1 4th Integrated Seminar 4.5.2005

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Determining the Tower Capability

Characteristic Curve MethodIt can be performed using the Demand

Curve tab, along with appropriate data supplied by the manufacturer and field test data.

Performance Curve MethodIt can be performed using the Mechanical

Draft Performance Curve tab, along with a set of performance curves supplied by the tower manufacturer or rebuilder and field test data.

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Cooling Tower Demand Curves

It is the numerical solution of the Merkel integral over the wide range of L/G values.

Use in designing cooling towers. Apply for analysis of test data. Evaluate for the prediction of cooling tower

performance. Plot with the thermal demand, KaV/L, and L/G. Contain the values of KaV/L, cooling ranges and

approaches

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Limitations on use of Demand Curve

Actual tower performance may deviate from predicted at water loading and air velocities. The followings affect it.

a)At very low water loadings

b)At very high water loadings

c)At very high air velocities

d)Changing the airflow due to air-side resistance

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Cooling Tower Characteristic Curves

• To analyze the thermal performance capability of a specified cooling tower.

This equation is used for each demand curve.

mGLCLKaV )/(/

where,

KaV/L = Tower characteristic,C = Constant related to the cooling tower design, L/G = Liquid and Gas ratio, andm or slope = Exponent related to the cooling tower design

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Illustration of Tower Demand Curve, Characteristic Curve and Design Point

Fig (1) – Tower Curve Descriptions

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Constructing the Demand and Characteristic Curves

Use the following data.

Wet bulb temperature = 75°F Hot water temperature (T1) = 100°F Cold water temperature (T2) = 80°F Temperature range = 20°F Approach = 5°F L/G ratio = 0.8662

Needs.

To construct the demand and characteristic curves of a specified cooling tower To plot the performance curve.

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Solving the Requirements by using Merkel Equation

4321

21 1111

4

1

2 hhhh

TT

hh

dT

L

KaV T

Taw

40.2)48.0(4

80100

L

KaV

T,°F hwater hair hw - ha

T2 = 80

T2 + 0.1(20) = 82

T2 + 0.4(20) = 88

T1 - 0.4(20) = 92

T1 - 0.1(20) = 98

T1 = 100

44.1

46.48

54.02

59.7

69.3

72.7

h1 = 39.1

h1 + 0.1(0.8662)(20) = 40.83

h1 + 0.4(0.8662)(20) = 46.03

h2 - 0.4(0.8662)(20) = 49.49

h2 - 0.1(0.8662)(20) = 54.69

h2 = 56.42

0.18

0.13

0.10

0.07

0.4861.14

21.10

99.7

65.5

4

3

2

1

h

h

h

h

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Checking Merkel Result by using CTI Toolkit v3.0 Software

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Creating the Demand Curve with CTI Toolkit v3.0 Software

Procedures Enter the values of wet bulb temperature, range and

altitude. Put in the fill or tower coefficients C and slope (m). Define the design point. Click on “Recalculate” brings up the appropriate

group of demand curves.

DEMONSTRATE THE SOLVING PROCEDURES IN OTHER WINDOW DIALOG BOX DETAILEDLY.

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Data Entering Dialog Box

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Result Dialog Box after clicking the Recalculate Tab

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The Complete Work Sheet Dialog Box

Characteristic Curve

Demand Curve

KaV/L Line

L/G Line

Design Point

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Constructing the Performance Curve of the Cooling Tower by using CTI Toolkit v3.0 Software

StepsSteps Recalculate the approach temperature by

changing the WBT again and again.Record each approach temperature.Then, calculate the CWT corresponding to

the WBT and approach.Plot CWT versus WBT.

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Plotting the Tower Performance Curve

Tower Performance Curve

7879808182838485

75 76 77 78 79 80

Wet Bulb Temperature

Co

ld W

ater

T

emp

erat

ure

20 DegreesRange

Fig (2) – Off-Design Performance Curve

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Conclusion

The performance curves are used to determine the cooling tower capability.

Tower demand curves are a graphical worksheet that facilities cooling tower calculations.

A set of performance curves of the standardized cooling towers is supplied by the tower manufacturers and companies.

Generally, it is experimentally determined by measuring field test data.

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