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# Cooling Towers - PowerPoint PPT Presentation

Cooling Towers. CM 4120 Julie King Original Presentation by Todd King and I edited it. Presentation Outline. Introduction Components Types Problems References. Introduction. Cooling Tower = boxed shaped collection of multilayered wooden slats called the ‘fill’

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### Cooling Towers

CM 4120

Julie King

Original Presentation by Todd King and I edited it.

Introduction

Components

Types

Problems

References

• 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

• 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.

• 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.

• 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.

• 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.

• 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)

• 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

• Fans used to create a draft

• Air forced in the bottom, and flows out the top

• Typically solid sides

• 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.

• 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

• Relative Humidity of air (want low RH)

• Temperature of air (want low air temperature)

• Wind Velocity

• 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.

• 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

• 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

• 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

• 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)

• pH of water

• total dissolved solids (TDS)

• inhibitor concentration

• chlorine or bromine concentration

• precipitant concentration

• filter and screen checks

• air temperature and humidity

• “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.