The Coolgardie Safe is a low-tech refrigeration unit which uses the heat transfer which occurs during evaporation of water. It was named after the place where it was invented — the small mining town of Coolgardie, Western Australia, near Kalgoorlie-Boulder.
Principles of operation - The Coolgardie Safe was made of wire mesh, hessian, a wooden frame and had a galvanised iron tray on top. The galvanised iron tray was filled with water. The hessian bag was hung over the side with one of the ends in the tray to soak up the water.
Gradually the hessian bag would get wet. When a breeze came it would go through the wet bag and evaporate the water. This would cool the air and in turn cool the food stored in the safe.
It was usually placed on a veranda where there was a breeze. The Coolgardie safe was a common household item in Australia up to the mid-twentieth century. Safes could be purchased ready-made or fairly easily constructed at home. Some of the metal panel safes are very highly decorated, showing the creativity of their makers.
Make a model Coolgardie safe that keeps things cool in hot weather. What you need:
- Washed milk carton
- Rubber bands
- Scissors
- Hessian material aka burlap
- Strips of cloth
- Stapler
- Sticky tape
- Thermometer
- Electric fan (optional)
- Water
What to do:
1 Cut the top from the carton at about 2 cm below the base of the sloping bit.
2 Staple the top, seal with sticky tape.
3 Cut 5cm x 10cm rectangles from each side of the carton.
4 Wrap a 30cm x 12cm rectangle of hessian around the sides, hold in place with rubber bands or staples.
5 Invert the top and sit it in the top of the safe.
6 Fill the top tray with water, use strips of cloth to draw water onto the hessian
7 Test the safe, using the fan and thermometer.
McCormick noticed that a wet bag placed over a bottle cooled its contents. He further noted that if this bottle was placed in a breeze, the bag would dry out more quickly, but the bottle would get colder. What McCormick had discovered was the principle of evaporation: ‘to change any liquid into a gaseous state requires energy. This energy is taken in the form of heat from its surroundings.’ Employing this principle, McCormick made a box for his provisions which he covered with a wet hessian bag. He then placed a tray on top, into which he poured water twice daily. He hung strips of flannel from the tray so that water would drip down onto the hessian bag, keeping it damp. As the water evaporated, the heat dissipated, keeping the food stored inside cool and fresh.
Something similar used here in the south. It was called a water cooling unit. Placed in a window to help survive the very hot summers in Texas. Though it did help considerably, after using it for years, home owners would find their carpets destroyed by the mold and mildew. Homes built in the fifties had the water cooling towers built along the side of the house. Our warming has always been difficult in summer. However, winter isn’t so bad.