theory and experiment procedure

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SOLTEQ ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152) 1.0 INTRODUCTION The SOLTEQ ® Basic Cooling Tower Unit (Model: HE152) has been designed to demonstrate 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 (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, students will then be able to determine the effects on the performance of the cooling tower by the following parameters: a) Temperature and flow rate of water b) Relative Humidity and flow rate of air c) Cooling load Additionally, a Packing Characteristics Column (optional) is available for SOLTEQ ® Basic Cooling Tower Unit (Model: HE152). This column is designed to facilitate 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. 1

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Page 1: Theory and Experiment Procedure

SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

1.0 INTRODUCTION

The SOLTEQ® Basic Cooling Tower Unit (Model: HE152) has been designed to demonstrate 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 (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, students will then be able to determine the effects on the performance of the cooling tower by the following parameters:

a) Temperature and flow rate of waterb) Relative Humidity and flow rate of airc) Cooling load

Additionally, a Packing Characteristics Column (optional) is available for SOLTEQ® Basic Cooling Tower Unit (Model: HE152). This column is designed to facilitate 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.

2.0 GENERAL DESCRIPTIONS

2.1 Components of the HE152 Basic Cooling Tower Unit

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

The unit comes complete with the following main components:

i) Load Tank

The load tank is made of stainless steel having a capacity of approximately 9 litres. The tank is fitted with two cartridge heaters, 0.5 kW and 1.0 kW each, to provide a total of 1.5 kW cooling load. A make-up tank is fixed on top of the load tank. A float type valve at the bottom of the make-up tank is to control the amount of water flowing into the load tank. A centrifugal type pump is supplied for circulating the water from the load tank through a flow meter to the top of the column, into the basin and back to the load tank. A temperature sensor and temperature controller is fitted to load tank to prevent overheating. A level switch is fitted to the load tank so that when a low level condition occurs, the heater and the pump will be switched off.

ii) Air Distribution Chamber

The stainless steel air distribution chamber comes with a water collecting basin and a one-side inlet centrifugal fan. The fan has a capacity of approximately 235 CFM of air flow. The air flow rate is adjustable by means of an intake damper.

iii) Column and Packing

One packed column is available. The column is a standard column that comes together with this unit. The column is made of clear acrylic with a square cross-sectional area of 225 cm2

and a height of 60 cm. It comes with eight decks of inclined packing. A top column that fitted on top of the column comes standard with a sharp edged orifice, a droplet arrester and a water distribution system.

Packed column: 110 m2/m3

iv) Measurements

Temperature sensors are provided to measure the inlet and outlet water temperatures as well as the make-up tank water temperature. In addition, temperature sensors have been installed to measure the dry bulb and wet bulb

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

temperatures of inlet and outlet of the air. The followings show the list of codes assigned to each temperature sensors.

T1 Wet Bulb Temperature of the Outlet AirT2 Dry Bulb Temperature of the Outlet AirT3 Inlet Water Temperature T4 Outlet Water TemperatureT5 Wet Bulb Temperature of the Inlet Air T6 Make-up Tank Water TemperatureT7 Dry Bulb Temperature of the Inlet Air

An inclined manometer is provided for the measurement of pressure drop across the packed column. On the other hand, the inclined manometer and the orifice are also used to determine the air flow rate.

A flow meter is provided for the measurement of water flow rate. The flow meter is ranged at 0.4 to 4 LPM.

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

1. Orifice 6. Air Blower2. Water Distributor 7. Differential Pressure Transmitter3. Packed Column 8. Make-up Tank4. Flow meter 9. Control Panel5. Receiver tank 10. Load tank

2.2 The Process Involved in the Operation

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i) Water Circuit

Water temperature in the load tank will be increased before the water is pumped through a control valve and flow meter to the column cap. Before entering the column cap, the inlet temperature of the water is measured and then the water is uniformly distributed over the top packing deck. This creates a large thin film of water, which is exposed to the air stream. The water gets cooled down, while passing downward through the packing, due to the evaporation process. The cooled water falls into the basin below the lowest deck and return to the load tank where it is re-heated before re-circulation. The outlet temperature is measured at a point just before the water flows back into the load tank. Evaporation causes the water level in the load tank to fall. The amount of water lost by evaporation will be automatically compensated by equal amount from the make-up tank. At steady state, this compensation rate equals the rate of evaporation plus any small airborne droplets discharged with the air.

ii) Air Circuit

A one-side inlet centrifugal fan draws the air from the atmosphere into the distribution chamber. The air flow rate is varied by means of an intake damper. The air passes a dry bulb temperature sensor and wet bulb temperature sensors before it enters the bottom of the packed column. While the air stream passes through the packing, its moisture content increases and the water temperature drops. The air passes another duct detector measuring its exit temperature and relative humidity, then through a droplet arrester and an orifice, and finally leaves the top of the column into the atmosphere.

2.3 Overall Dimensions

Height : 1.25 mWidth : 0.91 mDepth : 0.45 m

2.4 General Requirements

Electricity : 115VAC/1-phase/60HzWater Supply : Laboratory Water Supply

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

3.0 SUMMARY OF THEORY

3.1 Basic Principle

First consider an air stream passing over the surface of a warm water droplet or film. If we assume that 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. 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.

3.2 Evaporation from a Wet Surface

When considering evaporation from a wet surface into the surrounding air, the rate is determined by the difference between the vapour pressure at the liquid surface and the vapour pressure in the surrounding air. The vapour pressure at the liquid surface is basically the saturation pressure corresponding with the surface temperature, whereas the total pressure of the air and its absolute humidity determines the vapour pressure in the surrounding air. Such evaporation process in an enclosed space shall continue until the two vapour pressures are equal. In other words, until the air is saturated and its temperature equals the surface.

However, if unsaturated air is constantly supplied, the wet surface will reach an equilibrium temperature at which the cooling effect due to the evaporation equals the heat transfer to the liquid by conduction and convection from the air, which under these conditions; will be at a higher temperature. Under adiabatic conditions, this equilibrium temperature is the "wet bulb temperature".

For a cooling tower of infinite size and with an adequate air flow, the water leaving will be at the wet bulb temperature of the incoming air. Therefore, 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. Thus, "Approach to Wet Bulb", an important parameter of cooling towers, is the difference between the temperature of the water leaving the tower and the wet bulb temperature of the entering air.

3.3 Cooling Tower Performance

A study on the performance of a cooling tower can be done with the help of a bench top unit. Students shall be able to verify the effect of these factors on the cooling tower performance:

(i) Water flow rates(ii) Water temperatures(iii) Airflow rate

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(iv) Inlet Air Relative Humidity

The effect of these factors will be studied in depth by varying it. In this way, students will gain an overall view of the operation of cooling tower.

3.4 Thermodynamic Property

In order to understand the working principle and performance of a cooling tower, a basic knowledge of thermodynamic is essential to all students. A brief review on some of the thermodynamic properties is presented below.

At the triple point (i.e. 0.00602 atm 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

h=h f+v f ( p−psat ) (1)

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.

Specific heat capacity (Cp) is defined as the rate of change of enthalpy with respect to temperature (often called the specific heat at constant pressure). For the purpose of experiment using bench top cooling tower, we may use the following relationship:

Δh=C p ΔT (2)

and

h=C pT (3)

Where Cp = 4.18 kJ.kg-1

3.4.1 Dalton’s and Gibbs Laws

It is commonly known that air consists of a mixture of "dry air" (O2, N2 and other gases) and water vapour. Dalton and Gibbs law describes the behaviour of such a mixture as:

a) The total pressure of the air is equal to the sum of the pressures at which the "dry air" and the water vapour each and alone would exert if they were to occupy the volume of the mixture at the temperature of the mixture.

b) The dry air and the water vapour respectively obey their normal property relationships at their partial pressures.

c) The enthalpy of the mixture may be found by adding together the enthalpies at which the dry air and water vapour each would have as the sole occupant of the space occupied by the mixture and at the same temperature.

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

The Absolute or Specific Humidity is defined as follows:

Specific Humidity , ω=Mass of Water VapourMass of Dry Air (4)

The Relative Humidity is defined as follows:

Re lative Humidity , φ

=Partial Pr essure of Water Vapour in the AirSaturation Pr essure of Water Vapour at the same temperature (5)

The Percentage Saturation is defined as follows:

Percentage Saturation

=Mass of Water Vapour in a given Volume of AirMass of same vol of Sat Water Vapour at the same Temp (6)

At high humidity conditions, it can be shown that there is not much difference between the "Relative Humidity" and the "Percentage Saturation" and thus we shall regard as the same.

To measure the moisture content of the atmosphere, this bench top cooling tower unit is supplied with electronic dry bulb and wet bulb temperature sensors. The temperature readings shall be used in conjunction with a psychrometric chart.

3.4.2 Psychrometric Chart

The psychrometric chart is very useful in determining the properties of air/water vapour mixture. Among the properties that can be defined with psychrometric chart are Dry Bulb Temperature, Wet Bulb Temperature, Relative Humidity, Humidity Ratio, Specific Volume, and Specific Enthalpy. Knowing two of these properties, any other property can be easily identified from the chart provided the air pressure is approximately atmospheric.

In the Bench Top Cooling Tower application, the air inlet and outlet sensor show the dry bulb temperature and wet bulb temperature. Therefore, the specific enthalpy, specific volume, humidity ratio and relative humidity can be readily read from the psychrometric chart.

The psychrometric chart provided with this manual is only applicable for atmospheric pressure operating condition (1.013 bar). However, the error resulting from variation of local atmospheric pressure normally is negligible up to altitudes 500m above sea level.

3.5 Orifice Calibration

As mentioned above, the psychrometric chart can

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

be used to determine the value of the specific volume. However, the values given in the chart are for 1 kg of dry air at the stated total pressure.

However, for every 1 kg of dry air, there is w kg of water vapour, yielding the total mass of 1 + w kg.

Therefore, the actual specific volume of the air/vapour mixture is given by:

va=vab1+ϖ (7)

The mass flow rate of air and steam mixture through the orifice is given by

m=0 .0137√ xva (8)

Where,m = Mass flow rate of air/vapour mixtureva = Actual specific volume and x = Orifice differential in mmH20.

Thus,

m=0 .0137√ x (1+ϖ )vab (9)

The mass flow rate of dry air,

ma=11+ϖ

×Mass flow rate of air /vapor mixture

ma=11+ϖ

×0 .0137√ x (1+ϖ )vab

ma=0 .0137√ xvab

(1+ϖ )(10)

A simplification can be made since in this application, the value of ϖ is unlikely to exceed 0.025. As such, neglecting wb would not yield significant error.

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

3.6 Application of Steady Flow Energy Equation

Consider System A for the cooling tower defined as in Figure 1. It can be seen that for this system, indicated by the dotted line,

a) Heat transfer at the load tank and possibly a small quantity to surroundingsb) Work transfer at the pumpc) Low humidity air enters at point Ad) High humidity air leaves at point Be) Make-up enters at point E, the same amount as the moisture increase in the

air stream

Figure 2: System A

From the steady flow equation,

Q−P = H exit−H entry

Q−P = ( mahda+m shs )B−(mahda+mshs)A−mEhE

(11)

Note: The pump power, P is a work input. Therefore it is negative.

If the enthalpy of the air includes the enthalpy of the steam associated with it, and

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A

B

E

ma

ma

mE

Work, P Heat, Q

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

this quantity is in terms of per unit mass of dry air, the equation may then be written as:Q−P = ma (hB−hA )−mEhE (12)Note:

a) The mass flow rate of dry air (ma ) through a cooling tower is a constant, whereas the mass flow rate of moist air increases as the result of evaporation process.

b) The term mEhE can usually be neglected since its value is relatively small.

Under steady state conditions, by conservation of mass, the mass flow rate of dry air and of water (as liquid or vapour) must be the same at inlet and outlet to any system.

Therefore,

( ma )A = ( ma)B (13)

and

( ms)A+mE=( ms )B or

mE=(m s)B−(m s)A (14)

The ratio of steam to air (ϖ ) is known for the initial and final state points on the psychrometric charts. Therefore,

( ms)A=maϖA and (15)

( ms)B=maϖB (16)

Therefore,

mE=ma (ϖB−ϖA ) (17)

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Say, we re-define the cooling tower system to be as in Figure 2 where the process heat and pump work does not cross the boundary of the system. In this case warm water enters the system at point C and cool water leaves at point D.

Figure 3: System B

Again from the steady flow energy equation,

Q−P = H exit−H entry and

P =0

Q may have a small value due to heat transfer between the unit and its surroundings.

Q=mahB+mwhD−( mahA+mwhC+mEhE ) (18)

Rearranging,

Q=ma (hB−h A )+mw (hD−hC)−mEhE

=ma (hB−hA )+mwCp (tD−tC )−mE hE (19)

Again, the term mEhE can be neglected.

12

B ma

Cmw

A

ma

D

mE

E

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

3.7 Characteristics Column Study

In order to study the packing characteristics, we define a finite element of the tower (dz) as shown in Figure 3, the energy balances of the water and air streams in the tower are related to the mass transfer by the following equation:

C pWmW dT=K a dV (Δh )

(20)

where

C pW = Specific heat capacity of watermW = Mass flow rate of water per unit plan area of packingT = Water TemperatureK = Mass Transfer Coefficient a = Area of contact between air and water per unit volume of packingV = Volume occupied by packing per unit plan areaΔh = Difference in specific enthalpy between the saturated boundary layer and

the bulk air

Figure 4: Schematic Representation of the Air and Water Streams entering and leaving a Block of Packing

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 Equation 20,

K a dVmW

=C pW

dT

Δh (21)

Integrating Equation 21,

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z

WATER

INLET

WATER

OUTLET

AIR

OUTLET

AIR

INLET

dz

T2

H2

mw

T1

H1

mw

t2

h2

ma

t1

h1

ma

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

Ka VmW

=C pW∫T 1

T 2dT

hw−ha (22)

The numerical solution to the integral expression Equation 22 using Chebyshev numerical method gives,

Ka VmW

=C pW∫T 1

T 2dT

hw−ha

=T 2−T 14 ( 1Δh1 + 1

Δh2+ 1Δh3

+ 1Δh4 ) (23)

Where

Ka VmW = Tower CharacteristicΔh1 = value of hw−ha at T 2+0 .1(T1−T 2)Δh2 = value of hw−ha at T 2+0 .4 (T1−T 2)Δh3 = value of hw−ha at T 1−0.4 (T 1−T 2)Δh4 = value of hw−ha at T 1−0.1 (T1−T 2)

Thermodynamics state that the heat removed from the water must be equal to the heat absorbed by the surrounding air. Therefore, the following equation is derived:

L (T 2−T 1)=G (ha 2−ha 1) (24)

or,

LG

=(ha 2−ha 1)(T 2−T 1) (25)

Where,LG = Liquid to gas mass flow ratioT 1 = Cold water temperatureT 2 = Hot water temperatureha2 = Enthalpy of air-water vapour mixture at exhaust wet-bulb temperatureha1 = Enthalpy of air-water vapour mixture at inlet wet-bulb temperature

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Figure 5: Graphical Representation of Tower Characteristics

The following represents a key to Figure 5:

BA = Initial enthalpy driving forceAD = Air operating line with slope L/G

Referring to Equation 22, the tower characteristics could be found by finding the area between ABCD in Figure 4. Increasing heat load would have the following effects on the diagram in Figure 4:

1. Increase in the length of line CD, and a CD line shift to the right2. Increase in hot and cold water temperatures3. Increase in range and approach areas

The increased heat load causes the hot water temperature to increase considerably faster than does the cold water temperature. Although the area ABCD should remain constant, it actually decreases about 2% for every 10 0F increase in hot water temperature above 100 0F. To account for this decrease, an "adjusted hot water temperature" is used in cooling tower design.

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Air Operating Line

EnthalpyDriving Force(hw-ha)

hw2 (Hot water Temp)

hw1 (Cold water Temp)

ha2 (Air out)

ha1 (Air in)

Water Operating LineC

Approach Range

Temperature

Twb (In) T1 Twb (Out) T2

L/GA

D

B

Saturation Curve

Enthalpy

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

3.8 Useful Information

1. Orifice Calibration Formula:

Mass flow rate of air and vapour mixture,

m=0 .0137√ x (1+ϖ )vab

The mass flow rate of dry air,

ma=0 .0137√ xvab

(1+ϖ )

Where,

x = orifice differential in mmH20, vaB = specific volume of air at the outlet ϖ = humidity ratio of the mixture

2. Pump Work Input = 80W (0.08kW)

3. Column Inner Dimension = 150 mm x 150 mm x 600 mm

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4.0 EXPERIMENTAL PROCEDURES

4.1 General Operating Procedures

4.1.1 General Start-up Procedures

1. Check to ensure that valves V1 to V8 are closed.2. Fill the load tank with distilled or deionised water. It is done by first

removing the make-up tank and then pouring the water through the opening at the top of the load tank. Replace the make-up tank onto the load tank and lightly tighten the nuts. Fill the tank with distilled or deionised water.

3. Add distilled/deionised water to the wet bulb sensor reservoir to the fullest.

4. Connect all appropriate tubing to the differential pressure sensor.5. Install the appropriate cooling tower packing for the experiment.6. Then, set the temperature set point of temperature controller to 50°C.

Switch on the 1.0 kW water heater and heat up the water until approximately 40°C.

7. Switch on the pump and slowly open the control valve near the flow meter 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.

8. Fully open the fan damper, and then switch on the fan. Check that the differential pressure sensor is giving reading when the valve manifold is switched to measure the orifice differential pressure.

9. Let the unit run for about 20 minutes, for the float valve to correctly adjust the level in the load tank. Refill the makeup tank when required.

10. Now, the unit is ready for use.

Note: i. It is strongly recommended that ONLY distilled or deionised water be

used in this unit. The impurities existing in tap water may cause the depositing in cover tower.

ii. Check that the pressure tubing for differential pressure measurement is connected correctly. (Leave V3 to atmosphere; connect the column’s higher pressure tube to V4, orifice pressure tube to V5 and column’s lower pressure tube to V6.)

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

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

v. Always make sure that no water is in the pressure tubing for accurate differential pressure measurement.

4.1.2 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 is cooled down.

2.Switch off the fan and fully close the fan damper.

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3.Switch off the pump and power supply.4.Retain the water in reservoir tank for the following experiment.5.Completely drain off the water from the unit if it is not in use.

4.2 Experiment 1: General Observation of the Forced Draught Cooling Tower

Objective:To observe the process within a forced draught cooling tower

1. Perform the general start-up procedures and observe the forced draught cooling tower process.

2. As the warm water enters the top of the tower, it is fed into channels from which it flows via water distribution system onto the packing. The channels are designed to distribute the water uniformly over the packing with minimum splashing.

3. The packing surfaces are easily wetted and the water spreads over the surfaces to expose a large area to the air stream.

4. The cooled water falls from the lowest packing into the basin and then is pumped to the simulated load in the load tank.

5. During the process, some water is lost due to the evaporation. Thus, "make-up" water must be supplied to keep the amount of water in the cooling system constant. The make-up is observed flowing past the float-controlled valve in the load tank.

6. A “droplet arrester”, or “mist eliminator” is fitted at the tower outlet to minimize loss of water due to escape of droplets of water (resulted from splashing, etc.) which is entrained in the air stream. This loss does not contribute to the cooling, but must be made good by "make-up". The droplet arrester causes droplets to coalesce, forming drops that are too large to be entrained and thus the droplets fall back into the packing.

7. The fan drives the air upward through the wet packing. At air outlet, the air leaving the cooling tower is almost saturated, i.e. Relative Humidity is ~100%. The Relative Humidity at the air outlet is much higher than the Relative Humidity at the air inlet. The increase in the moisture content of air is due to the evaporation of water into steam and the "latent heat" for this account for most of the cooling effect.

8. When the cooling load is switched off and the unit is allowed to stabilize, it is found that the water leaves the basin at temperature close to the wet bulb temperature of the air entering. Wet bulb temperature is lower than the dry bulb temperature and this varies according to the local atmospheric conditions (i.e. pressure and relative humidity).

9. With no load, the water would be cooled to the incoming wet bulb temperature. However, the condition cannot be achieved since the work done by the pump transfers about 80W to the water.

4.3 Experiment 2: End State Properties of Air and Steady Flow Equations

Objective:To determine the “end state” properties of air and water from tables or chartsTo determine Energy and mass balances using the steady flow equation on the selected systems

Procedure:1. Prepare and start the cooling tower with according to Section 4.1.1.

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2. Set the system under the following conditions and allow stabilizing for about 15 minutes.Water flow rate : 2.0 LPMAir Flow : MaximumCooling load : 1.0 kW

3. Fill up the make-up tank with distilled water, record the initial water level and then start the stop watch.

4. Determine the make-up water supply in an interval of 10 minutes.5. In this 10 minutes interval, record a few sets of the measurements (e.g.

temperatures (T1–T7), orifice differential pressure (DP1), water flow rate (FT1) and heater power (Q1)), then obtain the mean value for calculation and analysis.

6. Determine the quantity of make-up water that has been supplied during the time interval by noting the height reduction in the make-up tank.

7. The observation may be repeated at different conditions:i. Different water flow ratesii. Different air flow ratesiii. Different load

Assignment:1. Calculate the make-up rate.2. Calculate the energy and mass balances by using the steady flow equation.

4.4 Experiment 3: Investigation of the Effect of Cooling Load on Wet Bulb Approach

Objective:To investigate the effect of cooling load on “Wet Bulb Approach”

Procedure:1. Prepare and start the cooling tower with according to Section 4.1.1.2. Set the system under the following conditions and allow stabilizing for about 15

minutes.Water flow rate : 2.0 LPMAir Flow : Maximum

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Cooling load : 0.5 kW 3. After the system stabilizes, record a few sets of measurements (e.g. air inlet

wet bulb and dry bulb temperature (T5 and T7), water outlet temperature (T4), orifice differential pressure (DP1), water flow rate (FT1) and heater power (Q1)), then obtain the mean value for calculation and analysis.

4. Without changes in the conditions, increase the cooling load to 1.0 kW. When the system stabilized, record all data.

5. Similarly, repeat the experiment at 1.5 kW.6. Finally, measure the cross sectional area of the column.7. The four tests may be repeated at another constant airflow. 8. The experiment may also be repeated at different:

i. Water flow ratesii. Air flow ratesiii. Load

Assignment:1. Calculate the “approach to wet bulb” and total cooling load.2. Plot a graph to show that the relationship between cooling loads and

approach to wet bulb temperature.

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4.5 Experiment 4: Investigation of the Effect of Air Velocity on Wet bulb Approach and Pressure Drop through the Packing

Objective:To investigate the effect of air velocity on:(a) Wet Bulb Approach(b) The pressure drop through the packing

Procedure:1. Prepare and start the cooling tower with according to Section 4.1.1.2. Set the system under the following conditions and allow stabilizing for about 15

minutes.Water flow rate : 2.0 LPMAir flow rate : MaximumCooling load : 1.0 kW

3. After the system stabilizes, record a few sets of measurements (i.e. temperature (T1-T5 and T7), orifice differential pressure (DP1), water flow rate (FT1), heater power (Q1) and pressure drop across packing (DP2)), then obtain the mean value for calculation and analysis.

4. Repeat the test with 3 different sets of orifice pressure drop values (75%, 50% and 25% of the maximum value) without changing the water flow rate and cooling loads. This can be done by adjusting the opening of the fan damper.

5. Finally, measure the cross sectional area of the column.6. The test may be repeated at another constant:

i. Loadii. Water flow rate

Assignment:1. Calculate the nominal velocity of air and find the “approach to wet bulb”.2. Plot a graph to show that the relationship between “approach to wet bulb” and

packing pressure drop versus nominal air velocity.

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

4.6 Experiment 5: Investigation of the Relationship between Cooling Load and Cooling Range

Objective:To investigate the relationship between cooling load and cooling range

Procedure:1. Prepare and start the cooling tower with according to Section 4.1.1.2. Set the system under the following conditions and allow stabilizing for about 15

minutes:Water flow rate : 2.0 LPMAir flow rate : MaximumCooling load : 0.5 kW

3. After the system stabilized, record a few sets of measurements (e.g. temperature (T1-T5 and T7), orifice differential pressure (DP1), water flow rate (FT1) and heater power (Q1)), then obtain the mean value for calculation and analysis

4. Without changes in the conditions, increase the cooling load to 1.0 kW. When the system stabilized, record all data.

5. Similarly, repeat the experiment at 1.5 kW.6. The tests may be repeated at other constant:

i. Water flow rateii. Air flow rate

Assignment:1. Plot a graph to show that the relationship between cooling loads and cooling

range.

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SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)

5.0 REFERENCES

Perry, R.H., Green, D.W. and Maloney, J.O., “Perry’s Chemical Engineering Handbook”, 6th Edition, McGraw Hill, 1984.

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