Beans in the Sun: Bag Mass, Projected Area, and the Hours Needed to Reach 50 Degrees in a Black Enclosure

Solar heating — sealing legumes in a black enclosure in strong sun — is a low-cost disinfestation method against bruchid beetles, but the exposure needed is usually given as 'a few hours' without reference to bag size. This study computes the heating time from an energy balance: irradiance 850 W/m2, absorptivity 0.85, losses of 38 percent, bean specific heat 1.9 kJ/kg K, and a target mean temperature of 50 C from a 30 C start. A 5 kg bag reaches the target in 2.14 hours, a 10 kg bag in 2.62 hours, a 25 kg bag in 3.68 hours and a 50 kg bag in 4.71 hours; adding a one-hour hold gives working schedules of 3.14, 3.62, 4.68 and 5.71 hours respectively. Time grows more slowly than mass because a bigger single-layer bag also presents more area to the sun. The practical limit is therefore the length of the clear midday window, and the originated rule is to treat bags of up to 25 kg in a single layer within one clear day, split 50 kg sacks before treatment, and start by mid-morning so the hold hour finishes inside the strong-sun period. This study is simulated, not measured. SIMULATED, NOT MEASURED.