The Humidity Ceiling of an Evaporative Cart Cooler: A Psychrometric Bound for Market Produce at 35 °C

Evaporative coolers — a wet charcoal or pad box through which air passes — are widely promoted for keeping market produce cool without electricity, and they work well in dry heat. This paper derives the condition under which they cannot work, at the scale of a cooler box mounted on a market cart or bicycle trailer. Using standard psychrometrics (Magnus saturation pressures, Stull wet-bulb temperatures) at a dry-bulb temperature of 35 °C, with a stated cooler effectiveness of 0.75, the achievable box temperature rises from 23.23 °C at 20 % relative humidity to 28.70 °C at 50 % and 33.91 °C at 90 %. Against stated market-day quality thresholds (leafy greens at or below 20 °C, tomatoes at or below 25 °C), the cooler holds the tomato threshold only below a relative humidity of 28.2 %, and the leafy-greens threshold only below 8.6 % — a humidity so low that, at 35 °C, the cooler effectively never holds leafy greens to threshold. Even a better cooler at an effectiveness of 0.85 moves the tomato ceiling only to 34.4 % and the leafy ceiling to 14.3 %. The negative result is therefore structural, not a matter of better construction: in a humid region, no evaporative cart cooler of any realistic effectiveness keeps produce at threshold through a 35 °C market day, and money spent building one there is spent on a box that cannot do the job. Where the air is dry enough, the design is cheap to run: at 35 °C and 20 % relative humidity the box holds 23.23 °C, the total cooling load for an 8-hour market day with 20 kg of produce is 1,748 kJ, of which cooling the produce itself is 918 kJ, and the water evaporated is 0.72 kg per day — a reservoir of 1.15 litres per day allowing for drainage and waste. The pad area required by the moisture balance is small (0.020 m² at a passive face velocity of 0.1 m/s), so a pad covering one full side of the box is governed by air distribution, not by moisture capacity.