Cooling Towers

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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.

Presentation Outline

Introduction

Components

Types

Problems

References

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 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.
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.