Evaporative cooling occurs when a liquid evaporates into the surrounding air. Objects in contact with the liquid become cooler as the heat surrounding the liquid change water into vapor and wick it away. The hotter the surrounding air is, the faster the water evaporates and the greater the cooling effect. If temperatures are more temperate, there will be less of a cooling effect.
You can feel this effect when you step out of the bathtub or become wet on a hot day. Waterfalls, lakes, rivers and oceans are other places where this difference in temperature exists.
WHAT'S HAPPENING DURING EVAPORATIVE COOLING?
"This happens because the temperature and the vapor pressure of the water and the air attempt to equalize. Liquid water molecules become gas in the dry air, a process that uses energy to change the physical state. Heat moves from the higher temperature of the air to the lower temperature of the water. As a result, the air is cooler. Eventually the air becomes saturated, unable to hold more water, and evaporation ceases."
Rural homemakers may not have explained it in those terms, but principles of evaporative cooling have been used for generations to keep perishable foods cooler.
A simple household item that employs this technology is the butter bell. In very warm climates, keeping butter at room temperature would result in a butter puddle. The butter bell is a piece of crockery, the size of a large coffee mug, that places a layer of water around the butter, keeping it at a cooler temperature as the water evaporates through the ceramic surface. This results in a spreadable pat of butter without the use of artificial refrigeration.
DID YOU KNOW? Butter that is made from raw milk does not go rancid when stored at room temperature. Rancidity is a phenomenon that occurs in milk and milk products made from homogenized and pasteurized milk. Modern dairy processing strips milk of natural qualities that prevent rancidity.
In contrast, raw milk and raw milk products will sour as they age and are still useful in cooking long after they begin to taste sour.
The difference is apparent when an animal is offered raw milk which has soured versus processed milk which has gone rancid. The rancid product has a non-food (chemical) odor and will be rejected.
"Indeed, naturally soured milk and cream are highly useful items. In fact, it can be argued that the soured versions are even more healthful than the "fresh from the cow variety" due to the higher level of enzymes and friendly bacteria present. Pasteurization destroys these naturally occurring enzymes and probiotics, which explains why processed milk goes rancid and does not sour."
Click on the video below to see Wardeh Harmon with GNOWFGLINS demonstrate how to use a butter bell.
A CANVAS WATER BUCKET?
Used by military and backwoodsmen, canvas bags provide a way to get a cool drink without carrying cumbersome buckets and canteens. If you have a properly made bag, it WILL hold water. When the bag is saturated, the canvas fibers swell and make the container watertight. The warm air surrounding the bag wicks water vapor through the canvass, keeping the drinking water relatively cool.
Zeer Pot image from: http://www.scienceinafrica.co.za/2004/september/refrigeration.htm
Nigeria's climate is relentlessly hot and dry. Without refrigeration, it is nearly impossible to keep food from spoiling.
Mohammed Bah Abba knew that cool storage could provide many opportunities for people in Nigeria: perishable items could be kept fresh longer, waste would be reduced, and profits for struggling farmers could increase.
Other cooling technologies were cost-prohibitive for villagers. Mohammed Bah Abba decided to use evaporative cooling, utilizing available materials, to create a low-cost solution. He designed a pot-in-pot cooling system, called Zeer pots.
Zeer pots are two unglazed clay bowls. The smaller bowl nests inside the larger. Between the two bowls, wet sand is packed. A heavy wet cloth is placed over the lid of the inner bowl, resting on the wet sand. This 'refrigerator' is placed in a shady spot, on a rack that allows air to circulate around the sides and under the outer bowl. The porous ceramic allows the water to be wicked through the pot and evaporate from the wet sand. This creates a lower temperature inside the inner pot.
Because of the low cost of materials, Mohammed has been able to sell his Zeer Pots for less than $1 per unit.
Source of photo: http://www.goselfsufficient.co.uk/potinpot-refrigerator-zeer.html
STATIC COOLING SYSTEM
In India, a similar technology is employed using bricks to build a larger cool storage container. Static cooling systems are constructed in a shaded spot with a thatched lid covered with a damp cloth.
"Construction is fairly simple. First the floor is built from a single layer of bricks, then a cavity wall is constructed of brick around the outer edge of the floor with a gap of about 75mm (3") between the inner wall and outer wall. This cavity is then filled with sand. About 400 bricks are needed to build a chamber of the size shown in [the illustration] which has a capacity of about 100kg. . .
"After construction the walls, floor, sand in the cavity and cover are thoroughly saturated with water. Once the chamber is completely wet, a twice-daily sprinkling of water is enough to maintain the moisture and temperature of the chamber. A simple automated drip watering system can also be added as shown in [the illustration]."
Build your own Pot-In-Pot Cooler.
You will need:
* One small unglazed terracotta pot
* One larger unglazed terracotta pot (large enough to hold the smaller with space between)
* If the pots have drain holes, a rock (or piece of crockery) to stop the sand from falling out of the larger container
* Wet sand
* Saucer to fit over the top of the smaller pot
* Two glass jars (identical) with lids -- The jars must be small enough that one of them can fit inside the small pot.
* A damp cloth
* A Thermometer
Rinse the two pots free of any film or debris. Place your rock or piece of crockery over the hole in the bottom of the larger pot (if there is a hole). Add a layer of wet sand. Place the smaller pot inside the larger pot and tamp the sand beneath it down securely. Adjust the amount of sand so that the top of the small pot is raised even to the top of the larger pot when placed inside. Once this is done, add sand around the sides, pressing it down firmly with your fingers or a spoon until the space is filled with wet sand. If necessary, add more water to the sand.
Fill your two jars with the same amount of water and secure their lids. Place one jar inside your new cooler and place the cooler in a shaded area. Cover the small pot with the saucer for a lid. Place the damp cloth over the lid, allowing it to touch the wet sand. Place the second jar of water outside your cooler.
After about four hours, use a thermometer to record the temperature of the water in the outside jar. Compare this to the temperature of the water in the jar inside your cooler. Is there a difference? Do you think your cooler works better/worse when the air is warmer/cooler? Do you think that the level of humidity in the air affected your results?
Variations of this Project:
* Try this again with the cooler placed up on a brick or some sort of rack that allows air to flow beneath the cooler. Did this change your results?
* Try this same project using a galvanized tub and an enamel sauce pan with lid. Did this cooler work better/worse than the first? Which method would you prefer?
* Try this experiment with differently sized pots. Do you see a difference in the cooling effect when there is more/less room for wet sand between the two containers?
* Take two more identical glass jars with lids. Fill them with the same amount of water. Replace the lids. Around one jar, wrap a wet towel. Leave both jars in a warm place for a couple of hours. When you return, measure the temperature of the water in the wrapped jar versus the unwrapped jar. Was there a difference?
CAN YOU DO THIS?
Use some surplus bricks to build your own static cooling system. How well did your system work? What sorts of items do you think would be good to store in this system?