Cooling Tower T P A
Content
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 changes 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.
Thermodynamics Properties
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 h=hf + vf (ppsat). 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 dh=CpdT where Cp=4.18kJ/kg. 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..
Psychometric Chart
The psychometric chart is a very useful chart used to determine the properties of air and water mixture. Example of properties that can be defined from this chart are dry bulb temperature, wet bulb temperature, relative humidity, humidity ratio, specific volume, and specific enthalpy. The
psychometric chart is applicable for condition at atmospheric pressure. However, the error resulting from the variation of local atmospheric pressure normally is negligible.
Orifice Calibration
As mentioned earlier, the psychometric chart can be used to determine the value of specific volume. However, the values given in the chart are for 1kg of dry air at the stated total pressure. For every 1kg of dry air, there is w kg of water vapour. Thus, the total mass is 1+w kg. Therefore, the actual specific volume of the air and water vapour mixture is flow rate of steam and air throughout the mixture is given by, √ . The mass flow rate of dry air, √ . √ √ . The mass . Thus,
X Mass flow rate of
air/vapour mixture =
Application of Steady Flow Equation
From the steady flow equation, Q – P = Hexit - Hentry Q – P = (mahda + mshs)a – (mahda + mshs)A –mEhE If the enthalpy of the iarincludes the enthalpy of the steam and the quantity is in terms of per unit mass of dry air, the equation may then be written as: Q – P = ma (hB – hA) – mEhE Under steady conditions, (ma)A = (ma)B and mE = (ms)B – (ms)A The ratio of steam to air is known for the initial and final state points from the psychometric charts. Therefore, (ms)A = mawA (ms)B = mawB
Thus, mE = ma (wB – wA)
Characteristics Column Study
In order to study the packing characteristics, we define a finite element of the tower (dz). The energy balances of the water and air streams in the tower are related to the mass transfer by the equation: CpwmwdT = KadV (∆h) Where, Cpw = Specific heat capacity of water mw = Mass flow rate of water per unit plan area of packing T = Water temperature K = Mass transfer coefficient a = Area of contact between air and water per unit volume of packing V = Volume occupied by packing per unit plan area ∆h = Difference in specific enthalpy, between the saturated boundary layer and the bulk air
In this equation, we assume that the boundary layer temperature is equal to the water temperature, T and the small change in the mass of water is neglected. Thus, from the above equation, we get:
By integrating this equation and foolowing the law of thermodynamics, the following equation is derived:
or
Where, L/G = Liquid to gas mass flow ratio T1 = Cold water temperature T2 = Hot water temperature ha1 = Enthalpy of air-water vapour mixture at exhaust wet-bulb temperature ha2 = Enthalpy of air-water vapour mixture at inlet wet-bulb temperature
PROCEDURES
General Start-Up Procedures 1. 2. 3. 4. 5. 6. 7. Ensure that valves V1 to V6 are closed and valve V7 is partially opened. Fill the load tank with distilled or deionised water. Fill the make-up tank with distilled or deionised water up to the zero mark on the scale. Add distilled/deionised water to the wet bulb sensor reservoir to the fullest. Install the appropriate cooling tower packing for the experiment. Connect all appropriate tubing to the differential pressure sensor. Then, set the temperature set point of temperature controller to 45o C. Switch on the 1.0 kW water heater and heat up the water until approximately 40o 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 : 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 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 Procedures 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.
Apparatus And Materials 1. Bench Top Cooling Tower Unit (HE152) 2. Deionised water
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 changes 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.
Thermodynamics Properties
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 h=hf + vf (ppsat). 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 dh=CpdT where Cp=4.18kJ/kg. 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..
Psychometric Chart
The psychometric chart is a very useful chart used to determine the properties of air and water mixture. Example of properties that can be defined from this chart are dry bulb temperature, wet bulb temperature, relative humidity, humidity ratio, specific volume, and specific enthalpy. The
psychometric chart is applicable for condition at atmospheric pressure. However, the error resulting from the variation of local atmospheric pressure normally is negligible.
Orifice Calibration
As mentioned earlier, the psychometric chart can be used to determine the value of specific volume. However, the values given in the chart are for 1kg of dry air at the stated total pressure. For every 1kg of dry air, there is w kg of water vapour. Thus, the total mass is 1+w kg. Therefore, the actual specific volume of the air and water vapour mixture is flow rate of steam and air throughout the mixture is given by, √ . The mass flow rate of dry air, √ . √ √ . The mass . Thus,
X Mass flow rate of
air/vapour mixture =
Application of Steady Flow Equation
From the steady flow equation, Q – P = Hexit - Hentry Q – P = (mahda + mshs)a – (mahda + mshs)A –mEhE If the enthalpy of the iarincludes the enthalpy of the steam and the quantity is in terms of per unit mass of dry air, the equation may then be written as: Q – P = ma (hB – hA) – mEhE Under steady conditions, (ma)A = (ma)B and mE = (ms)B – (ms)A The ratio of steam to air is known for the initial and final state points from the psychometric charts. Therefore, (ms)A = mawA (ms)B = mawB
Thus, mE = ma (wB – wA)
Characteristics Column Study
In order to study the packing characteristics, we define a finite element of the tower (dz). The energy balances of the water and air streams in the tower are related to the mass transfer by the equation: CpwmwdT = KadV (∆h) Where, Cpw = Specific heat capacity of water mw = Mass flow rate of water per unit plan area of packing T = Water temperature K = Mass transfer coefficient a = Area of contact between air and water per unit volume of packing V = Volume occupied by packing per unit plan area ∆h = Difference in specific enthalpy, between the saturated boundary layer and the bulk air
In this equation, we assume that the boundary layer temperature is equal to the water temperature, T and the small change in the mass of water is neglected. Thus, from the above equation, we get:
By integrating this equation and foolowing the law of thermodynamics, the following equation is derived:
or
Where, L/G = Liquid to gas mass flow ratio T1 = Cold water temperature T2 = Hot water temperature ha1 = Enthalpy of air-water vapour mixture at exhaust wet-bulb temperature ha2 = Enthalpy of air-water vapour mixture at inlet wet-bulb temperature
PROCEDURES
General Start-Up Procedures 1. 2. 3. 4. 5. 6. 7. Ensure that valves V1 to V6 are closed and valve V7 is partially opened. Fill the load tank with distilled or deionised water. Fill the make-up tank with distilled or deionised water up to the zero mark on the scale. Add distilled/deionised water to the wet bulb sensor reservoir to the fullest. Install the appropriate cooling tower packing for the experiment. Connect all appropriate tubing to the differential pressure sensor. Then, set the temperature set point of temperature controller to 45o C. Switch on the 1.0 kW water heater and heat up the water until approximately 40o 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 : 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 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 Procedures 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.
Apparatus And Materials 1. Bench Top Cooling Tower Unit (HE152) 2. Deionised water
