Cooling TowersAN EXTENSIVE APPROACH

Outline

Overview of heat transfer through evaporation

Overview of relevant terms related to cooling operations

Introduction to cooling towers

Basic working principle of a cooling tower

Classification of cooling towers

Major components of a cooling tower

Design of natural draft cooling tower with numerical example

Strategies to improve cooling tower performance

Importance of cooling towers in Chemical Process Industry

References

Evaporation

Evaporation is the conversion of a liquid phase of a material orsubstance to a vapor phase at a specific temperature and pressure.

At normal temperature and pressure, all liquids posses a liquid andvapor phase, which are in equilibrium with each other, if thetemperature and pressure conditions remain the same.

Rate of evaporation of a liquid is affected by the change inambient temperature and pressure conditions.

The change affects the vapor pressure of the liquid, causing anincrease or decrease in rate of evaporation.

Why evaporation causes cooling ?

Evaporation results in vapor escaping from the surface of the liquid.

The vapors require energy to escape.

That energy is heat.

As vapors escape the surface, they carry with them some heat

content of the liquid, and thus cause the liquid surface to cool.

Terminologies relevant to cooling

operations Humidity – Amount of vapor associated with a unit mass of dry gas.

Relative humidity – Ratio of partial pressure of vapor in gas to the partialpressure of vapor in same gas at saturation.

Humidification – Process of increasing the amount of vapor in a gasstream.

De-humidification – The opposite of humidification i.e. the process ofdecreasing the amount of vapor in a gas stream.

Wet-bulb temperature – The lowest temperature that watertheoretically can reach by evaporation.

Factors affecting rate of

evaporation

Some major factors include

Temperature and pressure of the liquid being evaporated.

Temperature and pressure of the ambient gas which will accept the

vapors from incoming liquid.

Humidity of the ambient gas.

Flow conditions for liquid and gas.

Weather conditions in case of open-air evaporation generally

describing the temperature, pressure, and velocity of ambient air e.g.,

water evaporating from a pond.

Cooling towers

As per the definition of Cooling Technology Institute (CTI), USA

“A cooling tower is a heat rejection device,

which extracts waste heat to the atmosphere

though the cooling of a water stream

to a lower temperature.”

Cooling towers are used in process industries to cool off effluent water from various heat transfer

equipment e.g., condensate from a condenser.

Cooling towers, in general, cool the water to a temperature below the dry-bulb and above the

wet-bulb temperature of air at the present conditions.

The cooled water is sent back to the process for reuse, thus emphasizing conservation of water.

Basic working principle of a cooling

tower

Considering an example of an air-water system, the basic working

principle of a cooling tower can be listed as,

1. Hot water and relatively cool ambient air enter the cooling tower.

2. Heat transfer between the air stream and the water stream occurs.

3. Hot water transfers its heat to the ambient air and becomes cool.

4. Cool water is removed from the cooling tower and sent back to the

process plant.

5. The resulting hot air rises and is, generally, removed from the top of the

tower by virtue of its low density.

Classification of cooling towers

Cooling towers are generally classified based on the following

factors,

1. Method by which air is introduced into the tower.

2. Flow configuration inside the tower.

3. Method of heat transfer / heat removal.

1. Air Introduction Method

NO. TYPE DESCRIPTION

1 Natural draft cooling

towers

Air movement is regulated without any help of

a mechanical fan or regulator and is

dependent on the height and size of the

tower

2 Mechanical draft

cooling towers

Air is regulated by means of mechanical fans.

It has two further types based on the

positioning of the fan

Induced mechanical draft where the fan is

positioned on the top side of the tower

Forced mechanical draft where the fan is

positioned at the bottom side of the tower

2. Flow configuration

NO. TYPE DESCRIPTION

1 Cross flow configuration The air stream enters the tower in a direction

perpendicular to that of flow of water e.g., air

entering from the sides of the cooling tower in

association with the water stream entering

from the top of the tower

2 Counter-current flow

configuration

The air stream and water stream flow in

parallel but opposite direction inside the

tower e.g., air entering from below and water

entering from the top of the tower

3 Co-current flow

configuration

The air stream and water stream flow in

parallel and same direction inside the tower

2. Flow Configuration (contd.)

3. Method of Heat Transfer

NO. TYPE DESCRIPTION

1 Dry cooling towers Transfer heat through a surface which

separates the working fluid from ambient air

e.g., tube to air heat exchanger. No

evaporation occurs in dry cooling towers

2 Wet cooling towers Transfer heat on the principle of evaporative

cooling e.g., hyperbolic natural draft cooling

towers used in power plants

3 Wet-dry hybrid cooling

towers

Combination of an air-cooled heat

exchanger and a wet cooling tower to cool

off the required fluid

4 Evaporative condenser

cooling tower

Principle of wet cooling tower is applied to

cool a process fluid which remains isolated

from the cooling tower fluids (usually air and

water)

3. Method of Heat Transfer (contd.)

Evaporative condenser cooling tower

Wet-dry cooling tower

Major components of a cooling

tower

NO. NAME OF

COMPONENT

FUNCTION MATERIAL OF

CONSTRUCTION

1 Frame and

casing

Supports exterior enclosures SS 316/304,

Concrete, Fiber glass

2 Fill Increases contact between air

and water, facilitating heat

transfer. Has two types; Splash fill

and Film fill

PVC, wood,

Polypropylene

3 Cold water basin Receives water at the bottom of

the tower

SS 316, Concrete

4 Drift eliminators Reduce loss of water due to

windage/drift

PVC, Polypropylene

Major components of a cooling

tower (contd.)

NO. NAME OF COMPONENT FUNCTION

5 Air inlet Enables air to enter the tower

6 Louvers Louvers equalize air flow into the fill and

retain the water within the tower

7 Nozzles / Spray tree Distributes water to wet the fill

8 Fans Regulate air flow in case of mechanical

draft towers

Design of a cooling tower

Design procedure of a hyperbolic natural draft cooling tower is

taken as reference.

Design of a cooling tower (contd.)

Important parameters prerequisite to design,

CLASS NO. NAME SYMBOL UNITS

MEA

SU

RED

PA

RA

METE

RS

1 Wet bulb

temperature of air

𝑇𝑤 K

2 Dry bulb

temperature of air

𝑇𝑑 K

3 Inlet water

temperature

𝑇𝑖𝑛 K

4 Outlet water

temperature

𝑇𝑜𝑢𝑡 K

5 Water mass flow

rate / Water load

𝑊𝐿 kg/sec

6 Enthalpy change

(air passing through

tower)

∆𝐻′ kJ/kg

Design of a cooling tower (contd.)

CLASS NO. NAME SYMBOL DESCRIPTION UNITS

PER

FO

RM

AN

CE P

AR

AM

ETE

RS

1 Range ∆𝑇 ∆𝑇 = 𝑇𝑖𝑛 − 𝑇𝑜𝑢𝑡 K

2 Approach ∆𝑇∗ ∆𝑇∗ = 𝑇𝑜𝑢𝑡 − 𝑇𝑤 K

3 Effectiveness 𝐸𝑐𝐸𝑐 =

∆𝑇

∆𝑇 + ∆𝑇∗∗ 100

%

4 Cooling capacity 𝑄 𝑄 = 𝑊𝐿 ∗ 𝐶𝑝 𝑤𝑎𝑡𝑒𝑟∗ ∆𝑇 kW

5 Performance

coefficient

𝐶𝑡 Value is usually 5.2 or

lower

--

Design of a cooling tower (contd.)

CLASS NO. NAME SYMBOL DESCRIPTION UNITS

DESIG

NP

AR

AM

ETE

RS

1 Duty coefficient 𝐷𝑡 𝑊𝐿

𝐷𝑡= 0.00369 ∗

∆𝐻′

∆𝑇∗ (∆𝑇∗ + 0.0752∆𝐻′)

0.5 --

2 Base area of tower 𝐴𝑏𝐷𝑡 =

19.5 ∗ 𝐴𝑏 ∗ 𝑧𝑡0.5

𝐶𝑡1.5

m2

3 Height of tower 𝑧𝑡 Assumed values are used during

calculations to confine to a height to

diameter ratio of 3:2; in case of

hyperbolic natural draft towers

m

4 Diameter of tower 𝑑𝑏𝑑𝑏 =

4 ∗ 𝐴𝑏𝜋

m

Design of a cooling tower –

Numerical example Numerical example, with reference to Coulson & Richardson, Chemical

Engineering Vol.1, 6th Edition.

Q. (a) Determine the diameter and height of a hyperbolic natural draftcooling tower handling 6500 kg/s of water under the followingconditions.

Inlet water temperature = 318 K

Outlet water temperature = 313 K

Dry bulb temperature of air = 301 K

Wet bulb temperature of air = 295 K

(b) Determine the effectiveness and cooling capacity for the specifiedtower.

Design of a cooling tower –

Numerical solution

Available values Required values

𝑇𝑤 = 295 K ∆𝐻′

𝑇𝑑 = 301 K ∆𝑇

𝑇𝑖𝑛 = 318 K ∆𝑇∗

𝑇𝑜𝑢𝑡 = 313 K 𝐷𝑡

𝑊𝐿 = 6500 kg/s 𝐴𝑏

𝑧𝑡

𝐸𝑐 𝑄

Design of a cooling tower –

Numerical solution (contd.)

1. Range = ∆𝑇 = 𝑇𝑖𝑛 − 𝑇𝑜𝑢𝑡 = 318 – 313 = 5 K

2. Approach = ∆𝑇∗ = 𝑇𝑜𝑢𝑡 − 𝑇𝑤 = 313 – 295 = 18 K

3. Mean temperature of water = 0.5 ∗ (𝑇𝑖𝑛 + 𝑇𝑜𝑢𝑡) = 0.5*(318 +313) = 315.5 K

4. Using a humidity-enthalpy chart, we shall calculate values of enthalpy of air

at the mean temperature of water and at the dry bulb temperature

Design of a cooling tower –

Numerical solution (contd.)

5. The corresponding values of enthalpies at mean water temperature and dry

bulb temperature are approximately 150 kJ/kg and 83 kJ/kg

6. Enthalpy change of passing air = ∆𝐻′ = 150 – 83 = 67 kJ/kg

7. Duty coefficient = 𝑊𝐿

𝐷𝑡= 0.00369 ∗ ∆𝐻

′

∆𝑇∗ (∆𝑇∗ + 0.0752∆𝐻′)

0.5= 27388

Design of a cooling tower –

Numerical solution (contd.)

8. Assuming height 𝑧𝑡 by hit and trial method and taking 𝐶𝑡 as 5.2, the

base area, diameter, and conformity of the height to diameter

ratio of the tower is calculated as,

No. Height

𝑧𝑡 (m) 𝐴𝑏 =𝐷𝑡 ∗ 𝐶𝑡

1.5

19.5 ∗ 𝑧𝑡0.5

(m2)

𝑑𝑏 =4 ∗ 𝐴𝑏𝜋

(m)

Height to diameter ratio

𝑧𝑡 / 𝑑𝑏 ≈ 1.5

1 95 1709 46.6 2.04 ≠ 1.5

2 90 1755 47.3 1.90 ≠ 1.5

3 85 1806 47.9 1.77 ≠ 1.5

4 80 1862 48.7 1.64 ≠ 1.5

5 75 1923 49.5 1.51 ≈ 1.5

Design of a cooling tower –

Numerical solution (contd.)

9. The acceptable value of height and diameter for the specified

tower is 75 m and 49.5 m, respectively

10. Effectiveness of tower = 𝐸𝑐 =∆𝑇

∆𝑇+∆𝑇∗∗ 100 =

5

5+18∗ 100 = 21.74%

11. Cooling capacity

𝑄 = 𝑊𝐿 ∗ 𝐶𝑝 𝑤𝑎𝑡𝑒𝑟∗ ∆𝑇 = 4500 (kg/s) * 4.1815 (kJ/kg K ) * 5 (K)

𝑄 = 94083.75 kW

Strategies to improve cooling tower

performance When designing a cooling tower, always use the highest wet-bulb temperature

as reference.

Monitor the range and approach carefully during the design.

High range and low approach leads to good performance.

Improve the quality of water to be cooled by the tower resulting in low utilization of make-up water.

Regularly monitor the tower for scale build-up and biological impurities.

Regularly monitor the flow of water and air inside the tower

Importance of cooling tower in CPI

Cooling towers are used to cool industrial processes and

applications to ensure that the correct temperature of the

environment and the process are maintained during manufacturingor large industrial processes.

Natural draft cooling towers require no power and are of key

importance in power plants.

References

Literature:

Dr. N.P. Cheremisinoff, Handbook of Chemical Processing Equipment, 1st

edition, Butterworth-Heinemann, USA, 2000.

J. M. Coulson, J.F. Richardson, J.H Harker, J.R. Backhurst, Chemical

Engineering Volume 1 – Fluid Flow, Heat Transfer and Mass Transfer, 6th edition, Butterworth-Heinemann, USA, 1999.

San Diego County Water Authority, Technical Information for Cooling

Towers Using Recycled Water, San Diego, USA, 2009.

Training Session on Energy Equipment, Cooling Towers, UNEP, 2006.

Websites / URL’s:

http://www.cti.org/whatis/coolingtowerdetail.shtml

http://www.engineeringtoolbox.com/

http://www.deltacooling.com/resources/principles-of-cooling-towers/