airah -types of ct

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Selecting a Cooling Tower Facilitator Guide Version 1.0 1

2. Types of cooling towers

Show this slide as an overview of the material to be covered in this topic.

Slide 1:Learning OutcomesTypes of CoolingTowers

2.1 Introduction

This topic looks at the different types of cooling towers available on the market. Italso looks at the principles of operation of each type of cooling tower.

2.2 Principles of operation

Activity 2.2 PowerPoint presentation / discussion 2 min

Refer participants to section 2-10 to 2-20 in theDA17 AIRAH/IRHACE Cooling TowersManual.

Show the following slide and review:

Slide 2:Two types of coolingtowers


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2 Selecting a Cooling Tower Facilitator Guide Version 1.0

2.3 Direct contact towers


Refer participants to section 2-20 in the DA17 AIRAH/IRHACE Cooling TowersManual.

Review the following then show the subsequent slide:

Evaporative cooling of recirculated water is achieved by it being spreadover a large surface area by spraying, splashing or film formation, or anycombination of these, within a tower. A once through air flow directlycontacts the surface of the water within the tower.

Slide 3:

Direct contact

evaporative coolingtower

Explain:

There are basically 2 types of direct contact cooling towers:

natural draft tower mechanical draft tower.

Mechanical draft towers dominate the market as they are available inany size and as their thermal performance is independent of windvelocity, it is possible to design for exacting conditions.


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Natural Draft


Slide 4:Natural draft tower

Mechanical Draft

Explain:

This type of cooling tower utilizes a motor -driven fan to move airthrough the tower, the fan being an integral part of the tower making the

thermal performance more stable than the natural draft tower.

Mechanical draft cooling towers are classified as either:

forced draft, or induced draft.

Forced Draft

Show the following slide, review then discuss the points below:

Slide 5:

Mechanical - Forceddraft tower


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Fans in forced draft towers are typically centrifugal blower type toovercome high air resistance.

Can be located in plantrooms or enclosures and their discharge can beducted away to minimise recirculation.

Generally quieter than propeller type fans.

Induced Draft

Show the following slide, review then discuss the points below:

Slide 6:Mechanical - Induced

draft tower

Fans are usually propeller type The location of the fan, motor and drive mechanism in the humid exit air

stream means that these components require weatherproofing to preventthe entering of moisture and protection against corrosion.

2.4 Types of air flow



Discuss:

Cooling towers are also categorised by the relative relationship of airand water flows. Towers are either:

counter flow or cross flow.


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Counter flow

Refer participants to figure 2-30A in theDA17 AIRAH/IRHACE Cooling TowersManual.

Slide 7:Counter flow coolingtower

Discuss:

In this type of tower the air moves vertically upwards and through thefill as the water falls downwards.

Water is usually delivered by means of fixed sprays, or in the case of thecircular or bottle shape tower, by means of rotating spray arms.

Eliminators are usually placed above the sprays to prevent loss of waterthrough drift.

Cross flow

Refer participants to figure 2-30B in the DA17 AIRAH/IRHACE Cooling TowersManual.

Explain:

One of the most advantageous characteristics of the cross flow design isits low internal pressure drop which means that greater air flow isachieved.



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Slide 8:Cross flow cooling

tower

Refer participants to the following two examples and discuss:

The following are two examples of cooling towers from B.A.C. sho win gcharacteristics and operative principles.

Characteristics

crossflow design induced draught configuration single side horizontal air entry vertical air discharge aluminium or steel axial fans gravity water distribution low fan power spacious interior for ease of maintenance low sound levels suitable for outdoor locations

Principles of operation

Water from the heat source enters a single inletconnection located on the louver face of the unit

and is distributed over the wet deck surfacethrough gravity flow diffuser nozzles.

Simultaneously, air is induced through air inletlouvers across the wet deck surface and causing asmall portion of the water to evaporate.

This evaporation removes heat from the remainingwater. The cooled water flows into the tower sump

where it is returned to the heat source.


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Characteristics

counterflow design low height forced draught configuration

horizontal air entry at one end vertical air discharge galvanized steel centrifugal fans with Corrosion

Protection System low pressure water distribution system BACount film fill with 15 mm spacing V-belt drive close capacity control options available suitable for indoor and outdoor locations low noise levels

Principles of operation

Water from the heat source is distributedover a wet deck surface by spray nozzles.

Air is simultaneously blown upward overthe wet deck surface, causing a smallportion of the water to evaporate. This

evaporation removes heat from theremaining water. The cooled water is

collected in the cooling tower sump and

returned to the heat source.

2.5 Indirect evaporative cooling towers



Show the following slide, review and elaborate using the notes below:

Slide 9:Two methods ofindirect evaporativecooling


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There are two methods of indirect evaporative cooling.

One cools air prior to it being passed through a dry air heat exchanger. The second comprises a fluid circulated within coils or heat exchanger

tubes, mounted within a cooling tower or fluid cooler.

? Cooling is achieved by the evaporation of cooling water whichis recirculated and distributed over the external surface of thetubes.

? A once through air flow is passed through the tower to contactthe water surfaces on the coil and above the tubes where the water isdistributed, as well as below the coil before it falls into the waterbasin.

Indirect evaporative coolers require a small quantity of water so the costof c ontrolling water quality and preventi ng bacteriologicalcontamination is lower than direct cooling tower systems.

However, capital cost is higher and the tubes must be kept clean to avoidcorrosion caused by solid deposits.


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Activity 2.6 Review Box

Participants may complete this Review box in their own time or at the discretion of

the facilitator. Answers are provided only within the facilitator guide.

Question Answer

How is evaporative cooling ofrecirculated water achieved?

Being spread over a large surface area byspraying, splashing or film formation, or

any combination of these, within a tower...

What are the 2 basic types ofdirect contact cooling towers?

Natural draft Mechanical draft of these are forced an

induced draft.

Name the 2 categories of air andwater flows in cooling towers.

Counter flow Cross flow

What is the most advantageouscharacteristic of cross flowdesign cooling towers?

Its low internal pressure drop whichmeans that greater air flow is achieved.

Describe the two methods ofindirect evaporative cooling.

One cools air prior to it being passedthrough a dry air heat exchanger

The second comprises a fluid circulatedwithin coils or heat exchanger tubes,

mounted within a cooling tower or fluidcooler.

They require smaller quantities of water soconserves water.

Summary

Review the learning objectives, summarise the key points of the topic and be

prepared to answer any questions from participants for clarification on the content.

Further reading

Section 2 of the DA17 AIRAH/IRHACE Cooling Towers manual.

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