In Part 1 of this series , we examined fundamental heat transfer calculations in cooling towers. In Part 2, we look at design features that maximize heat transfer, and particularly fill selection and the importance of selecting the proper design. Part 3 will outline correct chemistry control methods to maintain reliable operation. Cooling Tower Design Schematics of a counter-flow and cross-flow cooling tower.
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In Part 1 of this series , we examined fundamental heat transfer calculations in cooling towers. In Part 2, we look at design features that maximize heat transfer, and particularly fill selection and the importance of selecting the proper design.
Part 3 will outline correct chemistry control methods to maintain reliable operation. Cooling Tower Design Schematics of a counter-flow and cross-flow cooling tower. Source: Reference 1. For standard cooling towers, two types dominate industrial applications, the counter-flow and the cross-flow types. Of these, counter-flow towers are more numerous. The figures depict the general water and air flow paths in these towers. Both of these designs are of the induced-draft type, in which fans pull air through the towers.
This is common for large towers. The other type is the forced-draft design, in which fans push air through the towers. The key heat transfer concept with any cooling tower is to maximize as much as the water quality will permit interaction between the incoming air and the warm water being discharged above. Cooling tower fill increases the surface area of the incoming water and improves heat transfer. In the early days of cooling towers, splash fill was the configuration of choice.
The common design then was to use wooden bars or slats to break up the falling water into small droplets. Click to enlarge. Schematic of a typical cooling tower. Splash fill improves heat transfer, and in some cases is still used, albeit with plastic instead of wood as the construction material and for use with water with high fouling potential. However, in most cases film fills are the common choice. The figures illustrate three specific varieties of film fill.
Images courtesy of Rich Aull, Brentwood Industries. As the name implies, film fill induces the cooling water to form a film on the material surface. The filming mechanism maximizes liquid surface area. A guiding principle behind fill design and selection is to increase air-to-water contact, driving up convection and evaporative cooling while reducing pressure drop in the system.
The underlying design element that changes for each of the fill types is the flute geometry. Flute spacing is important, and for the designs shown may range from 19 mm for high-efficiency fill to perhaps 38 mm for low-fouling fill.
Flute size and flow path must be considered together when designing for the best combination of heat transfer and fouling resistance.
Some Process Physics The underlying goal of tower design is to supply the coolest water possible to power plant condensers and industrial plant heat exchangers.
Part 1 of this article outlined standard cooling tower heat transfer calculations, and notes that most heat is removed by evaporation of a slight amount of the inlet return water. However, if another thermometer was attached alongside the dry bulb thermometer with a soaked piece of cloth placed around the bulb and with both on a device that allows them to be swirled rapidly through the air. This second instrument is a device known as a sling psychrometer. Although the dry bulb thermometer will still read 90oF after it has been rotated for a while, the other thermometer will read No matter how efficient, a cooling tower can never chill the recirculating water to the wet bulb temperature, and at some point costs and space requirements limit cooling tower size.
As is evident, tower size becomes asymptotic as approach temperature decreases. Approach temperature plays an important role in the ongoing debate over wet cooling vs dry cooling. In arid areas of the world, an air-cooled condenser ACC may be the only logical selection because of the lack of makeup water for wet cooling.
However, ACCs are sometimes installed at power plants where water is not scarce, but where the designers wish to avoid large makeup due to cost, or to avoid regulatory issues related to cooling tower plume and blowdown discharges.
With this in mind, reconsider our earlier example with a wet cooling tower that has a 10oF approach. The water leaving the tower to cool a power plant condenser will have a temperature of 81oF. However, for an ACC operating at an ambient temperature of 90oF, the turbine exhaust steam will only be cooled to a temperature that relatively approaches 90o, but is likely to be higher. The effect on condenser performance and unit heat rate can be dramatic. Part 3 of this series will examine the reasons why cooling tower water treatment and chemistry are vitally important for maintaining system reliability.
References Post R. Wallis, J. Hensley, ed.
Cooling Tower Fundamentals
Dailkree Disinfectant and other chemical levels in cooling towers and hot tubs should be continuously maintained and regularly monitored. The chemistry of the make-up water, including the amount of dissolved minerals, can vary widely. The CDC recommends aggressive disinfection measures for cleaning and maintaining devices fnudamentals to transmit Legionellabut does not recommend regularly-scheduled microbiologic assays for the bacteria. In most cases, a continual low level oxidizing biocide is used, then alternating to a periodic shock dose of non-oxidizing biocides. A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a much larger area than hot water can distribute heat in a body of water.
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