1. CM 4120 Julie King Original Presentation by Todd King and I edited it.
Cooling Towers
CM 4120
Julie King
Original Presentation by Todd King and I edited it.

2. Presentation Outline
Introduction
Components
Types
Problems
References

3. Introduction
Cooling Tower = boxed shaped collection of multilayered wooden slats called the ‘fill’
Air from the atmosphere (so it is ‘free’) enters from the bottom of the cooling tower and flows upward
Warm water (typically about 120oF) flows in the top of the cooling tower thru a water distribution header (to break the water into droplets) and cooler water exits the tower at the bottom

4. Introduction
Hot water transfers heat to cooler air as it passes thru the cooling tower (counter current flow is typical)
Sensible heat (temp change but stays same phase) accounts for approx. 15% of the heat transfer in a cooling tower.
Evaporation (latent heat : phase change) of the liquid water to water vapor accounts for approx. 85% of the heat transfer in a cooling tower.

5. Introduction
When the liquid water changes to vapor, it takes heat energy with it, leaving behind cooler liquid water.
Evaporation removes approximately 1000 BTU’s for every lb of liquid water that evaporates.

6. Basic Components of a Cooling Tower
Water Distribution System: warm process water is sprayed or allowed to fall into the cooling tower and onto the fill
Fan: used to push of pull the air into or out of the cooling tower
Water Basin: located at the base of the cooling tower. Water is collected in the basin and then this cooler water is pumped back to be used again (say in heat exchangers)
Make Up Water: Liquid water is added to the cooling water system to account for water lost to evaporation.

7. Atmospheric Cooling Tower (Natural Draft)
Use natural forces (wind) to move air through cooling tower.
Air flows in through the sides, and out the top.
Drift eliminators (top) prevent liquid water from being blown or sucked out of the cooling tower.

8. Cooling Tower Classification
Classified by direction of air flow
crossflow (airflow is horizontal to the fill )
counterflow (airflow is vertical to the fill)
And, how the air flow is produced
Natural draft (atmospheric, etc)
Mechanically (forced draft or induced draft produced by fans)

9. Induced Draft, Cross Flow Cooling Tower

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

11. Forced Draft Cooling Tower
Fans used to create a draft
Air forced in the bottom, and flows out the top
Typically solid sides

12. Cooling Tower System
In a chemical plant the water is used to remove heat from a process fluid (oil stream, etc.). This is how the water gets ‘hot’ and then needs to be cooled off in the cooling tower.
Always want the water to enter the ‘bottom’ of the heat exchanger and leave out the ‘top’ of the heat exchanger so any vapors can get out.

13. Parallel vs. Series Flow

14. Definitions
Approach Temperature = T cool water out of cooling tower- T wet bulb of air in
Typically 5 to 15oF
Range = T warm water into cooling tower- T cool water out of cooling tower
Typically 10 to 30o F
HTU (height of transfer unit) typically 2 to 3 ft in a cooling tower

15. Factors that affect Cooling Tower Operations
Relative Humidity of air (want low RH)
Temperature of air (want low air temperature)
Wind Velocity
Water Contamination

16. Water Contamination
Water dissolves many things (especially hot water!)
When the hot water returns from the heat exchangers to the cooling tower, it is full of suspended solids.
As this hot water evaporates in the cooling tower, the solids are deposited which results in scale formation.

17. Problems Faced by Operators
Scale formation - suspended solids form deposits
Corrosion - electrochemical reactions with metal surfaces
Fouling - due to silt, debris, algae plug heat exchanger tubes
Wood (on the fill) decay - fungi

18. Water Composition Control
Suspended solids levels checked by operators (ppm)
Measured values compared to make-up (new) water concentrations
Problem controlled by “blowdown” (i.e., old water replaced with new water)
Note: 100 ppm = 100 lbs. suspended solids in 1,000,000 lb water
Often this work is ‘outsourced’ to another water specialist company such as Nalco and Betz

19. Water Composition Control (Solutions)
Scale formation
remove scale forming solids with softening agents
prevent scale forming materials by addition of chemicals
get scale to precipitate out so it can be removed

20. Water Composition Control (Solutions)
Corrosion
add chemical inhibitors to form a thin film that protects the metal)
Fouling
use filtering devices to remove silt, debris, algae, etc.
use dispersants (prevents solids from settling out) along with filtering devices
Wood decay on the fill
use biocides (often 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
air temperature and humidity

22. References
“Unit Operations of Chemical Engineering”, by McCabe, Smith, and Harriot, 6thed., McGraw Hill, New York, NY, 2001.
“The Process Technology Handbook”, by Charles E. Thomas, UHAI Publishing, Berne, NY, 1997.