Intro

cooling tower is a heat rejection device, which removes waste heat to the atmosphere though the cooling of a water stream to a lower temperature. The type of heat rejection in a cooling tower is termed "evaporative" in that it allows a small portion of the water being cooled to evaporate into a moving air stream to provide significant cooling to the rest of that water stream. The heat from the water stream transferred to the air stream raises the air's temperature and its relative humidity to 100%, and this air is discharged to the atmosphere. Evaporative heat rejection devices such as cooling towers are commonly used to provide significantly lower water temperatures than achievable with "air cooled" or "dry" heat rejection devices, like the radiator in a car, thereby achieving more cost-effective and energy efficient operation of systems in need of cooling. (Cooling Technology Institute, 2012). The cooling potential of a wet surface is much better than a dry one. . Common applications include cooling the circulating water used in oil refineries, petrochemical and other chemical plants, thermal power stations and HVAC systems for cooling buildings.

The generic term “cooling tower” is used to describe both direct (open circuit) and indirect (closed circuit) heat rejection equipment. A direct, open-circuit cooling tower is an enclosed structure with internal means to distribute the warm water fed to it over a labyrinth-packing or “fill”. The fill may consist of multiple, mainly vertical, wetted surfaces upon which thin film of water spreads. An indirect, or closed circuit cooling tower involves indirect contact of the air and the fluid, which usually water or a glycol mixture, being cooled (Southwest Thermal Technology,2015).

According to cleanenergy.org, cooling towers vary in size from small roof-top units to very large hyperboloid structures (as in the adjacent image) 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. The hyperboloid cooling towers are often associated with nuclear power plants, although they are also used to some extent in some large chemical and other industrial plants. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from air conditioning.

In this study, the cooling load effect, effect of different water flow rates and heater power also the effect of both flow rate on the wet bulb approach and pressure drop through the packing is determined. The manipulated variable in this study are the heater power in the first study and the water flow rate in the second study.

Theory

The basic principle of the cooling tower operation is that of evaporative condensation and

exchange of sensible heat. The air and water mixture releases latent heat of vaporization

which has a cooling effect on water by turning a certain amount of liquid into its gaseous

state thereby releasing the latent heat of vaporization. This is more effectively demonstrated

by wetting the back of your hand with water and blowing on it. This effect is what happens

inside the cooling tower. The air stream releases latent heat of vaporization thereby dropping

the temperature of the water on your skin. The liquid changing to its vaporous state consumes

heat which is taken from the water remaining, thus lowering its temperature.   There is a

penalty involved, and that is loss of water which goes up to the cooling tower and is

discharged into the atmosphere as hot moist vapour. Sensible heat that changes temperature is

also responsible for part of the cooling tower’s operation. When water is warmer that the air,

there is a tendency for the air to cool the water. The air then gets hotter as it gains the sensible

heat of the water and the water is cooled as its sensible heat is transferred to the air.

Approximately 25% of the sensible heat transfer occurs in the tower while the balance of the

75% cooling is due to the evaporative effect of latent heat of vaporization (Jalal Engineering,

2005).

Figure 4.1: Schematic diagram of cooling tower operation

(http://www.globalspec.com/learnmore/manufacturing_process_equipment/

heat_transfer_equipment/cooling_towers)

According to Jalal Engineering (2005), Cooling tower selection and performance is based on

water flow rate, water inlet temperature, water outlet temperature and ambient wet bulb

temperature and also the heat load. The heat load here is referring to the heater pump. The

heat load imposed on a cooling tower is determined by the process being served. The degree

of cooling required is controlled by the desired operating temperature level of the process. In

most cases, a low operating temperature is desirable to increase process efficiency or to

improve the quality or quantity of the product. In some applications (e.g. ), however, high

operating temperatures are desirable. The size and cost of the cooling tower is proportional to

the heat load. Process heat loads may vary considerably depending upon the process

involved. The heat load is supplied by the unit being served by the cooling tower. In the usual

circulating system the heat ‐load is independent of the cooling tower. The circulating water

flow is determined by the number of pumps running and the pressure drop in the overall

circulating water system. Therefore, it likewise is independent of the cooling tower. If heat

load, the constant, and the circulating water flow are all independent of the cooling tower,

then by mathematical deduction the range is likewise completely independent of the cooling

tower.

According to Sharon et. al., (1996) the operation of cooling tower s based on First Law of

thermodynamics which is conservation of energy. This law defines that the energy enters the

system must be equal to the energy exit the system, energy can neither be created nor

destroyed, just transformed from one to another. Energy that enters the cooling tower is in

form of hot water where this hot water was cooled from temperature T1 to a temperature

T2.The cooling of the hot water is in forced convection where the ambient air at T1 was

blown over the hot water and exited the cooling tower at some temperature T2. The energy

balance can be conducted when the temperature of air and water were recorded.

Energy balance is a form of book keeping that accounts for the energy entering and leaving

the system. The main component defined as

H = U – PV

Where H is enthalpy, U is internal energy, P is pressure and V is volume. Enthalpy can be

calculated or referred from tables of data for the fluid being used. Commonly table of data

that have enthalpy is for water in most engineering thermodynamics books. The enthalpy of

output cooled water can similarly be referred and an energy balance can be conducted for the

water. Since the temperature if initial and final temperature of input hot water and output cold

water were measured, the enthalpy can be calculated (Sharon et. al., 1996). Thus, equation

below displays the general method to conduct and energy balance:

in = out

Where H = H in - H out. The change in enthalpy can be determined from either of two

methods. Since the air operates at low pressure, thus it can be treated as ideal gas and the

enthalpy change can be calculated through following equation:

H = Cp T

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

heat with respect to constant pressure.

The water going into the cooling tower loses enrgy.the enthalpy of thw water going into the

tower can be determined by 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 steam tables are usually not given for enery temperature so linear interpolation must be

performed to determine the enthalpy desired temperature. The enthalphy if the water is

multiplied by the mass flow rate (Sharon et. al., 1996). 1 minute basis is chosen in order to

make the calculation easite. The change in enthalpy is detemuned by

Hair = Hair-out - Hair-in

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

of enthalpy values of the water stream. Thus psychrometric chart is used where the enthalpy

if air stream can be determined using wet bulb and the dry bulb of the air stream. Now the

mass flow rate of dry air is known,the enthalpy values of the in and out stream can be

determined.the change in enthalphy of the water should have a negative value, and the change

in enthalpy of the air should have a positive value. Theoritically, when this two value added,

the results must equal to zero (Sharon et. al., 1996) . Thus, the first law of thermodynamic

was applied where:

Hwater = air

And

Hwater + Hair = 0.

reference

Cooling Technology Institute, (2012). What is a (wet, atmospheric) cooling tower? retrieved on 21 April 2015 from http://www.cti.org/whatis/coolingtowerdetail.shtml

Southwest Thermal Technology, (2015). Cooling Towers: Counter Flow and Cross Flow. retrieved on 21 April 2015 from http://www.southwestthermal.com/cooling-towers.html

cleanenergy.org. Clean Energy Footprints. Identifying Nuclear Reactors in Google Earth Retrieved 5/19/2014

Jalal Engineering, (2005). Cooling Tower Basics and Common Misconceptions. retrieved on 21 April 2015 from http://www.jalal.com.pk/papers/Cooling%20Tower%20Basics%20and%20Common%20Misconceptions.pdf

Sharon Lewis Scott Daniels and Austin Newman (1996). Cooling Tower Experiments. University of Tennessee at Chattanooga College of Engineering Engineering 435. retrieved on 22 April 2015 from http://chem.engr.utc.edu/webres/435F/3T-CT/3T-CT.html