heat lab manual

Department of Chemical Engineering

ChE 442

Heat and Mass Transfer Laboratory

Laboratory Manual

Dr. Rami Jumah

http://www.just.edu.jo

September 2010

1

Table of Contents

Experiment # Title Page

1 Axial Heat Conduction in Solids 3

2 Radial Heat Conduction in Solids 16

3 Thermal Conductivity of Liquids and Gases 26

4 Saturation Pressure and Steam Quality 36

5 Diffusion of a Liquid 44

6 Forced Convection Heat Transfer from Flat, Finned and

Pinned Plates

53

7 Shell and Tube Heat Exchanger 62

8 Boiling and Condensation Heat Transfer 69

9 Convective Mass Transfer 88

2

REPORT WRITING

All reports in the Heat Transfer Laboratory require a formal laboratory report unless specified

otherwise. The reports should be simple and clearly written and free of spelling and grammatical

errors. Following is a summary of the key elements in a well written, formal laboratory report.

Details regarding each of these elements are also provided.

TITLE PAGE Experiment Number, Title of the Experiment, Name of the

Author, Name of Partners, the name of the instructor to whom

the report is submitted, Date the Experiment was Performed,

Due date for this Report.

ABSTRACT Brief summary of the objectives of the experiment, the most

important results obtained and do the results compare with the

theory. Main conclusions and recommendations. An abstract should not exceed two-thirds of a page.

INTRODUCTION Description of the problem, references, objectives.

THEORY AND ANALYSIS

Theory associated with this experiment, equation derivation. Sources of equations and derivations should be referenced and the

equations must be numbered.

EXPERIMENTAL:

SET-UP AND PROCEDURES

Describes the equipment used, equipment setup, model and/or

serial numbers, experimental procedure.

RESULTS AND DISCUSSION Sample Calculations: Complete step by step calculations using the

original data and showing the transformation of the experimental

data into calculated results.

Results should be presented in neat tabular or graphical form and

must be discussed and if possible compared with expected results. Sources of errors, if any, and their relation to the obtained data must

be stated. Different paragraphs must be used to discuss different

tables or graphs. Always refer to figures number when discussing

this figure.

CONCLUSIONS AND

RECOMMENDATIONS

Conclusions, observations, trends, and recommendations.

APPENDICES References, original data sheet, calibration curves.

3

Experiment # 1

Axial Heat Conduction in Solids

OBJECTIVES

1. To determine the steady-state temperature distribution along a simple thermally

conducting sample with uniform cross-sectional area and composition.

2. To study the conduction of heat along a composite bar and evaluate the overall heat

transfer coefficient.

3. To determine the thermal conductivity of a number of metallic materials.

4. To observe unsteady steady state conduction of heat.

5. To compare the experimental measurements to textbook or theoretical results and to

discuss and explain any differences.

INTRODUCTION

Conduction is the transfer of thermal energy from the more energetic particles of a substance to

the adjacent less energetic ones as a result of interactions between the particles. Conduction can

take place in solids or stationary fluids. In gases and liquids, conduction is due to the collisions

and diffusion of the molecules during their random motion. In solids, conduction of heat in solids

occurs by one of several mechanisms, depending upon the type of solid. In metals the primary

mode of heat conduction is due to the energy transport by free electrons in the valence bond of

the atoms. This is the same mechanism that is responsible for electrical conduction, so it is no

coincidence that good conductors of heat are also good conductors of electrical current. To a

lesser degree, heat is conducted in metals by vibratory motion of the atomic lattice structure. For

nonmetallic solids having a crystalline structure, such as quartz, the primary conduction

mechanism is the vibratory motion of the atomic lattice structure. In amorphous solids, i.e.,

solids that do not have a crystalline structure, such as plastic and rubber, heat conduction occurs

by random molecular collisions.

4

Fourier‟s law of heat conduction is given as:

𝑞 = −𝑘𝐴𝑑𝑇

𝑑𝑥 (1)

Where q (W) is the heat transfer rate in the x direction, A is the area normal the direction of heat

transfer, dT/dx is the temperature gradient, and k (W/m.K) is thermal conductivity of the

material, which is a measure of the ability of the material to conduct heat.

For a steady state, axisymmetric system with no generation, the heat conduction equation is

0z

Tk

zr

Tkr

rr

1

(2)

For purely axial conduction, temperature does not change in the radial direction and the relevant

governing equation is:

(3)

with corresponding boundary conditions requiring that T (z = 0) = T1 and T (z = L) = T2.

Integrating equation (3) results in a linear temperature profile:

112

TL

zTTzT (4)

Fourier's Law relates the heat transfer rate to the temperature profile as:

(5)

For steady heat transfer through a plane wall, the thermal resistance for conduction is therefore

given by

(6)

0

z

Tk

z

21

TTL

kA

z

TkAq

z

kA

L

q

TTR

z

cond

21

5

One-dimensional conduction through more complex systems, such as composite bars, is easily

analyzed using the thermal resistance concept. From the definition of thermal resistance, the

heat transfer rate qz can be expressed in terms of the overall temperature difference ΔToverall. For

example, for three successive slabs A, B and C,

(7)

For steady flow along the bar, the heat flow through successive slabs is the same for reasons of

continuity and hence from Fourier's Law:

(8)

The heat transfer rate can be expressed in terms of the overall heat transfer coefficient as follows:

overallz

TUAq (9)

A comparison of equations (7) and (9) reveals that

R

UA

1

(10)

Ak

L

Ak

L

Ak

L

T

R

Tq

C

C

B

B

A

A

overalloverall

z

C

CC

B

BB

A

AAz

L

Tk

L

Tk

L

Tk

A

q

6

EQUIPMENT

Measurements of axial conduction heat transfer will be obtained using an experimental setup

consisting of three components (Figures 1 and 2): a "P.A. Hilton Axial Heat Conduction Unit

(H111A)" for the study of one-dimensional conduction through solids, a universal “Heat

Transfer Service Unit (H111)”, and a computerized “Data Logger (HC111A)”.

The Axial Heat Conduction Unit (H111A) is cylindrical and mounted with its axis vertical to the

base plate. The test section consists of three segments: a heating brass section on top; a cooling

brass section on the bottom; and an intermediate removable sample. The heating section, cooled

section, and all intermediate sections are co-axially aligned inside plastic housings which provide

an air gap and insulate the section to minimize heat loss to the surroundings. The heated and

cooled sections incorporate centralizing “O” rings to ensure that each are held concentrically.

Similar “O” rings are fitted to the intermediate sections so that are installed in alignment. A pair

of toggle clamps ensures that the sections are held tightly together when in use. Loosen the

clamp tensioning screw before releasing the toggle clamps and re-apply tension after fitting a

new specimen, thus avoiding over-stressing the clamping device.

The heating section houses a 25 mm diameters brass rod with a nominally 65 Watt (at 240 V

AC) cartridge heater in the top end. An integral high temperature cut out (automatic reset)

prevents overheating. Power is supplied to the heater from the Heat Transfer Service Unit

(H111) via the 8-pole plug and lead. Three thermocouples (T1, T2 and T3) are positioned along

the heated section at uniform intervals of 15 mm.

The cooling section is also manufactured from 25 mm diameter brass rod to match the heated top

section and is cooled at its bottom end by water flowing through a chamber in the material.

Water flow rate can be regulated by manual control from a local tap and can be measured with

the attached flow rate sensor. The heating and cooling sections consist of 25 mm-diameter brass

rods. Three thermocouples (T6, T7 and T8) are positioned along the cooling section at uniform

intervals of 15 mm. These thermocouples measure the temperature gradient along the section to

which they are attached. Each type K thermocouple is fitted with a miniature plug for direct

connection to the front panel of the heat transfer service unit (H111) and an edge connector for

use with the HC111A Data Logger.

Four intermediate sections are available for study:

• Brass Specimen: 25 mm dia. 30 mm long (thermocouples T4 and T5 at uniform intervals of

15 mm). With the brass specimen clamped between the heated and cooled sections a uniform

25 mm diameter brass bar is formed with 8 uniformly spaced (15 mm intervals) thermocouples

(T1 to T8) (Figure 2).

• Stainless steel Specimen: 25 mm dia. 30 mm long (no thermocouples fitted).

• Aluminum Specimen: 25 mm dia. 30 mm long (no thermocouples fitted).

• Brass Specimen with Reduced Diameter: 13 mm dia. 30 mm long (no thermocouples

fitted).

7

The ends of the brass heater and cooler sections and the metallic samples have all been given a

fine surface finish to promote good thermal contact. Unless indicated, a heat-conducting

compound should also be applied at these surfaces to reduce thermal resistance. The paste should

be applied sparingly and smeared as thinly as possible over the entire contact area before

clamping the sections together. The voltage of the heating element is controlled through the heat

transfer service control unit. Voltage and current outputs can be read from the panel display.

Similarly, the thermocouple temperature readings can be read from the display panel by

adjusting the thermocouple selector switch. Alternatively, data can be recorded electronically

through the computerized Data Logger (HC111A) and the Hilton Data Logging Software

(HC111B).

In order to provide a basis for comparison of the experimental measurements obtained in this

laboratory, the nominal thermal conductivity for the materials tested are listed in the table below,

as provided by the equipment manufacturer. Note that for the materials listed, the exact value of

the thermal conductivity depends on the specific composition and temperature, those given are

only nominal values.

Figure 1: HT111A Axial Heat Conduction Unit, H111 Heat Transfer Service Unit, and HC111A Data Logger

Material Thermal Conductivity, k (W/m.K)

Brass 121

Stainless Steel 25

Aluminum 180

8

Figure 2: Schematic Diagram of the Axial Heat Conduction Unit

Thermocouple Distance from thermocouple T1, mm

T1 0

T2 15

T3 30

T4 45

T5 60

T6 75

T7 90

T8 105

9

EXPERIMENTAL PROCEDURE

Part A: Axial Conduction along a Simple Bar

1. Ensure that the Axial Heat Transfer Unit (H111A) has been connected to the Heat

Transfer Service Unit (H111) and the Computerized Data Acquisition System (HC111).

2. Ensure that the main switch is in the “OFF” position (the digital displays should not be

illuminated).

3. Turn the voltage controller anti-clockwise to set the AC voltage to minimum.

4. Ensure that the cold water supply and electrical supply are turned on at the source. Open

the water tap until the flow through the drain hose is approximately 1.5 liters/min. (25

g/s).

5. Release the toggle clamp tensioning screw and clamps. Ensure that the faces of the

exposed ends of the heated and cooled sections as well as the intermediate specimen are

clean.

6. Coat the faces of the heated and cooled sections with thermal conducting paste and clamp

them together with the 25 mm diameter Brass Intermediate Specimen in place.

7. Turn on the main switch and the digital displays should illuminate. Set the temperature

selector switch to T1 to indicate the temperature of the heated end of the bar.

8. On the computer desktop, click on the P.A. Hilton Data Loggers HDL icon.

9. On the „System Configuration” screen, click “Load” to be offered the pre-configured file

and then open the “HC111B” file.

10. Rename the “Data File Name”. This enables retrieval of data for a specific experiment.

11. Click “OK”. The “Main Menu” is the next screen when communication with data logger

is successful.

12. Click on “Channel Configuration” to check the pre-configured active channels. Then

click “OK” to get back to the Main Menu.

13. Select “Collect Data” and then “OK”, and then click “NO” in the offered box.

14. Choose data display method (e.g., Multiple Graph Display) and click “OK”.

15. Select the channel to view by use of the numbered tabs below the graph.

10

16. Rotate the voltage controller on Heat Transfer Service Unit (H111) to increase the

voltage to 150 V.

17. Click on “Start Recording” and examine the on-screen values to verify all channels are

functioning. Again ensure that the cooling water is flowing at approximately 1.5

liters/min. (25 g/s).

18. Monitor development of temperature profiles to steady state.

19. Record steady-state temperatures T1 through T8, the heater voltage and the heater current

in your lab datasheet by reading them on the computer monitor.

20. Click on “End Recording” to get back to the “Main Menu” and then exit the program.

21. When the experiment procedure is completed, it is good practice to turn off the power to

the heater by reducing the voltage to zero and allow the system a short time to cool

before turning off the cooling water supply.

22. Ensure that the water supply isolation valve is closed.

23. Turn off the main switch.

Part B: Axial Conduction along a Composite Bar

1. Following the basic operating procedure described in Part (A) smear the faces of the

heated and cooled sections with thermal conducting paste and clamp them together with

the Stainless Steel (or Aluminum) Intermediate Specimen in place.


voltage to 120 V.





5. Record steady-state temperatures T1, T2, T3, T6, T7, T8, the heater voltage and the

heater current in your lab datasheet by reading them on the computer monitor.


11

7. When the experiment procedure is completed, turn off the power to the heater by

reducing the voltage to zero and allow the system a short time to cool before turning off

the cooling water supply.

8. Shut down the system.

12

RESULTS


1. Data file name:

2. Steady-state values:

Intermediate

Section

T1

(oC)

T2

(oC)

T3

(oC)

T4

(oC)

T5

(oC)

T6

(oC)

T7

(oC)

T8

(oC)

Voltage

(V)

Current

(A)

Brass


1. Data file name:


Intermediate

Section

Voltage

(V)

Current

(A)

T1

(oC)

T2

(oC)

T3

(oC)

T6

(oC)

T7

(oC)

T8

(oC)

13

CALCULATIONS


1. Plot steady state temperature versus distance using data from all eight thermocouples. Do

all of the data points lie on a single straight line? Discuss the significance of this plot with

reference to contact resistance and discontinuities.

2. From a best straight line fit to the data, estimate the thermal conductivity of each brass

section, assuming that q = P = V × I. How does the average value compare with the

value provided by the manufacturer and with the published data?

3. What does the assumption q = P mean? Is it valid?

4. Calculate the individual and total thermal resistances involved.

5. Prepare two plots that illustrate the unsteady-state behavior of the system:

(1) Temperature versus time for each thermocouple, and

(2) Temperature versus distance at short, intermediate, and long times.

Comment on the behavior.

Figure 3: Temperature Profile with Brass Intermediate Section

14


1. Using heat balance (equation 8) , estimate the heater/sample interface temperature (TA)

and the sample/cooler interface temperature (TB).

2. Plot steady state temperature profiles. Do all of the data points lie on a single straight

line? Are the data similar to or different from those you obtained with the brass

midsection (Part A)? Why?

3. Estimate the conductivity of the intermediate section (Aluminum or Stainless Steel). How

does it compare with the value provided by the manufacturer and with the published

data?


5. Calculate the overall heat transfer coefficient, U using equation (9). The value of the

overall heat transfer coefficient obtained using this method should be compared with the

value calculated from the thermal geometry (equation 10).


(3) Temperature versus time for each thermocouple, and



Sample

Aluminum

or

Stainless Steel

TA TB

15

REFERENCES

1. Çengel, Y.A. Heat and Mass Transfer - A Practical Approach, 3rd

ed., McGraw –Hill,

New York, 2006.

2. Incropera, F. P.and Dewitt, D. P., Fundamentals of Heat and Mass Transfer, 5th ed., Wiley,

New York, 2002.

3. D. Kern, Process Heat Transfer, McGraw-Hill (1965).

4. J. P. Holman, Heat Transfer, 9th

ed., McGraw-Hill (2002).

5. C.J. Geankoplis “Transport Processes and Separation Process Principles”, 4th

Ed.,

Prentice Hall, NJ (2003).

6. W.L. McCabe, J.C. Smith and P. Harriot, “Unit Operations of Chemical Engineering” 7th

Ed., McGraw-Hill, New York (2005).

16

Experiment # 2

Radial Heat Conduction in Solids

OBJECTIVES

1. To determine the steady-state temperature distribution resulting from radial heat

conduction through a thick cylinder and to demonstrate the effect of a change in heat rate.

2. To determine the thermal conductivity of the disc material.

3. To observe unsteady steady state conduction of heat.

4. To compare the experimental measurements to textbook or theoretical results and to

discuss and explain any differences.

INTRODUCTION

A typical application where radial heat conduction occurs may be found in piping systems,

where it is common for conduction from the inside to the outside of the pipe wall to occur. In this

case the Fourier‟s law of heat conduction is given as:


𝑑𝑟== −𝑘 2𝜋𝑟𝐿

𝑑𝑇

𝑑𝑟 (1)

Where q (W) is the heat transfer rate in the radial direction, A is the area normal the direction of

heat transfer at any radius r, dT/dr is the temperature gradient, and k (W/m.K) is thermal

conductivity of the material.

For steady state condition with no generation, and assuming constant thermal conductivity with

no heat conduction in the axial direction, the heat transfer will occur only radially through the

cylindrical walls. The heat conduction equation, written in cylindrical coordinates, reduces to

17

𝒅

𝒅𝒓 𝒓

𝒅𝑻

𝒅𝒓 = 𝟎 (2)

with corresponding boundary conditions requiring that T (ri) = Ti and T (ro) = To. Integrating

equation (2) results in a logarithmic temperature profile:

𝑇 𝑟 = 𝑇𝑖 +ln 𝑟 𝑟𝑖

ln 𝑟𝑜 𝑟𝑖 𝑇𝑜 − 𝑇𝑖 (3)

Applying Fourier's law (equation (1)) to relate the heat-transfer rate to the temperature difference

across the cylindrical walls gives

𝒒 = 𝑻𝒊 − 𝑻𝒐

𝐥𝐧 𝒓𝒐 𝒓𝒊 𝟐𝝅𝒌𝑳

(𝟒)

If this equation is solved for 𝑻𝒊 − 𝑻𝒐 𝐥𝐧 𝒓𝒐 𝒓𝒊 and the result is substituted into equation

(3), the temperature distribution can also be written as

𝑻 𝒓 = 𝑻𝒊 −𝒒

𝟐𝝅𝒌𝑳𝐥𝐧 𝒓 𝒓𝒊 (5)

The thermal resistance for conduction is given by

𝑹𝒕𝒉 = 𝑻𝒊−𝑻𝒐

𝒒=

𝐥𝐧 𝒓𝒐 𝒓𝒊

𝟐𝝅𝒌𝑳 (6)

18

EQUIPMENT

Measurements of radial conduction heat transfer will be obtained using an experimental setup

consisting of three components (Figures 1 and 2): a "P.A. Hilton Radial Heat Conduction Unit

(H111B)" for the study of radial heat conduction through solids, a universal “Heat Transfer

Service Unit (H111)”, and a computerized “Data Logger (HC111A)”.

The Radial Heat Conduction Module (H111B) comprises an insulated disc of brass (3.2 mm

thick 110 mm diameter) with a brass core (14 mm diameter) and an electric heater at the center

(Figure 3). The brass disc is water cooled around its circumference. The central heater is

nominally rated at 100 Watt (at 240 V AC) and an integral high temperature cut out (automatic

reset) prevents overheating. Power is supplied to the heater from the Heat Transfer Service Unit

(H111) via the 8-pole plug and lead.

Six type K thermocouples (T1, T2, … T6) are positioned at increasing radii from the heated

center to record the temperature distribution across the disc. The thermocouple sensing tips are

located in drilled holes so that in each case the measured point is the center of the disc thickness.

Each thermocouple is fitted with a miniature plug for direct connection to the front panel of the

heat transfer service unit (H111) and an edge connector for use with the HC111A Data Logger.

Water for the cooled circumference is supplied from a local tap. Water flow rate can be

regulated by manual control of the supply tap and can be measured with the attached flow rate

sensor. After cooling the disc, the water is allowed to run to a drain via the outlet hose.

Voltage and current outputs can be read from the panel display. Similarly, the thermocouple

temperature readings can be read from the display panel by adjusting the thermocouple selector

switch. Alternatively, data can be recorded electronically through the computerized Data Logger

(HC111A) and the Hilton Data Logging Software (HC111B).

Useful Data:

Thermocouple Radial Position, m

T1 0.007

T2 0.010

T3 0.020

T4 0.030

T5 0.040

T6 0.050

19

Figure 1: HT111B Radial Heat Conduction Unit, H111 Heat Transfer Service Unit, and HC111A Data Logger

20

Figure 2: Schematic Diagram of the Radial Heat Conduction Unit

21

Figure 3: Radial Heat Conduction

Ri = 7 mm

Ro = 55 mm

L = 3.2 mm

22


1. Ensure that the Radial Heat Transfer Unit (H111A) has been connected to the Heat


2. Ensure that the main switch is in the “OFF” position (the digital displays should not be

illuminated).

3. Turn the voltage controller anti-clockwise to set the AC voltage to minimum.

4. Ensure that the cold water supply and electrical supply are turned on at the source. Open

the water tap until the flow through the drain hose is approximately 1.5 liters/min. (25

g/s).

5. Turn on the main switch and the digital displays should illuminate. Set the temperature

selector switch to T1 to indicate the temperature of the heated center of the disc.






is successful.

10. Click on “Channel Configuration” to check the pre-configured active channels. Then

click “OK” to get back to the Main Menu.





voltage to 100 V.





23

17. Record steady-state temperatures T1 through T6, the heater voltage and the heater current

in your lab datasheet by reading them on the computer monitor.


19. Repeat steps 6-18 at 150 or 200 V.



before turning off the cooling water supply.

21. Ensure that the water supply isolation valve is closed.


24

RESULTS

1. Data file name: run#1: run #2:


Run # Voltage

(V)

Current

(A)

T1

(oC)

T2

(oC)

T3

(oC)

T4

(oC)

T5

(oC)

T6

(oC)

1

2

25

CALCULATIONS

1. Plot the steady state temperatures versus distance of all thermocouples on one plot and

critique the result. Are there discontinuities?

2. Determine thermal conductivity k, assuming that q = P = V × I. How does the value

compare with the value provided by the manufacturer (k = 121 W/m.K) and with the

published data? Perform error analysis.

3. For each power setting, note down the temperature distribution corresponding to radial

distances. Consider these temperatures as experimental values. Now using the

temperature at node 1 (T1) and the theoretical thermal conductivity of Brass determine

the temperature at various radii and compare with the experimental values noted above.

Perform error analysis.



(1) Temperature versus time for each thermocouple on one plot, and



REFERENCES



New York, 2006.


New York, 2002.





Ed.,




26

Experiment # 3

Thermal Conductivity of Liquids and Gases

OBJECTIVES

1. To determine the thermal conductivity of air and acetone.

2. To verify Fourier‟s law of conduction heat transfer.

3. To compare the experimental data with the published data.

INTRODUCTION

The basis of conduction heat transfer is Fourier‟s law. This law involves the idea that the heat

flux, q, is proportional to the temperature gradient, T in any direction, n. Thermal

conductivity, k, is the constant of proportionality; a property of materials that is temperature

dependent, and A is the cross-sectional area normal to the heat flow,

𝑞 = n

TkA

(1)

Equation (1) can also be viewed as the defining equation for thermal conductivity. Thus the

thermal conductivity of a material can be defined as the rate of heat transfer through a unit

thickness of the material per unit area per unit temperature difference. The thermal conductivity

of a material is a measure of how fast heat will flow in the material. A large value for thermal

conductivity indicates that the material is a good heat conductor and a low value indicates that

the material is a poor heat conductor or insulator.

Temperature is a measure of the kinetic energies of the particles such as the molecules or atoms

of a substance. In a liquid or gas, the kinetic energy of the molecules is due to their random

transitional motion as well as their vibrational and rotational motions. When two molecules

possessing different kinetic energies collide, part of the kinetic energy of the more energetic

(higher-temperature) molecule is transferred to the less energetic (lower-temperature) molecule.

The higher the temperature, the faster the molecules move and the higher the number of such

collisions, and the better the heat transfer. The kinetic theory of gases predicts and the

27

experiments confirm that the thermal conductivity of gases is proportional to the square root of

the absolute temperature, and inversely proportional to the square root of the molar mass.

The mechanism of heat conduction in a liquid is complicated by the fact that the molecules are

more closely spaced, and they exert a stronger intermolecular force field. The thermal

conductivities of liquids usually lie between those of solids and gases. The thermal conductivity

of a substance is normally highest in the solid phase and lowest in the gas phase. Unlike gases,

the thermal conductivities of most liquids decrease with increasing temperature. Like gases, the

conductivity of liquids decreases with increasing molar mass.

Gas and Liquid Thermal Conductivity Measurements

There are several experimental techniques used to determine the thermal conductivity of gases

and liquids at steady state such as the hot wire method, the coaxial-cylinder method, the

horizontal parallel flat-plate method, and the concentric sphere and sphero-cylinder method. The

main principle of these methods is the employment of a thin layer of a test fluid enclosed

between two surfaces that maintained at different temperatures.

For precise thermal conductivity measurement, the account must be made of energy loss by test

fluid convective heat flow. An apparatus with the smallest gap width between the two surfaces to

employ the test fluid is recommended. Thus, coaxial-cylinder method takes an intermediate

position between the hot-wire method and the flat-plate method.

The apparatus consists of two coaxial cylinders vertically placed and leaving a very small

annular gap that is charged with the test fluid. The inner cylinder is heated with the electrical

heater.

Figure 1: Heat Conduction in Coaxial Cylinders

28

Fourier‟s law of heat conduction in the radial, one dimensional configuration can be written as:


𝑑𝑟 (2)

The cross-sectional area is given by

𝐴 = 2𝑟𝐿

where r is the radial coordinate, and L is the length of the cylinder. Substituting into Equation (2)

and integration gives

𝑞𝑟 = 2𝑘𝐿 𝑇1−𝑇2

ln 𝑅2/𝑅1 (3)

Where T1 is the inner cylinder temperature, T2 is outer cylinder temperature, R1 is the outer radius

of the inner cylinder, R2 is inner radius of the outer cylinder, and k is the thermal conductivity of

test fluid.

Solving for k, equation (3) can be written as:

𝑘 = ln 𝑅2/𝑅1

𝑇1−𝑇2 2𝐿 𝑞𝑟 (4)

Taking into account the heat losses or “incidental” heat transfer, the conduction heat transfer rate

is given by:

𝑞𝑟 = 𝑄 − 𝑞𝑙𝑜𝑠𝑡 (5)

Where Q = P = power supplied to the system.

The “incidental” heat transfer (qlost) is proportional to the temperature difference (T1-T2) between

the plug and the jacket and can be estimated from calibration graph shown in Figure (2).

29

T (o

C)

0 10 20 30 40 50

qlo

st (

W)

0

2

4

6

8

10

Figure 2: Incidental Heat Transfer vs Temperature Difference.

30

EQUIPMENT

The SOLTEQ® Thermal Conductivity of Liquids and Gases Unit (Model: HE 156) consists

two coaxial concentric cylindrical plugs with a thin radial clearance in between (Figures 3 &4).

The clearance is made extremely small which is 0.3 mm to reduce the natural heat convection.

The heat sourced from the centre of the coaxial concentric cylindrical plugs.

The plug is made of copper and has two ports for introducing and venting the test fluid. The plug

is placed in the middle of the water jacket. The jacket has water inlet and drain connections.

Three type K thermocouples are positioned in the heating and cooling cylindrical plugs,

respectively. The positioning of the thermocouples and the high thermal conductivities of the

materials involved allows measuring the temperatures of the hot and cold faces of the test fluid.

The test module is connected to the control panel for the heater power supply. Power input and

temperature readings are digitally displayed on the control panel. A potentiometer on the control

panel allows student to vary the heating power of the heating elements.

Equipment Specification:

The test module consists of two cylindrical plugs assemble with 0.3 mm gap and a cylindrical

water-cooled jacket.

Inner cylinder plug

Outer diameter : 33.3 mm

Length : 100 mm

Material : Copper

Outer cylinder plug

Inner diameter : 33.9 mm

Length : 100 mm

Material : Copper

Cylindrical water jacket

Material : Stainless steel

31

Figure 3: Thermal Conductivity of Liquids and Gases Unit (Model: HE 156)

1. Thermocouple Sensors 5. Cooling Water inlet

2. Sample Port (Top) 6. Heater

3. Cooling Water Control Valve 7. Sample Port (Bottom)

4. Cooling Water Outlet

Cooling Water Control

Valve

V1

Top Sample Port Valve V2

Bottom Sample Port Valve V3

1

2

5

3

4

6

7

32

Thermocouples

Heater

Cold

Water

Inlet

Hot

Water

Outlet

Sample

Inlet

Sample

Discharge

Outlet Cylinder

0.3 mm gap

Inner Cylinder

Figure 4: Construction of Thermal Conductivity of Liquids and Gases Unit

Outer radius of the inner cylinder, R1 (m) 0.01665

Inner radius of the outer cylinder, R2 (m) 0.01695

Length of the cylinder, L (m) 0.10

33


Part A: Determination of Thermal Conductivity of Air

1. Turn the power regulator fully anti-clockwise to set the power to minimum.

2. Ensure that the cold water supply is connected and electrical supply is switch on.

3. Open the main water supply and gradually regulate the cooling water flow to allow

sufficient cooling to the system.

4. Turn on the main switch.

5. Make sure that the temperature controller is set to 100 oC.

6. Make sure there is cooling water supply to the water jacket.

7. Turn on the heater switch, and then adjust the power regulator to about 20 W.

8. Record the power and temperature readings (T1 and T2) when all readings stabilised

for about ten minutes.

9. Repeat with heating power of 30 and 40 W.

10. Turn the power regulator on the control panel to minimum by turning the knob fully

anti-clockwise and switch off the heater switch. Keep the cooling water flowing for at

least 5 minutes through the module to cold down the test module.

11. Switch off the main switch and power supply.

12. Close the water supply.

Part B: Determination of Thermal Conductivity of Acetone

Repeat the experimental procedure described in Part A by substituting the air with the

acetone with the heating power of 20, 30 and 40 W.

34

RESULTS


Q

(W)

T1

(°C)

T2

(°C)


Q

(W)

T1

(°C)

T2

(°C)

35

CALCULATIONS


1. Calculate the “incidental” heat transfer rate (qlost) using Figure (2).

2. Calculate the conduction heat transfer rate (qr) using equation (5).

3. Calculate the thermal conductivity of air using equation (4).

4. Compare with the published thermal conductivity data.


1. Calculate the “incidental” heat transfer rate (qlost) using Figure (2).

2. Calculate the conduction heat transfer rate (qr) using equation (5).

3. Calculate the thermal conductivity of acetone using equation (4).

4. Compare with the published thermal conductivity data.

REFERENCES



New York, 2006.

2. Incropera, F. P.and Dewitt, D. P., Fundamentals of Heat and Mass Transfer, 5th ed.,

Wiley, New York, 2002.





Ed.,


6. W.L. McCabe, J.C. Smith and P. Harriot, “Unit Operations of Chemical Engineering”

7th


36

Experiment # 4

Saturation Pressure and Steam Quality

OBJECTIVES

1. To study the relationship between saturation pressure and temperature of a water-steam

mixture.

2. To use steam tables to determine the thermodynamic state of a liquid-vapor mixture.

3. To estimate the quality of a liquid-vapor mixture via throttling.

INTRODUCTION

Liquid-vapor phase change (evaporation and condensation) are extremely important to many,

industries. Processes such as distillation and separation in petroleum refineries, electrical power

generation in steam power plants, and refrigeration cycles all depend upon control of evaporation

and condensation. Evaporation (and boiling) is the process in which liquid becomes vapor and in

doing so absorbs a measure of thermal energy known as latent heat. The process of evaporation

occurs at a constant temperature. The cooling effect arises from the loss of thermal energy; that

is, the transfer of latent heat. The temperature at which evaporation and condensation occurs is

known as the saturation temperature. The corresponding pressure is known as the saturation

pressure. The temperature at which evaporation or boiling occurs varies with pressure.

Figure 1 illustrates the relationship between pressure and temperature for the solid, liquid, and

vapor phases of a substance. The triple point is the temperature and pressure at which all three

phases can coexist. The line separating the solid-liquid regions represents a set of temperatures

and pressures at which the solid and liquid phases (ice and water) may coexist. Similarly, the line

separating the liquid-vapor regions represents a set of temperatures and pressures at which the

liquid and vapor phases (water and steam) may coexist. The critical point is the pressure-

temperature state beyond which there is no distinction between liquid and vapor phases.

37

Figure 1: General Pressure-Temperature Relationship

Quality of Steam

The thermodynamic state of a single phase fluid (gas or liquid) can be determined if two

properties are known. So, if the pressure and temperature are measured and the system is in

thermal equilibrium, then all of the other properties at this state can be determined. If two phases

are present (vapor and liquid), then three thermodynamic states must be known. Any three

properties may be used in specifying the thermodynamic state of a two-phase mixture. One

property typically used, in addition to temperature and pressure, is quality. Quality, x, is the

ratio of vapor mass, mg, to mixture mass, mg + mf :

𝑥 =𝑚𝑔

𝑚𝑔 + 𝑚𝑓 (1)

By convention, a subscript f is used to denote the liquid phase and a subscript g to denote the

vapor phase.

Quality has significance for saturated mixtures only and must have a value between zero and one

(0 x 1). It has no meaning in the compressed liquid or superheated vapor regions. The

quality of saturated liquid is 0 and the quality of saturated vapor is 1. The thermodynamic

properties of the mixture which are dependent upon mass can be expressed using quality. For

example, the specific volume (m3/kg) of the system is v = (1 − x)vf + x vg. Other properties

dependent upon mass such as internal energy, enthalpy, and entropy can be determined in a

similar manner.

38

Measuring Quality

Saturated liquid-vapor mixture at high pressure is allowed to expand through a throttling valve

(flow-restricting device) resulting in a sharp decrease in pressure. Some familial examples are

ordinary adjustable valves, capillary tubes, porous plugs, and orifices). If a wet steam is throttled

though a large enough pressure drop, the steam becomes superheated. In this state the steam is

defined by pressure and temperature. The pressure drop in the fluid is often accompanied by a

drop in temperature, and for that reason throttling devices are commonly used in refrigeration

and air conditioning applications.

Figure 2: Throttling process for a liquid-vapor mixture.

The throttling process can be analyzed by applying conservation of energy (the First Law of

Thermodynamics) to a control volume surrounding the throttle in Fig. 2. Assuming steady and

uniform flow, negligible change in kinetic energy and potential energy, no heat transfer

(adiabatic) and no work crosses the control surface, and then the conservation of energy is

reduced to:

𝑕1 𝑕2 (2)

The enthalpy at the exit, h2, can be determined if the pressure and temperature are known. This

is a single phase so only two properties need to be measured to define the thermodynamic state.

The value of the enthalpy at the inlet, h1, is defined by the amount of fluid in the liquid state and

vapor state.

𝑕2 = 𝑕1 = (1 − 𝑥1)𝑕𝑓 ,1 + 𝑥1𝑕𝑔 ,1 (3)

Thus, by combining equations (2) and (3), the quality can be found:

𝒙𝟏 =𝒉𝟐 − 𝒉𝒇,𝟏

𝒉𝒈,𝟏 − 𝒉𝒇,𝟏 (𝟒)

The enthalpies, hf,1 and hg,1 are estimated from steam tables at P1. The enthalpy h2 can be found

from steam tables at T2 and P2.

P1

T1

P2 < P1

vapor T2

39

Empirical Correlation for Saturation Pressure and Temperature

An empirical correlation between saturation pressure and temperature is:

𝑃𝑎𝑏𝑠 = 𝑏𝑒𝑎 𝑇𝑎𝑏𝑠 (5)

Where Pabs is the absolute pressure, Tabs is the absolute temperature and a and b are empirically

determined coefficients. Equation (5) can be linearized to give:

𝐥𝐧𝑷𝒂𝒃𝒔= 𝐥𝐧 𝒃 + 𝒂 𝟏

𝑻𝒂𝒃𝒔 (6)

Thus, plotting 𝐥𝐧𝑷𝒂𝒃𝒔 vs 𝑻𝒂𝒃𝒔 results in a straight line and hence the constants a and b can be

determined.

40

EQUIPMENT

A bench top unit designed to introduce students to the characteristics of saturated water vapor

and throttling. The saturation pressure apparatus consists of a boiler vessel, a separator, a

throttling valve and a condenser.

If the steam is quite wet then pure throttling may not be enough to ensure that the steam is

superheated. Thus, it is necessary to partially dry the steam. This is done by passing the liquid-

vapor mixture to a separator. Here, the mixture is made to change direction suddenly. As water

is denser than steam, it is separated out. The separated water quantity is measured. The

remaining steam now goes to the throttle. It is then passed to a condenser and the condensate is

collected and measured.

The analog signals for various thermocouples and pressure transducers are acquired by a data

acquisition system connected to a personal computer for digitized data gathering. The data

acquisition system consists of a data logger (Pico Technology) and Pico

software package for

data acquisition and monitoring.

41


Part A: Saturation Pressure Experiment:

1. Place a bucket under the drain pipe and open the drain valve.

2. Close the steam valve and let any remaining moisture to drain out of the vessel.

3. Close the drain valve.

4. Open the steam valve and pressurize the vessel to the steam supply main pressure and

then close the steam valve.

5. As the pressure decreases, record the gauge pressure and steam temperature.

Part B: Throttling Experiment

1. Pass the steam through the throttling orifice to warm up the unit.

2. Circulate cooling water through the condenser during operation.

3. Open the separator valve to drain any condensate.

4. Close the separator valve and open the steam main valve.

5. Keep the steam main valve open until a reasonable amount of condensate is collected

then close the steam main valve.

6. Record pressure and temperature data.

42

RESULTS

Part A: Saturation Pressure Experiment:

Data file name:

T1 (°C)

P1,gauge (bar)

Part B: Throttling Experiment:

Volume of condensate =

Volume of separated water =

T1 (°C) P1,gauge (bar) T2 (°C) P2,gauge (bar)

43

CALCULATIONS

1. Plot the measured saturation temperature against the measured saturation pressure. Verify

your measured data with published data (steam tables) by overlaying the data on the same

graph. How well does the experimental data compare to the published data over the

temperature and pressure range covered by the experiment? Provide qualitative and

quantitative comparisons.

2. Prepare a semilog plot [ln(Pabs) vs (1/Tabs)] of your experimental data and derive an

empirical correlation for the vaporization curve data using least squares methods.

Compare this fit with your data by plotting on the original linear Tsat vs Psat graph.

3. Calculate the steam quality using data from the throttling.

4. Find Tsat at P1 and P2 from steam tables and compare with T1 and T2.

5. Comment on your findings.

REFERENCES

1. Çengel, A.Y. and Boles, A.M. Thermodynamics: An Engineering Approach. 6th

edition,

McGraw-Hill (2007).

2. Smith, J.M., Van Ness, H.C. and Abbott, M.M. Introduction to Chemical Engineering

Thermodynamics, 6th

edition, McGraw-Hill (2005).

44

Experiment # 5

Diffusion of a Liquid

OBJECTIVES

1. To determine the diffusivity of sodium chloride solution in de-ionized water at ambient

temperature.

2. To compare the result with results from generalized correlations.

INTRODUCTION

Diffusion of solutes in liquids is very important in many industrial processes, especially in such

separations as liquid-liquid extraction, gas absorption, and distillation. Diffusion in liquids also

occurs in many situations in nature, such as oxygenation of rivers and lakes by air and diffusion

of salts in blood.

It should be apparent that the rate of molecular diffusion in liquids is considerably slower than in

gases. Since the molecules in a liquid are packed together much more closely than in gases, the

density and resistance to diffusion in a liquid are much greater. Also, because of this closer

spacing of the molecules, the attractive forces between molecules play an important role in

diffusion. In general, the diffusion coefficient in a gas will be on the order of magnitude of

about 105 times greater than in a liquid. However, the flux in a gas is not that much greater,

being only about 100 times faster, since the concentrations in liquids are higher than in gases.

In diffusion in liquids an important difference from diffusion in gases is that the diffusivities are

often dependent on the concentration of the diffusing components due to the changes in viscosity

with concentrations and changes in the degree of ideality of the solution.

Certain molecules diffuse as molecules, while others that are designated as electrolytes ionize in

solutions and diffuse as ions. For example, sodium chloride, NaCl, diffuses in water as the ions

Na+ and Cl

-. Though each ion has a different mobility, the electrical neutrality of the solution

indicates that the ions must diffuse at the same rate; accordingly, it is possible to speak of a

diffusion coefficient for molecular electrolytes such as NaCl. The average diffusivity of the

electrolyte is a combination of the diffusion coefficients of the two ions. Its value is in between

the diffusivity values for the two ions. However, if several ions are present, the diffusion rates

of individual cations and anions must be considered.

45

The well-known Nernst-Haskell equation [1] for dilute, single-salt solutions can be used at 25 oC

to predict the overall diffusivity DAB of the salt A in the solvent B:

(1)

Where o

ABD is in cm2/s, n+ is the valence of the cation, n- is the valence of the anion, and + and

- are the limiting ionic conductances in very dilute solutions in (A/cm2)(V/cm)(g equiv./cm

2).

Values of the conductances are given in Table A1 (Appendix A) at 25 oC. The value of T =

298.2 K when using values of + and - at 25 oC. To correct o

ABD for temperature, first calculate o

ABD at 25 oC from Eq. (1) then multiply this value by T/(334w), where T is the new

temperature in K and w is the viscosity of water in cp at the new T.

The diffusion coefficient of an individual ion i at 25 oC can be calculated from

(2)

Then Eq. (1) becomes

(3)

Several methods are used to determine diffusion coefficients experimentally in liquids [1-3]. In

this experiment a small volume of concentrated solution is placed on one side of the honeycomb

of capillaries, inside the glass diffusion cell, whilst the other side consists initially of a large

volume of pure solvent (water). As diffusion of the solute occurs, the concentration within the

larger volume increases and is monitored with a conductivity meter. The mixture is continuously

agitated with a magnetic stirrer to ensure uniform concentration within the bulk liquid.

The rate of diffusion for a liquid can be expressed by the equation

(4)

where *

AzJ is the molar diffusion flux of component A in the z – direction in gmol A/s.cm2

DAB is the molecular diffusivity of the molecule A in B in cm2/s

Ac

z

is the concentration gradient in the z-direction in (gmol/cm

3)/cm

10

1/ 1/8.928 10

1/ 1/

o

AB

n nD T

/ /

o

AB

n nD

n D n D

72.662 10 ii

i

Dn

* AAz AB

cJ D

z

46

To restrict diffusion in one dimension, a small vertical capillary is used in the present

experiment. Hence, the concentration at the lower ends is taken to be constant. The

concentration at the top ends is effectively zero during the experiment.

Thus

(5)

Therefore (6)

Where

V = volume of water (cm3)

z = length of capillaries (cm)

d = diameter of capillaries (cm)

N = number of capillaries

M = Molarity of the salt solution in diffusion cell

CM = the electrical conductivity change per unit molarity change for dilute solutions (-1

M-1

or

Siemens M-1

)

dk/dt = the rate of change of conductivity with time (-1

sec-1

or Siemens sec-1

)

Thus by plotting conductivity against time the diffusion coefficient can be calculated using the

slope of the graph and Eq. (6).

2

4AB

M

V dk d MD N

C dt z

2

4AB

M

Vz dkD

d NMC dt

47

EQUIPMENT

A simple apparatus to measure the liquid diffusivity of sodium chloride in water is shown in

figure 1. The apparatus consists of a specially designed diffusion cell containing a high

concentration of sodium chloride, a stirred reservoir containing a low concentration of sodium

chloride, which at the beginning of the experiment will be deionized water, a variable speed

magnetic stirrer with stirring bar and a conductivity cell connected to a conductivity meter. The

salt concentration in reservoir is measured with the conductivity cell and read on the conductivity

meter.

Diffusion Cell Specifications:

Number of capillaries = 317

Length of capillaries = 5 mm

Diameter of capillaries = 1 mm (to restrict diffusion to one dimension).

Figure 1: Diffusion of a Liquid Apparatus

CHEMICALS/MATERIALS

- NaCl solutions.

- Deionized water.

48


I. Relation between the Concentration of a Saline Solution and the Conductivity:

1. Prepare NaCl solutions at different concentrations: 0.001, 0.0012, 0.0014, 0.0016, 0.0018

and 0.002 M.

2. Make sure that conductivity meter and magnetic stirrer is working properly.

3. Record the readout of the conductivity meter for all the standards prepared in step 1.

II. Determination of Diffusivity:

1. Make sure the conductivity meter and magnetic stirrer are working properly.

2. Clean the diffusion cell (reservoir (1)) and the test vessel (reservoir (2)) thoroughly in

deionized water.

3. Clamp the reservoir (1) on the side of reservoir (2) so the top of the capillaries are

parallel with graduation mark of reservoir (2) and 5 mm below this mark.

4. Fill the reservoir (1) with the salt solution (2M NaCl solution) so the liquid just reaches

the tops of the capillaries.

5. Fill reservoir (2) containing magnetic stir bar with deionized water from a squeeze bottle

to the graduation mark. The capillary tops should be submerged 5 mm below the surface

of the water.

6. Record the volume of the deionized water needed.

7. Switch on the magnetic stirrer at the lowest setting possible and at the same time start the

stopwatch. The speed must be sufficient to give good mixing within the vessel but not to

allow disturbance such as a vortex to form.

8. Record the conductivity reading. Take the data every 60 seconds. Data should be

collected for 1 hour.

9. Turn off the conductivity meter and remove reservoir (1) from the reservoir (2). Rinse the

cell and test vessel several times with deionized water to remove all traces of the salt.

49

RESULTS


Concentration (M) Conductivity (µS)

50

II. Determination of Diffusivity:

Volume of water =

T =

Time

(sec.)

Conductivity

(µS)

Time

(sec.)

Conductivity

(µS)

Time

(sec.)

Conductivity

(µS)

51

CALCULATIONS


1. Plot the conductivity (Siemens) against the concentration of NaCl (M).

2. Estimate the slope of this line, CM.

II. Calculation of Diffusivity:

1. Plot the conductivity as a function of time and determine the slope dk/dt.

2. Calculate the diffusivity using Eq. (6).

3. Compare the experimental value for the diffusivity with the estimated diffusivity using

the generalized correlation, Eq. (1), and discuss any differences.

4. Calculate diffusion coefficient of the individual ions Na+ and Cl

- using Eq. (2) and use

Eq. (3) to calculate DAB.

5. Compare between the diffusivity of the salt and that of the two ions.

REFERENCES


Ed., Prentice

Hall, NJ (2003).

2. R.E. Treybal, “Mass-Transfer Operations”, 3rd


3. E.L . Cussler, “Diffusion: Mass Transfer in Fluid Systems”, 3rd

Ed., Cambridge University

Press, Cambridge, UK (2009).

52

APPENDIX A

Table A1. Limiting Ionic Conductances in Water at 25 oC [1]

Anion - Cation +

OH- 197.6 H

+ 349.8

Cl- 76.3 Li

+ 38.7

Br- 78.3 Na

+ 50.1

-

3NO 71.4 K+ 73.5

-

3 3CH CO 40.9 +

4NH 73.4

2-

41 2SO 80 2+1 2Ca 59.5

-

4CLO 68.0 2+1 2Zn 53

53

Experiment # 6

Forced Convection Heat Transfer from Flat, Finned, and

Pinned Plates

OBJECTIVES

1. To calculate the forced convection heat transfer coefficients and compare these results to the

calculated values from empirical relationships.

2. To demonstrate the relationship between air velocity and surface temperature in forced

convection heat transfer from flat, finned and pinned plates.

3. To demonstrate the use of extended surface to improve heat transfer from the surface.

4. To determine the temperature distribution along an extended surface.

INTRODUCTION

Convection is the mode of heat transfer between a solid surface and the adjacent liquid or gas

that is in motion and it is comprised of two mechanisms. In addition to energy transfer due to

random molecular motion (diffusion), energy is also transferred by the bulk, or macroscopic,

motion of the fluid. Convection is called forced convection if the fluid is forced to flow over the

surface by external means such as a fan, pump, or wind. In contrast, convection is called free (or

natural) convection if the fluid motion is caused by buoyancy forces that are induced by density

differences due to the variation of temperature in the fluid. Convection may be classified further

by specifying whether the fluid flows over a surface or through a pipe or duct. These two

situations correspond to external and internal convection, respectively.

Convection occurs in a variety of heat transfer applications. In heat exchangers, thermal energy

is transferred by convection between two fluids that pass each other in tubes or channels. Ovens

used in industrial processes such drying and curing materials use convection. Quenching of

metals during heat treating is primarily a convection process. Electrical heating elements in

54

boilers transfer energy to water by convection to make steam. Fans and blowers in electronic

equipment move air over heat producing components to keep them cool.

The rate of convection heat transfer is expressed by Newton‟s law of cooling as

𝒒 = 𝒉𝑨 𝑻𝒔 − 𝑻∞ (1)

Where h is the convective heat transfer coefficient in W/m2.K, A is the surface are through which

convection heat transfer takes place, Ts is the surface temperature, and T is the temperature of

the fluid far from the surface. The heat transfer coefficient is a fluid and a flow parameter. It is

an experimentally determined parameter whose value depends on all the variables affecting

convection such as the surface geometry, the nature of fluid motion, type and properties of the

fluid, and the bulk fluid velocity.

An inspection of equation (1) reveals that the only way to increase heat transfer rate, q, when the

temperature difference 𝑻𝒔 − 𝑻∞ is fixed by design considerations is to increase the heat

transfer coefficient, h, the surface area, A, or both. Increasing h may require the installation of a

pump or fan, or replacing the existing one with a larger one, but this approach may or may not be

practical. The most viable approach for increasing heat transfer is to the increase the surface area

by attaching to the surface extended surfaces called fins made of highly conductive materials

such as aluminum. The car radiator is an example of a finned surface. Fins are classified

according to their geometry. A longitudinal fin is flat plate that protrudes from a plane or

cylindrical surface. A pin fin is a rod of circular, square, or rectangular cross section whose end

is a attached to the surface. A radial fin is an annular disk whose inside edge is attached to a

cylindrical object such as pipe.

55

EQUIPMENT

The P.A. Hilton H111P convection heat transfer apparatus (Figure 1) consists of a vertical

rectangular duct supported by a bench-mounted stand. A flat plate, pinned, or finned exchanger

can be installed in the duct and secured by a quick release catch on each side. The H111P

apparatus is designed to be used with the Heat Transfer Service Unit (H111) and the

computerized Data Logger (HC111A).

The accessory comprises a rectangular duct (6) mounted on the discharge of a base mounted

variable speed centrifugal fan. Air velocity in the duct is measured on a portable hot wire

anemometer held in bracket on the duct wall and displayed (m/s) on the air velocity display (4)

below the base. The air velocity is controlled by the use of an intake air throttle (9). At the

center of duct is an aperture that allows any of the three plates (8) supplied to be installed. Each

plate incorporates an electric heater mat with thermostat protection against overheating. The

surface temperature (T1) of each plate is continuously monitored and displayed by the

temperature indicator when plugged in to the console. The pinned plate is fitted with three extra

thermocouples (T3,T4,T5) to measure the temperature of extended surfaces. Also, The finned

plate is fitted with three extra thermocouples (T6,T7,T8) to measure the temperature of extended

surfaces. The air temperature (T9) is measured by sensor (7) located at the base of the duct and

records the temperature of the air flowing over the heated plate. Thermocouple attachment

points on the plates are protected by a covering of adhesive.

Flat Plate Specifications:

Length = 15 cm, width = 12.5 cm

Pinned Plate Specifications:


number of pins = 16 (each pin is 12.5 mm in diameter and 45 mm in length)

Finned Plate Specifications:


Thermocouple Location

T1 Plate (flat, finned or pinned) surface

T3 Pin, 10 mm from prime surface



T6 Fin, 10 mm from prime surface



T9 Air temperature

56

Figure 1: The P.A. Hilton H111P Convection Heat Transfer Apparatus

57


Part A: Forced Convection over a Flat Plate

1. Ensure that the Convection Heat Transfer Unit (H111P) has been connected to the Heat


2. Ensure that the instrument console main switch is in the “OFF” position (the digital

displays should not be illuminated). Ensure that the fan is switched off.

3. If the flat plate is not in position, open the toggle clamps. Replace with the flat plate and

close the toggle clamps. Note that with the plate the power leads exit from the top of the

plate. Refer to the diagram (Figure 1).

4. Turn on the main switch (1) and the digital displays should illuminate. Set the air

velocity to a low value by closing the air throttle (9).






is successful.

9. Click on “Channel Configuration” to see the pre-configured active channels. Then click

“OK” to get back to the Main Menu.





voltage to 200 V. Ensure that the surface temperature (T1) does not exceed 100 oC.

14. Set the air velocity at 5 m/s by opening the air throttle.


functioning.

58

16. Monitor development of the plate temperature to steady state.

17. Record steady-state plate temperature (T1), the air temperature (T9), the air velocity, the

heater voltage and the heater current in your lab datasheet by reading them on the

computer monitor.


19. Repeat steps 5-18 at 10 m/s.



before turning off the fan.


Part B: Forced Convection over Extended Surfaces (Finned or Pinned Plate)

1. Ensure that the Convection Heat Transfer Unit (H111P) has been connected to the Heat


2. Ensure that the instrument console main switch is in the “OFF” position (the digital

displays should not be illuminated). Ensure that the fan is switched off.

3. If the pinned or finned plate is not in position, open the toggle clamps. Replace with the

pinned or finned plate and close the toggle clamps. Note that with the plate the power

leads exit from the top of the plate. Refer to the diagram (Figure 1).

4. Turn on the main switch (1) and the digital displays should illuminate. Set the air

velocity to a low value by closing the air throttle (9).






is successful.

9. Click on “Channel Configuration” to see the pre-configured active channels. Then click

“OK” to get back to the Main Menu.

59





voltage to 200 V. Ensure that the surface temperature (T1) does not exceed 100 oC.

14. Set the air velocity at 5 m/s by opening the air throttle.


functioning.

16. Monitor development of the plate temperature to steady state.

17. Record steady-state surface temperature (T1), pin temperatures (T3, T4, T5) for the

pinned plate or fin temperatures (T6, T7, T8) for the finned plate, the air temperature

(T9), the air velocity, the heater voltage and the heater current in your lab datasheet by

reading them on the computer monitor.




before turning off the fan.


60

RESULTS


1. Data file name: run#1: run #2:


Run # Air Velocity

(m/s)

Voltage

(V)

Current

(A)

T1

(oC)

T9

(oC)

1

2

Part B: Forced Convection over Extended Surfaces (finned or pinned plate)

1. Data file name:


Plate Air velocity

(m/s)

Voltage

(V)

Current

(A)

T1

(oC)

T3

(oC)

T4

(oC)

T5

(oC)

T6

(oC)

T7

(oC)

T8

(oC)

T9

(oC)

Pinned _ _ _

Finned _

_ _

61

CALCULATIONS


1. Plot plate surface temperature (T1) versus time to illustrate the unsteady-state

behavior of the system.

2. Calculate the forced convection heat transfer coefficients and compare the results

with the published correlations.

3. Construct a graph of the steady state temperature difference, Ts - T∞= T1 - T9, as a

function of velocity.

4. Discuss the effect of air velocity on the surface temperature and the heat transfer

coefficient.

Part B: Forced Convection over Extended Surfaces

1. On the same graph, plot temperature versus time for each thermocouple to

illustrate the unsteady-state behavior of the system.

2. Plot steady state temperature profile along the fin (or pin).

3. Compare the values of the heat transfer coefficient for both the flat plate (Part A)

and the finned (or pinned) surface.

4. Comment on your results.

REFERENCES



New York, 2006.

2. Incropera, F. P.and Dewitt, D. P., Fundamentals of Heat and Mass Transfer, 5th ed.,

Wiley, New York, 2002.





Ed.,


6. W.L. McCabe, J.C. Smith and P. Harriot, “Unit Operations of Chemical Engineering”

7th


62

Experiment # 7

Shell and Tube Heat Exchanger

OBJECTIVES

1. To investigate the heat transfer capabilities of a shell and tube heat exchanger.

2. To investigate the effect of flow rate on the overall heat transfer coefficient

3. To investigate the differences and similarities between co-current and counter-current

operation.

4. To investigate the effect of heat losses on the performance of a heat exchanger.

INTRODUCTION

In many parts of the chemical process industry it is essential to add or remove heat from a

process stream. In these it is necessary to alter the amount of energy in the system in a rapid,

controllable and reliable way. A great deal of attention has been paid to practical methods for

adding or removing heat and to the design of practical heat exchangers.

A wide range of heat exchangers are used in industrial processes. The actual type being a

function of what specific function is required e.g. air-cooling, liquid-cooling etc. The most

common type of heat exchanger used in industry contains a number of parallel tubes enclosed in

a shell and is thus called a shell and tube heat exchanger. These heat exchangers are employed

when a process requires large quantities of fluid to be heated or cooled. Due to their compact

design, these heat exchangers contain a large amount of heat transfer area and also provide a

high degree of heat transfer efficiency.

63

EQUIPMENT

The heat exchange unit (Figure 1) consists of three different types of heat exchangers: (i) plate

(HE1), (ii) coil in shell (HE2) and (iii) shell and tube (HE3), all processing a heat transfer area of

0.5 m2. All the exchangers are arranged for independent operation and evaluation using either

hot water or low pressure steam on one side of the heat exchanger and cold water on the other

side. Additionally, when operating with hot water and cold water, the direction of flow of the

cold water can be reversed to permit study of both co-current and countercurrent operation in any

of the heat exchangers. In this experiment the shell and tube heat exchanger is used.

Shell and Tube Heat Exchanger (HE 3) Specifications:

Data Acquisition and Instrumentation:

The analog signals for various thermocouples and pressure transducers are acquired by a data

acquisition system connected to a personal computer for digitized data gathering. The data

acquisition system consists of a data logger (Pico Technology) and Pico

software package for

data acquisition and monitoring.

Diameter 80 mm

Tube length 1000 mm

Tube diameter, internal 12 mm

Tube wall thickness 1.0 mm

Number of tubes 7

Number of baffles 3

Baffle spacing 236 mm

Internal heat transfer area (Ain) 0.5 m2

External heat transfer area, (Aex) 0.254 m2

64

Figure 1: Schematic of the Heat Exchangers

65


1. Choose the shell and tube heat exchanger (HE3) and complete the pre-start up checks.

2. Ensure that the shell and tube heat exchanger has been connected to the computerized

data acquisition system.

3. Start up cold water circuit for co-current operation with a flow rate of 2 L/min.

4. Start up the hot water circuit with a suitable flow rate assigned by the lab. Instructor.

5. Using the data acquisition software, monitor development of temperature readings to

steady state.

6. Record steady-state temperature and flow rate readings in your experiment datasheet by

reading them on the computer monitor.

7. Repeat for increased cold water flow rates up to 10 L/min. Take 5 sets of readings in

total.

8. Change the cold water flow direction to have a counter-current operation with a flow rate

of 2 L/min.

9. Repeat steps 4 to 7.

10. Shut down the heat exchanger.

66

RESULTS

I. Co-current Operation:

1. Data file name:


Ambient air temperature, T∞ = oC.

COLD WATER

Flow rate, L/min 2 4 6 8 10

Tin, oC

Tout, oC

HOT WATER

Flow rate, L/min

Tin, oC

Tout, oC

II. Counter-current Operation:

1. Data file name:


Ambient air temperature, T∞ = oC.

COLD WATER

Flow rate, L/min 2 4 6 8 10

Tin, oC

Tout, oC

HOT WATER

Flow rate, L/min

Tin, oC

Tout, oC

67

CALCULATIONS

At a minimum, your report should include at least the following information for both co-current and

counter-current operations:

1. Calculate the rate of heat transferred into the cold fluid, Qc and the rate of heat transferred

out from the hot fluid, Qh for each experimental result.

cicoccc TTCpmQ

hohihhh TTCpmQ

2. Draw a graph of Qc against Qh.

3. Draw the temperature profile and calculate the log mean temperature difference (LMTD).

4. Calculate the overall heat transfer coefficient.

Theoretically, Qc should equal Qh, but for various reasons (what are they?) they may be

slightly different, and we choose to define a mean rate of heat transfer as

2/hc QQQ

and note that this obeys the relation

LMTUAQ

U = overall heat transfer coefficient (W/m2°C)

A = total surface area for heat transfer = 0.5 m2

TLM = log mean temperature difference (oC)

5. Draw a graph of U against cold water flowrate for both co- and countercurrent operations.

Comment on your results.

6. Calculate the overall heat transfer coefficient between shell-side fluid and surrounding air

using:

TUAQQ LMexch

Aex = the heat exchanger external surface area = 0.254 m2

TLM = the log mean temperature difference between shell-side fluid and air.

7. Prepare two graphs that illustrate the unsteady-state behavior of the system for both co-

current and counter-current operations (i.e., plot temperature vs. time using data from all

thermocouples). Comment on the graphs.

68

REFERENCES



New York, 2006.


New York, 2002.





Ed.,




69

Experiment # 8

Boiling and Condensation Heat Transfer

OBJECTIVES

1. To observe the different pool boiling regimes.

2. To determine the boiling heat flux and surface heat transfer coefficient as functions of the

excess temperature.

3. To demonstrate the two modes of surface condensation: film condensation and dropwise

condensation.

4. To study the effect of dropwise verses film condensation on the heat transfer coefficient.

5. To compare the experimental results with published relationships.

INTRODUCTION

Boiling and condensation are special convection processes associated with a phase change of a

fluid. Like the standard convection processes, boiling and condensation occur at the boundary

between a solid and a fluid. Unlike standard convection processes which rely on sensible heat,

however, the primary driving force for heat transfer in phase-change processes is not a

temperature difference across a boundary layer but rather the latent heat, often as heat of

vaporization, or enthalpy of vaporization, is the amount of thermal energy required to vaporize a

unit mass of liquid at a given temperature and pressure. Likewise, latent heat is also the amount

of energy required to condense a unit mass of vapor. Since the latent heat is relatively large,

particularly for water (~ 2.5 × 106 J/kg), the associated heat transfer rates and hence the

associated heat transfer coefficients are also usually high. Moreover, because the primary heat

transfer mechanism in boiling and condensation is a phase change, high heat transfer rates may

be achieved in the absence of large temperature differences. Condensation processes require that

the latent heat be a removed by a coolant, and boiling processes require that this latent heat be

supplied from an energy source. In most industrial processes, the vapor and liquid phases both

flow through the heat exchanger. Thus, the heat transfer to the phase interface is essentially a

convective process.

Condensation occurs when the surface temperature of the solid is lower than the saturation

temperature of the fluid at a specified pressure, resulting in the formation of condensate on the

surface and heat transfer from the vapor to the solid surface. Conversely, boiling occurs when the

surface temperature of a solid is higher than the saturation temperature of a fluid at specified

70

pressure, resulting in heat transfer from the surface to the fluid. Two parameters of particular

importance in phase-change processes are the difference in densities of the liquid and vapor

phases and surface tension at the liquid-vapor interface. Density differences in the liquid and

vapor phases are responsible for buoyancy forces that drive convection currents, whereas surface

tension affects the formation and development of bubbles.

Boiling and condensation occur in a variety of heat transfer systems. For example, liquid water

is vaporized in boilers to provide steam for heating buildings, driving turbines or processing

materials. Refrigeration systems utilize special heat exchangers, called condensers and

evaporators, for condensing and vaporizing refrigerant. In an oil refinery, oil is evaporated in a

distillation column to give a variety of vapor products, which are eventually condensed into

liquid fuels such as gasoline and kerosene.

I. Boiling Heat Transfer [1]

Boiling is a liquid-to-vapor phase change process just like evaporation, but there are significant

differences between the two. Evaporation occurs at the liquid–vapor interface when the vapor

pressure is less than the saturation pressure of the liquid at a given temperature. Boiling, on the

other hand, occurs at the solid–liquid interface when a liquid is brought into contact with a

surface maintained at a temperature Ts sufficiently above the saturation temperature Tsat of the

liquid. At 1 atm, for example, liquid water in contact with a solid surface at 110°C boils since the

saturation temperature of water at 1 atm is 100°C. The boiling process is characterized by the

rapid formation of vapor bubbles at the solid–liquid interface that detach from the surface when

they reach a certain size and attempt to rise to the free surface of the liquid.

As a form of convection heat transfer, the boiling heat flux from a solid surface to the fluid is

expressed from Newton‟s law of cooling as:

excess

Thsat

Ts

Thq (1)

Where Texcess = Ts - Tsat is called the excess temperature, which represents the temperature

excess of the surface above the saturation temperature of the fluid.

Boiling is classified as pool boiling or flow boiling, depending on the presence of bulk fluid

motion. Boiling is called pool boiling in the absence of bulk fluid flow and flow boiling (or

forced convection boiling) in the presence of it. In pool boiling, the fluid body is stationary, and

any motion of the fluid is due to natural convection currents and the motion of the bubbles under

the influence of buoyancy. The boiling of water in a pan on top of a stove is an example of pool

boiling. Pool boiling of a fluid can also be achieved by placing a heating coil in the fluid. In flow

boiling, the fluid is forced to move in a heated pipe or over a surface by external means such as a

pump. Therefore, flow boiling is always accompanied by other convection effects.

71

Pool Boiling Regimes and the Boiling Curve

Boiling takes in different forms, depending on the value of the excess temperature Texcess. Four

different boiling regimes are observed: natural convection boiling, nucleate boiling, transition

boiling, and film boiling. These regimes are illustrated on the boiling curve in boiling (Figure 1)

which is a plot of boiling heat flux versus the excess temperature.

Figure 1: Typical Boiling Curve for Water at 1 atm Pressure.

Natural Convection Boiling (to Point A on the Boiling Curve)

We know from thermodynamics that a pure substance at a specified pressure starts boiling when

it reaches the saturation temperature at that pressure. But in practice we do not see any bubbles

forming on the heating surface until the liquid is heated a few degrees above the saturation

temperature (about 2 to 6°C for water). Therefore, the liquid is slightly superheated in this case

(a metastable condition) and evaporates when it rises to the free surface. The fluid motion in this

mode of boiling is governed by natural convection currents, and heat transfer from the heating

surface to the fluid is by natural convection.

72

Nucleate Boiling (between Points A and C)

The first bubbles start forming at point A of the boiling curve at various preferential sites on the

heating surface. The bubbles form at an increasing rate at an increasing number of nucleation

sites as we move along the boiling curve toward point C. The nucleate boiling regime can be

separated into two distinct regions. In region A–B, isolated bubbles are formed at various

preferential nucleation sites on the heated surface. But these bubbles are dissipated in the liquid

shortly after they separate from the surface. The space vacated by the rising bubbles is filled by

the liquid in the vicinity of the heater surface, and the process is repeated. The stirring and

agitation caused by the entrainment of the liquid to the heater surface is primarily responsible for

the increased heat transfer coefficient and heat flux in this region of nucleate boiling.

In region B–C, the heater temperature is further increased, and bubbles form at such great rates at

such a large number of nucleation sites that they form numerous continuous columns of vapor in

the liquid. These bubbles move all the way up to the free surface, where they break up and

release their vapor content. The large heat fluxes obtainable in this region are caused by the

combined effect of liquid entrainment and evaporation.

At large values of Texcess, the rate of evaporation at the heater surface reaches such high values

that a large fraction of the heater surface is covered by bubbles, making it difficult for the liquid

to reach the heater surface and wet it. Consequently, the heat flux increases at a lower rate with

increasing, Texcess, and reaches a maximum at point C. The heat flux at this point is called the

critical (or maximum) heat flux, maxq . For water, the critical heat flux exceeds 1 MW/m2. This

point is often called the burn out point. This term is used because after the burnout point, the

element temperature increases rapidly, by several hundred degrees or more. This will often

damage the element. Nucleate boiling is the most desirable boiling regime in practice because

high heat transfer rates can be achieved in this regime with relatively small values of Texcess,

typically under 30°C for water.

In the design of boiling heat transfer equipment, it is extremely important for the designer to

have knowledge of the maximum heat flux in order to avoid the danger of burnout. The

maximum (or critical) heat flux in nucleate pool boiling is expressed as:

412 ρρρσ vlvfgcrmax ghCq

(2)

Where

Ccr a constant whose value depends on the heater geometry, but generally it is about 0.15.

hfg enthalpy of vaporization, J/kg

l density of the liquid, kg/m3

v density of the vapor, kg/m3

surface tension of liquid–vapor interface = 0.0589 N/m for water @ Tsat = 100 oC.

73

Transition Boiling (between Points C and D)

As the heater temperature and thus the Texcess is increased past point C, the heat flux decreases,.

This is because a large fraction of the heater surface is covered by a vapor film, which acts as an

insulation due to the low thermal conductivity of the vapor relative to that of the liquid. In the

transition boiling regime, both nucleate and film boiling partially occur. Nucleate boiling at point

C is completely replaced by film boiling at point D. Operation in the transition boiling regime,

which is also called the unstable film boiling regime, is avoided in practice. For water, transition

boiling occurs over the excess temperature range from about 30°C to about 120°C.

Film Boiling (beyond Point D)

In this region the heater surface is completely covered by a continuous stable vapor film. Point

D, where the heat flux reaches a minimum, is called the Leidenfrost point, in honor of J. C.

Leidenfrost, who observed that liquid droplets on a very hot surface jump around and slowly boil

away. The presence of a vapor film between the heater surface and the liquid is responsible for

the low heat transfer rates in the film boiling region. The heat transfer rate increases with

increasing excess temperature as a result of heat transfer from the heated surface to the liquid

through the vapor film by radiation, which becomes significant at high temperatures.

A typical boiling process does not follow the boiling curve beyond point C. When the power

applied to the heated surface exceeded the value at point C even slightly, the surface temperature

increased suddenly to point E. When the power is reduced gradually starting from point E the

cooling curve follows Figure 2 with a sudden drop in excess temperature when point D is

reached. Note that the boiling process cannot follow the transition boiling part of the boiling

curve past point C unless the power applied is reduced suddenly.

Figure 2: The actual boiling curve obtained as the heat flux is increased and then decreased.

C

E

D

74

II. Condensation Heat Transfer [1]:

Condensation occurs when the temperature of a vapor is reduced below its saturation pressure.

The condensation process of most practical importance is the type that involves direct contact of

a warm vapor with a cool surface, commonly referred to as surface condensation. In surface

condensation, the cool surface extracts latent heat from the vapor, resulting in a vapor-to-liquid

phase change and the formation of condensate on the surface. The mode of condensate

formation on a surface is classified as either filmwise or dropwise. In either condensation, the

condensate tends to form in droplets on the surface, but in film condensation, however, the

droplets readily spread, forming a layer or film that covers the entire surface and the film

thickness will grow as it flows down the surface under the action of gravity. In dropwise

condensation, however, the condensate does not spread but remains as droplets on the surface.

In both film and dropwise condensation, the condensate presents a thermal resistance to heat

transfer between the vapor and the surface. Dropwise condensation yields higher heat transfer

coefficients than film condensation because there are areas of the surface that are not covered

with condensate. To achieve drop condensation in practice requires special surface treatment or

coatings that inhibit wetting. Teflon, silicones, waxes, silver, gold, and a variety of other

materials have been employed to obtain dropwise condensation conditions. Such surface

modifications are usually costly and cannot be effectively sustained over the life time of the

equipment due to corrosion, erosion, or other processes. Consequently, dropwise condensation

eventually reverts to film condensation. Film condensation is generally assumed in the design of

condensers and other heat transfer equipment; because the associated heat transfer coefficients

are smaller than those associated with dropwise condensation, thereby yielding conservative

results.

The average heat transfer coefficient for laminar film condensation on vertical wall or on the

inside or outside surfaces of vertical cylinders is given by:

4

13

943.0

wsatl

lvllL

TTL

kgh

(3)

Where:

Lh : the average heat transfer coefficient, w/m2.K

Tsat: the saturation temperature, K

Tw: the surface temperature, K

l: the liquid density, kg/m3

v: the vapor density, kg/m3

75

l : the liquid dynamic viscosity, Pa.s

kl: the liquid thermal conductivity, w/m.K

L: The height of the surface, m

: the latent heat, J/kg

Liquid properties are evaluated at the film temperature, Tf = (Tw + Tsat)/2, whereas and v are

evaluated at Tsat.

The heat transfer rate is given by:

wsatL TTAhQ (4)

The Nussel number is given by:

l

LL

k

LhuN (5)

The condensation rate is equivalent to the mass flow rate of the condensate film and may found

using the relation

Qm

(6)

76

EQUIPMENT

The TecQuipment boiling and condensing heat transfer apparatus (TE78) consists of a special

heat-resistance glass vessel (Figure 3). It contains a heater coil and two high current insulated

supports for the wire specimens. The cover of the vessel holds two cylinder specimens each with

thermocouples. It also has a filling plug, a split clamp for the connecting wires for the wire

specimens, and water pipe connections. A hole in the center of the cover allows vapor to leave

the vessel and down a temperature-resistant pipe into the water tank. The glass vessel holds d-

ionized water, separate from the circulating water for the condensing heat transfer experiment.

For safety, to the side of the vessel, a capacitive water level sensor switches off the heater coil if

the water level in the vessel becomes low. Also, a „bund‟ (drip tray) surrounds the bottom of the

vessel, to hold any escaping water, in the unlikely event of someone accidently damaging the

glass vessel.

A water tank on the main unit holds the circulating water for the condensation heat transfer

experiments. A peristaltic pump circulates water around either of the cylinder specimens,

selected by two-way valve. A flow meter inside the main unit measures the circulating water

flow regulated with a pump speed control, so a small overflow pipe at the back of the water tank

allows excess water to drain away. For condensing heat transfer experiments, two K-type

thermocouples measure the temperature of the cooling water going into (Tin) and out (Tout) of

each specimen cylinder. Two thermocouples at a time connect to K-type sockets on the front of

the control and instrumentation unit. Two drain taps at the side of the main unit allow draining

away the water from the glass vessel and the water tank after each experiment. The display on

the front of the control and instrumentation unit shows:

- The wire specimen voltage and current (for the boiling heat transfer experiments).

- The cooling water flow rate.

- The thermocouple temperatures and difference between them (for the condensation

experiments).

Switches on the front of the control cabinet allow the user to turn on the pump, the heater and the

power to the wire specimen. Anther control allows the user to vary the voltage supplied to the

wire specimen. The control and instrumentation unit has a connection socket for connection to

TecQuipmen‟s Versatile Data Acquisition System (VDAS) (Figure 4).

77

Figure 3: Boiling and Condensation Heat Transfer Apparatus

Figure 4: Versatile Data Acquisition System (VDAS)

78

Wire Specimens:

For boiling heat transfer experiments the equipment includes two wire specimens made of

Nickel, fitted with two copper sleeves (Figure 5). One is fore nucleate boiling experiment and

the other is for the film boiling experiment. The specimens fit on the top of the high current

supports inside the glass vessel. The specimens are identical except for the wires that connect

them. The resistance of the copper sleeves affect the accuracy of the voltage readings in the

lower temperature and power of nucleate boiling, so this specimen uses the inner voltage

measurement connection points. At the higher temperature and voltages of film boiling the

resistance of the copper sleeves is not significant and can be ignored.

Figure 5: Details of the Wire Specimens

79

Cylinder Specimens:

For condensation heat transfer experiments, the cover of glass vessel has two cylinder specimens

(Figure 6). One has oxidized copper surface to help film condensation. The other specimen has

gold-plating, which lowers the surface tension and allows drops to form (dropwise

condensation).

Figure 6: Cross Section of a Cylinder Specimen

Specifications:

Nickel wire specimens

(Boiling heat transfer experiments)

1 mm nominal diameter 99.98% pure Nickel

Maximum current: 120 A

Maximum wire voltage: 5 V

Cylinder specimens

(Condensation heat transfer experiments)

1 gold plated (3 m thick)

1 oxidized copper

Outside diameter: 15 mm

Effective length: 100 mm

80


I. Boiling Heat Transfer:

1. Make sure that the apparatus has been drained, is cooled to the touch and is disconnected

from the electric supply.

2. On the computer desktop, click on the VDAS icon and select the boiling heat transfer

experiment.


3. Find the proper wire specimen (Figure 5).

4. Check that the copper sleeving is securely crimped to the wire.

5. Undo the thumbscrews at the top of the glass vessel and carefully remove the lid.

6. Using gloves carefully push the specimen in through the hole in the lid.

7. Use the hexagon tool to remove any used specimens and fit the new specimen into the

grooves in the tops of the pillars.

8. Fill the vessel with de-ionized water to about 20 mm above the level of the wire

specimen.

81

9. Fit the lid, tightening it in position with the thumbscrews. Fit the split clamp around the

voltage measurement cables, sliding it into its hole.

10. Plug the specimen voltage measuring cables into the sockets at the rear of the apparatus.

11. Make sure all switches on the control and instrumentation cabinet are off and the voltage

adjust knob is set to minimum.

12. Turn on the power to the control and instrumentation cabinet, then switch on the bulk

water heater.

13. Wait until the water boils then, switch off the water heater and switch on the specimen

heater.

14. Gradually increase the voltage drop across the resistance wire in steps of about 0.1 volts.

Leave for about 15 seconds after each step to stabilize, then switch on the heater for a few

seconds to make sure the water is at Tsat.

15. Record the specimen voltage and current in your results table as well as the VDAS.

16. At each step note what is physically happening at the wire specimen, in terms of the type

of boiling observed.

17. Continue increasing the current up to 140 A.

82

18. As you enter the nucleate region, increase the voltage carefully and gradually, watching

what happens to the wire. Wait at least 30 seconds before increasing the voltage to the

next step, allowing time for the wire temperature to stabilize.

19. You notice when unstable film boiling starts to occur, large bubbles of vapor start to

surround the wire.

20. If you are very careful, you may be able to create stable film boiling, where the

bubbles stabilize and a film surrounds the wire. When this happens, the current

will quickly drop as the resistance of the wire increases dramatically with

temperature. At this point you must quickly reduce the power or the ire will glow

bright orange and burn.

83

II. Condensation Heat Transfer:

1. Make sure the apparatus has been drained, is cool to touch and is disconnected from the

electric supply. Wipe out any water that has been collected in the bund. (drip tray).

2. Undo the two thumbscrews and carefully remove the cover.

3. Remove the wire specimen (if fitted). Clean the cylinder specimens with suitable

degreasing agent.

4. Fill the vessel with de-ionized water to just above the level maker of the level sensor on

the side of the glass vessel. Do not overfill – otherwise when the water starts to boil, the

water will splash onto the specimen surfaces and effect the results.

5. Refit the cover and make sure the split clamp is firmly in its hole.

6. Half fill the water tank with de-ionized water. Ensure all the pipes are connected up

correctly.

7. Set the two way valve so that the water re-circulate through the chosen test specimen.

8. Connect all the power and signal cables to the control and instrumentation cabinet.

Switch off all switches on the cabinet and turn the specimen voltage o minimum (fully

anticlockwise). Connect the thermocouple wires to the specimen you are to test.

9. On the computer desktop, click on the VDAS icon and select the condensing heat transfer

experiment.

10. Choose the material (copper or gold-plated).

11. Select condensation type (filmwise for copper or dropwise for gold-plated specimen )


13. Turn on the control and instrumentation unit and switch on the water heater. Allow the

water to boil vigorously for at least two minutes so the vapor pushes out the air from the

vessel.

14. Switch on the circulating pump. For initial experiment, manually adjust the flow rate

control to give around 250 mL/min flow rate through the specimen.

15. As the cooling water heats up, record the specimen inlet water temperature, the specimen

outlet water temperature and the water flow rate.

16. Stop the experiment when the specimen inlet water temperature reaches 60 oC.

17. Drain the vessel and water tank.

18. Repeat the experiment for the other cylinder specimen, using fresh cooling water and

switching the two-way valve to the other cylinder specimen.

84

RESULTS


Data file name:

Voltage (V) Current (A) Observations

85


A. Oxidized Copper Cylinder Specimen:

Data file name:

B. Gold Plated Cylinder Specimen:

Data file name:

Notes:

86

CALCULATIONS


1. Calculate the heat flux (W/m2):

l d

I V

A

I Vq

s π

V: voltage.

I: current

As: surface area of the wire.

d: wire diameter; l: the effective length of the nickel wire (Figure 5).

2. Calculate the evaporation rate (kg/min).

3. Calculate the electrical resistance (Re) and the electrical resistivity (e) of the nickel wire:

)( I

V Re

Ac: cross-sectional area

4. Estimate the surface temperature of the wire (Ts) using the following relationship

between nickel electrical resistivity and temperature:

5. Calculate the heat transfer coefficient h (kW/m2K) between liquid and metal:

excess

Thsat

Ts

Thq

6. Make a plot of heat flux verses excess temperature difference and compare qualitatively

this plot with Figure 1.

7. Make a plot of the heat transfer coefficient as a function of the excess temperature.

)m( l

dπ

R

l

A Rρ

eCe

e .4

2

8

e 6.99 10 1 0.0059 T-20 ( .m)

87

8. Use equation (2) to calculate the maximum (or critical) heat flux. Comment on your

result.

9. Discuss your results. Are they reasonable? Discuss any errors that might occur within the

equipment or methodology of the experiment.


1. For each experiment calculate,

- The heat transfer rate: waterp TcmQ

- The heat transfer coefficient (equation 4)

- The Nusselt number (equation 5)

- The condensation rate (equation 6).

2. Discuss the effect of dropwise verses film condensation on the heat transfer coefficient

3. Use equation (3) to calculate the heat transfer coefficient and compare the results with the

experimental data for film condensation.

REFERENCES



New York, 2006.


New York, 2002.





Ed.,




88

Experiment # 9

Convective Mass Transfer

(Wetted-Wall Gas Absorption)

OBJECTIVES

1. To estimate the liquid film mass transfer coefficient.

2. To determine the power-law relationship between the liquid film mass transfer coefficient

and the mass flow rate of water and to compare the results with published relationships.

INTRODUCTION

In view of the complexity of mass transfer in actual mass transfer devices such as packed towers,

sieve trays and bubble cap columns, fundamental equations for mass transfer are rarely available,

and empirical methods, guided by dimensional analysis, are relied upon to give workable

equations. This problem has been approached using experimental devices in which the area of

contact between phases is known and where boundary-layer separation does not take place. The

wetted wall column, which is sometimes used in practice, is one device of this type.

A wetted-wall gas absorption column is essentially a vertical tube in which a liquid flows down

the inner surface of the tube section while a gas flows upward. The liquid which comes in

contact with flowing gas partially diffuses into the gaseous stream by means of convective mass

transfer. This process is commonly used in examining the mass transfer between components in

two phases because the wetted wall column, unlike other mass transfer processes, has a well

defined interfacial between the two phases. The surface area is assumed to be that of the inner

tube surface area provided the liquid flows in a smooth, thin and laminar layer.

The absorption of pure CO2 into water eliminates the gas-film resistance, permitting study of the

liquid-film mass transfer coefficient. The following empirical equation has been used to

correlate mass transfer data in wetted wall towers:

1 41

6 1020.433 ReL L L LSh Sc Ga (1)

89

where

L

LL

D

zkSh ,

LL

LL

DSc

,

2

L

32

LL

gzGa

,

4 4Re

LL

L L

m

d

and kL: liquid film mass transfer coefficient, m s-1

z: height of the column, m

DL: diffusivity of CO2 in water, m2 s

-1

L: viscosity of water, N s m-2

L: density of water, kg m-3

Lm : mass flow rate of water, kg s-1

d: column diameter, m

: mass flow rate of water per unit wetted perimeter of the column, kg s-1

m-1

That is, for a given gas/liquid system:

LSh n

LRe (2)

Thus, construction of a logarithmic graph ( LSh vs LRe ) for various flow rates a power law can

be determined and compared to equation 1. This will require calculation of kL using the

following equation:

LM

Lc A

jk

where ,int22 COoutCO ccVj

A = area for mass transfer = z d

ΔcLM

= log mean concentration difference =

outCOiCO

inCOiCO

outCOiCOinCOiCO

cc

cc

cccc

,,

,,

,,,,

22

22

2222

ln

LV volumetric flow rate of water, m3 s

-1

2CO ,i

3 o

2

c equilibrium concentration

= solubility of CO in water = 0.0336 kmol/m solution at 25 C and 1 atm

90

EQUIPMENT

The experimental set-up is shown in Figure 1. The unit comprises glass wetted wall absorption

column, water tank, compressor, and a control console housing flow meters and pump controls.

Carbon dioxide is admitted to the column from a cylinder with regulator through a valve.

Equipment Specification:

Wetted wall column: height = 90.0 cm, internal diameter = 3.16 cm.

CHEMICALS/MATERIALS

1. Carbon dioxide gas.

2. 0.05 M sodium hydroxide solution

3. Phenolphthalein indicator solution.

91

Figure 1: The Wetted-Wall Gas Absorption Column

92


1. Fill the water supply tank with ordinary tap water until the level is about 80 mm above

the pump suction connections.

2. Start the water feed pump (pump1).

3. Set the water flow rate to 150 cm3/min on the flow meter.

4. Feed CO2 to the wetted wall column and regulate its flow rate until it reads 1000 cm3/min

on the flow meter.

5. The water will gradually fill the top section of the wetted wall column and overflow

down the inside wall. A complete wetted wall around the whole perimeter can be

achieved by cleaning the inside of the tube with special long handled brush provided. At

this point, fine adjustments can be made in the positioning of the column to achieve the

required all round water film.

6. After 5 min of operation, collect a sample and determine its CO2 concentration following

the directions in the Analysis Procedure (Appendix A).

7. Repeat steps (7-9) for water flow rates of 200 and 250 cm3/min.

8. Repeat steps (6-10) for CO2 flow rates of 2000 and 3000 cm3/min.

93

RESULTS

I. Water flow rate = 150 cm3/min:

CO2 flow rate, cm3/min Titre of NaOH, ml

1000

2000

3000

II. Water flow rate = 200 cm3/min:


1000

2000

3000

III. Water flow rate = 250 cm3/min:


1000

2000

3000

94

CALCULATIONS

1. Calculate j, kL, ShL and ReL for each run.

2. Prepare log-log plots of Sh vs. ReL for various CO2 flow rates and determine the power

law relationship, i.e. the exponent n in equation (2).

3. Compare your results with the empirical equation (eqn.1).

4. Comment and on your results.

REFERENCES


Ed., Prentice

Hall, NJ (2003).



3. R.E. Treybal, “Mass-Transfer Operations”, 3rd


95

APPENDIX A

Analysis Procedure for CO2 in Water

1. Collect 25 mL with a graduated cylinder from the liquid outlet point.

2. Place the sample in a 125 mL Erlenmeyer flask.

3. Titrate the sample with 0.05 M NaOH solution. The end point is reached when a definite

pink color persists for about 30 seconds - record volume of the NaOH added.

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