cooling tower humidification/cooling tower saddawi 2013

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COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

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Page 1: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

COOLING TOWER 

HUMIDIFICATION/COOLING TOWERSaddawi

2013

Page 2: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

The goal of this experiment is to determine heat and mas balance for countercurrent air-water system in a Packed Cooling Tower.To find the Characteristic equation, Number of Transfer Units NtoG and Number of Heights Transfer Units HtoG

Murphree gas phase stage efficiency and the Overall cooling tower effectiveness efficiency

The Goal of the Experiment

Page 3: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

Experimental Setup

Page 4: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

Base unit components include:1. Air distribution chamber.2. A tank with heaters to simulate cooling

loads of 0.5, 1.0 and 1.5kW.3. A makeup tank with gauge mark and float

operated control valve.4. A centrifugal fan with intake damper to give

0.06kg s-1 max. air flow.5. A water collecting basin.6. An electrical panel

Note

Use distilled water to fill the makeup tank . Monitor and record the amount of water evaporated during all of the test operations of the cooling tower. Check wet bulb thermocouple reservoir for water. Add if necessary.This can be done by measuring the time needs to spend by added amount of water to the make-up tank.After the system reach to study sate,Record all temperatures, dry and wet bulb temperatures of the air and water temperature of all sections, mass flow-rate of ware and air.

Page 5: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

The basic function of a cooling tower is to cool water by intimately mixing it with air. This cooling is accomplished by a combination of: Sensible heat transfer between the air and the water (Conduction and Convection) and it controlled by temperature differences and area of the contact between air and water. And the evaporation of a small portion of the water.In the cooling towers, the evaporation is the most effective part in the cooling process

Some background theory

Page 6: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

Mass Balance and Enthalpy Balance on Cooling Tower*Please see page 12 equations (1,2,&3)

*Because the latent heat of water is a big value, so a small amount of water evaporation will produce large cooling effect.Therefor we can assume the mass velocity of the water falling down through the tower is constant with out large consequences error Please see equation (4) on page 12

Take mass balance over a differential section (see the fig.)

*Mass velocity of dry air remain constant through the cooling tower

Take enthalpy balance over the same differential section

(1)

(2)

(3)

(4)

dz

T2

t2

H2

h2

mw

ma

 

Air outlet

WaterInlet

T1

t1

H1

h1

mw

ma

 

Air Inlet

WaterOutlet

z1’

2

1

Page 7: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

Equation (4) can be rewritten in term of heat balance as in equation (5)

(5)

Take the integral of eq (5) over entire Column

(6)

Eq (6) represent Air Bulk Operating Line by plotting air enthalpies versus water temperatures.

Page 8: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

Cooling Tower Operating line (Air bulk operating line)

h1

h2

T1 T2

Water Temperature

En

th

alp

y o

f A

ir

O

N

Page 9: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

Saturated Air Operating line

Water bulk at temp T

Saturated Air water vapor Film

Air bulk at temp t

Heat movement

If you assume that the drops of water falling through the tower are surrounding by a thin air film,* This film must be saturated with water vapor.* The heat and mass transfer take place between the film and the upstream air bulk

Where there is no resistance to heat flow in the interface between the saturated air film and water. In other words, the interface temperature can be assumed to be equal to the bulk water temperature (Merkel assumption)T(wart temperature) ≈ ti (interface temperature)

By plotting the enthalpies of the saturated air–water vapor mixture (film) and water bulk temperatures will produce a curve, please see the Figure.This carve represent Saturated Air Operating line or can be called Water Operating line

Page 10: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

 

H1

H3

H2

h1

h3

h2

T1 T3 T2

Water TemperatureE

nth

alp

y

Driving Force Diagram

The relation between the temperature and enthalpy of the saturated air

This curve applies to the air film surrounding the waterIt called Water Operating Line And limited for hot and cold water temp (T2 and T1)

Air Operating Line or Tower Operating LineRepresent Air condition through the column Enthalpy Driving Force

H2-h2

Cooling RangeT2-T1

Page 11: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

Mass Balance and Enthalpy balance on Cooling TowerIn terms of mass and heat transfer coefficients.

*Please see page 15-19

(5)

(7)

By rearrange eq 7 pleas see eq 11&12 on page 17

Take integral over entire Tower

(8)

(9)

Page 12: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

= HtoG Heights of Transfer Units

NtoG = Number of Air Enthalpy Transfer Units

Page 13: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

Merkel’s Equation

This equation is commonly referred to as the Merkel equation. The left-hand side of this equation is called the ”Tower Characteristic,” which basically indicates the 'degree of difficulty to cool' the water or the 'performance demand' of the tower.

The tower characteristic and the cooling process can be explained on a Psychrometric Chart

By combing eqs (5 &9)

Page 14: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

Please note that V=Z =Volume occupied by packing per unit plan area

To obtain mean driving force (∆hm) Carey and Williamson method can be used. This depends upon the application of correction factor f to the observed value of Hm- h3 (at the arithmetic mean of inlet and out let water temps T1 & T2)

Page 15: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

Characteristic Cooling Tower Equation

Page 16: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013

The cooling tower effectiveness .ε. is defined as the ratio of the actual energy transfer to the maximum possible energy transfer

Murphree gas phase stage efficiency Yas

Y2

Y1

tas t2 t1

Air TempsSatu

ratio

n lin

e

Page 17: COOLING TOWER HUMIDIFICATION/COOLING TOWER Saddawi 2013