Cooling Tower(1)

ABSTRACT The aim of this experiment is to study the effect of cooling load and also the effect of water flow rate to the cooling water. Two parameters been manipulated in this experiment which are; power used and the water flow rate. On the first place, allow the cooling tower to operate for about 10 minutes until temperature reached 42°C with fully open blower. For the first part of experiment, the cooling loads used are set to be 0.5 kW, 1.0 kW and 1.5 kW. The water flow rate is set to be constant at 1.0 LPM. The temperatures from T1, T2, T3, T4, T5 and T6, as well as the heater power, orifice and column differentials are recorded after 10 minutes. The experiment is run for the next two cooling loads. For the second part of using different water flow rate, the cooling load is set to be constant at 0.5 kW. The water flow rates chose is 1.0 LPM, 1.5 LPM and 2.0 LPM. Same as the first part, the temperatures from T1, T2, T3, T4, T5 and T6, as well as the heater power, orifice and column differentials are recorded after 10 minutes for water flow rate of 1.0 LPM. After that, the other two values of water flow rate are used to run the next experiment. As the result, the average temperature at 0.5 kW, 1.0 kW and 1.5 kW are 30.80°C, 33.15°C and 36.25°C, while the feed are 0.0004 kg/s, 0.0061 kg/s and 0.0064 kg/s respectively. While values of vapour calculated in the second experiment are -0.0021 kg/s, -0.0054 kg/s and -0.0094 kg/s at 1.0 LPM, 1.5 LPM and 2.0 LPM respectively.

TABLE OF CONTENT CONTENTS

PAGE NUMBER

1.0 Introduction

3

2.0 Objective

5

3.0 Theory

5

4.0 Apparatus

9

5.0 Procedure

9

6.0 Results

11

7.0 Sample Calculation

14

8.0 Discussion

18

9.0 Conclusion

19

10.0 Recommendation

19

11.0 References

20

12.0 Appendices

20

2

1.0

INTRODUCTION Cooling towers are very important part of many chemical plants. Cooling tower is

equipment that used to reduce the temperature of a water stream by extracting heat from water and emitting it to the atmosphere. Cooling towers make use of evaporation whereby some of the water is evaporated into a moving air stream and subsequently discharged into the atmosphere. As a result, the remainder of the water is cooled down significantly. Cooling towers are able to lower the water temperatures more than devices that use only air to reject heat, like the radiator in a car, and therefore more cost-effective and energy efficient. The primary task of a cooling tower is to reject heat into the atmosphere. They represent a relatively inexpensive and dependable means of removing low-grade heat from cooling water.

Common applications for cooling towers are providing cooled water for air-conditioning, manufacturing and electric power generation. The smallest cooling towers are designed to handle water streams of only a few gallons of water per minute supplied in small pipes like those might see in a residence, while the largest cool hundreds of thousands of gallons per minute supplied in pipes as much as 15 feet (about 5 meters) in diameter on a large power plant.

The basic components of a cooling tower include the frame and casing, fill, cold-water basin, drift eliminators, air inlet, louvers, nozzles and fans. Frame and casing - Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. With some smaller designs, such as some glass fiber units, the casing may essentially be the frame. Fill - Most towers built with fills (made of plastic or wood) to maximize heat transfer by maximizing water and air contact. There are two types of fill: Splash fill is where water falls over successive layers of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill surface. The other one is film fill which consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the splash fill. Cold-water basin - The cold-water basin is usually located at or near the bottom of the tower, and it receives the cooled water that flows down through the tower and fill. Drift eliminators - Capture water droplets entrapped in the air stream that otherwise would be released to the atmosphere. 3

Air inlet - This is the point of entry for the air entering a tower. The inlet may take up an entire side of a tower (cross-flow design) or be located low on the side or the bottom of the tower (counter-flow design). Louvers - Generally, only cross-flow towers have inlet louvers. The purpose of louvers is to equalize air flow into the fill and retain the water within the tower. Many counter flow tower designs do not have louvers. Nozzles - These spray water to wet the fill. Uniform water distribution at the top of the fill is essential to achieve proper wetting of the entire fill surface. Fans - Both axial (propeller) and centrifugal fans are used in towers. Generally, propeller fans are used in induced draft towers and both propeller and centrifugal fans are found in forced draft towers.

Basically, cooling towers fall into two main categories: Natural draft and Mechanical draft cooling tower. Natural draft towers use very large concrete chimneys to introduce air through the media. Due to the large size of these towers, they are generally used for water flow rates above 45,000 m3/hr. These types of towers are used only by utility power stations. On the other hand, mechanical draft towers are a type of cooling tower that utilize large fans to force or suck air through circulated water. The water falls downward over fill surfaces, help increase the contact time between the water and the air - this helps maximize heat transfer between the two. Cooling rates of mechanical draft towers depend upon their fan diameter and speed of operation. Hence, by comparing those two types, mechanical draft cooling towers are much more widely used in industry.

Cooling tower can be divided into two designs of cooling tower which is counter flow and cross flow. The counter-flow and cross flows are two basic designs of cooling towers based on the fundamentals of heat exchange. It is known that counter flow heat exchange is more effective as compared to cross flow or parallel flow heat exchange. Cross-flow cooling towers are provided with splash fill of concrete, wood or perforated PVC. Counter-flow cooling towers are provided with both film fill and splash fill.

The laboratory cooling tower that used allows complete control of the speed of the fan used in cooling the warm return water and the pump used to return the cooled water to the water heater. Experiments can be conducted which to study how adjustment of one or both of these parameters affects the amount of heat removed from the water provided to the water heater. The laboratory cooling tower unit is supplied with a packed column having packing density of approximately 110 m2/m3. The unit mainly consists of a load tank with a

4

total of 1.5 kW electric heaters, an air distribution chamber, a make-up tank and a test column.

Figure 1: Schematic diagram of a general cooling water system

2.0

OBJECTIVES

-

To study the basic principles and characteristics of evaporative water cooling tower system.

-

To estimate the evaporation rate of water (water loss) for the tower.

-

To investigate cooling tower performance and key design factors.

-

To study the performance at different range of cooling loads and inlet temperature

-

To determine the correlation of water to air mass flow ratio with increasing water flow rate.

3.0

THEORY Cooling tower is a device used to reduce the temperature of water. The water is then

recycled back into the many processes and industries that use it. Some industries use the water to control the temperature of a process like a car radiator. The Basic Principle of Operation Evaporating some of the circulating water and cools the majority of the water in a cooling tower. The evaporation process only takes place on the surface of a liquid and needs latent heat of vaporization to happen. Sensible heat is drawn from the body of the water to

5

the surface to supply the energy needed for the latent heat. It can be seen that for a little evaporation a lot of sensible heat will be needed therefore the main body of the circulating water is cooled for very little loss of water. Warm to hot water from the cooling process is pumped to the top of the cooling tower and into the sprays where the water is broken up into droplets and distributed over the Fill. The water droplet spreads out as it slides down the Fill creating the surface area necessary for evaporation. The evaporation rate of the water is restricted by the amount of moisture already in the air around it. In order to maintain evaporation, the moistened air must be replaced with dry air, usually by fans blowing into the tower. Types of Towers Two basic types: a) Natural Draft Towers: rely on the heat of the water to generate the air movement inside the tower. They are only used for very large capacity systems such as Electricity Generation Plants, where they are called Hyperbolic Towers. b) Mechanical Draft: are fitted with fans to improve the airflow through the tower which increases the evaporation rate of the water which increases the capacity of the tower. Mechanical Draft Towers are used just about everywhere. Generally a centrifugal fan is used to force the air into a tower, and tube axial (propeller) fans are used to induce the air out the water. Orifice Constant

√

Where

)

6

Figure 2: Cooling Tower Cooling tower consists of water circuit and air circuit. Water circuit is where warm water is pumped from the load tank through the control valve and water flow meter to the column cap. After its temperature is measured, the water is uniformly distributed over the top packing deck and, as it spreads over the plates, a large thin film of water is exposed to the air stream. During its downward passage through the packing, the water is cooled, largely by the evaporation of a small portion of the total flow. The cooled water falls from the lowest packing deck into the basin, where its temperature is again measured and then passes into the load tank where it is re-heated before re-circulation. Due to evaporation, the level of the water in the load tank tends to fall. This causes the float operated needle valve to open and transfer water from the make-up tank into the load tank. Under steady condition, the rate at which the water leaves the makeup tank is equal to the rate of evaporation plus any small airborne droplets in the air discharge. For air circuit, air from the atmosphere with temperature, enters the fan at a rate which is controlled by the intake damper setting. The fan discharge into the distribution chamber and the air passes wet and dry bulb sensors before entering the packed column. As the air flows through the packing, its moisture content increase and the water is cooled. On leaving the top of the column the air passes through droplet arrester, which traps most of the entrained droplets and returns them to the packing. The air is then discharged to the atmosphere via the air measuring orifice and further wet and dry bulb sensors.

7

Figure 3: Basic Principle of Cooling Tower Assuming that the water is hotter than the air, it will be cooled: 1. By radiation – this effect is likely to be very small at normal condition and may be neglected. 2. By conduction and convection – this will depend on the temperature difference, the surface area, air velocity. 3. By evaporation – this is by far the most important effect. Cooling takes place as molecules of H2O diffuse from the surface into the surrounding air. These molecules are then replaced by others from the liquid and the energy required for this is taken from the remaining liquid. Evaporation from a Wet Surface The rate of evaporation from a wet surface into the surrounding air is determined by the difference between the vapor pressure at the liquid surface. The latter is determined by the total pressure of the air and its absolute humidity. In an enclosed space, evaporation can continue until the two vapor pressures are equal. However, it unsaturated air is constantly circulated, the wet surface will reach an equilibrium temperature at which the cooling effect due to the evaporation is equal to the heat transfer to the liquid by conduction and convection from the air, which under these conditions, will be at a higher temperature. The equilibrium temperature reaches by the surface under adiabatic condition in the absence of external heat gains or losses.

8

4.0

APPARATUS

Figure 4: Cooling Tower Equipment, H892 The equipment used for this experiment are:

5.0

-

cooling tower unit (induced-draft counterflow cooling tower)

-

graduated cylinder

-

distilled or deionised water

-

Packing installed: A

-

Packing density: 110 m2 per m3

-

Measuring instruments for dry bulb and wet bulb temperatures

PROCEDURES

Experimental Procedure 1. Ensure that valve V1 to V6 are closed and V7 is partially closed. 2. Load tank was filled with distilled or deionised water. 3. Make-up tank was filled with distilled or deionised water up to zero mark on the scale. 9

4. Distilled/deionised water was added to the wet bulb sensor reservoir till the fullest. 5. Appropriate cooling tower packing was installed for the experiment. 6. All appropriate tube was connected to the differential pressure sensor. o

7. After that, temperature was set to the set point of temperature controller till 45 C. o

Switch on the 1.0 kW water heaters and heat up the water until approximately 40 C. 8. Switch on the pump and slowly open the control valve V1 and set the water flow rate to 2.0 LPM. Until the steady operation obtained, where the water was distributed and flowing uniformly through the packing. 9. Fully open the fan damper, and then switch on the fan. Check that the differential pressure sensor is giving reading: i.

To measure the differential pressure across the orifice, open valve V4 and V5: close valve V3 and V6.

ii.

To measure the differential pressure across the column, open valve V3 and valve V6: close valve V4 and V5.

10. Steps ( 7 – 9 ) was repeated with ( 0.5 kW ) water heaters, water flow rates with ( 1.4 LPM and 1.2 LPM ) and fan damper for ( semi-open and fully closed ). 11. After the units run for about 5 minutes to stable, data obtained was recorded in the following table. Then, change the heaters power with different flow rates also different air flow rates (fan damper).

General Start-up Procedure

1. Ensure that valve V1 to V6 are closed and V7 is partially closed. 2. Fill the load tank with distilled or deionised water. 3. Fill the make-up tank with distilled or deionised water up to zero mark on the scale. 4. Add distilled/deionised water to the wet bulb sensor reservoir to the fullest. 5. Install the appropriate cooling tower packing for the experiment. 6. Connect all appropriate tubing to the differential pressure sensor. o

7. Then, set the temperature set point of temperature controller to 45 C. Switch on the o

1.0 kW water heaters and heat up the water until approximately 40 C. 8. Switch on the pump and slowly open the control valve V1 and set the water flow rate to 2.0 LPM. Obtain a steady operation where the water is distributed and flowing uniformly through the packing. 9. Fully open the fan damper, and then switch on the fan. Check that the differential pressure sensor is giving reading:

10

i.

To measure the differential pressure across the orifice, open valve V4 and V5: close valve V3 and V6.

ii.

To measure the differential pressure across the column, open valve V3 and valve V6: close valve V4 and V5.

10. Let the unit run for about 20 minutes, for the float valve to correctly adjust the level in the load tank. Refill the make-up tank as required. 11. Now, the unit is ready for use. General Shut-Down Procedure

1. Switch off heaters and let the water to circulate through the cooling tower system for 3-5 minutes until the water cooled down. 2. Switch off the fan and fully close the fan damper. 3. Switch off the pump and power supply. 4. Retain the water in the reservoir tank for the following experiment. 5. Completely drain off the water from the unit if it is not in used.

6.0

RESULT

Variable

: Power (heater)

Table 1

: Experimental data for constant water flow rate at 1LPM Unit

Heater (kW) 1.0 31.7

T1

⁰C

0.5 31.9

T2

⁰C

29.1

28.9

29.1

T3

⁰C

28.0

28.8

30.2

T4

⁰C

27.8

28.5

29.8

T5

⁰C

34.3

38.4

44.0

T6

⁰C

27.3

27.9

28.5

Heater power

W

433

828

1244

DP Orifice

Pa

93

92

90

DP Column

Pa

10

10

10

Blower

1.5 32.0

Half-opened

11

Variable

: Flow rate (water)

Table 2

: Experimental data for constant heater power at 0.5kW Unit

Water Flow Rate (LPM) 1.5 31.4

T1

⁰C

1.0 31.5

T2

⁰C

29.1

28.9

29.2

T3

⁰C

28.4

28.6

28.3

T4

⁰C

28.1

28.4

28.4

T5

⁰C

36.2

33.7

32.7

T6

⁰C

28.0

27.6

27.7

Heater power

W

434

438

430

DP Orifice

Pa

87

96

91

DP Column

Pa

11

11

10

Blower

Table 3

2.0 31.8

Half-opened

:The calculated correlation of water to air mass flow rate ratio at constant heater power (0.5kW) and increasing water flow rates Water Flow Rate (LPM)

L/G

1.0

0.2895

1.5

0.1951

2.0

0.3808

Table 4: The „wet bulb approaches‟ with its respective cooling loads. Cooling loads (kW)

‘wet bulb approach’ (ºC)

0.5

0.5

1.0

1.0

1.5

0.6

12

Table 5: Rate of vaporized steam for water flow rate constant at 1 LPM Heater Power (kW)

The average temperature

Rate of vaporized steam, V

(ºC)

(kg/s)

0.5

30.80

0.0004

1.0

33.15

0.0061

1.5

36.25

0.0064

Table 6: Rate of vaporized steam for heater power constant at 0.5 kW Water flow rate

The average temperature

Rate of vaporized steam, V

(LPM)

(ºC)

(kg/s)

1.0

32.10

-0.0021

1.5

30.65

-0.0054

2.0

30.20

-0.0094

Rate of vaporized steam VS heater power Rate of vaporized steam (kg/s)

0.007 0.006 0.005 0.004 0.003 0.002 0.001 0 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Heater Power (kW)

Graph 1: Graph of rate of vaporized steam versus heater power

13

0 -0.001

Rate of vaporized steam VS water flow rate 0

0.5

1

1.5

2

2.5

Rate of vaporized steam (kg/s)

-0.002 -0.003 -0.004 -0.005 -0.006 -0.007 -0.008 -0.009 -0.01 Water flor rate (LPM)

Graph 2: Graph of rate of vaporized steam versus water flow rate

7.0

SAMPLE OF CALCULATIONS

i) To calculate the correlation of water to air mass flow rate ratio at constant heater power (0.5kW) and different water flow rates: Thermodynamic rule indicates that the heat removed from the water must be equal to the heat absorbed by the surrounding air: L(T1-T2) = G(h2-h1) L/G = (h2-h1) / (T1-T2) Where L/G = liquid to gas mass flow rates ratio T1 = hot water temperature (ºC) T2 = cold water temperature (ºC) h2 = enthalpy of air-water vapor mixture at outlet wet-bulb temperature h1 = enthalpy of air-water vapor mixture at inlet wet-bulb temperature *the enthalpy of water is determined in table of saturated water properties (attached in appendices)

14

T1 = T5 = 36.2 ºC

h1 (T2 = 29.1 ºC) = 2431.942

T2 = T6 = 28.0 ºC

h2 (T4 = 28.1 ºC) = 2434.3158

Therefore,

The correlation of water to air mass flow rate ratio for increasing water flow rate of 1.5LPM and 2.0LPM is calculated and tabulated in result section. ii) To determine the the cooling load effect, effect of different air flow rates and the effects of different flow rates on the „wet bulb approach‟ and the pressure drop through the packing.

For cooling load (heater power) at 0.5kW „

‟

| –

|

–

|

–

|

–

|

|

For cooling load (heater power) at 1.0 kW „

‟

| –

|

|

For cooling load (heater power) at 1.5kW „

‟ |

| –

|

15

TO CALCULATE THE EVAPORATION RATE OF THE COOLING TOWER Water flow rate constant at 1 LPM V = 1.0 LPM

Q = 0.5 kW

Assume that heat capacity of liquid water at 101.325 kPa, by using table A.2-5, (Transport Process and Separation Process Principles). Therefore, Cp value at temperature 30.8 °C is 4.181 kJ/kg°C.

⁄ Density of water, ρ = 1000 kg/m3 m = (1.67 x 10-5 m3/s) (1000 kg/m3)

L = m = 0.0167 kg/s

Therefore, the rate of vaporized steam:

V=F–L V = (0.0171 – 0.0167) kg/s V = 0.0004 kg steam/s

16

Heater power constant at 0.5 kW V = 1.0 LPM

Q = 0.5 kW

Assume that heat capacity of liquid water at 101.325 kPa. By using table A.2-5, (Transport Process and Separation Process Principles). Therefore, the Cp value at temperature 32.1 °C is 4.181 kJ/kg°C.

⁄ Density of water, ρ = 1000 kg/m3 m = (1.67 x 10-5 m3/s) (1000 kg/m3)

L = m = 0.0167 kg/s

Therefore, the rate of vaporized steam:

V=F–L V = (0.0146 – 0.0167) kg/s V = -0.0021 kg steam/s

17

8.0

DISCUSSIONS The cooling tower experiment is conducted in order to determine i) the correlation of

water to air mass flow rate ratio with increasing water flow rate, ii) the cooling load effect, effect of different air flow rates and the effects of different flow rates on the „wet bulb approach‟ and the pressure drop through the packing, iii) the evaporation rate of water (water loss) for the tower. The experiment is divided into two part which are constant water flow rates (1LPM) with increasing heater power and constant heater power (0.5kW) with increasing water flow rates. The correlation of water to air mass flow rates ratio (L/G) is calculated at increasing water flow rates with a constant heater power (0.5kW). For water flow rate of 1.0, 1.5, and 2.0 LPM, the L/G calculated are 0.2895, 0.1951, and 0.3808 respectively. The L/G ratio of a cooling tower is basically the ratio between the water and the air mass flow rates. In order to get the best cooling tower effectiveness, the cooling towers itself requires adjustment and tuning of water and air flow rates. Adjustments can be made by water box loading changes or blade angle adjustments. At L/G < 1, the contact area between air and water is large and better heat transfer rate will be achieved. The cooling loads (or heat load) is defined as the amount of heat energy removed from the water cooling system. The cooling load can be also referred to the heater powers which are 0.5kW, 1.0kW and 1.5kW. The „wet bulb approach‟ is calculated for each cooling loads which gives 0.5 ºC, 1.0 ºC and 0.6ºC respectively. The „wet bulb approach‟ is usually known as an indicator of cooling tower performance. Generally, due to increased size, the closer the approach to the wet bulb, the more expensive the cooling tower will be (Electrical Energy, 2006). Besides, the blower is half-opened throughout the experiment for the purpose of cooling down the hot water from the heater. The evaporation rates are calculated for both part of experiment. At constant water flow rate (1LPM), the rate of vaporised steam is calculated and a graph of rate of vaporized steam versus heater power is plotted in figure 1. The vaporised steam is calculated to be 0.0004, 0.0061 and 0.0064 for heater power of 0.5, 1.0, and 1.5kW respectively. Based on figure 1, it can be seen that the rate of vaporised steam is increased as heater power increased. This is because more energy is removed as the heater power increased and thus the system more steam will be vaporised. On the other hand, at constant heater power of 0.5 kW, the rate of vaporised steam for different water flow rate is calculated and a graph of rate of vaporised steam versus water flow rate is plotted in figure 2. For water flow rates of 1.0, 1.5 and 2.0 LPM, the vaporised 18

steam is calculated to be -0.0021, -0.0054, and -0.0094 respectively. Based on figure 2, the rate of vaporised is observed to be decreased as the water flow rate increases. This is because, the higher the water flow rate, the smaller the area of heat transfer would takes place. On top of that, the average temperature of water seems to be decreasing as water flow rates increase. Which means as the water flow rates increase, the lower the energy is removed from the system. 9.0

CONCLUSION

The objectives of the experiment to determine the correlation of water to air mass flow rate ratio with increasing water flow rate, define the cooling load effect and the pressure drop through the packing as well as the evaporation rate of water for the tower were fully achieved. For the first experiment, water flow rate was kept constant and heater power was varied. Meanwhile, for the second experiment, the heater power was kept constant and water flow rate was varied. The correlation of water to air mass flow rate ratio at constant heater power and wet bulb approaches were both fluctuated at increasing water flow rate and heater power respectively. The rate of vaporized steam at increasing heater power increased while the rate of vaporized steam at increasing water flow rate decreased with time. 10.0

RECOMMENDATIONS

Several recommendations have been generated as a result of the experiences with the cooling tower: -

The auxiliary heaters must 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.

-

Only a few experiments can be planned due the time needed for the system to reach steady state which is approximately 30 minutes, making it is insufficient to run all of the experiments.

-

Recalibrated the humidity recording devices so that more accurate and timely measurements of humidity can be made.

19

11.0 

REFERENCES Perry H.R, Don W.G (1998). Perry‟s Chemical Engineers‟ Handbook, 6th Edition, McGraw Hill.



United Nations Environment Programme. 2006. “Electrical Energy Equipment: Cooling Towers”. What is a Cooling Tower? Pp 1 – 2. Retrieved from www.energyefficiencyasia.org. on 9th November 2013.



Cooling

Tower

Experiment.

Retrieved

http://www.me.iitb.ac.in/~matrey/PDF's/cooling%20tower.pdf

on

from November

17th,2013.



C.J. Geankoplis “Transport Processes and Unit Operations”, 3rd Ed., Prentice Hall, Englewood Cliffs, NJ (1993).

12.0

APPENDICES

20