THEORY

The theory behind the cooling tower operation is the First Law of Thermodynamics, which states that the energy can neither be created nor destroyed. It can only change from one form to another. The energy entering the cooling tower is in the form of hot water. The hot water is cooled from its initial temperature, T1 to a lower temperature, T2. The method of cooling the hot water is by a forced convection, where ambient air at T3 was blown at the hot water. An energy balance is done for each temperature recorded. The most important component, the enthalpy, is defined as H = U + PV. There are various factors affecting the readings obtained. The factors are the energy input, blower power and the water flow rate. The effects of these factors are studied by varying it. By doing so, students will gain an overall view of the operation of cooling tower.In order to understand the working principle and performance of a cooling tower, a basic knowledge of thermodynamics is essential. At the triple point, the specific enthalpy of saturated water is assumed to be zero. The specific enthalpy of compressed liquid is given as . The correction pressure for the operating condition of a cooling tower is negligible, thus, h is approximately equal to hf at given temperature. Specific heat capacity (Cp) is defined as the rate of change of enthalpy with respect to the temperature. For the purpose of experiment using bench top cooling tower, the relationship used is where Specific humidity is defined as the mass of water vapor over the mass of dry air. The relative humidity is defined as the partial pressure of water vapour in the air divided by the saturation pressure of water vapour at the same temperature, while the percentage saturation is the mass of water vapour in a given volume of air over the mass of same volume of saturated vapour at the same temperature.Cooling towers use the evaporative cooling principle to cool the circulated water, and they can achieve water temperatures below thedry bulb temperature-tdb-of the air cooling air, or they are in general smaller and cheaper for the same cooling loads than other cooling systems.

Cooling towers are rated in terms ofapproachandrange,where;a) theapproachis the difference in temperature between the cooled-water temperature and the entering-airwet bulb - twbtemperature, and;b) therangeis the temperature difference between the water inlet and exit statesSince a cooling tower is based on evaporative cooling the maximum cooling towerefficiencyis limited by thewet bulb temperature.The efficiency of the cooling tower is limited by the wet bulb temperature-twb-of the cooling air since it is based on the evaporative cooling. The cooling tower efficiency can be expressed as;

where= cooling tower efficiency - common range between 70 - 75%ti= inlet temperature of water to the tower (oC)to= outlet temperature of water from the tower (oC)twb= wet bulb temperature of air (oC)The temperature difference between inlet and outlet water(ti- to)is normally in the range10 - 15oF.

APPARATUS AND MATERIALS

Figure 1: SOLTEQ Model HE1521. SOLTEQ Model HE 152, bench top cooling water unit2. Deionised water3. Stopwatch

PROCEDURE

General start up procedures

1. Valve V1 to V6 were checked so that they are closed and valve V7 was partially opened.2. The load tank was filled with deionised water. The make up tank was removed and water was poured in the load tank through the opening at the top. The make up tank was replaced onto the load tank and all nuts were placed.3. Deionised water was added in the wet bulb sensor 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. The temperature set point was set of temperature controller of 50C.7. The pump was switched on and the control valve V1 was slowly opened and water flow rate was set to 2.0 LPM.8. The fan damper was fully opened, and the fan was switched on. The differential pressure sensor was checked whether it giving reading when the valve manifold was switched to measure the orifice differential pressure.9. The unit was run for about 20 minutes and the float valve was adjusted to the correct level in the load tank.10. The unit was ready to use.

Note:1) The pressure measurement was ensured to be connected correctly. (Orifice pressure tapping point to V4: columns lower pressure tapping point to V6: columns higher pressure tapping point to V3: V5 leave to the atmosphere).2) The differential pressure was measured across the orifice, valve V4 and V5 were opened, and valves V3 and V5 were closed.3) The differential pressure was measured across the column, valve V3 and V6 were opened, and valves V4 and V5 were closed.4) It was ensured that there was no water is in the pressure tubing for accurate differential pressure measurement.

General shut down procedures

1. The heater was switched off and water was let to circulate through the cooling tower system for 3-5 minutes until the water cooled down.2. The fan was switched off and the fan damper was fully closed.3. The pump and the water supply were switched off.4. The water in the reservoir was retained for the next experiment.5. Water was completely drained off from the unit if it is not used.

First Experiment:

1. The cooling tower was prepared and started.2. The system was set under the following conditions and was stabilized for 15 minutes;

Water flow rate : 1.0 LPM.Damper Air Inlet : Fully openedCooling load : 0.5 kWColumn : A

3. After the system stabilized, a few sets of measurement were recorded. They were temperatures (T1-T6), orifice differential pressure (DP1), water flow rate (FT1) heater power (Q1) and pressure drop across packing (DP3), and then obtain the mean value for calculation and analysis.4. The test was repeated with 3 different condition of the damper (half open and fully closed) without changing the water flow rate and cooling loads.5. Finally, the cross sectional area of the column was measured.

Second Experiment:

1. The cooling tower was prepared and started.2. The system was set under the following conditions and was stabilized for 15 minutes;

Water flow rate : 1.0 LPM.Damper Air Inlet : Fully OpenCooling load : 0.5 kWColumn : A

3. After the system stabilized, a few sets of measurement were recorded. They were temperatures (T1-T6), orifice differential pressure (DP1 , water flow rate (FT1) heater power (Q1) and pressure drop across packing(DP3), then the mean value was obtained for calculation and analysis.4. Without changing in the condition, the cooling load was increased to 1.0 kW. When the system stabilized, all data were recorded.5. Similarly, the experiment was repeated at 1.5kW.