Cooling Towers: Overview

1. Cooling Towers: Overview CM4120 Spring 2008

2. Topics • Introduction • Definitions • Operating Conditions • Basic Components • Water Cooling Systems • Types

3. Introduction • Boxed shaped collection of multilayered wooden slats • Air flow breaks up water as it falls • Design ensures good contact between water and air • Used to remove heat from water

4. Key Definitions • Wet-bulb temperature = air temperature measured by a wet-bulb thermometer • simulates effect of evaporative cooling • Dry-bulb temperature = air temperature measured by a dry-bulb thermometer • Approach = difference in wet-bulb temperatures between inlet and outlet called “the approach to the tower” • Latent heat = heat associated with change in state of matter (e.g., liquid to gas phase)

5. Operating Conditions • 10-20% of heat (sensible heat) removed from contact between water and air • 80-90% of heat removed following evaporation • Evaporation is most critical factor affecting tower efficiency!

6. Operating Conditions • Factors which affect cooling tower performance: • relative humidity • temperature • wind velocity • tower design • water contamination • equipment problems (pump failure) Let’s discuss these!

7. Basic Components • Water distribution system = includes header which distributes (sprays) water from top of tower over splash bars • Fan = induced and forced draft towers use fans to push or pull air • Air intake louvers = louvers on side of towers which direct air into tower (fixed or movable) • Water basin = collects water at bottom of tower prior to discharge

8. Basic Components • fill = material inside a tower which redirects air flow and water • column = wooden or metal post which supports tower • stack = hyperbolic towers and chimney towers have huge stacks located at top • make-up water = water which is added due to evaporation and blowdown • splash bars = used to redirect the downward flow of water

9. Parallel vs. Series Flow

10. Classification of CTs • By direction of air flow • crossflow (airflow is horizontal ) • counterflow (airflow is vertical) designs • By how the air flow is produced • naturally (hyperbolic or chimney towers) • mechanically (forced draft or induced draft)

11. Induced Draft, Cross Flow CT

12. Atmospheric Cooling Tower (Natural Draft) • Use natural forces (wind) to move air through CT • Air flows in through the sides, and out the top • Drift eliminators on the top

13. Hyperbolic Cooling Tower • Also called chimney CT • Often seen at power plants • Very high flowrates • Air flows up, creating a draft

14. Forced Draft Cooling Tower • Fans used to create a draft • Air forced in the bottom, and flows out the top • Typically solid sides • Some recirculation of air possible, harming efficiency

15. Induced Draft Cooling Towers • Fans located at the top of the CT • Lifts air out of the CT, preventing recirculation • Probably the most common type used in chemical plants and refineries

16. Troubleshooting • Water dissolves many things (especially hot water!) • Water is cooled and results in deposits in tower • Solids concentrate in cooling tower basin Trivia Question: Are Cooling Towers equipped with automatic sprinklers?

17. Problems Faced by Operators • Scale formation - suspended solids form deposits • Corrosion - electrochemical reactions with metal surfaces • Fouling - due to silt, debris, algae • Wood decay - fungi

18. Water Composition Control • Suspended solids levels checked by operators (ppm) • Measured values compared to make-up water concentrations • Problem controlled by “blowdown” (i.e., old water replaced with new) • Note: 100 ppm = 100 lbs. suspended solids/1,000,000 lb water

19. Water Composition Control (Solutions) • Scale formation • remove scale forming solids with softening agents • prevent scale forming materials by addition of chemicals • precipitate scale for removal

20. Water Composition Control (Solutions) • Corrosion • add chemical inhibitors (adds thin film to metal) • Fouling • use filtering devices • use dispersants with filtering devices • Wood decay • use biocides (chlorine or bromine)

21. Water Testing (by Operators) • pH of water • total dissolved solids (TDS) • inhibitor concentration • chlorine or bromine concentration • precipitant concentration • filter and screen checks • temperature and humidity

22. Humidity Measurements

23. Humidity -- Background • Humidity is the amount of water vapor in the air • Humidity is described in different ways • "relative humidity," which is the term used most often in weather information meant for the public • Relative humidity is the amount of water vapor in the air compared with the amount of vapor needed to make the air saturated at the air's current temperature • Dewpoint temperature gives a much better estimate of the amount of moisture actually present in the air • very important in determining precipitation amounts and even how comfortable you feel

24. Definitions • Absolute humidity: Mass of water vapor in a given volume of air( i.e., density of water vapor in a given parcel, usually expressed in grams per cubic meter) • Dewpoint: Temperature air would have to be cooled to in order for saturation to occur (Assumes there is no change in air pressure or moisture content of the air).

25. Definitions • Wet bulb temperature: Lowest temperature that can be obtained by evaporating water into the air at constant pressure. • Name comes from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity.

26. Definitions • Relative humidity: The amount of water vapor actually in the air divided by the amount of water vapor the air can hold. Relative humidity is expressed as a percentage and can be computed in a variety of ways. • One way is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent.

27. Sling Psychrometer

28. Humidity Determination • From wet and dry bulb temperatures • Use psychrometric charts • find intersection of wet and dry bulb temperature lines • can read humidity from chart (y-axis)

29. END LECTURE!