1

Cooling Towers: Overview

CM4120

Spring 2008

2

Topics

Introduction Definitions Operating Conditions Basic Components Water Cooling Systems Types

3

Introduction

Boxed shaped collection of multilayered wooden slats

Air flow breaks up water as it falls

Design ensures good contact between water and air

Used to remove heat from water

4

Key Definitions Wet-bulb temperature = air temperature measured by a

wet-bulb thermometer– simulates effect of evaporative cooling

Dry-bulb temperature = air temperature measured by a dry-bulb thermometer

Approach = difference in wet-bulb temperatures between inlet and outlet called “the approach to the tower”

Latent heat = heat associated with change in state of matter (e.g., liquid to gas phase)

5

Operating Conditions

10-20% of heat (sensible heat) removed from contact between water and air

80-90% of heat removed following evaporation

Evaporation is most critical factor affecting tower efficiency!

6

Operating Conditions

Factors which affect cooling tower performance:– relative humidity

– temperature

– wind velocity

– tower design

– water contamination

– equipment problems (pump failure)

Let’s discuss these!

7

Basic Components

Water distribution system = includes header which distributes (sprays) water from top of tower over splash bars

Fan = induced and forced draft towers use fans to push or pull air

Air intake louvers = louvers on side of towers which direct air into tower (fixed or movable)

Water basin = collects water at bottom of tower prior to discharge

8

Basic Components

fill = material inside a tower which redirects air flow and water

column = wooden or metal post which supports tower

stack = hyperbolic towers and chimney towers have huge stacks located at top

make-up water = water which is added due to evaporation and blowdown

splash bars = used to redirect the downward flow of water

9

Parallel vs. Series Flow

10

Classification of CTs

By direction of air flow– crossflow (airflow is horizontal )

– counterflow (airflow is vertical) designs

By how the air flow is produced – naturally (hyperbolic or chimney towers)

– mechanically (forced draft or induced draft)

11

Induced Draft, Cross Flow CT

12

Atmospheric Cooling Tower (Natural Draft)

Use natural forces (wind) to move air through CT

Air flows in through the sides, and out the top

Drift eliminators on the top

13

Hyperbolic Cooling Tower

Also called chimney CT

Often seen at power plants

Very high flowrates

Air flows up, creating a draft

14

Forced Draft Cooling Tower

Fans used to create a draft

Air forced in the bottom, and flows out the top

Typically solid sides

Some recirculation of air possible, harming efficiency

15

Induced Draft Cooling Towers

Fans located at the top of the CT

Lifts air out of the CT, preventing recirculation

Probably the most common type used in chemical plants and refineries

16

Troubleshooting

Water dissolves many things (especially hot water!)

Water is cooled and results in deposits in tower

Solids concentrate in cooling tower basin

Trivia Question: Are Cooling Towers equipped with automatic sprinklers?

17

Problems Faced by Operators

Scale formation - suspended solids form deposits

Corrosion - electrochemical reactions with metal surfaces

Fouling - due to silt, debris, algae

Wood decay - fungi

18

Water Composition Control

Suspended solids levels checked by operators (ppm)

Measured values compared to make-up water concentrations

Problem controlled by “blowdown” (i.e., old water replaced with new)

Note: 100 ppm = 100 lbs. suspended solids/1,000,000 lb water

19

Water Composition Control (Solutions)

Scale formation– remove scale forming solids with softening agents

– prevent scale forming materials by addition of chemicals

– precipitate scale for removal

20

Water Composition Control (Solutions)

Corrosion– add chemical inhibitors (adds thin film to metal)

Fouling– use filtering devices

– use dispersants with filtering devices

Wood decay– use biocides (chlorine or bromine)

21

Water Testing (by Operators)

pH of water total dissolved solids (TDS) inhibitor concentration chlorine or bromine concentration precipitant concentration filter and screen checks temperature and humidity

22

Humidity Measurements

23

Humidity -- Background

Humidity is the amount of water vapor in the air

Humidity is described in different ways– "relative humidity," which is the term used most often in

weather information meant for the public– Relative humidity is the amount of water vapor in the air

compared with the amount of vapor needed to make the air saturated at the air's current temperature

Dewpoint temperature gives a much better estimate of the amount of moisture actually present in the air– very important in determining precipitation amounts and even how

comfortable you feel

24

Definitions

Absolute humidity: Mass of water vapor in a given volume of air( i.e., density of water vapor in a given parcel, usually expressed in grams per cubic meter)

Dewpoint: Temperature air would have to be cooled to in order for saturation to occur (Assumes there is no change in air pressure or moisture content of the air).

25

Definitions

Wet bulb temperature: Lowest temperature that can be obtained by evaporating water into the air at constant pressure.

Name comes from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity.

26

Definitions

Relative humidity: The amount of water vapor actually in the air divided by the amount of water vapor the air can hold. Relative humidity is expressed as a percentage and can be computed in a variety of ways.

One way is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent.

27

Sling Psychrometer

28

Humidity Determination

From wet and dry bulb temperatures

Use psychrometric charts– find intersection of wet and dry bulb

temperature lines– can read humidity from chart (y-axis)

29

END LECTURE!