FACULTY OF ENGINEERING
Department of Chemical Engineering
PO Box 1906
Bellville
7535
TEL: (021) 959 6083
Practical Manual
Year: 2019
Subject Name:
Chemical engineering Laboratory
Subject Code:
CEL261S
ND: Chemical Engineering
NQF level:
6
Lecturer:
Dr. Mahabubur Chowdhury
Revision No
Approved
Date
Purpose Statement for Qualification
This qualification is intended for process or chemical engineering technicians working in the process
or metallurgical related industries. Learners achieving this qualification have the competence to
apply existing process technology to chemical engineering related problems, process design and
will illustrate technical competence thus contributing to the needs of the chemical profession.
1
1)
Introduction:
The main objectives of the practicals are to reinforce some of the concepts covered in theory
and to practically demonstrate their applications. At the same time these sessions present the
opportunity to exercise and improve deductive and observational skills and to cultivate those
habits of accuracy, neatness and thoroughness – qualities that are important for good
experimentation.
2)
Preparation for a Practical Session:
In the practical manual provided to you, the procedures for the experiments as well as some
theory and background to the experiments have been included. Additional background will be
provided during the pre-practical session, but the onus is on you to read as much about the
practical/subject matter as possible. Sources of information include your prescribed and
recommended textbooks as well as books available in the library. Be sure to read the
experimental directions in advance of your scheduled laboratory session. You will be required
to familiarize yourself with the equipment and procedures before you enter the laboratory –
this will ensure that you do not do the experiments “cook-book” or “recipe” style. Further, the
practical write-up should be completed and handed in to the technical officer no later than 2
weeks after completion of practicals.
3)
RULES FOR ATTENDING THE PRACTICAL SESSIONS
The following is important:

One practical per semester will be performed per subject.

The contribution of the practical mark to the year mark is equivalent to 15 % of the final
mark.

Students are required to get a minimum of 50% for the practicals to pass the subject.

It is expected of each student to do all the practicals and each must be handed in as a
complete or a short report as allocated by the technical officer (For the refrigeration
practical all students must submit a short report). If a student does not attend a
practical, a medical certificate must be shown, or else a mark of NIL will be allocated to
the student in question.
2

A student may not hand in a report if the student did not attend the practical.

Data generated by groups other than the student’s group may not be used in the report
unless provided by the Technical Officer.

Students should understand plagiarism and the various types. Students should also
heed the seriousness of the offence and that such an offence will carry the minimum
penalty of receiving a mark of zero for parts of the report plagiarized. A mark of zero can
be awarded.

The report cover page should clearly indicate the report title, student’s full name, student
number, group number, date performed and date submitted. It should also include the
mark allocation for a short report as shown under section 7 below.

The due date for practical reports is two weeks after the practical was performed unless
the due date falls in the holidays. In such a case the due date falls on the first day of
term that the technical officer is at the campus.

All reports are submitted to the laboratory technicians and not the subject lecturer.

If a practical is handed in later than the due-date, 10 % will be deducted from the
student's mark for each day the practical is handed in late. After 3 days overdue, the
report will receive “zero” mark.
3
4)
GENERAL INFORMATION
Practical Scheduling
The schedule of practicals for each group is provided. Students are expected to perform each
practical on the day and between the hours specified, and will not be permitted to work outside
these hours unless prior agreement has been granted by the Practical Supervisor. Any
problems with regard to the practical schedule should be taken up with the Practical
Coordinator as soon as possible. If scheduling changes are made with the agreement of the
Coordinator, hand-in dates will also be changed; otherwise the original dates will remain in
force.
Location of Equipment
The equipment is located in the Chemical Engineering laboratory.
5)
DECLARATION
While all experimental work will be performed in groups, the practical report must be the
student's own individual work. The following undertaking is incorporated in the cover page
of each report and must be signed by the student:
“I certify that this report is my own unaided work, except for the assistance received from the
teaching staff. I undertake not to pass this report onto any other student”.
6)
LAB SAFETY RULES
a.
Keep books, briefcases, and other property (especially jewellery) away from your work
bench.
b.
DO NOT TASTE anything and AVOID INHALING toxic or noxious fumes.
c.
Clean up spills as soon as possible. Wash with water to dilute.
d.
When working with concentrated acids, take special care to AVOID SKIN CONTACT.
Also work over the sink to avoid any spillage on the bench.
e.
When DISPOSING of LIQUIDS, do so into RUNNING WATER in the sink. Wash down
with lots of water.
f.
Any broken apparatus must be reported immediately.
g.
Smoking, eating or drinking will not be tolerated in the laboratory.
4
h.
Unauthorized experiments are strictly forbidden.
i.
Students are reminded that many of the experiments performed have inherent dangers
associated with them and due precaution should be observed at all time. These dangers
include the effects of rotating equipment, electrical machinery, high pressure air and
steam. First aid equipment is available at chemical engineering laboratory. Fire hoses,
fire extinguishers and safety showers are placed on all levels of the building. An eye
bath is located in the analytical laboratory. Students should check to see where these
items are located. Students must wear appropriate footwear and ladies should wear
jeans in place of skirts while performing experimental work. Smoking and eating is not
permitted in the laboratory.
j.
7)
REPORT ANY ACCIDENT, however small, to your supervisor immediately.
MARK ALLOCATION
Short Report
Title Page
5
Structure
5
Executive Summary
10
Procedure
15
Results
45
Discussion
15
Conclusion
5
Total
8)
Mark allocation
Student’s mark
100
Guide to Report Writing
a) Introduction:
Engineers who are unable to communicate effectively with their superiors or colleagues will
never receive the credit for their work. Information must be transferred to enable decisions to
be made, money to be allocated or work to be started. The most used method of technical
communication is the REPORT.
Before starting a report, certain points should be considered:
5
i.
The reason the report is written. Has the contents been planned so that the reader will
understand what is being said?
ii.
Who will the reader be? This decides the level of technical information that is to be
included.
The writer must be absolutely clear as to what information is to be conveyed. The reader
studies a report for what he can gain from it.
Some suggestions are given below to improve and simplify the presentation of reports.
i.
Choose a short meaningful title.
ii.
Include as many sketches and diagrams as you think are appropriate. Remember that
a single diagram will often clarify what would otherwise be a very confusing paragraph.
iii.
Describe what you are about to discuss in general terms before giving the details.
Nothing is more confusing or frustrating than reading a very detailed description of
some piece of equipment when the reason for its use is not yet known.
iv.
Clearly separate facts from opinions. Opinions certainly have their place but can cause
a report to be misleading if not clearly stated so.
v.
Use a simple, clear style of writing. Long and involved sentences are a hindrance to
understanding and frequently contain grammatical errors.
vi.
Be precise. For example, a fuzzy statement referring to a “large tank” immediately
raises questions: how much does it hold?
vii.
Ensure that the report is legible.
b) Logical Progression
The structure of a report can be likened to the structure of a joke or a story. There is a
section that puts the work into context followed by a detailed description or the body of the
work. Lastly there is the punch line that can be compared to a report’s conclusion that wraps
up the work.
Story Structure:
c)
Background
Detail (Core)
Conclusion
6
c) Arrangement of the Material:
Typically the material presented should be arranged in the following sequence:
Title
X
Synopsis
L
Table of Contents
L
List of Symbols
A
Introduction
X
Theoretical Background
L
Body of Experiment
X
Results
X
Discussion
X
Conclusions
X
Recommendations
A
References
X
Appendices
X
X
A
L
required
where appropriate
for long reports
The reports done this semester can be assumed to take the format of a long report. The
following structure should be adhered to. The mark allocation is also given.
d) Mark Allocation
Overall impression
[8]
–
Title page
1
–
Table of contents
3
–
List of symbols
2
–
Structure
2
-
Synopsis
[5]
1. Introduction
[5]
2. Literature Review / Theory
[15]
3. Experimental Procedure
[10]
4. Results and Calculations
[25]
5. Discussion of Results
[25]
6. Conclusion and Recommendations
[5]
7
7. References
[2]
8. Appendices
e) Contents of Each section
e.1) TITLE PAGE:
The title page should include the following:
•
Name of the practical (can be larger font)
•
Subject
•
Name of the student
•
Date practical was performed
•
Date report handed in
•
Declaration that the report is the author’s own work
•
Signature
It is the first page (i) but it is not numbered. This page should not contain unusual font and
colours. Use black font and a standard font type. Avoid adding pictures or excessive
decoration such as borders. A single border can look good but keep it to a minimum.
e.2) SUMMARY
There are two types of summaries and the choice between the two depends on what is
required. These are the ABSTRACT/SYNOPSIS and the EXECUTIVE SUMMARY.
e.3) SYNOPSIS:
The synopsis is also sometimes referred to as the summary or abstract and acts as a
standalone section. The synopsis should be intelligible on its own and should give a clear
outline of the contents of the report stating the purpose, methodology, results and conclusion.
Academics find the abstract useful for sifting through research papers to find relevant
information. Only the abstract needs to be read to check if the research paper is of use to
them. In industry the abstract allows managers to prioritise various projects before having to
delve into the report. Sometimes it is all the manager needs to read.
Care should be taken not to use unfamiliar terms, acronyms, trade names, abbreviations or
symbols without explanation. More than one paragraph may be used but the summary should
not exceed 2% of the total of the contribution and should be limited to no more than 250 words.
About a quarter of a page to half a page should be sufficient for the reports in this course.
8
Here is an example. Try to identify the separate sections in the abstract.
“Polyacrylic acid (PAA) and starch represent two of the most widely used commercial
flocculants in the mineral processing industry. Both are known to behave as selective
flocculants in the beneficiation of iron ore slimes. This paper compares these two types of
flocculants in achieving selective separation of iron oxide from clay. Our results indicate that
PAA requires a lower flocculent dosage and yields bigger flocs. Starch is found to be more
selective than PAA. Important differences in the behaviour of these two flocculants, as
investigated with the help of a set of statistically designed experiments, are explained in terms
of the proposed mechanisms of interactions of starch and PAA with iron oxide and clay.
Electrostatic interactions play an important role in achieving selectivity.” Ravishankar et al
(1995)
e.4) EXECUTIVE SUMMARY
The executive summary is similar to the abstract but it is more detailed in every aspect. It aims
to sell the idea to the reader (usually management) who may have less technical background
on the subject matter. The main report is usually reserved for the core technical detail. A
motivation should be presented and therefore the aim should feature prominently. The main
findings should be clearly stated, along with pertinent discussions in line with the aim.
The executive summary is usually in the region of 10% of the total report but effort should be
made not to exceed this length. The format of the executive summary is in strategic
paragraphs. Headings could be used but not in the sense of “Literature Review” etc. Tables
and figure are allowed to detail the main findings.
e.5) TABLE OF CONTENTS AND LIST OF FIGURES/TABLES:
The table of contents should contain the main and sub-paragraph and page numbers:
TABLE OF CONTENTS
CONTENT
PAGE
Synopsis
ii
List of symbols
iv
1. INTRODUCTION
1
2. LITERATURE REVIEW
2
9
2.1.
Heat Exchangers
2
2.2.
Heat transfer coefficients (calculations)
4
3. EXPERIMENTAL PROCEDURE
6
3.1.
APPARATUS
6
3.2.
SAMPLE PREPARATION
7
MS Word has a handy tool for creating a Table of Contents (TOC). On the Toolbar go to:
INSERT – REFERENCES – INDEX AND TABLES – TABLE OF CONTETNS
Various customisations can be performed here including setting the number of levels in the
TOC. In the above example there are two levels. The headings to appear in the TOC need to
be identified. Make sure the OUTLINING toolbar is visible.
VIEW – TOOLBARS – OUTLINING
Scroll though your report and select the first heading you want to have shown in the TOC.
Select which level the heading should be.
To update the TOC either right click on the TOC or use the OUTLINING toolbar.
Similarly a list of tables and figures can be compiled. The only difference being how the caption
is produced and the how the captions are linked to the list entries.
e.6) LIST OF SYMBOLS:
The list may appear immediately after the TABLE OF CONTENTS section or it may appear
after the CONCLUSIONS section. The list can be named Notation or Nomenclature as well.
The list should contain all symbols used in the text and appendices, arranged in ascending
alphabetical order, along with a brief description and their units:
Nomenclature
Ga
Galileo number
dimensionless
x
wall thickness
m
µ
viscosity
Pa.s
ρ
density
kg.m-3
Greek Letters
10
Sometimes many subscripts are used and if need be an additional section can be made to
accommodate them, e.g.
Subscripts
d
dynamic conditions
e
equilibrium conditions
p
particle
i
i’th screen
Internationally standardised or recognised symbols should be used and if yours are not in
general use a list of their meanings should be included.
Mistakes some students make in this section include omitting symbols, including equations
and describing units, e.g.
W
Watts
e.7) INTRODUCTION:
The principal object of the introduction is to acquaint the reader with the problem and point out
the purpose and significance of the report as well as putting the work in context. The
Introduction should be logically developed and should also provide the rationale for the
present study. Only those references that supply the most salient background should be used
rather than providing an exhaustive review of the topic.
As a guideline the following should be specifically included:
a) A brief description of the Unit Operation or general class (e.g. ‘heat transfer’) to which
the experiment belongs, and of which the particular aspect (e.g. ‘forced convection’)
which you investigated, in a few short sentences.
b) A summary of the relevant theory based on your notes, textbook, Perry, and books and
other references given in lectures. What guidance can theory, empirical correlations,
etc., give us in designing and analysing the equipment? How reliable are these
methods? Try to summarise what you know about the operation.
c) The object of the work.
11
Remember to cite work correctly. Citing work serves two purposes. It gives credit to people
whose work is being used and acts as a way to substantiate the information used in the report.
Certain sources are more reliable than others. Information not cited is regarded as the author’s
own work/findings and if found otherwise can result in serious consequences. The best that
can happen is getting zero marks for that section.
e.8) THEORETICAL BACKGROUND/LITERATURE REVIEW:
This section summarizes and evaluates the literature that you have used in your study by
considering how that literature has contributed to your area of research. It also comments on
the strengths and weaknesses of previous studies from a technical point of view. It documents
the core technical details of the work and the methodology the writer adopts. The introduction
and theory should supply sufficient theoretical background knowledge to allow the reader to
understand and evaluate the results of the experimental work without needing to refer to
previous publications on the topic.
As an example, imagine the writer was challenged with calculating the volume of an object.
The different methods of this calculation would be explained in detail. Examples thereof would
be liquid displacement, dimension measurements, from density and mass relationships and
from pressure volume relationships. The writer would then introduce their own technique with
all the necessary technical details.
Equations used in the calculations can be given here and explained. An example of how to
present an equation is shown below. The equations should be numbered according to the
section numbering or as it is done in journal papers, starting at (1) for the first equation in the
report.
Vsphere 

6
d3
..(1)
e.9) BODY OF EXPERIMENT OR EXPERIMENTAL PROCEDURE:
The experimental section should include sufficient technical information so that a competent
worker could repeat the experiments that are described.
As a guideline the following should be specifically included.
a) A description and diagram of the apparatus.
12
b) A list of the apparatus used.
c) A brief description of the operation of the apparatus (i.e. ‘method’) and the procedure.
Do not waste time in a lengthy description of standard procedures (e.g. titrations) but
provide sufficient detail so that the reliability of your experimental work can be
assessed.
Use the past tense and passive voice; “the thermometer was placed in the water”.
e.10) RESULTS:
The results should visually and textually represent the experimental findings. In the results
section, include only the results of the experiment; reserve extensive interpretation of the
results for the discussion section. The explanatory text should point out the most significant
portions of research findings and indicate trends or relationships. It should also highlight
expected and/or unexpected findings. These can also be included in the discussion.
Present the results in as a concise way as possible. Figures and tables should be selected to;
illustrate the points being made if they cannot be described in the text, to summarise or present
repetitive data or to record quantitative results. Results may also be referred to in the
Appendices. Reserve the Appendices for results that are voluminous or less significant and
would otherwise bulk up the Results section. The results obtained directly from the experiment
should be separated from those calculated. Clearly label a graph or a table with the associated
chapter number and provide a brief description. A table should be presented with the heading
above the table:
Table 4.1 Experimental and theoretical mass over time
Time
Mass
Experimental Theoretical
(s)
(g)
(g)
0
0
0
0.20
0.04
0.04
0.30
0.09
0.10
0.45
0.20
0.22
When plotting graphs, it is neater to have a white background with the legend in the graph
area to save space. The theoretical results are normally represented continuously (as a line)
and the experimental results as discrete points.
13
0.06
0.05
0.04
10°C Theoretical
+
[Na ] 0.03
10°C Experimental
20°C Theoretical
0.02
20°C Experimental
0.01
0
0
50
100
150
Time (Minutes)
Figure 4.1 Experimental and theoretical sodium concentration over time
Please note, the sample of calculations is not meant to be presented in the results section. It
should appear in the appendices.
A hint: When doing graphs one can often not include the ° sign for °C in the legend and
the axis labels. While pressing ALT, first press 2, then 4 and then 8 in sequence and
see what happens. These are called ASCII codes. Do a search for ASCII codes on
Google and see what other signs are easy to make. (NB: ALT + numbers)
e.11) DISCUSSION:
The Discussion should provide an interpretation of the results in relation to the theoretical
and/or previously published work and should not contain extensive repetition of the Results
section or reiteration of the Introduction section.
As a guideline the following points should be considered:
How reliable were the results? Compare estimates of accuracy on an analysis of the
experimental procedure with those based on an analysis of the results. If possible, use your
knowledge of statistics to its full extent. No experimental result has any meaning or use until
its reliability has been established.
Where applicable, discuss the effect of the experimental variables under your control (e.g. flow
rate) as well as those that are not (e.g. ambient temperature). Express these quantitatively in
the most effective manner. Compare calculated results for the various runs with each other so
that effects of system parameters can be evaluated. Try to give explanations thereof.
14
Compare these effects and the numerical results with published figures and with ones
predicted from theory. Try to assess the reasons for the differences (in terms of type of
experimental equipment, test system, ranges of experimental conditions, scale effects).
Did the experiment represent a good (or possible, or the best) way of obtaining the required
information?
Make your discussion and description as quantitative as possible (always quote errors
numerically never refer to them as merely ‘large’, ‘reasonable’ etc.) Avoid vague and hopeful
generalisations.
Back up your statements, either from the literature or from the experimental results you are
discussing.
Clearly refer to your graphs and tables. E.g. “… Figure 4.1 vindicates the use of …” or “… the
graph of concentration against time, Fig 4.1, shows that …”)
e.12) CONCLUSIONS:
The conclusion should briefly outline what has been learned from performing the experiment.
It should state concisely:
i.
what is shown by the work
ii.
the significance of the findings
e.13) RECOMMENDATIONS:
If, as of a result of the work done, it is possible to make any worthwhile recommendations this
should be done – one should make recommendations for further experimental work necessary
to confirm and/or extend them. The conclusion and the recommendations can usually be
combined.
If making recommendations for the elimination of experimental error, try to be specific when
stating reasons for experimental error. Simply stating parallax error is not enough. It is evident
that some students do not even know what parallax error is and mention it as if it were a
mystical force that takes a perverse pleasure in ruining experiments.
15
e.14) REFERENCES:
Cited papers and books must be acknowledged to clearly differentiate between published fact
and the author’s own ideas. References are cited in the text by quoting the authors name and
year of publication, while at the end of the report a full list of references is given in alphabetical
order. The Cape Peninsula University of Technology makes use of the Harvard method for
referencing purposes. Here follows examples of references from the most frequently used
sources:
BIBLIOGRAPHY
TEXTUAL REFERENCE
(Appears before the Appendices at the end of the (Appears in the text of the report where the
report)
material is used)
BOOKS
Books: one author
Chase, J. 1979. Advertising in the modern
world. New York: Simon & Schuster.
Chase, 1979:page numbers cited
Books: two authors
Ellis, R. & Peters, J.P. 2000. Writing about
literature. London: Macmillan.
Ellis & Peters, 2000:page numbers cited
Books: multiple authors (three or more)
Henderson, R.S., Smith, P.G., Rossiter, I. &
Henderson, Smith, Rossiter & King,
King, P.Q. 1987. The tenets of modern philosophy. New 1987:page numbers cited
York: Van Nostrand.
Subsequent citations use et al.
Henderson et al., 1987:page numbers
cited
INTERNET
Cape
Peninsula University of Technology. n.d. Cape Peninsula University of Technology, n.d.
Intellectual property policy.
[No date indicated on document.]
http://www.cput.ac.za/polic/ippolicy.html
[15 November 2004]. [Date indicated in square
brackets is date downloaded.]
New Media Publishing. 2005. New Media Publishing New Media Publishing, 2005.
scoops
prime
position
for
2005.
http://www.newmediapub.co.za
[16
November
2005].
Look at the reference in the document as an example.
Mistakes sometimes made by students include simply citing Google as the internet site. Google is a
search engine that finds relevant sites for you. Sources cited should have some credibility. Do NOT use
16
“Wikipedia.com” as a reference. Even though material being published on the online encyclopaedia has
been assessed to be reliable using stricter criteria than before, it is still not good enough because it can
be authored by anyone and is not peer reviewed.
17
e.15) APPENDICES:
The Appendices should contain:
i.
Tables of results where graphs are given in the main part of the report.
ii.
Sample of calculations. This is extremely important because it indicates how the calculations
were done. A large portion of the marks for the “Results” section will be come from here.
iii.
Physical constants used.
iv.
Equipment list: Type, Make, Model and Serial number.
v.
Theoretical derivations that are not pertinent to the theory or which would break the flow of the
report if included in the main body.
e.16) EDITORIAL AND GENERAL:
e.16.1) Conventions
i.
It is usual to report in the third person, past tense, passive voice and indicative
ii.
All drawings and graphs should be labelled in such a way that they are read with the page in
the normal position or with the page turned clockwise through 90
iii.
mood.
degrees.
Text relating to the drawings or tables should come before drawing and should clearly indicate
which object it is referring to.
iv.
Use the appropriate number of significant figures. Writers own discretion but not more than 4.
e.16.2) Units and Quantities:
Metric (SI) units should be used.
e.16.3) Paragraphs:
A decimal system of numbering paragraphs should be used where the report is long and there is a need
to reference other parts of the report.
e.16.4) Proofreading:
It is essential that all the content (layout, content, references, diagrams and tables) of the report be read
and checked carefully and objectively. It is a common mistake to gloss over near completed report and
assume that everything is in the correct place and makes sense. Sometimes expressing oneself is
challenging and when describing something technical, the reader could get lost in text that the writer
finds perfectly clear because it is so familiar. When writing, try and see it from the reader’s perspective.
The real proof of how coherently the report has been written is to allow somebody else to proofread it.
18
PRACTICAL:
TERMINAL SETTLING VELOCITY
Objectives:
This practical has two objectives:
(1) To determine the dynamic and kinematic viscosity of a fluid using a falling sphere
viscometer.
(2) To identify a fluid from its physical properties (i.e. viscosity and density).
Theoretical Background:
A balance of forces acting on a falling sphere through a fluid gives:
𝐹
=𝐹
+𝐹
Which may be written as:
𝐶 =
2𝑔 𝜌 − 𝜌 𝑉
𝜌 𝑣 𝐴
Where CD is the drag coefficient; v is the falling velocity of the sphere; Ap is the projected
particle area in the direction of motion; Vs is the volume of the sphere; g is gravitational
acceleration; s and f is the density of the sphere and fluid, respectively. The drag coefficient
in the second equation is a function of the Reynolds number, NRe. At low Reynolds number,
Stokes law gives:
𝐶 =
24
𝑁
𝑁
< 0.1
In the intermediate range (0.1<NRe<1,000), the drag coefficient may be estimated by:
𝐶 =
24
(1 + 0.14𝑁
𝑁
.
)
0.1 < 𝑁
< 1,000
In the flow regime 1,000<NRe<350,000, the drag coefficient may be estimated by CD = 0.445.
Beyond NRe = 1  106, the drag coefficient is given by:
𝐶 = 0.19 −
8 × 10
𝑁
𝑁
> 1 × 10
Apparatus:
19
Falling sphere viscometer
Sets of balls of different sizes ( = 7850 kg/m3)
Venial calliper
Measuring tape
Stopwatch
Thermometer
Method:
The unit consists of two cylindrical tubes, each one containing a different liquid. The density
of each liquid is known. There are also spheres of known diameter and specific weight.
(a) Put liquid into the measuring cylinder.
(b) Set the stopwatch to zero.
(c) Release a spherical object of known size and density near the top of one of the tubes.
(d) Determine the terminal velocity by letting the object come to its steady velocity and,
then, measuring the time required for it to fall a known distance.
(e) Remove the ball with the valve.
20
Results:
Fluid Type
Ball
Drop
Type
distance
Drop time (s)
Rate of fall
NRe
(mm/s)
(mm)
FLUID A
A
(𝜌 = 1𝑔/𝑐𝑚 )
B
FLUID B
A
(𝜌 = 1.1132𝑔/
𝑐𝑚 )
B
Analysis:

Determine the Reynolds number, NRe

Determine the drag coefficient, CD

Determine the dynamic viscosity, 

Determine the kinematic viscosity
21
PRACTICAL: REFRIGERATION CYCLE
Refrigeration cycles are used to remove heat from a confined space. The biggest applications
for this kind of cycle are food preservation and air condition. Refrigeration also has many
important chemical engineering applications such as:

Control of chemical reactions by keeping reactants at reduced temperature as is done is
some of the acid treating processes used in an oil refinery.

Separation of wax from lubricating oils by a combined chilling and final centrifuging.

Separation by distillation of normally gaseous mixtures, as is done in fuel production of
oxygen and nitrogen from liquefied air.
Refrigeration cycles use special fluids called refrigerants as the working fluid. The refrigeration
system in this practical is using refrigerant R-134a.
The functioning of the majority of refrigerating plants is based on the vapour-compression
cycle and such is made up of the following basic processes:
Evaporation:
Liquid at low pressure is evaporated at a low temperature and absorbs
heat from a cold environment.
Compression:
The low pressure vapour at 1 is compressed to a higher pressure.
Condensation:
High pressure vapour is condensed and loses heat to the cooling
medium (water or air).
Expansion valve: High pressure liquid at 3 is expanded to a lower pressure through the
use of an expansion valve or a capillary tube.
2
Condenser
4
Evaporator
Compressor
Expansion
valve
3
1
In this practical, the AMATROL T7082 Thermal Systems Trainer (Figure 1) will be used as the
refrigeration system.
Aim(s):
 To provide the student with experience and confidence in the operation of the
refrigeration system.

To calculate the heat duties for both the evaporator and the condenser.

To calculate the compressor power.
22

To assess the efficiency of the AMATROL T7082 Thermal Systems Trainer by comparing
the actual COP to the theoretical maximum COP.
Theory:
Refer to class notes and literature.
Apparatus and Materials:
Figure 1 below is the schematic representation of the experimental setup.
Capillary tube
Condenser
Evaporator
Figure 1: AMATROL T7082 Thermal Systems Trainer
Operation and experimental procedure:
1. Locate the power switch and make sure that it is in the OFF position.
2. Locate the mode switch and make sure that it is set on STAND-BY.
3. Perform the following to make sure the dual pressure controller is properly set:
a.
Locate the low-pressure cut-out scale and make sure it is set to 5 psi. If it is not, use
a standard screwdriver to turn its adjusting screw.
b.
Locate the cut-in scale and make sure it is set to 45 psi. Adjust if necessary.
23
c.
Locate the high-pressure cut-out setting on the right side of the pressure controller
and make sure it is set to 200 psi. Adjust if necessary.
4. Reset the refrigerant valves to the following positions (if necessary):
V1: Open
V4: Open
V2: Closed
V5: Closed
V3: Closed
V6: Closed
5. Locate the electric cord and plug into a standard wall outlet.
6. Shut the evaporator damper ¾ closed (notch 6 of 8).
7. Turn the power switch to the ON position. You should see the LCD on the electronic
temperature controller display the room (ambient) temperature.
8. Record the room temperature being displayed by the electronic temperature controller.
This is the HOT RESERVOIR TEMPERATURE.
9. Perform the following to ensure that the electronic temperature controller is set properly.
a.
Press the button marked SET on the controller.
b.
Use the up and down arrow keys to select degrees Fahrenheit (°F) on the LCD.
c.
Press the SET button once again to enter into the set temperature mode. Use the
arrow keys to select 55°F for the set point. The compressor will disengage when this
temperature is reached. This is set lower that the room temperature so that the unit
will run constantly.
d.
Press the SET button once more to enter the temperature differential mode. Use the
arrows to set this to 10°F. This tells the controller at what temperature above set point
to engage the compressor (65°F in this case).
e.
Press the SET button again to enter into the heating / cooling mode selection. Use the
arrow keys to select “C1” on the LCD. This sets the controller for cooling mode.
f.
Finally, press the SET button once more and check the LCD to make sure the room
temperature is being displayed.
10. Locate the flow meter and make sure that the knob is turned fully counter-clockwise (full
open).
11. Turn the mode switch to the COOLING position. The blowers should come on and the
compressor should engage as well.
12. Allow the trainer to operate until it comes to steady state conditions (there should be no
bubbles flowing through the flow meter). The trainer may take 5 – 20 minutes to attain
steady state conditions.
13. Record the flow in the flow meter (height of the ball). Read the meter from the middle of
the ball against the scale.
14. Record the pressures and temperatures on the system gauges. Convert the indicated
gauge pressure to absolute pressure.
PS-1
TS-1
PS-2
TS-2
PS-3
TS-3
PS-4
TS-4
24
15. Before shutting down the trainer, record the air temperature coming out of the evaporator
(evaporator outlet temperature). This is displayed on the Electronic Temperature control
panel. This is the COLD RESERVOIR TEMPERATURE.
16. To shut down the trainer, turn the mode switch to STAND-BY and then turn the power
switch to the OFF position.
Calculations:
1. Calculate the mass flow rate of the refrigerant in kg/s.
2. Calculate the heat duty of the evaporator (Capacity, Q) in kW.
3. Calculate the heat duty of the condenser in kW.
4. Calculate the compressor power.
5. Calculate the maximum theoretical coefficient of performance.
6. Calculate the actual coefficient of performance.
Reference(s):
Cengel, YA, Boles MA. “Thermodynamics: An Engineering Approach” 8th edition, 2015,
McGraw-Hill Education.
Appendices:
Table 1: Chart used to determine Volumetric Flow Rate.
Figure 2: P-h diagram for Refrigerant R-134a
Table 1: Determination of the Volumetric Flow Rate
25
PRACTICAL: SEDIMENTATION OF FINE PARTICLES
1. Objectives
1. To observe the sedimentation of concentrated suspensions during type 1 and type 2
settling.
2. To perform a batch settling test and construct its settling curve.
3. To use the Kynch method to determine the cross-sectional area of a settling tank
required for a given underflow concentration at a given feed-rate.
2. Theory
A: clear liquid
A: clear liquid
B: constant composition zone
C zone of
variable
composition
C zone of variable composition
D: Sediment
D: Sediment
Type
Type II
Figure 8.1 Concentrated suspensions of fine particles will settle in one of two ways
Type I suspension: After an initial brief acceleration period, the interface between the clear
liquid and the suspension moves downward at a constant rate and a layer of sediment builds
up at the bottom of the container. When this interface approaches the layer of sediment,
its rate of fall decreases until the “critical settling point is reached when a direct interface is
formed between the sediment and the clear liquid. Sedimentation then results solely from a
consolidation of the sediment, with liquids being forced upwards around the solids, which
are then forming a loose bed with the particles in contact with one another. Since the flow
cross-sectional area is being gradually reduced, the rate of settling gradually diminishes.
Type II suspensions: This obtains when the range of particle size varies greatly. In this
case there is no zone of constant composition. The sedimentation rate progressively
decreases throughout the whole operation. Zone C extends from the top interface to the
layer of sediment.
26
The Kynch method of calculating cross-sectional area of sedimentation tank:
Kynch’s theory: The basic assumptions of his theory are that (i) particle concentration
is uniform across any horizontal layer, (ii) wall effects can be ignored, (iii) no differential
settling of particles as a result of differences in shape and size iv) the velocity of fall of any
particle depends only on the local concentration of particles.
The flux ψ defined as the volumetric rate of sedimentation per unit area. It is given by
𝜓 = 𝐶𝑢𝑐
where C is the volumetric concentration of particles and uc is the sedimentation velocity.
A
T
Interface Height
B
(H)
C
O
Time (t)
Figure 8.2 Interface plotted against time with the tangents drawn at various points
Because the concentration rate of the particles is initially uniform and the sedimentation
rate depends only on the particle concentration, line AB will be straight having a slope equal
to uc. After point B the sedimentation curve has a decreasing negative slope, reflecting
the
increasing
the concentration of solids at the interface. It can be shown that the
concentration C at any level is given by:
𝑐=𝑐
𝑘𝑔/𝑚
8.1
and therefore the corresponding solids flux by,
𝑐=𝑐
𝑢
𝑘𝑔/𝑚 𝑠
8.2
So by drawing the tangent at a series of points on the curve ABC and measuring the
corresponding slope –uc and intercept OT, it is possible to establish the solids flux ψ
27
for any concentration. To calculate the required cross-sectional area of a settling tank for a
specified concentration of overflow, it is necessary to establish, often by experiment, the slurry
concentration at which the total flux is minimum. This is the concentration that we use in the
design calculation. According to the Kynch theory, the required area can be calculated from:
𝐴=𝑄 𝑐
8.3
3. Procedure
1. Prepare two samples of slurry, sample I and sample II, of concentration 200kg/m3 by
measuring out the necessary mass of ore and mixing it with the required amount of water
to make 1 liter of the suspension..
2. Into sample II, poor 5 ml of the flocculent provided using a pipette.
3. Vigorously shake each sample and observe how the solids settle in each case. Compare
the sampling pattern in the two cylinders. Record your observations and classify the type
of settling observed in each cylinder.
4. Vigorously shake sample I until all the sediment has returned into suspension.
Using a stopwatch record the height of the constant composition/clear liquid zones until
the height of the sediment stops changing over a 10 minute period.
5. Plot a graph of height of interface against time.
4. Calculation
1. The initial constant slope of the curve gives the sedimentation velocity (uc)o. Use graph
paper to determine (uc)o.
2. The slope of the tangent to the curve gives the sedimentation velocity uc at a given
instant.
3. The concentration of solids at that instance is given by where OA is the initial height of
the interface and OT is the height of interface at the instant in question.
4.
The solids flux in kg/m2is given by:
𝒄
𝑘𝑔
𝑐𝑚
1
× 𝒖𝑐
×
𝑚3
𝑚𝑖𝑛
100 × 60
28
5. for each height tabulate the value 1000
−
6. Use the tabulated values in (5) above to calculate the total flux
𝜓𝑇
values:
𝑢
1
1
𝑐− 𝑐
7. Tabulate the values of the reciprocal of the total flux
gives you the limiting total flux
𝜓𝑇 . The maximum of these values
𝜓𝑇𝐿 .
8. The cross-sectional area of the sedimentation tank required is given by equation 1.
29
PRACTICAL: COOLING TOWER
AIM
Familiarise students with the working of a cooling tower and to complete mass and energy
balances around the cooling tower in order to determine the mass of water lost due to
evaporation.
LIST OF SYMBOLS
Symbol
Description
Unit
h
ṁ
n
p
PT
Q
R
T
VT
l
Enthalpy
Mass flow rate
moles
Pressure
Total Pressure
Flow rate
Ideal gas constant
Temperature
Total Volume
Height
kJ/kg
m3/hr
mol
Pa
Pa
m3/hr
Density
Efflux coefficient (0.63)
Kg/m3
J .K -1 .mol 1
K
m3
m
Greek Symbols

ɛ
Subscripts
1
2
a
atm
s
w
o
Bottom boundary of Column
Top boundary of Column
Air
Atmospheric
Property of the superheated vapour at the dry bulb
temperature
Water
Orifice
THEORY
Cooling towers operate with the principle of cooling water by exposing its surface to air, in
so doing, cooling towers use the evaporation of water molecules in order to remove process
heat, allowing the working fluid temperature to be in similar to the wet- bulb air temperature.
Although cooling towers are found in different shapes and sizes, the principle by which it
operates is the same.
Heat transfer involves latent heat transfer due to the evaporation of some water as well as
sensible heat transfer due to the temperature difference between the air and water. The
theoretical amount of heat that can be removed depends on temperature and moisture
30
content of the air. Therefore, the wet bulb temperature of the air is the minimum temperature
that the water can be cooled. This value is never attained, because not all the water can be
contacted with fresh air. The extent to which this value is attained, is determined by (i) airto-water contact time, (ii) amount of fill surface, and (iii) the extend of droplet formation.
Cooling towers are used in a vast amount of industries such as oil refineries, petrochemical
plants, thermal power stations as well as HVAC systems. The type of cooling tower used is
dependent upon the type of air induction into the tower; the main types are natural draft and
mechanical draft cooling towers.
The natural draft type is usually utilised by large power plants. Hot water is introduced to
the bottom third of the tower and spread evenly through the tower cross-section. Packing
is used to increase the surface area between the water and the air. As the heat is transfer
from the water to the air, the air temperature rises, density decreases and it starts to rise.
Colder more dense air replaces the hot air and a natural draft is created.
The mechanical draft towers utilise fans to force the air through the tower. These towers
can be subdivided into two classes, namely (i) forced-draft and (ii) induced-draft towers. In
the former tower, the fan is mounted at the bottom of the tower, whereas the latter the fan
is at the top.
Performance of cooling towers
The performance of a cooling tower is dependent on the following factors:
1. Air flow
2. Water flow
3. Water temperature
4. Temperature and humidity of air at the outlet
5. Type of packing used
6. Area and volume of packing
Cooling tower operation
Warm water from the heat source is pumped to the water distribution system at the top of
the tower. The water is distributed over the wet-deck fill by means of nozzles.
Simultaneously, air is drawn through air-inlet louvers and through the wet-deck surface
causing a small portion of the water to evaporate. The evaporative process removes heat
from the water. The warm, moist air is drawn out of the top of the tower. The resulting cold
water is then recirculated back through the heat source in a continuous cycle.
31
Mass Balance
In many engineering textbooks giving calculations concerning cooling towers, the water loss
from the tower is presented simply as the product of air mass flow and change in specific
humidity of the air across the tower. The result of this calculation is taken to be the quantity
of ‘make up’ water required to make good the evaporative loss.
In actual practice this result might prove to be an embarrassing and misleading
approximation. ‘Drift’ or ‘Free moisture carry-over’ are terms used to describe free droplets
of water being carried out of the Tower by entrainment with the air. The actual amount of
moisture carry over is governed by the following:
(a) The velocity of the air passing through the Tower.
(b) The temperature of the hot water entering the tower
Ideal gas laws
Dalton and Gibbs law
The behaviour of air, which is composed of a mixture of “dry air” (oxygen, nitrogen and other
gases) with steam, can be explained through Gibbs laws, from where the following
conclusions can be reached:
-
Pressure of air is equal to the sum of pressures of dry air and steam.
-
Dry air and steam, respectively, follow their normal relations of partial pressure.
-
Enthalpy of the mixture can be obtained by adding the enthalpies of dry air and of steam if
each one took up the whole space occupied by the mixture, being both to the same
temperature
PT VT  nRT
Where,
PT  Porifice  Patm
R = 8.314 J .K -1 .mol 1
This relationship and the specific humidity at the top of the column are used to calculate the density
of the air at that point in order to obtain the mass flow rate. The change in the orifice pressure can
be deemed negligible.
Energy Balance
32
By applying the equation Q-P = HOutlet - HInlet and assuming that P= 0 and where Q’s value is limited
due to heat transfer between the unit and its surrounding. The equation becomes:
HOutlet = HInlet
The derived energy balance below can be used in order to determine if the energy taken in by the
air is equal to that supplied by the water.
m a 2 ha 2  m a1ha1   m s 2 hs 2  m s1hs1   g l 2 m a 2  m s 2   l1 m a1  m s1 
 m w 2 hw 2  m w1 hw1   g l 2 m w 2  l1 m w1 
33
EXPERIMENTAL SET UP AND APPARATUS
Figure 2: Schematic of Bench Top Cooling Tower
PRE-PRACTICAL
SIMILAR INDUSTRIAL SCALES COOLING TOWERS ARE AVAILABLE AT CPUT. Use
Microsoft Visio to draw a detailed process and instrumentation flow diagrams and
brief general operation of the cooling towers. The pre-practical must be submitted on
the day of the Practical.
EXPERIMENTAL PROCEDURE
1. Ensure that the level of water in the tank is sufficiently above the level sensor.
2. Check Wet Bulb Temperature reservoir water levels.
3. Select a set point temperature for the water temperature.
4. Allow the water tank to reach the set point temperature.
5. Record the level in the tank
6. Choose an air flow and water flow rate and wait till it is reached
7. Once steady state is reached record the level change after 15 minutes
8. Repeat the exercise with a different set-point temperature, water flow rate or air flow
rate.
INSTRUCTIONS
34
1. Do a complete mass balance over the cooling tower and determine the mass of water
that evaporates. Compare this with the measured mass of evaporated water.
1.1. Psychrometric charts (Determine specific humidity)
1.2. Ideal gas laws (Determine total volume; Determine density of air)
1.3. Orifice calculations (Determine flow rate of air)
2. Perform energy balance calculations to ensure that the energy taken in by the air is
equal to that supplied by the water.
35
29