An evaporative cooler is a porous surface kept wet in moving air: the water that leaves it takes heat from whatever is behind it. In dry air a zeer pot, two clay pots with wet sand between them, drops the inside by 10 to 15 °C below the air, while in humid air it does little, so it is a hot-dry-climate tool and not a substitute for a refrigerator.
Part of Refrigeration
Cooling stored food by evaporating water from a porous clay, cloth or charcoal surface, and the limits that the humidity of the air puts on it.
Evaporation Cools Because Water Leaving Takes Its Heat With It
Water that turns to vapour needs energy, and it takes that energy from the nearest warm thing, which here is the pot wall and the food behind it. The corpus gives about 2,260 joules per gram of water evaporated in Cold Storage: Passive Cooling for Perishables, which is large compared with the 334 joules per gram needed to melt ice. The method is described alongside earth pits and ice in Cold Storage.
Two conditions have to hold. The surface must stay wet, and air must be able to carry the vapour away. Dry, moving air does both; still or humid air does neither, which is why the carrier for this tree calls the method useless in humid conditions. In dry air the water evaporates freely, and the cooler can fall a long way below the air temperature. As the air approaches saturation, the drop shrinks toward zero.
What You Need
- Two unglazed clay pots, one fitting inside the other with a 2 to 3 cm gap between them (the carrier’s dimension). Glazed pots do not work, because water cannot reach the outside air.
- Clean sand, to fill the gap and hold water against the inner pot.
- A wet cloth to cover the top, and a lid.
- A shaded, breezy place and a supply of clean water.
Method: A Zeer Pot
Step 1: Set the inner pot inside the outer pot. Keep the gap even all the way round so the sand layer is the same thickness everywhere.
Step 2: Pack the gap with sand and soak it. Pour water until the sand is wet through and the outer pot’s wall is damp. When it is working you can feel the outer wall cooler than the air.
Step 3: Put the food in the inner pot and cover it with a wet cloth. The cloth is a second evaporating surface and also keeps flies off.
Step 4: Re-wet the sand two or three times a day. The carrier gives two to three times daily. If it dries out, the cooling stops; if it stays dripping, it wastes water but still works.
Step 5: Check by touch and thermometer. The inside should be clearly below the air temperature. If it is not, the air is probably too humid, the pot too glazed, or the place too sheltered from wind.
Worked Example: How Much Water A Zeer Pot Uses
Assume, as a round figure and not a measurement, that the pot has to remove 100 W of heat leaking in from the hot air and the sun.
- Heat per second: 100 W = 100 J/s.
- Water evaporated per second: 100 ÷ 2,260 = 0.0442 g/s.
- Per hour: 0.0442 × 3,600 = about 159 g.
- Per day: 159 × 24 = about 3.8 kg, which is about 3.8 litres.
So a pot working that hard drinks several litres a day. That is why the sand must be re-wet repeatedly, and why the water supply matters as much as the pot. If the real heat leak is half, the water use halves too.
The scale-up the carrier mentions is a charcoal cooler: a wooden frame with walls of charcoal held in wire mesh, kept wet, so that air drawn through the wet charcoal is cooled. It follows the same arithmetic, with a larger wet surface.
Common Mistakes
- Using it in humid air. The surface cannot evaporate, and the pot is just a damp pot.
- Sealing the pots in a closed room or cupboard. The vapour has nowhere to go and the cooling stops.
- Glazed or painted pots. Water cannot pass the wall.
- Treating the drop as refrigeration. A 10 to 15 °C drop from a hot day can still leave the inside well above the temperatures food safety needs. The corpus cold-chain article puts the zeer-pot interior at 15 to 20 °C and says plainly that this is not cold enough for most vaccines.
Safety
A damp, mid-temperature box grows bacteria
A zeer pot is cooler than the air but warm enough for spoilage organisms, and its wet sand and cloth are themselves a place for them to grow. Use clean water, wash the cloth and the inner pot regularly, and never rely on the pot to make meat, dairy or any medicine safe. The refrigeration carrier’s own table gives meat only 3 to 4 days in a zeer pot against 1 day without. Food Safety covers what happens when it goes wrong.
What’s Next
- Learn next: The Refrigeration Cycle — evaporation again, but with the vapour recaptured and reused
- Combine with: Pre-Mechanical Cooling — choosing between evaporation, ice and earth