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 120 o F) 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 BTUs 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 15 o F Range = T warm water into cooling tower- T cool water out of cooling tower –Typically 10 to 30 o 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 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, 6 th ed., McGraw Hill, New York, NY, The Process Technology Handbook, by Charles E. Thomas, UHAI Publishing, Berne, NY, 1997.