COOLING
TOWERS
Cooling process
Cooling involves the transfer of heat from one substance to another. As this “give and take” transaction occurs, the substance that gives or loses heat is said to be cooled, and the one that take or receives the heat is referred to as the coolant.
Evaporation
Another way that cooling occurs is through a process called evaporation. Many cooling systems use evaporation to remove heat from cooling water that has absorbed heat from process fluids.
Water molecules move faster when they are heated, and they are constantly trying to break away from the main body of water. When a molecule gains enough heat and does break away, it is said to have evaporated. In a cooling system, the main body of water has been heated by absorbing heat from the process, so the molecules are primed for evaporation if they are exposed to the right conditions. The cooling tower provides those conditions. First the tower fill breaks the water into the smaller units (droplets in many cases). Creating more escape routes for the water molecules to break away. Second, the rapid flow of air removes the water molecules that have escaped, allowing even more molecules to escape.
Water
There are serval reasons why water is the most widely used industrial coolant. First, there is plenty available. Water covers three-fourth of the earth’s surface and is easy to obtain and much less expensive than any other liquid coolant. Water is also easy to handle and transport from one location to another.
Finally, pound for pound (gram for gram). Water can hold more heat than almost any other liquid, and it is non-toxic and environmentally safe.
Although water is one of the best coolants available, it is not perfect. Unless it is properly treated, water cam promote serious problems such as corrosion, fouling, scale, and microbiological growth, which can seriously reduce heat transfer efficiency, waste energy, and in some cases, even cause a total system failure.
Once inside a plant’s cooling system, these contaminants quickly begin their destructive work.
Water used for cooling purposes contains many impurities that can cause serious problems in cooling water systems. These problems include corrosion, safe, microbiological growth, and fouling. A total cooling water treatment program that includes both chemical and mechanical methods is necessary to keep these problems under control. This section present a general overview of the cooling water system in use today, the problems commonly encountered, and the role of cooling water treatment programs.
TYPES OF COOLING WATER SYSTEMS
Although no two cooling water systems are alike, there are really only three basic designs:
Once – through
systems
Closed recirculating
systems
Open recirculating
systems
The name once-through describes how this type of system works. Cooling water is drawn from a large, nearby water source, passed one time through the system to absorb process heat, and then discharged back into the original water source.
In recirculating systems, the cooling water is used over and over again. First, it acts as a coolant, receiving heat from the production process. Then, before it can be used again as a coolant, it must release the heat it received. Two types of systems are available to complete this cooling cycle: Closed recirculating systems & Open recirculating systems.
In closed recirculating systems, heat absorbed by the cooling water is either transferred to a second coolant or released into the atmosphere in a dry cooling tower. The word dry is used because the water is never exposed to the air, and as a result, very little water is lost.
The open recirculating cooling water system is the most widely used industrial cooling design. Just as in closed recirculating systems, the open system uses water over and over again. Its trademark is the large open cooling tower that uses evaporation to release heat from the cooling water. The system consists of three main pieces of equipment : the recirculation pumps, the heat exchanges, and cooling tower.