ABSTRACT
This experiment was conducted to perform energy and mass balance on the cooling tower system and to observe the effects of one of the process variables on the exit temperature of water. For water cooling tower experiment, there are several parameters that can be adjusted to observe its effects on the evaporation of water. The parameters are temperature and flow rate of water, relative humidity and flow rate of air and cooling load. In this experiment, we choose the cooling load as variable while water flow rate and flow rate as constant parameters. The steady flow equations which is energy and mass balances were employed in order to provide an insight on the amount of energy transferred between phases under different conditions. The energy transfer calculated from the experiment for cooling load of 0.5 kJ/s , 1.0 kJ/s and 1.5 kJ/s.
INTRODUCTION
The laboratory cooling tower is a cooling tower unit from a commercial air conditioning system used to study the principles of cooling tower operation. It is used in conjunction with a residential size water heater to simulate a cooling tower used to provide cool water to an industrial process. In the case of the laboratory unit, the cooling tower process load is provided by the water heater. The laboratory cooling tower allows for 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 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 remainder of this report will explain the theory behind the operation of a cooling tower and how the laboratory cooling tower is operated. An example of a mass and energy balance on the laboratory cooling tower will be presented along with the results of experiments in which the rate of heat dissipated by the tower was calculated at full capacity and when the pump speed and fan speed were varied independently.
OBJECTIVE
To study the performance at different ranges cooling load and inlet temperature of cooling tower.
THEORY
The cooling tower experiment operate according to the First Law of Thermodynamics which is the conservation of energy. Energy can neither be created nor destroyed, just transformed from one form to another. The energy that enter the system must exit the systemas it can diffuse through the system. Energy that enters the cooling tower is in the form of hot water. (Other energy contributions such as heat generation fromfriction of both air and water, energy losses from pipes, etc. are ignored.) This hot water was cooled from temperature T1to a temperature of T2. The cooling of the hot water was in the form of forced convection 3 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 enterence 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 V = is volume
This equation is related to the heat as it is use to calculate the enthalpy of the system. Enthalpy can be calculated or referenced from tables of data for the fluid being used. In this experiment we used the air and wateras the fluids in the cooling tower. Enthalpy values can be obtained from a thermodynamics textbook. For example: Since both the initial and finaltemperatures of the input hot water and the output cool water were measured, the temperature T in can be referenced and the enthalpy (BTU/lbm, or KJ/kg) can be recorded. The enthalpy of the output cooled water can be similarly referencedand an energy balance
can be conducted for the water.The equation below displays the general method to conduct an energy balance:
in = out
where H = H in - H out.
The change in enthalpy for air can be determined form 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 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 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 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
.
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
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 a positive value. Theoretically, when the two values are added together, the result should be zero. This can be shown by the first law of thermodynamics where
APPARATUS AND SET UP
Stopwatch
Deionized water
SOLTEQ Bench Top Cooling Tower Unit (Model: HE152)
1. Orifice 2. Water distributor 3. Packing column 4. Flow meter 5. Receiver tank 6. Air blower 7. Make-up tank
8. Differential Pressure Transmitter 9. Load Tank
10. Control Panel
PROCEDURE
1. Valves V1 and V6 were checked and ensured to be closed and valve V7 to be partially opened.
2. The load tank was filled with deionized water. Firstly, the make-up tank was removed and deionized 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 were lightly tightened. Then, the tank was filled with deionized water up to the zero mark and the scale. 3. Deionized water was added to the wet bulb sensor reservoir to the fullest.
4. All appropriate tubing was connected to the differential pressure sensor. 5. The appropriate cooling tower packing was installed for the experiment.
6. Temperature set point of temperature controller was set to 45o-C. The 1.0kW water heater was switch on and water was heated up to 40oC.
7. The pump was switched on and the control valve V1 was slowly opened and the water flow rate was set to 2LPM.
8. The damper was fully opened and the fan was switched on.
9. Blower switch was switched on after the water already went through the cooling tower.
10. The unit was run for 20 minutes to ensure float valve correctly adjusted the level in the load tank. The make-up tank was refilled as required.
11. The damper and the flow rate were set to be constant.
12. The 1.0kW water heater was switched off to set the power as 0kW.
13. Record all the data required after 10 minute to ensure the unit stabilized for first trial and another 10 minutes for second trial.
14. To measure the differential pressure across the orifice , valves V4 and V5 were opened while valves V3 and V6 were closed.
15. To measure the differential pressure across the column, valves V3 and V6 were opened while V4 and V5 were closed.
16. The water heater then was set to 0.5kW, 1.0kW and 1.5kW.
17. After all the experiments were done, the heaters were switched off and the water was let to circulate through the cooling tower system for 3-5 minutes until the water cooled down.
18. The fan was switched off and the fan damper was closed fully 19. The pump and power supply were switched off.
RESULT
Heater Column B
Water flow rate : 1.0 LPM Blower: fully opened
Heater (kW) 0.5 1.0 1.5 Air inlet dry bulb, T1(°C) 26.8 26.7 26.9 Air inlet wet bulb, T2(°C) 27.5 27.3 27.3 Air outlet dry bulb, T3(°C) 25.6 26.1 27.4 Air outlet wet bulb, T4(°C) 25.9 26.6 28.0 Water inlet temperature,
T5(°C)
36.6 38.7 44.9
Water outlet temperature, T6(°C)
25.1 25.6 26.1
Heater power (W) 425 801 1232
Dp orifice 108 108 108
Dp column 0 0 0
Water flow rate Column B Heater: 1.0 KW
Water flow rate (LPM) 0.8 1.0 1.2 1.4 1.6 Air inlet dry bulb, T1(°C) 27.3 27.6 27.9 27.4 27.2 Air inlet wet bulb, T2(°C) 27.8 28.0 28.1 27.9 27.5 Air outlet dry bulb, T3(°C) 28.1 26.8 26.1 26.2 26.0 Air outlet wet bulb, T4(°C) 29.7 26.7 23.3 21.5 20.2 Water inlet temperature,
T5(°C)
48.2 39.9 30.1 26.0 23.5
Water outlet temperature, T6(°C)
26.9 25.9 24.8 23.5 22.9
Heater power (W) 829 812 801 796 784
Dp orifice, (Pa) 111 109 107 105 102
Dp column, (Pa) 0 0 0 0 0
CALCULATION Experiment 1
Water flow rate constant = 1.0LPM Variable : heater
Change in temperature for each power supply, ∆T (cooling range) = water inlet temperature,T5 – water outlet temperature, T6 Power = 0.5 kW ∆ T =T5−T6 ¿36.6−25.1 ¿11.5 Power =1.0 kW ∆ T =T5−T6 ¿38.7−25.6
¿13.1 Power =1.5 kW ∆ T =T5−T6 ¿44.9−26.1 ¿18.8 Experiment 2
At water flow rate 0.8 LPM = 0.013 kg/s
Cooling range, ∆T = water inlet temperature,T5 – water outlet temperature,T6 = 48.2 – 26.9
=21.3°C Heat load Q = mCp∆T
= 0.013kg/s x 4.186 x 21.3°C = 1.1591kW
Water flow rate (LPM) Heat load, kW Cooling range, °C 0.8 1.1591 21.3 1.0 0.9962 14.0 1.2 0.4526 5.3 1.4 0.1494 2.5 1.6 0.3302 0.6
DISSCUSSION
From this experiment, the instrument was used in this experiment is Water Cooling Tower HE152 unit. All cooling towers operate on the principle of removing heat from water by evaporating a small portion of the water that is recirculated through the unit. The heat that is removed is called the latent heat of vaporisation. This experiment consist of three experiment. Experiment 1: Investigation of the effect of different the power of heater toward cooling range of cooling tower. Experiment 2: Investigation of the effect of cooling range toward different water flow rate.
There are several term in principle of cooling tower need to be focused when conducting this experiment as a basic knowledge to perform experiment perfectly. First, cooling range. The difference in temperature between the hot water entering the tower and the cold water leaving the tower is the cooling range. Second is approach. The difference between the temperature of the cold water leaving the tower and the wet-bulb temperature of the air is known as the approach. Establishment of the approach fixes the operating temperature of the tower and is a most important parameter in determining both tower size and cost. Others is heat load and wet-bulb temperature. Heat Load is the amount of heat to be removed from the circulating water within the tower. Heat load is equal to water circulation rate (gpm) times the cooling range times 500 and is expressed in BTU/hr. Heat load is also an important parameter
in determining tower size and cost. Wet-Bulb Temperature is the lowest temperature that water theoretically can reach by evaporation. Wet-Bulb temperature is an extremely important parameter in tower selection and design and should be measured by a psychrometer.
For experiment 1, Investigation of the effect of different the power of heater toward cooling range of cooling tower based on three different value of power of heater which is 0.5 kW, 1.0 kW and 1.5 kW give the result of three different cooling range. When we used the value of heater power above it will resulting the value of cooling range 11.5°C, 13.1°C, and 18.8°C respectively. It show that increasing the value of heater power will increase the temperature of cooling range in cooling tower by constant of air blower and constant water flow rate which is 1.0 LPM (liter per minute).
For experiment 2, we have investigate of the effect of cooling range toward different water flow rate. We choose to investigate the relationship between the flow rate and the cooling effect at 0.8 (LPM), 1.0 (LPM), 1.2(LPM), 1.4(LPM) and 1.6(LPM). From the calculation, the value of cooling range where T5-T6 will show the decresing value which is 21.3°C, 14.0°C, 5.3°C, 2.5°C and 0.6°C respectively. From this data we can conclude that the faster water flow rate will give small value of mass transfer which in this term of heat transfer in this cooling system. In this experiment also we can calculate the value of heat load based on different water flow rate. From calculation, the heat load show the value 1.1591 kW, 0.9962 kW, 0.4526 kW, 0.1492 kW and 0.3302 kW respectively. It prove that the heat that released is become decrease when the water flow rate is increase.
CONCLUSION
For the conclusion of this experiment, we can said that this experiment was successfully conducted because the objective of the experiment had achieved. This experiment consist of two part which is experiment 1 and 2. For experiment 1 we can conclude that if the value of heater power is increasing the temperature of the cooling range in the cooling tower will increase. In experiment 2, based on the result obtain, we can conclude that the higher the water flowrate , the lower the energy in the form of heat transfer or released and the higher the power the lower the energy transfer.
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 devices were not working properly. So,be recalibrated or
replaced so that more accurate and timely measurements of humidity can be made. 3. Use appropriate safety PPE when conducting the experiment
4. Make consultation with lab assistance before run the experiment 5. Make sure student know how to use the equipment.
6. Avoid any of mistake and error when conducting the experiment to get best result. Stay alert to the time taken of every ten minutes running.
REFFERENCE
1. http://www.towercomponentsinc.com/operation-cooling-tower.php 2. http://www.baltimoreaircoil.com/english/what-is-evaporative-cooling 3. http://www.matangi.in/principle-operation.html
