COOLING TOWERS




(Image copyright SPX Cooling Technologies, Overland Park, KS)


MAJOR TYPES


HSND - Hyperbolic Stack Natural Draft



This section discusses hyperbolic stack-natural draft cooling towers, such as the one shown above in the animation.


GENERAL INFORMATION

Above is an animation of a hyperbolic stack-natural draft cooling tower.


Air flows into the tower through openings in the bottom, and cools 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 air flow through the column. No fans or other devices are needed.


EQUIPMENT DESIGN

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 air flow rate.


The tower cools 480,000 gallons of water per minute.


Since the water vapor is discharged at a high elevation, there is rarely a problem of fogging or recirculation.


In addition to enhancing the air flow 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, natural draft cooling towers.




(Image copyright SPX Cooling Technologies, Overland Park, KS)

USAGE EXAMPLES

Natural draft hyperbolic cooling towers are most often used in the power industry. Operating costs are minimal, and flow rates can be large.



(Image copyright SPX Cooling Technologies, Overland Park, KS)

The picture shown above is an example of a fan-assisted natural draft tower. It has the outward appearance of a natural draft cooling tower, but also is equipped with mechanical draft fans to help with air flow. This design allows the cooling tower to minimize horse power and required costs.


ADVANTAGES

DISADVANTAGES

  • 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.

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COUNTERFLOW


This section describes counterflow cooling towers, the most common type of cooling tower used in industry.



Counterflow Cooling Tower

(Image copyright SPX Cooling Technologies, Overland Park, KS)


GENERAL INFORMATION

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 a greater thermal efficiency than crossflow designs.


(Picture copyright Brentwood Industries, Reading, PA)

Shown above, to the right, are counterflow cooling towers in action.


EQUIPMENT DESIGN

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.



(Picture copyright Brentwood Industries)


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.



(Image copyright SPX Cooling Technologies, Overland Park, KS)


Shown here are single-cell, counterflow cooling towers.



(Image copyright SPX Cooling Technologies, Overland Park, KS)


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.


USAGE EXAMPLES

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)

ADVANTAGES

DISADVANTAGES

  • 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 - leads to structural and noise problems.

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CROSSFLOW


This section discusses crossflow cooling towers, such as the ones shown here.



(Image copyright SPX Cooling Technologies, Overland Park, KS)


GENERAL INFORMATION

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.



(Picture copyright Brentwood Industries, Reading, PA)


EQUIPMENT DESIGN

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 are often used in industry because they are relatively easy to maintain.



(Image copyright SPX Cooling Technologies, Overland Park, KS)


Shown below is a basic diagram that shows the flow through a crossflow cooling tower.


(Image copyright SPX Cooling Technologies, Overland Park, KS)


USAGE EXAMPLES

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


(Image copyright SPX Cooling Technologies, Overland Park, KS)


ADVANTAGES

DISADVANTAGES

  • 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 - leads to lower thermal efficiency.
  • Coldest air does not contact the hottest water - leads to lower thermal efficiency.

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MINOR TYPES


HSFD - Hyperbolic Stack Forced Draft


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.


A 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.



(Image copyright SPX Cooling Technologies, Overland Park, KS)


ADVANTAGES

DISADVANTAGES

  • 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.

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ATMOSPHERIC


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. 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.



(Picture copyright Butterworth-Heinemann, Newton, MA)


ADVANTAGES

DISADVANTAGES

  • 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.

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WET/DRY


GENERAL INFORMATION

Wet/Dry cooling towers combine the effects of evaporative and nonevaporative cooling methods. The hot water first passes through the nonevaporative (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.




EQUIPMENT DESIGN

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 nonevaporative 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.




ADVANTAGES

DISADVANTAGES

  • 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 minimized by modifying the tower exhaust air condition.
  • Expensive.
  • Coil scaling and restricted air flow are possible.

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ACKNOWLEDGEMENTS


Brentwood Industries, Reading, PA

SPX Cooling Technologies, Overland Park, KS


REFERENCES


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.

Walas, Stanley M. Chemical Process Equipment: Selection and Design. Stoneham:      Butterworth-Heinemann, 1990: 283. Print.


DEVELOPERS


Jeff Scramlin

Mike Africa

Alex Wozniak

Joseph Palazzolo

Steve Wesorick

Keith Minbiole


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