Cooling Towers

CM 4120

Julie King

Original Presentation by Todd King and I edited it.

Presentation Outline

IntroductionComponentsTypesProblemsReferences

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

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.

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.

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.

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.

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)

Induced Draft, Cross Flow Cooling Tower

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

Forced Draft Cooling Tower

Fans used to create a draft

Air forced in the bottom, and flows out the top

Typically solid sides

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.

Parallel vs. Series Flow

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

Factors that affect Cooling Tower Operations

Relative Humidity of air (want low RH) Temperature of air (want low air

temperature) Wind Velocity Water Contamination

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.

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

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

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

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)

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

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.