Why Cooling Towers Are Used
What is the purpose of a cooling tower?
In the United States, there are federal laws and regulations that require reducing water use and improving water efficiency. Federal agencies are required by the Federal Leadership in Environmental, Energy and Economic Performance by virtue of the Executive Order 13514 for an annual 2% reduction of water use for agency potable water consumption and 2% reduction of water use for landscaping, agricultural, and industrial applications.
What can be a significant source of water for both industrial and potable water use at federal facilities are cooling towers. They are an integral part of many refrigeration systems designed to provide comfort or facilitate cooling across a wide array of applications.
They regulate temperature through the dissipation of heat from recirculating water that has been used to cool air conditioning units, chillers, or other process equipment. And because evaporation rejects heat from the cooling tower, by design the cooling towers use up significant amounts of water.
Primary Function Of Cooling Towers
While cooling tower structures vary so much in design and size, they all perform the same function – to provide the same thing and that’s to liberate waste heat extracted from a process or building system through evaporation of water. Technically, cooling towers are designed and engineered based on a specified cooling load – determined by the amount of heat needed to be extracted from a given peak or process comfort cooling demand. In order for the cooling tower to reject the same amount of heat to the atmosphere, it must be sized appropriately.
Outside of complying with the federal requirements to reduce water use and improve water efficiency, other applications for cooling towers include providing cooled water for electric power generation, manufacturing and air conditioning. There are small cooling towers designed to handle water streams of only a few gallons of water per minute supplied in small pipes usually seen in houses. There are also large cooling towers designed to handle hundreds of thousands of gallons per minute which are supplied in pipes as big as 15 feet in diameter seen on large power plants.
When used in residential applications, cooling towers are usually installed either in the basement or on the roof on a structural platform with appropriate spring vibration isolators to prevent excessive movement and vibration.
Types of Towers
There are five types of towers depending on application.
- Cooling Towers – these are classified by the air flow direction (counter-flow or cross-flow) and the type of draft (if mechanical or natural)
- Mechanical Draft Towers – these have air forced through the structure using a fan. Airflow is pushed through by fans which are installed at the base of the tower.
- Natural Draft Towers – these are designed to move air up through the structure without using fans. The natural law of differing densities between the ambient air and warm air in the tower take care of this function.
- Cross-Flow Towers – these are structured to allow air to flow horizontally across the falling water.
- Counter-Flow Towers – these towers are designed to have upward air flow directly opposing the downward flow of water to provide optimal thermal efficiency as the coolest air contacts the coolest water.
Ways To Maintain Water Efficiency
Federal agencies can maintain water efficiency across facilities through a number of retrofit options, including:
- Installing a side stream filtration system consisting of a rapid sand filter or high-efficiency cartridge filter to cleanse the water. The system draws water from the sump, filters out and remove sediments, and returns the filtered water to the tower enabling the system’s more efficient operation with less water and chemicals. This particular filtration system is especially helpful if the system is exposed to dusty atmospheric conditions. A troublesome system can be converted into a more trouble-free system using side stream filtration system.
- Installing a side stream softening system or make-up water side stream system when hardness (calcium and magnesium) is the limiting factor on cycles of concentration. Hardness is removed by water softening using an ion exchange resin. This can allow operation at higher cycles of concentration.
- Installing covers to block sunlight penetration. Biological growth (e.g. algae) is significantly reduced when the amount of sunlight on tower surfaces is reduced.
- Considering other water treatment options such as ionization or ozonation to reduce the chemical and water usage. However, you should check the life cycle cost impact of using such systems.
- Installing automated chemical feed systems on cooling tower systems that are over 100 tons. The automated feed system should control bleed-off or blowdown by conductivity and then add chemicals based on make-up water flow. Water and chemical use are minimized by the chemical feed systems while at the same time optimizing control against corrosion, biological growth and scale.
Do you see any cooling towers in your area?