Usage---Format of print

Usage

Desalination

People can drink fresh water, but not salt water. If we drink salt water we become very ill, as our body cannot cope with the salts. In places where fresh water is in short supply, such as in desert area near the equator, desalination plants provide an importance source of fresh water. Desalination is a process by which the salt is removed from the seawater. The fresh water can then be used for drinking or irrigate crops. In the past, most desalination plants relied on distillation to remove the salt. The salt water was placed in a vessel and heated so that it boiled. The water vapor was drawn off and then cooled. This causes the water vapor to condense, allowing pure water to be collected. Impurities in the water such as salt were left in the vessel. The water that was produced was ever purer than normal fresh water. This method was quite expensive, because it required a large amount of energy to boil off the water. But, in the 1970s, a process called reverse osmosis was developed. In this process, an artificial membrane is used to remove the molecules of salt from the water.

Water for cooling

It takes a lot of heat energy to change water from its liquid state into water vapor. This physical fact is commonly made use of in cooling systems. Engines, for example, generate a lot of excess heat energy. In order to prevent an engine from overheating, this heat must be removed by cooling. Water is one of the most common forms of coolant, because it is plentiful and uses up lots of heat energy in the evaporative process.

Natural Cooling

As the sun beats down upon the earth, land surfaces absorb the heat and quickly release it back into the air again. Water, on the other hand, has a larger capacity than land for absorbing heat. Heat is not released into the air as quickly, which helps to keep the earth's atmosphere from becoming too hot. Many of the other planets in our solar system have wide ranges of temperature-at times varying by hundreds of degrees. Water in the earth's atmosphere and on its surface helps prevent this from happening on our planet.

The human body can be thought of as a form of engine. The food we eat is digested and absorbed into our body. Once it reaches a cell, the chemical from the food can be released and used for the processes that take place in the cell. This process is called respiration. However, quite a lot of heat energy is also released. Most of the time, this helps to keep our bodies at our normal temperature of 37°C (98.6°F). But, if we do strenuous physical exercise, or we are in a hot environment, excess heat energy has to be dissipated (got rid of) or the body will overheat. The body uses water to help it keep cool by sweating. The skin contains millions of tiny sweat glands that produce water with a little urea and some salts dissolved in it. The liquid travels up a dust from the sweat glands, which actually lie quite deep in the skin, to the surface of the skin. As the sweat lies on the surface, the water takes heat energy from the skin and evaporates, thereby cooling down the skin.

Sometimes it is difficult for the sweat to evaporate. For example, in a hot and humid climate, sweat often lies on the surface of the skin and does not evaporate .This is because the air already contains so much water vapor that no more water can evaporate . Instead, it stays on the skin and makes the person feel sticky and uncomfortable. In a hot, dry climate, however, the air contains little water vapor, so any sweat quickly evaporates, making us feel cooler. A similar effect is felt when it is windy. Wind carries the water vapor away from the skin more rapidly, and replaces it with dry air capable of absorbing more moisture .

Very few mammals are able to sweat. Most mammals have evolved other ways of keeping cool in hot weather. Kangaroos have a network of tiny capillaries very near the surface of the shin along their forearm. They lick the fur above this area of skin, and the blood is cooled as the saliva evaporates and heat is drawn from the capillaries below. Dogs hang their tongues out and pant. These quick, shallow breaths bring cool dry air into body and allow lots of water to evaporate from the tongue and the rest of the mouth, thus cooling the cooling the blood. Cats lick themselves in order to spread a thin layer of saliva over their fur. As the saliva evaporates, it cools the body.

Camels live in dry desert areas where water is in short supply. They cannot afford the luxury of being able to lose water by sweating. Nevertheless, camels still need some means of keeping cool. While most mammals keep their body temperature within a very narrow range day and night (in humans within a fraction of a degree above or below 37°C [98. 6°F]), camels allow their body temperature to fluctuate. During the heat of the day, they allow their body temperature rise. By the end of the day their body temperature may be as must as 6°C (11°F) higher than it was in the morning. At night, deserts are quite cold as there are few clouds to retain the heat. So as nighttime temperatures drop, the body temperature of the camel falls back to its lowest lever. It has been estimated that by allowing the body temperature to rise and fall naturally in this way, the camel saves approximately six liters (12. 5 pts.) of water that would otherwise have been lost as sweat. A further adaptation of the camel, designed to save even more water, is the production of very concentrated urine.

Cooling engines and power stations

Water is used as a coolant in car engines. Cool water is pumped around the cylinders of the engine where the fuel is burned, removing excess heat. From there it is passed through the radiator where it loses its heat energy to the air. A car radiator is said to be a form of heat exchanger. The hot water in the tubes of the radiator comes into contact with cooler air from outside and the heat energy is transferred to the air. The water in the radiator is therefore cooler than when it entered the radiator. When the car is moving, air is forced over the radiator. However, engines could overheat while the car is stationary or moving very slowly. Therefore a fan is often provided to force air over the radiator if the car is stationary. The cool water from the radiator then passes back around the cylinders. Some of the hot water passes through a smaller heat exchanger from which hot air can be used to heat the passenger compartment of the car in winter. In cold weather, an antifreeze is added to the water in the coolant system to stop it from freezing and cracking the pipes of the radiator.

Water is frequently used as a coolant in power stations. Most power stations are built beside a river or by the sea from which an adequate supply of water can be pumped easily and cheaply. A coal-fired power station burns coal inside a furnace. The heat is used to turn water into high pressure steam. This steam turns huge fans, called steam turbines, that are coupled to electromagnets called rotors. The rotors revolve inside a stationary coil of copper wire and generate electricity. The whole process involves two transformations of energy: the heat energy in the steam is transformed into kinetic (movement) energy in the rotating turbines and rotors. This kinetic energy is then converted into electrical energy.

Although the steam is cooler than it leaves the turbine, it is not cool enough to condense back into water. To reduce its temperature and make it return to its liquid state, the steam passes through enormous cooling towers where it comes into contract with tubes containing cold water from the river or sea. The steam thus cools further and condenses. It is then returned to the furnace. The sea or river water in the tubes, now several degrees warmer, will be pumped back to where it came. In some places, the warm water leaving the power station is sufficient to keep the nearby water ice-free all winter. For example, in northern Canada many birds are attracted to the ice-free lakes beside power station because it allows them to feed all through the winter.

In some power stations, where there are factories and building nearly, the warm waste water from the power station is not released into the river but is used as a cheap source of heat to warm the buildings during the winter. This makes the power station more efficient.

Water for heat storage

A large body of water will absorb heat energy quiet slowly. It will also release this heat energy slowly, so it takes a long time to cool down. The high heat capacity of water has many uses. Since the earliest times, water has been used to keep homes cool in summer. For example, people would take blocks of ice from lakes in the winter and pack them away in sawdust and then use them to cool food during the summer. Now that way of producing energy are becoming more expensive, many small scale heating and cooling projects are being developed all around the world.

Scientists are developing ways of storing the heat energy of summer sunlight and using it for winter heating. In the United States, the University of Massachusetts at Amherst is building a central solar heating plant designed to capture the heat energy of the sun and to store it in the ground for use later in the year. During the summer months, south-facing solar collectors will absorb the heat energy and transfer it, via a heat exchanger, to a mixture of water and alcohol (the alcohol acts as an antifreeze). The warm water and alcohol mix is pumped through thousands of plastic pipes sunk deep in the ground, releasing heat energy that warms the clay soil. By midsummer the clay will reach temperatures of 50°C (122°F ) or more. Some heat energy will be diverted to a spare water tank to provide hot water at night when the solar collectors are not working. During the winter months, cold water flowing from the clay. The warm water will be circulated around pipes in the university buildings, keeping them warm.

The future

Water supply is one of the most important factors controlling the colonization of desert areas. Scientists have recently rediscovered an old method of obtaining water using rocks. In this remarkable method, stoned are piled into a pyramid shape, forming a structure called an aerial well. It works best in areas that experience a wide fluctuation in temperature with hot days and cold nights. In the daytime the stones warm up slowly, but at night when the temperature drops they lose much of the heat and become cooler than the surrounding air. The stones in the middle remain cool even on the hottest days. Even dry air contains a small amount of water vapor, so at night when warm air passes over the cooler stones the vapor in the air condenses. This water can then be collected. Using these devices, enough moisture could be captured from the dry air to supply water to a small community. The ancient Greek city of Feodosiya, now in the Ukraine, had a water supply system consisting of 13 aerial wells, each nearly 14 meters (about 46 ft.) tall. Scientists estimate that the system could have produced as much as 20,000 liters (6,760 gal.) of water a day.