Evaporative cooling

In short

• Evaporation cools at most to the wet-bulb temperature. On a dry day (30 °C, 30–40 % humidity) that is 18–20 °C; on a muggy one (70 %) only 25–26 °C.
• Two thermometers, one with a wet wick, show in advance how much cooling you can get. A difference below 4 °C: do not bother.
• In a typical British or Irish summer the difference is often only 3–6 °C; in the dry American West and the Prairies 10–15 °C.
• It needs shade, a draught and a surface that is always wet. A closed, muggy space does not cool.
• Evaporation is for vegetables, fruit, greens, water and drinks. Not for meat, milk or cooked food (H57-01).
• Wet surfaces must be dried and cleaned: mould, mosquitoes and stale water are the price of this method. Water in sprayers and tanks is changed every 2–3 days.

What's inside

Chapters

1. How evaporation cools
2. Wet-bulb table
3. The psychrometer: measure before you build
4. Water and drinks: bottle, bag and jar
5. The charcoal cooler
6. A wet-hessian cabinet
7. Vegetables after harvest: field cooling
8. Cooling a pantry or summer kitchen
9. When evaporation does not work or does harm
10. Common mistakes
11. Printable checklist

Worksheets

• Water and materials calculator
• Psychrometer log
• Comparison test of devices
• Charcoal cooler sketch
• The evaporative cooling season
• Cost sheet and buying