report cooling tower.docx

1.0 Abstract Water cooling tower laboratory was conducted to perform heat and mass balance on the system. The effects of the process variables were observed on the exit temperature of water. This experiment had several parameters that can be adjusted to observe the effects on the evaporation of water. The parameters are flow rate of water, temperature, power of heater, relative humidity and flow rate of air and cooling load. For this experiment, we had selected the blower as the constant parameter while water flow rate and heater power as the variable. Thus, the equation of steady flow which are energy and mass balances were used in order to provide an insight on the amount of heat transferred between phases under different conditions. The power transfer was calculated from this experiment of cooling load for 0.5 kW, 1.0 kW and 1.5 kW.

2.0 Introduction The SOLTEQ® Basic Cooling Tower Unit (Model: HE152) has been designed to show students the construction, design and operational characteristics of a modern cooling system. The unit resembles a full size forced draught cooling tower and it is actually an "open system" through which two streams of fluid which in this case air and water pass and in which there is a mass transfer from one stream to the other. The unit is self-contained supplied with a heating load and a circulating pump. Once energy and mass balances are done, then students will be able to determine the effects on the performance of the cooling tower by several parameters which are temperature and flow rate of water, relative humidity and flow rate of air, and cooling load. Packing characteristics column which is optional (A, B, C, D) are available for SOLTEQ® Basic Cooling Tower Unit (Model: HE152). The column is designed to assist study of water and air conditions at three additional stations (I, II and III) within the column. This enables driving force diagrams to be constructed and the determination of the Characteristic Equation for the Tower. (Experimental Manual, Water Cooling Tower; MODEL: HE 152)

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The experiment is started with the general start-up procedure. The column C had been chosen in conducting this experiment to observe and determine the objectives. For the first experiment, the water flowrate will be the constant value which is 2.0 LPM, while the power of heater as variable parameter for 0.5 kW, 1.0 kW, and 1.5 kW. This is to determine the change of temperature for each power supply. Then, for the second experiment, we had chosen different water flow rate as the variable parameter which in value of 1.0 LPM, 2.0 LPM, and 2.5 LPM, while the heater as a constant value at 1.0 kW. This was conducted to determine the heat load, Q and cooling range, ΔT.

3.0 Aims  To observe the different ranges cooling load and inlet temperature of water cooling tower.

4.0 Theory The cooling range means the difference between the water temperature at entry to and exit from the tower. Then, the cooling load is the rate at which heat is removed from the water. This may be expressed in kW, Btu/h or k Cal/h. The quantity of fresh water which must be supplied to the water circuit to make good the losses due to evaporation and other causes is called make-up. Drift or carry is droplets of water which are entrained by the air stream leaving the tower. Over packing or fill means the material over which the water flows as it falls through the tower, so that a large surface area is presented to the air stream. Approach to wet bulb means the difference between the temperature of the water leaving the tower and the wet bulb temperature of the air entering.

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Basic principles consider the surface of a warm water droplet or film in contact with an air stream. The first consider an air stream passing over the surface of a warm water droplet or film. An assumption had been made which that if the water is hotter than the air, then the water temperature will be cooled down by radiation, conduction and convection, and evaporation. The radiation effect is normally very small and may be neglected. Then, the conduction and convection depend on the temperature difference, the surface area, air velocity, etc. The effect of evaporation is the most significant where cooling takes place as water molecules diffuse from the surface into the surrounding air. During the evaporation process, the water molecules are replaced by others in the liquid from which the required energy is taken. (Experimental Manual, Water Cooling Tower; MODEL: HE 152)

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, which is the saturation pressure corresponding with the surface temperature, and the vapor pressure in the surrounding air. For the final 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, which is until the air is saturated and at the same temperature as the surface. However, if 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. Then, the equilibrium temperature will reach by the surface under adiabatic conditions, which is in the absence of external heat gains or losses, that called the "wet bulb temperature". In a cooling tower of immeasurable size and with a sufficient air flow, the water leaving will be at the wet bulb temperature of the incoming air. The difference between the temperature of the water leaving a cooling tower and the local wet bulb temperature is an indication of the effectiveness of the cooling tower. The "Approach to Wet Bulb" is one of the important parameters in the design, testing, specification, and selection of cooling towers. For the conditions within a cooling tower packing are complex due to the changing air temperature, humidity and water temperature as the two fluids pass through the tower usually in a counter flow. (Experimental Manual, Water Cooling Tower; MODEL: HE 152)

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To understand the working principle and the performance of a cooling tower, a basic knowledge of thermodynamic is important. There is a brief review on some of the thermodynamic properties. At the triple point (i.e. 0.00602atm and 0.01°C), the specific enthalpy of saturated water is assumed to be zero, which is taken as datum. The specific enthalpy of saturated water (hf) at a range of temperatures above the datum condition can be obtained from thermodynamic tables. The specific enthalpy of compressed liquid is given by p− p sat h=hf + v f ¿ ) The correction for pressure is negligible for the operating condition of the cooling tower; therefore, we can see that h ≈ hf at a given temperature. The specific heat capacity (Cp) is defined as the rate of change of enthalpy with respect to temperature which often called the specific heat at constant pressure. For the purpose of experiment using bench top cooling tower, we may use the following relationship: ∆ h=C p ∆ T and h=C p T Where Cp = 4.18 kJ.kg -1 (Experimental Manual, Water Cooling Tower; MODEL: HE 152)

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5.0 Apparatus 7

1 s 2 3 4

8

9

5 10 6

Figure 5.0-1: Parts Identification and Equipment Set-up of Bench Top Cooling Tower

1 2 3 4 5

Orifice Water Distributor Packed Column Flow Meter Air Blower

 

6 7 8 9 10

Receiver Tank Differential Pressure Transmitter Make-up Tank Control Panel Load Tank

Stopwatch Deionized water

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6.0 Procedure General Start-up Procedures 1) Valves V1 to V6 were checked to ensure it were closed and valve V7 is partially opened. 2) The load tank is filled with distilled or deionized water. It is done by first the make-up tank was removed and then the water was poured through the opening at the top of the load tank. The make-up tank was replaced onto the load tank and the nuts lightly tightened. The tank was filled with distilled or deionized water up to the zero mark on 3) 4) 5) 6)

the scale. Distilled or deionized water was added to the wet bulb sensor reservoir to the fullest. All appropriate tubing was connected to the differential pressure sensor. The appropriate cooling tower packing was installed for the experiment. Then, the temperature was set point of temperature controller to 50°C. The 1.0 kW

water heater was switched on and the water heated up until approximately 40°C. 7) The pump was switch on and the control valve V1 was slowly opened and the water flowrate was set to 2.0 LPM. A steady operation was obtained where the water is distributed and flowing uniformly through the packing. 8) The fan damper was fully opened, and then the fan was switched on. The differential pressure sensor is giving reading was checked when the valve manifold is switched to measure the orifice differential pressure. 9) The unit was let ran for about 20 minutes, for the float valve to correctly the level in the load tank was adjusted. The makeup tank was refilled as required.

Experiment 1 1) Temperature set point of temperature controller was set to above 45ºC. The 0.5kW water heater was switched on. 2) The pump was switched on and the control valve V1 was slowly opened and the water flow rate was set to 2.0LPM. 3) After 15 minutes, T1, T2, T3, T4, T5, and T6 were obtained from control panel. 4) To measure the differential pressure across the orifice, valves V4 and V5 were opened while valves V3 and V6 were closed. 5) Then, to measure the differential pressure across the column, valves V3 and V6 were opened while valves V4 and V5 were closed. 6) The step was repeated from (1-5) for different heater power 1.0 kW and 1.5 kW. Page | 6

Experiment 2 1) Temperature set point of temperature controller was set to above 45ºC. The 1.0kW water heater was switched on. 2) The pump was switched on and the control valve V1 was slowly opened and the water flow rate was set to 2.0LPM. 3) After 15 minutes, T1, T2, T3, T4, T5, and T6 were obtained from control panel. 4) To measure the differential pressure across the orifice, valves V4 and V5 were opened while valves V3 and V6 were closed. 5) Then, to measure the differential pressure across the column, valves V3 and V6 were opened while valves V4 and V5 were closed. 6) The step was repeated from (1-5) for different water flow rate 1.0LPM and 2.5 LPM.

General Shut-Down Procedure 1) Heaters were switched off and the water was let to circulate through the cooling tower 2) 3) 4) 5)

system for 3-5 minutes until the water cooled down. The fan was switched off and the fan damper was fully closed. The pump and power supply were switched off. The water in reservoir tank was retained for the following experiment. The water completely drained from the unit if it is not in used.

7.0 Results Experiment 1    

Heater Column C Water flow rate = 2.0 LPM Blower: Fully opened Page | 7

Heater (kW) Air inlet dry bulb, T1(ºC) Air inlet wet bulb, T2(ºC) Air outlet dry bulb, T3(ºC) Air outlet wet bulb, T4(ºC) Water inlet temperature, T5(ºC) Water outlet temperature, T6(ºC) Different pressure (Pa) Dp orifice Dp column

0.5 30.7 26.9 26.6 27.7 30.1 26.4 5 69 64

1.0 31.3 27.6 29.2 30.1 34.5 28.2 4 67 63

1.5 31.1 27.4 30.8 32.0 38.0 28.8 3 66 63

1.0 32.4 28.2 29.5 30.5 41.9 27.5

2.0 32.1 28.0 29.5 30.6 35.0 28.5

2.5 31.8 27.9 29.5 30.5 33.8 28.6

Experiment 2    

Water flow rate Column C Heater = 1.0 kW Blower: Fully opened

Water flow rate (LPM) Air inlet dry bulb, T1(ºC) Air inlet wet bulb, T2(ºC) Air outlet dry bulb, T3(ºC) Air outlet wet bulb, T4(ºC) Water inlet temperature, T5(ºC) Water outlet temperature, T6(ºC)

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Different pressure (Pa) Dp orifice Dp column

17 70 53

3 68 65

3 65 68

8.0 Calculations Experiment 1 Water flow rate constant = 1.0 LPM Variable: heater Change in temperature for each power supply, = water inlet temperature, T5 – ΔT (cooling range)



water outlet temperature, T6

Power = 0.5 kW ΔT = T5 – T6 = 30.1 -26.4 = 3.7 ºC Page | 9



Power = 1.0 kW ΔT = T5 – T6 = 34.5 – 28.2 = 6.3 ºC



Power = 1.5 kW ΔT = T5 – T6 = 38.0 – 28.8 = 9.2 ºC

Experiment 2 At water flow rate = 1.0 LPM Cooling range, ΔT = water inlet temperature, T5 – water outlet temperature, T6 = 41.9 – 27.5 = 14.4 ºC m=1.0

L kg 1 M ×1 × M L 60 s

= 0.0167 Heat load, Q ¿ mCp ∆ T

=

0.0167

kg × 4.186 ×14.4 ℃ s

= 1.0046 kW Page | 10

Water Flow Rate (LPM) 1.0 2.0 2.5

Heat Load, kW 1.0046 0.9070 0.9070

Cooling Range, ºC 14.4 6.5 5.2

9.0 Discussion The SOLTEQ Water Cooling Tower (Model: HE 152) is used to run this experiment. This experiment was conducted to study the effects of variables changes on the temperature of the water. There are three variables that will affect the water temperature which are heater power, water flow rate, and the blower. For this experiment we were focused on the heater power and water flow rate as the changing variable in different experiment, meanwhile the blower was set as constant which is fully opened for both experiments. For every change in heater power energy and water flow rate, the time interval for the process to occur is 15 minutes. This was to ensure that the system is in steady state operation. Heat is transferred from a body with a higher temperature to lower temperature. As for this cooling tower experiment, there is need to produce a product which has a lower temperature which is cooler than the media being used for the transfer of heat in the system. For the water cooling tower, the temperature of the outlet water can be lower than the temperature of the cooling air. The cooling of the hot water was in the form of forced convection by which ambient air was blown over the hot water. Page | 11

Water in the cooling tower is cooled by the process known as evaporation. In the process, heat energy is being transferred between the water and air which having different temperature. As the energy in the water molecules is transfer to the air flowing through the water, the bond of the water molecules becomes weaker then it will slowly evaporate to the air. This was proved from the result of this experiment, which the water outlet temperature is lower than the inlet temperature. It can be seen from the results section above on experiment 1 and 2. The example on experiment 1 for 0.5 kW; water outlet=26.4ºC, while water inlet=30.1ºC. The fill is used in order to increase the time contact of the water and the air flowing in the system. From the data obtained at the end of the experiment, the changes in heater power has affect the energy being transferred within the system and surrounding. Larger heater power may cause higher temperature in water. Then, in this experiment, the temperature of the water is assumed higher than the air flowing through the system without considering the changes in heater power. For the example, based on results from the experiment 1 for 1.0 kW, the value of water inlet=34.5ºC, while air inlet dry bulb=31.3ºC and air inlet wet bulb=27.6ºC.

In experiment 2, we had determined the effect of cooling range, ΔT based on the differences of water flow rate. From the calculation, the experiment had showed higher water flow rate will cause the decreases cooling range. For example, water flow rate: 1.0 LPM, 2.0 LPM and 2.5 LPM, while cooling range: 14.4ºC, 6.5 ºC and 5.2 ºC. Other than that, we also had determined the heat load, kW based on the differences value of water flow rate. From the calculation, the heat load value: 1.0046 kW, 0.9070 kW and 0.9070 kW. Therefore, it can prove that as the water flow rate increases the heat released decreases.

10.0

Conclusion

The laboratory water cooling tower is conducted to study the performance at differences range of cooling load and inlet temperature of cooling tower. From the data obtained, we have calculated the changes in temperature for each power supply for experiment 1 and the cooling range and heat load for experiment 2. As the increases of power supply in experiment Page | 12

1, the temperature of cooling range in the cooling tower increases. Then, as the water flow rate increases, the cooling range decreases which decreases the heat load. Hence, the experiment has been a success since the objectives of the experiment were achieved and we also have gained knowledge regarding to the water cooling tower operation.

11.0

Recommendations

In this experiment there are several recommendations that can be deduced to improve the results of this experiment. First, the water that used in the unit only deionized water or distilled water. Then, when using the tap water in the system, the impurities exist in the tap water may cause the depositing in the cover tower and then lower the efficiency of the cooling tower. Other than that, make sure there is no water in the pressure tubing for accurate differential pressure measurement. When installing the cooling tower column or make-up tank, the nuts must ensure not tighten too hard because it may cause the crack. Also, not too tight lightly which can cause spilling of water out from the column. Furthermore, ensure that the pressure of tubes for differential pressure measurement are connected correctly which orifice pressure tapping point to V4, column’s lower pressure tapping to V6, column’s higher pressure tapping point to V3 and V5 leave to atmosphere. Besides that, after completed the experiment, ensure that the water inside the tower was cooled down first before draining it off because if water still hot when draining off the water, the unit’s efficiency will decrease.

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12.0 References 1) Richard M. Felder, Ronald W. Rousseau. (2005). Elementary Principles of Chemical Process, 3rd Ed. John Wiley & Sons, Inc. 2) Frank P. Incropera, David P. Dewitt, Theodore L. Bergman, Adrienne S. Lavine. 2007. Fundamentals of Heat and Mass Transfer, 6th Ed. John Wiley & Sons (Asia) Pte Ltd. 3) http://www.me.iitb.ac.in/~matrey/PDF's/cooling%20tower.pdf 4) Experimental Manual, Water Cooling Tower; MODEL: HE 152

13.0

Appendices

Figure 13.0-1: Different packing in another column.

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Figure 13.0-2: Packed Column C with 200 m2/m3.

Figure 13.0-3: The control panel showed the data.

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