Report Cooling Tower

8/3/2019 Report Cooling Tower

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Cooling Tower CHE523

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ABSTRACT

We have conducted an experiment of cooling tower apparatus on 28th October 2011 at pilot

plan. The objectives of this experiment is to analyze and identify the water cooling tower

system. We run this experiment three times for which the first experiment is conducted with

fully opened blower. We varied the heating load with the value of o.5 kW, 1.0 kW and 1.5

kW. The second experiment is to compare the system in the difference of the opening of the

blower. In the third experiment, we varied the water flow rate in value of 1.0 LPM, 1.2 LPM

and 1.4 LPM. The data of this experiment is jotted down in the result sheet.


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INTRODUCTION

Cooling towers are heat removal devices used to transfer process waste heat to

the atmosphere. Cooling towers may either use the evaporation of water to remove process

heat and cool the working fluid to near the wet-bulb air temperature or in the case of closed

circuit dry cooling towers rely solely on air to cool the working fluid to near the dry-bulb air

temperature. Common applications include cooling the circulating water used in oil

refineries, chemical plants, power stations and building cooling. The towers vary in size from

small roof-top units to very large hyperboloid structures that can be up to 200 metres tall and

100 metres in diameter, or rectangular structures that can be over 40 metres tall and 80 metres

long. Smaller towers are normally factory-built, while larger ones are constructed on site.

They are often associated with nuclear power plants in popular culture, although cooling

towers are constructed on many types of buildings.

With respect to the heat transfer mechanism employed, the main types are:

Wet cooling towers or simply open circuit cooling towers operate on the principleofevaporation. The working fluid and the evaporated fluid (usually water) are one and the

same.

Dry cooling towers operate by heat transfer through a surface that separates the working

fluid from ambient air, such as in a tube to air heat exchanger, utilizing convective heat

transfer. They do not use evaporation.

Fluid coolers or closed circuit cooling towers are hybrids that pass the working fluidthrough a tube bundle, upon which clean water is sprayed and a fan-induced draft applied.

The resulting heat transfer performance is much closer to that of a wet cooling tower, with

the advantage provided by a dry cooler of protecting the working fluid from

environmental exposure and contamination.
http://en.wikipedia.org/wiki/Atmospherehttp://en.wikipedia.org/wiki/Evaporationhttp://en.wikipedia.org/wiki/Wet-bulb_temperaturehttp://en.wikipedia.org/wiki/Dry-bulb_temperaturehttp://en.wikipedia.org/wiki/Dry-bulb_temperaturehttp://en.wikipedia.org/wiki/Oil_refinerieshttp://en.wikipedia.org/wiki/Oil_refinerieshttp://en.wikipedia.org/wiki/Chemical_planthttp://en.wikipedia.org/wiki/Power_stationhttp://en.wikipedia.org/wiki/Hyperboloid_structurehttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Evaporative_coolinghttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Evaporative_coolinghttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Hyperboloid_structurehttp://en.wikipedia.org/wiki/Power_stationhttp://en.wikipedia.org/wiki/Chemical_planthttp://en.wikipedia.org/wiki/Oil_refinerieshttp://en.wikipedia.org/wiki/Oil_refinerieshttp://en.wikipedia.org/wiki/Dry-bulb_temperaturehttp://en.wikipedia.org/wiki/Dry-bulb_temperaturehttp://en.wikipedia.org/wiki/Wet-bulb_temperaturehttp://en.wikipedia.org/wiki/Evaporationhttp://en.wikipedia.org/wiki/Atmosphere


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In a wet cooling tower (or open circuit cooling tower), the warm water can be cooled to a

temperature lower than the ambient air dry-bulb temperature, if the air is relatively dry

(see dew point and psychrometrics). As ambient air is drawn past a flow of water, a small

portion of the water evaporate, the energy required by that portion of the water to evaporate is

taken from the remaining mass of water reducing his temperature (approximately by 970 BTU

for each pound of evaporated water). Evaporation results in saturated air conditions, lowering

the temperature of the water process by the tower to a value close to wet bulb air temperature,

which is lower than the ambient dry bulb air temperature, the difference determined by the

humidity of the ambient air.

To achieve better performance (more cooling), a medium called fill is used to increase the

surface area and the time of contact between the air and water flows. Splash fillconsists of

material placed to interrupt the water flow causing splashing. Film fill is composed of thin

sheets of material (usually PVC) upon which the water flows. Both methods create increased

surface area and time of contact between the fluid (water) and the gas (air).
http://en.wikipedia.org/wiki/Dew_pointhttp://en.wikipedia.org/wiki/Psychrometricshttp://en.wikipedia.org/wiki/Wet_bulbhttp://en.wikipedia.org/wiki/Wet_bulbhttp://en.wikipedia.org/wiki/Psychrometricshttp://en.wikipedia.org/wiki/Dew_point


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With respect to drawing air through the tower, there are three types of cooling towers:

Natural draft, which utilizes buoyancy via a tall chimney. Warm, moist air naturally risesdue to the density differential to the dry, cooler outside air. Warm moist air is less dense

than drier air at the same pressure. This moist air buoyancy produces a current of air

through the tower.

Mechanical draft, which uses power driven fan motors to force or draw air through thetower.

Induced draft: A mechanical draft tower with a fan at the discharge which pulls airthrough tower. The fan induces hot moist air out the discharge. This produces low

entering and high exiting air velocities, reducing the possibility of recirculation in

which discharged air flows back into the air intake. This fan/fin arrangement is also

known as draw-through. (see Image 3)

Forced draft: A mechanical draft tower with a blower type fan at the intake. Thefan forces air into the tower, creating high entering and low exiting air velocities. The

low exiting velocity is much more susceptible to recirculation. With the fan on the air

intake, the fan is more susceptible to complications due to freezing conditions.

Another disadvantage is that a forced draft design typically requires more motor

horsepower than an equivalent induced draft design. The forced draft benefit is its

ability to work with high static pressure. They can be installed in more confined

spaces and even in some indoor situations. This fan/fill geometry is also known

asblow-through. (see Image 4)

Fan assisted natural draft. A hybrid type that appears like a natural draft though airflow isassisted by a fan.
http://en.wikipedia.org/wiki/Stack_effecthttp://en.wikipedia.org/wiki/Lighter_than_air#Water_vapourhttp://en.wikipedia.org/wiki/Lighter_than_air#Water_vapourhttp://en.wikipedia.org/wiki/Stack_effect


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Hyperboloid (sometimes incorrectly known as hyperbolic) cooling towers (Image 1) have

become the design standard for all natural-draft cooling towers because of their structural

strength and minimum usage of material. The hyperboloid shape also aids in accelerating the

upward convective air flow, improving cooling efficiency. They are popularly associated

with nuclear power plants. However, this association is misleading, as the same kind of

cooling towers are often used at large coal-fired power plants as well. Similarly, not all

nuclear power plants have cooling towers, instead cooling their heat exchangers with lake,

river or ocean water.
http://en.wikipedia.org/wiki/Hyperboloidhttp://en.wikipedia.org/wiki/Hyperbolichttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Nuclear_power_planthttp://en.wikipedia.org/wiki/Nuclear_power_planthttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Hyperbolichttp://en.wikipedia.org/wiki/Hyperboloid


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Crossflow

Crossflow is a design in which the air flow is directed perpendicular to the water flow (see

diagram below). Air flow enters one or more vertical faces of the cooling tower to meet the

fill material. Water flows (perpendicular to the air) through the fill by gravity. The air

continues through the fill and thus past the water flow into an open plenum area.

A distribution or hot water basin consisting of a deep pan with holes or nozzles in the bottom

is utilized in a crossflow tower. Gravity distributes the water through the nozzles uniformly

across the fill material.


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Counterflow

In a counterflow design the air flow is directly opposite to the water flow (see diagram

below). Air flow first enters an open area beneath the fill media and is then drawn up

vertically. The water is sprayed through pressurized nozzles and flows downward through the

fill, opposite to the air flow.

Common to both designs:

The interaction of the air and water flow allow a partial equalization and evaporation ofwater.

The air, now saturated with water vapor, is discharged from the cooling tower. A collection or cold water basin is used to contain the water after its interaction with the

air flow.


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Both crossflow and counterflow designs can be used in natural draft and mechanical draft

cooling towers.


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OBJECTIVES

1. Determine the correlation of the water to air mass flow ratio with increasing waterflow rate.

2. Determine the cooling load effect, effect of different air flow rates and also the effectof different flow rates on the wet bulb approach.


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THEORY

The driving force for moisture transfer at a given water temperature T is the difference

between the saturated boundary layer enthalpy H and the enthalpy h of the bulk air

Figure 5.2 shows the equilibrium curve and the operating line relationship for a typical

cooling tower.

At the water outlet of a cooling tower

1 = H1 - h1

The water inlet-air outlet at T2

2 = H2 = h2

And at the numerical average water temperature T3 = (T1 + T2)/2

m= H3 - h3

= H - h

DRIVING FORCE DIAGRAM SPECIFIC ENTHALPY (kJ/kG DRY AIR)


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It may be shown that, for the finite element dz of the tower in Figure 3, energy balances of the

water and air streams in the tower are related to the mass transfer by the equation

Cpw mw dTw = Ka(H-h)dV

where H is the specific enthalpy of the saturated boundary layer per unit mass of dry air,

assumed to be at the local water temperature within dz, and h is the specific enthalpy of the

bulk air per unit mass of dry air. K is the mass transfer coefficient per unit plane area, a is the

area of contact between the air and water per unit volume of packing, and V is the volume

occupied by the packing, Cpw is the specific heat, mw is the mass flow-rate, and Tw is the

water temperature. The small change in the mass of the water is neglected.

Equation below is integrated down the column to give an average mass transfer coefficient


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SCHEMATIC REPRESENTATION OF THE AIR AND WATER STREAMS ENTERING

AND LEAVING A BLOCK OF PACKING.

One numerical solution of this equation can be written in terms of a corrected arithmetic mean

driving force

(H - h)m

Ka/Vmw= Cpw (T2 - T1)(H - h)m

Using the driving forces 1, 2 and mobtained in the previous calculation the correction factor

f to estimate the corrected mean driving force (H - h)m can be obtained

Thus,

(H - h)m = fm= f(H3 - h3)


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The range of cooling water is given by:

Twater inTwater out

The approach of cooling tower is given by:

Twater outTwb air

The effectiveness of cooling tower is given by:

The total cooling load is given by:

Pump input + heating load

The air mass flowrate per unit area is given by:


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APPARATUS

Laboratory cooling water unit


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PROCEDURE

General start up procedure

1. Valve V1 to V6 are ensured to be closed. While valve V& is partially closed.2. The load tank is filled with distilled water or deionised water.3. The make-up tank is filled with distilled water or deionised water up to zero mark on

the scale.

4. Distilled water or deionised water is added to the wet bulb sensor reservoir to thefullest.

5. The appropriate cooling tower is installed for the experiment.6. All appropriate tubing to the differential pressure sensor are connected.

7. The temperature set point of temperature controller is set to 45 oC. the 1.0 kW waterheaters is switched on and the water is heated up until approximately 40

o

C.

8. The pump is switched on and the control valve V1 is slowly opened. the water flowrate is set to 2.0 LPM. a steady operation where the water is distributed and flowing

uniformly through the packing is obtained.

9. The fan damper is opened fully, and the fan is switched on. Check that the differentialpressure sensor is giving reading:

i. To measure the differential pressure across the orifice, open valve V4and V5: close valve V3 and V6.

ii. To measure the differential pressure across the column, open valve V3and valve V6: close valve V4 and V5.

10.The unit is let run for about 20 minutes, for the float valve to correctly adjust the levelin the load tank. Refill the make-up tank as required.

11.Now, the unit is ready for use.


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General Shut-Down Procedure:

1. Heaters are switched off and the water is let to circulate through the cooling towersystem for 3-5 minutes until the water cooled down.

2. The fan is switched off and the fan damper is closed fully.3. The pump and power supply are switched off.4. The water in the reservoir tank is retained for the following experiment.5. The water is drained completely from the unit if it is not in used.

EXPERIMENT 1:EFFECT OF HEATING LOAD

1. The blower is opened fully.2. The 0.5 kW of water is switched on.3. The water flow rate is set to 1.0 LPM.4. The fan and pump are switched on.5. The unit is let to run for 10 minutes.6. After the steady operation is obtained, all the readings are taken.7. The operation is run three times to take the average.8. The above steps are repeated twice for varied heating load of 1.0 and 1.5 kW.


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EXPERIMENT 2:BLOWER OPENED.

1. The blower is opened partially.2. The 0.5 kW of water is switched on.3. The water flow rate is set to 1.0 LPM.4. The fan and pump are switched on.5. The unit is let to run for 10 minutes.6. After the steady operation is obtained, all the readings are taken.7. The operation is run three times to take the average.

Note:for fully opened blower, no need to run the experiment because all the varied properties

are just the same with experiment 1 for 0.5 kW of heating load.

EXPERIMENT 3:WATER FLOWRATE

1. The blower is opened fully.2. The 0.5 kW of water is switched on.3. The water flow rate is set to 1.2 LPM.4. The fan and pump are switched on.5. The unit is let to run for 10 minutes.6. After the steady operation is obtained, all the readings are taken.7. The operation is run three times to take the average.8. The above steps are repeated one more time for 1.4 LPM of water flowrate.

Note:no need to run the operation for 1.0 LPM of water flowrate because all the varied

properties are just the same with experiment 1 for 1.0 LPM of water flowrate and 0.5 kW of

heating load.


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RESULT

Cross sectional area:

High:

Pack column:

EXPERIMENT 1:EFFECT OF HEATING LOAD

Fixed variable:

a) Fully opened blower

b) 1.0 LPM

0.5 kW 1.0 kW 1.5 kW1 2 3 Average 1 2 3 Average 1 2 3 Average

T1 27.3 27.4 27.6 27.4 27.4 27.5 27.4 27.4 27.5 27.7 27.7 27.6

T2 24.5 24.4 24.3 24.4 24.1 24.0 24.0 24.0 24.1 24.1 24.2 24.1

T6 24.6 24.3 24.3 24.4 25.0 25.0 24.9 24.9 25.5 25.7 26.0 25.7

D.P 131 129 128 130 121 117 119 119 123 117 120 120

Q 434 430 441 435 818 831 821 823 1230 1237 1252 1242

EXPERIMENT 2:BLOWER OPENED

Fixed variable:

a) 0.5 kW

b) 1.0 LPM

Fully opened Half opened

1 2 3 Average 1 2 3 Average

T1 27.3 27.4 27.5 27.4 28.3 28.2 28.3 28.3

T2 24.5 24.4 24.3 24.4 24.3 24.2 24.2 24.2

T6 24.6 24.3 24.3 24.4 25.3 24.6 24.4 24.8

D.P 133 129 128 130 112 106 112 110

Q 434 430 441 435 428 437 430 432


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EXPERIMENT 3:WATER FLOWRATE

Fixed variable:

a) 0.5 kW

b) 1.0 LPM

1.0 LPM 1.2 LPM 1.4 LPM

1 2 3 Average 1 2 3 Average 1 2 3 Average

T1 27.3 27.4 27.5 27.4 28.6 28.5 28.3 28.5 28.2 28.3 28.4 28.3

T2 24.5 24.4 24.3 24.4 24.3 24.2 24.1 24.2 24.0 24.0 24.1 24.0

T6 24.6 24.3 24.3 24.4 24.6 24.3 24.1 24.3 24.1 23.9 24.0 24.0

D.P 131 129 128 130 116 111 105 111 117 115 116 116

Q 434 430 441 435 435 430 430 432 43.7 43.8 43.0 43.5


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CALCULATION

EXPERIMENT 1

Heating load:0.5 kW

Water flowrate:1 LPM

Blower:fully opened

1) Range of cooling tower

2) Approach of cooling tower

3) Effectiveness of cooling tower

4) Total cooling load

( )


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5) Air mass flowrate per unit area

From psychometric chart,

Hence, the air mass flowrate per unit area =


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EXPERIMENT 2

Heating load:0.5 kW

Water flowrate:1 LPM

Blower:half opened

1) Range of cooling tower

2) Approach of cooling tower

3) Effectiveness of cooling tower

4) Total cooling load

( )


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5) Air mass flowrate per unit area

From psychometric chart,

Hence, the air mass flowrate per unit area =

Note:calculation for experiment 3 is just the same with experiment 1 since the variables are

the same.

Report Cooling Tower

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