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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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