Cooling Tower

Cooling Tower

HUMIDIFICATION/COOLING TOWER

Saddawi

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

The Goal of the Experiment

Experimental Setup

Base unit components include: Air distribution chamber. A tank with heaters to simulate cooling loads of 0.5, 1.0 and 1.5kW.A makeup tank with gauge mark and float operated control valve.A centrifugal fan with intake damper to give 0.06kg s-1 max. air flow.A water collecting basin. An electrical panel

NoteUse 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. This can be done by measuring the time needs to spend by added amount of water to the make-up tank.Check wet bulb thermocouple reservoir for water. Add if necessary.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.

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

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

T

2

t

2

H

2

h

2

m

w

m

a

Air

outlet

Water

Inlet

T

1

t1H1 h1mw ma 

Air

Inlet

WaterOutlet

z

1’

2

1

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.

Cooling Tower Operating line (Air bulk operating line)

h

1

h

2

T

1

T

2

Water Temperature

Enthalpy of Air

O

N

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

H

1

H

3

H

2

h

1

h

3

h

2

T

1

T

3

T

2

Water Temperature

Enthalpy

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 H

2

-h

2

Cooling Range

T

2

-T

1

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)

H

toG

= Heights of Transfer Units

NtoG = Number of Air Enthalpy Transfer Units

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)

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

To obtain mean driving force (∆

h

m) 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)

Characteristic Cooling Tower Equation

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

Y

as

Y

2

Y

1

t

as

t

2

t

1

Air Temps

Saturation line