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What Is a What Is a Cooling Tower? | Process Engineering Glossary
What Is a Cooling Tower?
In piping engineering and process engineering, a cooling tower is a heat rejection device that removes waste heat from process plant cooling water by transferring it to the atmosphere through evaporation. Warm water returning from process heat exchangers and equipment enters the top of the tower and cascades down through fill media while air flows across or counter to the water. A fraction of the water evaporates, absorbing latent heat from the remaining water and cooling it. The cooled water collects in the basin at the base of the tower and the recirculation pumps return it to the process users.
Evaporation accounts for approximately 75 to 95 percent of the heat removed in a typical cooling tower. This high efficiency of evaporative heat transfer makes the cooling tower far more compact than an air-cooled heat exchanger of equivalent capacity. The cooling tower can reduce the water temperature to within a few degrees of the ambient wet bulb temperature, which is the thermodynamic limit of evaporative cooling.
Cooling towers are a central element of the cooling water utility system in almost every process plant, refinery, petrochemical facility, and power station. Their reliable operation underpins the operation of every piece of process equipment that requires cooling water.
Applications of Cooling Towers
Process Plant Cooling Water Systems
Every process plant uses cooling water to condense overheads from distillation columns, cool compressor intercoolers and aftercoolers, cool reactor product streams, cool lube oil circuits, and maintain safe operating temperatures in process vessels. The cooling tower provides the central heat sink that makes all of these individual cooling duties possible.
Power Generation
Power stations use cooling towers to condense exhaust steam from turbines back into liquid water for recirculation to the boiler. This condensation step recovers the maximum work from the steam cycle by achieving the lowest possible condensing pressure. The cooling tower must reject the entire heat load from the turbine exhaust, which is typically the largest heat rejection duty of any industrial cooling system.
Refrigeration and HVAC
Large commercial buildings and industrial refrigeration systems use cooling towers to reject heat from the condenser side of the refrigeration cycle. Water-cooled condensers are more efficient than air-cooled condensers because the wet bulb temperature is always lower than the dry bulb temperature, allowing the refrigerant to condense at a lower pressure and reducing the compressor energy consumption.
Data Centres
Large data centres use cooling towers as part of their thermal management system to reject the heat generated by servers and electrical equipment. The reliability requirements for data centre cooling are extremely high because loss of cooling causes immediate equipment shutdown. Cooling tower systems for data centres typically have full N+1 or N+2 redundancy in pumps, fans, and tower cells.
Benefits of Cooling Towers
Efficient Heat Rejection at Low Approach Temperatures
Evaporative cooling achieves supply water temperatures within a few degrees of the ambient wet bulb temperature, which is significantly lower than the dry bulb temperature that air-cooled equipment uses as its thermodynamic limit. This lower supply temperature allows process heat exchangers to operate at a lower cooling water inlet temperature, improving their efficiency and reducing the surface area required for a given cooling duty.
Lower Capital Cost Than Air-Cooled Systems
For equivalent heat rejection capacity, a cooling tower with its associated water-cooled heat exchangers requires less total heat transfer area than an air-cooled fin fan system because water-side heat transfer coefficients are much higher than air-side coefficients. This compact, efficient heat transfer translates into lower capital cost for the overall cooling system.
Central Water Quality Management
A recirculating cooling tower system treats all the water in one central basin. The chemical dosing system doses inhibitors and biocides into the single basin and they distribute throughout the entire system. This centralised treatment is simpler and more cost-effective than treating individual cooling circuits independently.
Limitations to Consider
Water Consumption
Evaporation, blowdown, and drift losses all consume water continuously. A typical large cooling tower evaporates approximately one percent of its circulating flow per 10 degrees Celsius of cooling range. In regions where water supply is constrained or expensive, this consumption is a significant operating cost and environmental consideration. Air-cooled heat exchangers avoid water consumption entirely but at the cost of higher capital cost and higher condensing temperatures.
Legionella Risk
The warm, humid environment of a cooling tower basin is ideal for the growth of Legionella bacteria. Cooling tower drift, the fine water droplets carried out of the tower with the exit air, can carry Legionella to surrounding areas if the tower is not properly maintained. Effective biocide treatment, regular tower cleaning, and drift eliminator maintenance are essential to control Legionella risk and comply with public health regulations.
Freeze Protection in Cold Climates
In cold climates, cooling towers require freeze protection during winter operation and during shutdowns. Ice formation on the fill media reduces the air flow area and can cause structural damage. Operators cycle fans on and off to manage the air flow and prevent ice buildup. Electric or steam heat tracing on the basin and the exposed piping prevents freezing during cold weather or extended shutdowns.
Cooling Tower FAQ
What is a cooling tower in process engineering? A cooling tower is a heat rejection device that removes waste heat from plant cooling water by evaporating a fraction of the water into the ambient air. Warm water from process heat exchangers enters the tower and flows over fill media in contact with air. Evaporation carries away heat as latent heat, cooling the remaining water, which collects in the basin below and returns to the process. Cooling towers reject 75 to 95 percent of their heat load through evaporation and can cool water to within a few degrees of the ambient wet bulb temperature.
What is the difference between a natural draft and a mechanical draft cooling tower? A mechanical draft cooling tower uses fans to force or induce air through the fill media. Fans are electrically driven and allow the operator to control the airflow and therefore the cooling performance. Mechanical draft towers are compact and suit most process plant applications. Natural draft towers use the buoyancy of warm humid air rising inside the tower shell to induce airflow without a fan. They require very tall hyperbolic concrete shells to generate sufficient draft and are only economical at the very large scale of major power stations.
Why is blowdown necessary in a cooling tower system? As water evaporates in the cooling tower, dissolved salts and minerals remain in the basin water and progressively concentrate. High salt concentrations cause scale to form on heat exchanger surfaces and in the tower fill, reducing efficiency. Blowdown removes a continuous stream of concentrated basin water and replaces it with fresh make-up water, maintaining the dissolved solids concentration below the level at which scaling occurs. The blowdown rate is set by the target cycles of concentration, which balances the water conservation benefit of higher concentration against the increased scaling and corrosion risk.
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