INTRODUCTION

Cooling tower is a heat rejection device, which rejecting the waste hot air into the

atmosphere through the cooling of water stream to a lower temperature. 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 closed circuit dry cooling towers, rely solely on the air to cool the

working fluid to near the dry-bulb air temperature. The cost for this process is inexpensive and

very dependable by removing a low amount of heat from the process. Commonly, the

applications of the cooling towers included; oil refineries, petrochemical, thermal power stations

and chemical plants. Basically, the cooling process in the industry will involve the cooling tower

system.

There are many types of cooling towers that has been used in many applications. We can

see the difference of the cooling towers by knowing how the air and water interact in open

cooling tower and closed cooling towers. Open cooling towers is also called the direct cooling

tower in which is allows the water to come into contact with outside air. Some water must be

added when cooled water is returned from the cooling tower to be used again. During the

process, the pollutant will able to enter into the tower, thus this pollutant need to filtered out.

The laboratory cooling tower allows the speed of fans (blower or damper) to be

controlled for cooling the warm return water and the pump is used to return the cooled water to

the water heater. This experiment is conducted to show the mass and heat transfer in the system

and also the mass and the energy balance for the closed system and to study how the adjustment

of difference parameters can a affect the performance of the system in removing the heat from

the process.

In a counter current cooling tower, the water stream is introduced at the top of the tower

and falls over or circulated in the packing materials which are used to increase the surface area

for the heat transfer process. This water stream is exposed to the air that is flowing upward

through the tower in order to remove the hot air. The gas-liquid interface in contact with each

other will evaporate the water into the air stream. At this stage, the latent heat of evaporation is

carried into the bulk air by the water vapour, and then heat will remove from water as its

temperature decreases. Latent heat of evaporation is the energy needed to change the state water

from liquid into gas at constant temperature.

In this experiment, energy enter and leaving the system involving the study of the first

law of thermodynamics at a work in the system. Enthalpy can be define as thermodynamic

potential or is define in equation:

H=U+PV -------- EQUATION 1

Where,

H = enthalpy

U= internal energy

P= pressure

V= volume

The combination of the term U+PV is the form of heat or energy. The enthalpy of the

system depends on the fluid as the working fluid. In this experiment, the fluid used is air and

water. Thus, the enthalpy can be determined by referring to the table at certain temperature and

pressure. The temperature of the inlet and outlet of the system is recorded, thus the temperature

is used as reference to determine the enthalpy of water. The enthalpy of outlet cooled water can

be determined by using equation:

∑H ¿=∑H out ---------EQUATION 2

The enthalpy of the air can be determined by two methods. The first method is by

assuming the air is ideal gas since the pressure of the gas is low. Thus, the enthalpy of the air can

be calculated by equation:

∆ H=CpdT ----------------- EQUATION 3

Where, Cp is the specific heat with respect to constant pressure and dT is difference in

temperature.

The second method is by using psychrometric chart. The information needed to use this

chart is dry bulb and wet bulb temperature of inlet and outlet of the system, that can be recorded

from the experiment.

THEORY

The cooling tower experiment operates according to the First Law of Thermodynamics

which is the conversation of energy. Energy can neither be destroy nor created, just transformed

from one another to another. Energy that enters the cooling tower is in the form of hot water.

This hot water was cooled from temperature T1 to a temperature T2. The cooling of the hot

water was in the formed of forced convection by which ambient air at T1 was blown over the hot

water and exited the cooling tower at some temperature T2. The data of both the entrance and the

exit temperature was recorded.

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

H = U + PV

H = enthalpy

U = internal energy

P = pressure

This equation is related to the heat as it is used to calculate the enthalpy of the system. Enthalpy

can be calculated or reference from tables of data for the fluid being used. In this experiment we

used the air and water as the fluids in the cooling tower. Enthalpy values can be obtained from a

thermodynamic textbook. The enthalpy of the output cooled water can be similarly reference and

an energy balance can be conducted for the water. The equation below displays the general

method to conduct an energy balance:

∑∆H in = ∑∆H out

Where ∆H = H in – H out

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

low pressure, it can be treated as an ideal gas and the enthalpy changed can be calculated through

the use of the following equation:

∆H = CP ∆ T(3)

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

with respect to constant pressure.

As water going into the cooling tower it loses energy. The enthalpy of the water going into

the tower can be determined using the enthalpy of saturated liquid water in a steam table. The

enthalpy of the water coming out of the tower can be determined in the same way. The data in

the steam tables are usually not given for every temperature so linear interpolation must be

performed to determine the enthalpy at the desired temperature. Then the enthalpy of the water is

multiplied by the mass flow rate. A basis of an operation of 1 minute was chosen to make the

calculation easier. The change in enthalpy for the water is determined by:

∆H water = ∆H water-out - ∆H water-in

The change in energy of the air can be determined using the same methodology as was used for

water. The enthalpy change is shown as

∆Hair = ∆Hair-out - ∆Hair-in

However, the determination of the enthalpy of air is more complicated than the determination of

the enthalpy values of the water stream. Now that the mass flow rate of dry air is known, the

enthalpy values of the in and out streams can be determined. The change in enthalpy of the water

should have a negative value, and the change in enthalpy of the air should have positive value.

Theoretically, when two values are added together, the result should be zero. This can be shown

by the first law of thermodynamics where :

∆H water = ∆H air

and

∆H water + ∆H air = 0

RECOMMENDATION

In order to obtain better results, there are a few methods or recommendations that may be

considered.

1. The auxiliary heaters always be used during experiments in order to increase the

temperature difference between the return water from the water heater and the cool

supply water. This increase in temperature difference will allow for a larger enthalpy

difference and will decrease the possibility of the enthalpy difference being negligible.

2. The humidity recording device was not working properly. So, be recalibrated or replaced

so that more accurate and timely measurement of humidity can be made.

3. Use appropriate safety PPE when conducting the experiment.

4. Avoid any mistake and error when conducting the experiment to get best result. Stay alert

to the time taken every ten minutes running.

REFFERENCE

1. The Engineering ToolBox, retrieved from http://www.engineeringtoolbox.com/fluids-evaporation-latent-heat-d_147.html.

2. Cooling Tower, retrieved from http://en.wikipedia.org/wiki/Cooling_tower.3. cooling tower, retrieved from https://www.scribd.com/doc/52403241/cooling-tower,

written by Durrah Khaz.4. Cooling Tower, retrieved from https://www.scribd.com/doc/178317145/Cooling-Tower,

written by sankarsuper83.5. cooling tower heat and mass, retrieved from

https://www.scribd.com/doc/49526932/cooling-tower-heat-and-mass, written by  Chris Mark.

APPENDICES