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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

Cooling Towers

CM 4120

Julie King

Original Presentation by Todd King and I edited it.


Presentation outline
Presentation Outline

Introduction

Components

Types

Problems

References


Introduction
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


Introduction1
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.


Introduction2
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
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
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
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 cooling towers
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
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
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.



Definitions
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
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 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
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
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
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 solutions1
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
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
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.


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