Cooling towers remove heat from a warm liquid by contacting it with a cool, dry gas. Some of the liquid vaporizes and enters the gas stream. The vaporization removes heat from the liquid stream. The outgoing streams are a humid gas containing the vaporized liquid feed component and a cooled liquid. This cooled liquid can be used for other cooling needs in a chemical process. Cooling towers are most commonly used for air-water systems.
Cooling towers differ in shape, directions of flow, and the way the gas stream enters. Cooling towers can be hyperbolic or cylindrical. The gas and liquid flow can be in the countercurrent or crossflow directions. Cooling towers can also either be natural draft, meaning that gas naturally flows into the tower, or mechanical draft, meaning that devices, typically fans, bring gas and liquid into the device at set rates. These differences are described in the types of cooling towers below. The table of contents below link to different types of cooling towers. Each type of equipment usually has sections on general information, information about equipment design, usage examples, and advantages/disadvantages.
Hyperbolic Stack Natural Draft (HSND)
This section discusses hyperbolic stack-natural draft cooling towers, such as the one shown above in the animation.
Above is an animation of a hyperbolic stack-natural draft cooling tower.
Air flows into the tower through openings in the bottom, cooling the entering hot water. Cooled water flows out the bottom and warm, moist air exits out the top.
The density difference between the cold entering air and the warm exiting air results in a natural draft that causes the airflow through the column. No fans or other devices are needed.
The cooling tower is constructed of a thin concrete shell with strong air resistance. The hyperbolic shape of the tower enhances the aerodynamic lift due to the wind passing over it, which increases the airflow rate. In addition to enhancing the airflow rate, the hyperbolic shape of this cooling tower provides superior strength, so that fewer materials are needed in its construction relative to other models. The opening at the bottom of the tower allows air to enter. The two diagrams below show the design differences between crossflow, on the right, and counterflow, on the left, of natural draft cooling towers. The tower can cool up to 480,000 gallons of water per minute.
Since hyperbolic stack-natural draft cooling towers are such high towers, the outgoing vapor stream exits at such a sufficiently high elevation that there is rarely a problem of fogging or recirculation.
Natural draft hyperbolic cooling towers are most often used in the power industry. Operating costs are minimal, and flow rates can be large.
- Superior strength provides a close match to a natural flow of air through the tower shell.
- Minimal operating costs
- Only effective for large quantities of utility water
- Sensitive to climatic changes
- Physical appearance may be negative in the public eye
- High cost of utility water
This section describes counterflow cooling towers, the most common type of cooling tower used in industry.
The picture above shows a typical counterflow-induced draft cooling tower in operation.
The draft is induced by a fan placed at the top of the tower, drawing the cooling air.
Counterflow means that the flow of air is parallel and opposite in direction to the flow of the water being cooled, as shown in the schematic below. This results in greater thermal efficiency than crossflow designs.
Cooling towers are often made out of wood or fiberglass composites. The fill media in the mechanical draft, counterflow cooling tower shown below is made from rigid, corrugated PVC sheets.
This schematic shows the elements of a general counterflow cooling tower. The cooling air drawn by the fans flows upward against the downward flowing water.
Shown here are single-cell, counterflow cooling towers.
The outside shell is made of both steel and fiberglass. The internal packing (also called fill) is made up of PVC (polyvinyl chloride), a polymer.
Using fiberglass and PVC as construction materials ensures good corrosion resistance.
Counterflow cooling towers are used for air conditioning, process cooling, and power generation. They can be seen in steel industries, automotive foundries, and waste-to-energy plants.
(Images copyright SPX Cooling Technologies, Overland Park, KS)
- Highly efficient – designed to cool within 5°F of the wet-bulb temperature.
- Design allows air to flow at a relatively high velocity preventing the backflow of humid air.
- More economical than natural draft towers for water flow rates less than 19,200 gallons/min.
- Fan power is required (this is the largest operational cost for a cooling tower).
- Induced air design places the fan at the top of the tower – leading to structural and noise problems.
This section discusses crossflow cooling towers, such as the ones shown here.
In this crossflow, two-side air inlet cooling tower, the flow of air is perpendicular to the flow of the water being cooled.
Crossflow towers use an induced draft: a fan, placed at the top of the tower, draws in the cooling air.
Notice that the entering air in crossflow cooling towers is perpendicular to the flow of water. Hot water is evenly distributed over the wet deck surface. The air drawn through the inlet louvers causes a small portion of the water to evaporate. This evaporation removes the heat from the remaining water. The cooled water then flows into the tower sump and exits the cooling tower.
Above is a crossflow, single-side air inlet cooling tower.
Crossflow towers shown below are often used in industry because they are relatively easy to maintain.
Shown below is a basic diagram that shows the flow through a crossflow cooling tower.
Crossflow cooling towers are used extensively, such as in:
- Air conditioning and refrigeration systems
- Chemical and industrial processes
- Plastic industry processes
- Dairy, citrus, and other food industry processing
- Jacket water cooling for engines and air compressors
- Batch and welder cooling
- Can operate at higher velocities than counterflow towers – lower power consumption.
- Constructed wider and shorter than counterflow towers – leads to lower pumping costs.
- Relatively easy to maintain.
- Air travels through a shorter path than in counterflow towers – leading to lower thermal efficiency.
- Coldest air does not contact the hottest water – which leads to lower thermal efficiency.
Hyperbolic Stack Forced Draft (HSFD)
General Information/Equipment Design
Hyperbolic Stack – Forced Draft (HSFD) cooling towers combine the hyperbolic shape of a natural draft cooling tower with the use of large motor-driven fans.
An HSFD cooling tower has a diameter and height that are two-thirds and one-half, respectively, the size they would be for a natural draft cooling tower designed for the same performance.
- Fans result in greater control of the air movement than natural draft cooling towers.
- Recirculation and fogging are more of a problem for the fan-assisted hyperbolic cooling towers.
- Power to run the fans and maintenance can be cost-ineffective
General Information/Equipment Design
Atmospheric cooling towers are similar to natural draft cooling towers in many ways. The main difference between the two lies in the mechanism of air movement. In an atmospheric cooling tower, natural wind currents provide the air supply. These towers are narrow but tall so that enough wind can enter the tower. Louvers on the sides prevent water from being blown out and allow air to enter in any direction. Hot, moist air rises in the tower, drawing in colder outside air.
- Low power requirement.
- Performance varies greatly due to its dependence on wind direction and velocity.
- Require a considerable amount of clear ground space in the area surrounding them as well.
- Need more area per unit of cooling than most towers.
Wet/Dry cooling towers combine the effects of evaporative and non-evaporative cooling methods. The hot water first passes through the non-evaporative (dry) cooling section, composed of heat exchangers. The water then flows over the evaporative (wet) section by gravity, where it is cooled through contact with dry air.
The cooling air, drawn by mechanical fans, is divided into two parallel streams. One of these streams passes through the evaporative section, the other through the non-evaporative section. The exiting streams combine in a common chamber upstream of the fans. The combined air is discharged into the atmosphere by the fans, creating a less visible plume than in a typical cooling tower.
- Take advantage of a lower heat-sink temperature attainable with evaporative cooling when there is enough water available, and still allow the plant to operate, although at a reduced efficiency, when cooling water is scarce.
- Cooling tower fog formation during cold weather is minimized by modifying the tower exhaust air condition.
- Coil scaling and restricted airflow are possible.
- Burger, Robert. Cooling Tower Technology Maintenance, Upgrading, and Rebuilding. 3rd ed. Lilburn: Fairmont Press, 1995. Print.
- Cheremisinoff, Nicholas P. and Paul N. Cheremisinoff. Cooling Towers: Selection, Design, and Practice. Michigan: Ann Arbor Science Publishers, 1981. Print.
- Hill, G. B., E. J. Pring and Peter D. Osborn. Cooling Towers: Principles and Practice. 3rd ed. Stoneham: Butterworth-Heinemann, 1990. Print.
- McKelvey, K.K. and Maxey Brooke. The Industrial Cooling Tower. Amsterdam: Elsevier Publishing Company, 1959. Print.
- Perry, Robert H., and Don W. Green. Perry’s Chemical Engineers’ Handbook. 7th ed. New York: McGraw-Hill, 1997: 12-20 – 12-21. Print.
- Puckorius, Paul R. “Consider Recycled Water for Your Cooling Tower Makeup.” Chemical Engineering Progress 109.2 (2013): 24-29.
- Walas, Stanley M. Chemical Process Equipment: Selection and Design. Stoneham: Butterworth-Heinemann, 1990: 283. Print.
- Wenzel, L. A. 2000. “Simultaneous Heat and Mass Transfer.” Kirk-Othmer Encyclopedia of Chemical Technology.
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