provide quantitative design guidelines for optimizing tubular solar air heaters for sustainable drying.