A Cold Box Sized for One Day of Trading: An Ice Sizing Rule per Kilogram of Produce per Trading Hour

A trader selling soft produce from an insulated box must decide each morning how much ice to buy. Too little leaves produce warm; too much ties up cash, space and carrying weight. This paper originates a sizing rule in units a trader can use: kilograms of ice per kilogram of produce per trading hour, as a function of ambient temperature, trading duration and insulation. The rule is derived from a heat balance on a stated reference box — expanded polystyrene with 25 mm walls, mean surface area 1.50 m² and target interior 4 °C — carrying 20 kg of produce. For an 8-hour day it gives 10.417 kg of ice at 20 °C ambient, 15.47 kg at 30 °C and 20.524 kg at 40 °C. At 30 °C that is 0.7735 kg per kilogram of produce, or 0.09669 kg per kilogram per hour. The ice at 30 °C costs $6.19 and, under the stated spoilage model, prevents $13.91 of value loss, for a net benefit of $7.72 on a $60.00 lot, or $2.25 saved per dollar spent on ice. At 20 °C the net benefit is only $0.98. The largest single heat load is not conduction through the walls but cooling the produce itself from ambient temperature to 4 °C. All results are computed from stated assumptions. SIMULATED, NOT MEASURED.