Final Cooling Tower

1.0 ABSTRACT

The purpose of cooling tower experiment is to investigate the effect of air velocity on wet

bulb approach and also the pressure through the packing. This cooling tower system utilizes a

series of baffles inside a tower in order to allow a heat transfer between the atmospheric air and

the water been cooled. Other than that, this experiment was carried out to find the different

parameters for the analysis of the system’s performance. To determine the power required for the

system’s performance, the parameters such as heater power and blower were used. Next, in order

to find the effectiveness of the system, the temperatures of the water and air entering and

exciting were used to calculate the thermodynamic balance. The measurements taken are include

the air entry and exit, temperatures measurements of the system’s water at difference points and

making a velocity profile for the cooling tower fan. In addition, through the experiment also, we

were contact with the equipment which is cooling tower and enriches the learning experience

using a hand on approach.

This experiment is a study on heat and mass transfer coefficients in packing of wet

cooling towers. This cooling tower was supplied with a packed column having packing density

of approximately 110 m2/m3. Two column were used in this experiment which is column C and

E. During conducting the experiment, heater is set to 0.5kW and 1.0kW when the air pump is

both fully open and half open for each column. Blower is functioned to deliver air into the air

distribution chamber. It is set to be open fully and partially for each 0.5kW and 1.0kW and for

both column C and column E. The air passes wet and dry bulb thermometers before entering the

column. As the air passes up the column, its moisture content increases and the water is cooled.

At the top of the column, the air passes a mist water falls into the basin before going back into

the load tank where it is re-heated and re-circulated. The load-tank water level is maintained by

means of a make-up tank. The water flowrate is set to 2.0 litter per minutes (LPM) throughout

the experiment. The number of thermocouples installed along the height packed is vary among

the two column as the height of each column is different hence leading to variation of the

temperature reading recorded.

2.0 INTRODUCTION

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. Common applications for cooling

towers are providing cooled water for air-conditioning, manufacturing and electric power

generation. The generic term "cooling tower" is used to describe both direct (open circuit) and

indirect (closed circuit) heat rejection equipment. A direct, or open-circuit cooling tower is an

enclosed structure with internal means to distribute the warm water fed to it over a labyrinth-like

packing or "fill." The fill may consist of multiple, mainly vertical, wetted surfaces upon which a

thin film of water spreads. An indirect, or closed circuit cooling tower involves no direct contact

of the air and the fluid, usually water or a glycol mixture, being cooled. In a counter-flow cooling

tower air travels upward through the fill or tube bundles, opposite to the downward motion of the

water. In a cross-flow cooling tower air moves horizontally through the fill as the water moves

downward. Cooling towers are also characterized by the means by which air is moved. Because

evaporation consists of pure water, the concentration of dissolved minerals and other solids in

circulating water will tend to increase unless some means of dissolved-solids control, such

as blow-down, is provided. Some water is also lost by droplets being carried out with the exhaust

air (drift).

The cooling tower experiment was done to study the principles of a cooling tower

operation and show the heat and mass transfer as well as the mass and energy balance in a closed

system. In industrial and energetic installations the water plays very significant role. The main

reason for it is its wide presence in the nature as well as good thermodynamic properties. There

is in many countries the lack of industrial water. The quantity of available water is defining the

kind of cooling system, which can be the conventional once-through condenser arrangement and

the circulation one. When the condenser cooling water is available in adequate quantities then

the once-through system comes into use, in contrary the designer must provide an alternate

cooling system such as a circulation water cooling system with cooling tower.

Many chemical processes require utility cooling to lower the temperature of the process

stream. As it passes through a heat exchanger, the temperature of the cooling water is increased.

Before this water can be reused to cool the process stream, its temperature must first be lowered.

The most common unit used is a cooling tower. In our experiment, the industrial process load

heat from process stream is provided by the water heater which heats up the water. The

laboratory cooling tower allows the speed of the fan blower or damper to be controlled for

cooling the warm return water and the pump used to return the cooled water to the water heater.

This experiment was conducted to show the mass energy balance in a closed system as well as

study how the adjustment of one or more parameters can affect the amount of heat removed from

the water. The remainder of this report explains the theory behind the operation and workings of

a cooling tower and how the laboratory cooling tower is operated

.

A cooling tower is used as opposed to a heat exchanger because in a heat exchanger, the

outlet cooled water cannot be cooled below the temperature of the inlet air. In the experiment,

the various thermocouple equipped on the tall tower can measure the temperature of the water

and dry and wet bulb temperature of the air at specific heights of the column which will be

needed to calculate the change in enthalpies of both the water and air to determine the mass

energy balance of the system. In the water circuit, the flow of water is regulated by a gate valve

and is monitored by a flow meter. The water is pumped from a load tank to the distribution cap

where the temperature of the water is taken and the water is evenly distributed over the packing

using a rotating showerhead. This water flows over the packing material to increase the surface

area exposed to the cooling air stream. The water is then cooled by evaporation into the air

stream. At the bottom of the tank, the water falls through one last thermometer and into the load

tank where it is reheated and re-circulated through the column.

In the air circuit, the air is pulled from the atmosphere by a fan blower and passes through

a fan into the column. A switch is used to control the speed of the fan to vary the flow rate of air

through the tower column. The wet and dry bulb temperature of the air is taken at various points

along the length of the column. The air then pass by a droplet arrestor and its temperature is

taken again before exiting to the atmosphere through a orifice. The pressure drop through the

orifice can be used to estimate the air flow rate.

In a cooling tower, the theory behind the whole operation of the unit is the First Law of

Thermodynamics which is the conservation of energy. In simpler terms, energy entering the

system must exit the system; energy can neither be destroyed nor created, it just transform from

one form to another. Energy enters the cooling tower in the form of hot water. This hot water

was cooled from an initial temperature of T1 to a temperature of T2. The water is cooled by the

upward moving air stream through forced convection with ambient air at T1 which then gets

heated and exits at some temperature of T2. Both enter and exit temperature of water and air is

recorded. An energy balance can then be calculated for the system once the data is recorded.

An energy balance is a form of boo keeping account for the energy entering and leaving

the system to study the First Law of Thermodynamics at work in the system. We define the

enthalpy which is the main component of energy balance as:

H = U + PV. (1)

Where H is the enthalpy, U is internal energy, P is pressure and V is volume.

The combined term of U + PV is enthalpy which means heat. We can determine the enthalpy by

referencing from the tables of value for the fluid being used. The fluid used in the cooling tower

is air and water, whose enthalpy value can be obtained from a thermodynamic book. Since both

initial and final temperature of the inlet water and the outlet cool water were measured, the

temperature of water in can be referenced and the enthalpy can be determined. The enthalpy of

the outlet cooled water can also be referenced and an energy balance can be calculated for water.

The equation for the energy balance is as below:

∑Hin= ∑Hout

Where ∆H = Hin -Hout.

We employ a similar method to calculate the energy balance for air entering and leaving the

system. For air, there are two methods to determine the change in enthalpy of air. Because the air

is at low pressure, it can be treated as an ideal gas and the enthalpy change can be calculated

through the use of the equation as below:

∆H = Cp∆T

Where H is the change in enthalpy, T is the change in temperature and Cp is the specific heat

with respect to constant pressure.

However, the process of heat and mass transfer between the air and the water in the tower

is very complicated. The influence factors include the interior structure, the direction and amount

of spraying water, environmental factors, the air mass flow rate, wind velocity, the mass flow

rate and temperature of inlet water and so on. The cooling tower performance is predicted by

using heat and mass transfer between water and air to drive the solution to steady-state

conditions. The second law is used to take account of energy distributions of water and air in the

cooling tower. An investigation of the calculated results can be used to further understand the

details of energy in shower cooling towers. Heat and mass transfer coefficients are constant

within the tower. Both the cool air and hot water have constant physical properties.

3.0 AIMS/OBJECTIVE

1. Determine at various ratios of water to air mass flow rate, the tower characteristic for the

cooling tower located in Jarvis 116.

2. Determine at a water to air mass flow ratio of ~1, how the range, and the approach vary

with increasing water flow rate.

3. Estimate the evaporation rate of water (water loss) for the tower.

4.0 THEORY

First Law Thermodynamics which is the conservation of energy is the theory that involve

in the operation of the cooling tower. Conservation energy is the energy that enters the system is

same as the energy that exit the system and energy can be either created or destroyed, it just

transformed from one form to another.

Hot water that enters the cooling tower is energy. Then, this hot water was cooled from

temperature 1, T1 to temperature 2, T2. The cooling of the hot water was in the form of forced

convection by which ambient air at T1 was blown over the hot water and exited the cooling tower

at some temperature T2. Both the entrance and exit temperatures of the air and water were

recorded. An energy balance can be conducted on the system once this data is recorded.

The main component of the energy balance is enthalpy which is defined as:

H = U + PV........................................................(1)

Where

H= enthalpy

U= internal energy,

P = pressure

V= volume.

Enthalpy is heat. Thus, the combined terms U+PV is also heat. Enthalpy can be

calculated by using that equation 1 .The enthalpy also can be refer to the referenced from tables

of data for the fluid being used. In the Engineering 435 laboratory, the fluids used by the cooling

tower are air and water, whose enthalpy values can be obtained from thermodynamics textbook.

For instance : Since both the initial and final temperatures of the input hot water and the output

cool water were measured, the temperature Tin can be reference and the enthalpy (BTU/lbm, or

KJ/kg) can be recorded. The enthalpy of the output cooled water can be similarly referenced and

an energy balance can be conducted for the water.

The equation below displays the general method to conduct an energy balance:

in = out ...................................(2)

Equation 2 can be obtained when H = 0

Where H = H in - H out. A method is used for conducting the energy balance for air

entering and leaving the system.

The change in enthalpy for air can be determined forms either of two methods. Since the

air is at low pressure, it can be treated as an ideal gas and the enthalpy change can be calculated

through the use of the following equation:

H = Cp T .........................................(3)

Where

H = change in enthalpy

T = change in temperature

Cp = specific heat at constant pressure.

A psychrometric chart is used to determine the enthalpy change between the entrance and

exit air since the specific heat relation does not take into account the percent of water in the air.

Additionally, some information is needed about the input and output air in order for the

psychrometric chart to be used effectively.

The information needed is the dry bulb and wet bulb temperatures of the inlet and outlet

air to reference the psychrometric chart. A sling psychrometer used to measure both the input

and output air flow. The sling psychrometer is an instrument that has two thermometers. For

measuring the wet bulb temperature, the thermometer that has a wetted cotton sleeve over the

bulb end is used, while a regular thermometer is for measuring the dry bulb temperature. Each

can be referenced on the psychrometric chart and the enthalpies obtained once the wet and dry

bulb temperatures of the inlet and outlet air have beenmeasured. Energy balance can be

conducted on the system once the enthalpies for the inlet and outlet water and air conditions are

known.

5.0 APPARATUS

1. Solteq® Cooling Tower Model HE 152.

2. Dionised Water.

6.0 PROCEDURES

1. Column C was setup in the cooling tower

2. The main switch is on and the water flow rate was set to 2 LPM

3. The main blower was adjusted until it is fully open

4. The 1.0 kW heater is switched on and the temperatures in the system is allowed to reach

a steady state which is 40 C

5. Wait until 10-15 minutes and the temperatures T1-T6 and pressure of orifice and column

was record

6. Step 4 and 5 is repeated and blower is set to partially open.

7. Step 3 to 5 is repeated and heater is adjusted with 0.5kW.

8. Step 2 to 7 is repeated by using column E.

7.0 RESULT

Column: Column C

Water flow rate: 2.0 LPM

Heater: 0.5 kW

Air pump Fully open Half open

Temperature T1, (0C) 33.2 33.3






Difference pressure orifice, (Pa) 64 46

Difference pressure column, (Pa) 68 49

Heater Power, (kW) 0.43 0.42

Column: Column C


Heater: 1.0 kW











Column: Column E


Heater: 0.5 kW





















Column: Column E


Heater: 1.0 kW





















8.0 CALCULATIONS

Sample calculation of experiment 1

Power input = 0.5kW

Water flow rate= 2.0 LPM (1.9LPM actual)

Blower = Half Open

Specific volume of air at outlet (from the Psychometric Chart) = 1.071 m3kg-1

Air mass flow rate, ṁ = 0.0137√ xV B

(x= 88 Pa x 1 mmH2O/10.13 Pa) = 8.69 mmH2O

ṁ = 0.0137√ 8.691.071

= 0.039 kgs-1

Air volumetric flow rate = m V B

= 0.039 x 1.071

= 0.042 m3s-1

Cross sectional area of column A = 0.15 m x 0.15 m

= 0.0225 m2

Air Velocity = VA

= 0.042

0.0225

= 1.867 ms-1

Power input = 0.5kW

Water flow rate= 2.0 LPM (1.9LPM actual)

Blower = Fully Open

Specific volume of air at outlet (from the Psychometric Chart) = 1.067 m3kg-1

Air mass flow rate, ṁ = 0.0137√ xV B

(x= 93 Pa x 1 mmH2O/10.13 Pa) = 9.18 mmH2O

ṁ = 0.0137√ 9.181.07

= 0.04 kgs-1

Air volumetric flow rate = m V B

= 0.04 x 1.067

= 0.043 m3s-1

Cross sectional area of column A = 0.15 m x 0.15 m

= 0.0225 m2

Air Velocity = VA

= 0.043

0.0225

= 1.911 ms-1

9.0 DISCUSSION

10.0 CONCLUSSION

11.0 RECOMMENDATIONS

12.0 REFERRENCE

1. Boles, M. A. and Y. A. Gengel, Thermodynamics, Engineering Approach , 7th ed.,McGraw Hill Book Company, St. Louis, MO, 2011, p. 8-12.

2. Harriot, P., W. L. McCabe, and J. C. Smith, Unit Operations of Chemical Engineering,7th ed., McGraw-Hill Book Company, St. Louis, MO, 2011, p. 330-340.

3. http://www.eng.buffalo.edu/Courses/ce427/Fall05cooling%20tower.pdf

4. http://www.engr.usask.ca/classes/CHE/424/experiments/424_Manual_2012_T2.pdf

http://www.engr.usask.ca/classes/CHE/424/experiments/424_Manual_2012_T2.pdf

http://www.eng.buffalo.edu/Courses/ce427/Fall05cooling%20tower.pdf

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Final Cooling Tower