Saturation Pressure Instruction Manual TH3

Saturation Pressure
Instruction Manual
TH3
ISSUE 15
November 2012
Table of Contents
Copyright and Trademarks ...................................................................................... 1
General Overview ....................................................................................................... 2
Equipment Diagrams................................................................................................... 3
Certificate of Conformity.............................................................................................. 6
Important Safety Information....................................................................................... 7
Introduction.............................................................................................................. 7
The COSHH Regulations ........................................................................................ 7
Description .................................................................................................................. 9
Overview.................................................................................................................. 9
Installation ................................................................................................................. 11
Advisory................................................................................................................. 11
Electrical Supply .................................................................................................... 11
Installing the optional PC software ........................................................................ 12
Installing the Equipment ........................................................................................ 12
Commissioning ...................................................................................................... 13
Electrical Wiring Diagram ...................................................................................... 14
Operation .................................................................................................................. 15
Operating the optional PC Software ...................................................................... 15
Operating the Equipment....................................................................................... 15
Equipment Specifications.......................................................................................... 17
USB Channel Numbers ......................................................................................... 17
Environmental Conditions...................................................................................... 18
Routine Maintenance ................................................................................................ 19
Responsibility ........................................................................................................ 19
General.................................................................................................................. 19
Laboratory Teaching Exercises................................................................................. 20
Index to Exercises ................................................................................................. 20
Nomenclature ........................................................................................................ 20
Data Sheet 1.......................................................................................................... 22
ii
Table of Contents
Data Sheet 2.......................................................................................................... 23
Data Sheet 3.......................................................................................................... 25
Data Sheet 4.......................................................................................................... 26
Data Sheet 5.......................................................................................................... 27
Data Sheet 6.......................................................................................................... 28
Data Sheet 7.......................................................................................................... 30
Exercise A - Characteristic behaviour of a two phase fluid ....................................... 34
Exercise B - Principles of saturation pressure measurement ................................... 38
Exercise C - Concept of a saturation line.................................................................. 42
Exercise D - Steam tables......................................................................................... 45
Exercise E - Use of the steady flow energy equation................................................ 47
Contact Details for Further Information ..................................................................... 52
iii
Disclaimer
This document and all the information contained within it is proprietary to Armfield
Limited. This document must not be used for any purpose other than that for which it
is supplied and its contents must not be reproduced, modified, adapted, published,
translated or disclosed to any third party, in whole or in part, without the prior written
permission of Armfield Limited.
Should you have any queries or comments, please contact the Armfield Customer
Support helpdesk (Monday to Thursday: 0830 – 1730 and Friday: 0830 - 1300 UK
time). Contact details are as follows:
United Kingdom
International
(0) 1425 478781
(calls charged at local rate)
+44 (0) 1425 478781
(international rates apply)
Email: support@armfield.co.uk
Fax: +44 (0) 1425 470916
Copyright and Trademarks
Copyright © 2012 Armfield Limited. All rights reserved.
Any technical documentation made available by Armfield Limited is the copyright
work of Armfield Limited and wholly owned by Armfield Limited.
Brands and product names mentioned in this manual may be trademarks or
registered trademarks of their respective companies and are hereby acknowledged.
1
General Overview
The TH range is designed to introduce the fundamental principles of thermodynamics
to the student. The range of equipment starts at basic concepts such as temperature
and pressure measurement and leads on to introducing the relationships between
these fundamentals, the first and second law of thermodynamics, the principles of
reversibility, entropy, enthalpy etc.
The equipment allows the student to gain a true understanding of these principles.
The small scale of the equipment allows the relevant teaching exercises to be carried
out in a relatively short period of time.
This instruction manual describes the operation of the TH3 'Saturation Pressure'
apparatus that has been designed by Armfield to introduce students to the concept of
saturation pressure and how different techniques can be employed to measure this
variable.
The electrical output from the sensors are available as a voltage signal for direct
connection to a PC via an optional interface device (a PC interface and Windows
based Educational Software is available to support the TH3 Saturation Pressure
apparatus).
2
Equipment Diagrams
Figure 1: Top View of TH3 Saturation Pressure Apparatus
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Armfield Instruction Manual
Figure 2: Side View of TH3 Saturation Pressure Apparatus
4
Equipment Diagrams
Figure 3: Front and Rear View of Electrical Console for TH3 Saturation Pressure Apparatus
5
Certificate of Conformity
Our Ref:
EES/AK/Dec
PRESSURE EQUIPMENT REGULATIONS
CERTIFICATE OF CONFORMITY
Supplier:
Armfield Limited
Product Code: TH3-A/B/G (Saturation Pressure Apparatus)
Declaration:
We certify that the boiler vessel and pipework installed on the above Armfield product
has been designed, manufactured and hydraulically pressure tested in accordance
with the Pressure Equipment Regulations 1999 (Statutory Instruments, 1999 No
2001) Category 1.
The boiler vessel has been classified Category 1 as follows:
Type of equipment:
Steam Generator
Volume of vessel:
2.4 litres
Maximum allowable pressure:
8 bar gauge
Classification chart:
5
PS.V:
19.2 (<50)
Conformity assessment module: A
The detailed design of the pressure vessel is in accordance with BS5500:1997
(British Standard Specification for unfired fusion welded pressure vessels).
The following information is duplicated on a plate attached to the boiler vessel:
Manufactured by:
See plate on equipment
Armfield index no:
See plate on equipment
Maximum working pressure (PS):
8 bar gauge
Maximum working temperature (Tmax):
180ºC
Hydraulic test pressure:
12 bar gauge
Date of manufacture:
See plate on equipment
For and on behalf of Armfield Ltd.
E E Sansom
Technical Director Date:
14 June 2004
6
Important Safety Information
Introduction
All practical work areas and laboratories should be covered by local safety
regulations which must be followed at all times.
It is the responsibility of the owner to ensure that all users are made aware of
relevant local regulations, and that the apparatus is operated in accordance with
those regulations. If requested then Armfield can supply a typical set of standard
laboratory safety rules, but these are guidelines only and should be modified as
required. Supervision of users should be provided whenever appropriate.
Your TH3 Saturation Pressure Apparatus has been designed to be safe in use
when installed, operated and maintained in accordance with the instructions in this
manual. As with any piece of sophisticated equipment, dangers exist if the equipment
is misused, mishandled or badly maintained.
Before proceeding to install, commission or operate the equipment described in this
instruction manual we wish to alert you to potential hazards so that they may be
avoided.
Although designed for safe operation, any laboratory equipment may involve
processes or procedures that are potentially hazardous. The major potential hazards
associated with this particular equipment are listed below.

INJURY THROUGH MISUSE

INJURY FROM ELECTRIC SHOCK

INJURY FROM INCORRECT HANDLING

BURNS FROM COMPONENTS AT HIGH TEMPERATURES

SCALDING FROM BOILING WATER AND STEAM

DAMAGE TO CLOTHING

RISK OF INFECTION DUE TO LACK OF CLEANLINESS
Accidents can be avoided provided that equipment is regularly maintained and
staff and students are made aware of potential hazards. A list of general safety
rules is included in this manual, to assist staff and students in this regard. The list is
not intended to be fully comprehensive but for guidance only.
Please refer to the notes below regarding the Control of Substances Hazardous to
Health Regulations.
The COSHH Regulations
The Control of Substances Hazardous to Health Regulations (1988)
The COSHH regulations impose a duty on employers to protect employees and
others from substances used at work, which may be hazardous to health. The
regulations require you to make an assessment of all operations that are liable to
expose any person to hazardous solids, liquids, dust, vapours, gases or micro-
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Armfield Instruction Manual
organisms. You are also required to introduce suitable procedures for handling these
substances and keep appropriate records.
Since the equipment supplied by Armfield Limited may involve the use of substances
which can be hazardous (for example, cleaning fluids used for maintenance or
chemicals used for particular demonstrations) it is essential that the laboratory
supervisor or some other person in authority is responsible for implementing the
COSHH regulations.
Parts of the above regulations are to ensure that the relevant Health and Safety Data
Sheets are available for all hazardous substances used in the laboratory. Any person
using a hazardous substance must be informed of the following:
Physical data about the substance
Any hazard from fire or explosion
Any hazard to health
Appropriate First Aid treatment
Any hazard from reaction with other substances
How to clean/dispose of spillage
Appropriate protective measures
Appropriate storage and handling
Although these regulations may not be applicable in your country, it is strongly
recommended that a similar approach is adopted for the protection of the students
operating the equipment. Local regulations must also be considered.
8
Description
Where necessary, refer to the drawings in the Equipment Diagrams section.
Overview
The equipment is a bench top unit designed to introduce students to the
characteristics of saturated water vapour, ie. how the temperature of water behaves
at its boiling point with variation in the absolute pressure.
The equipment consists of a rectangular pipe loop incorporating a boiler (2) in one
vertical limb. Pure water in the boiler is heated to its boiling point using a pair of
cartridge heaters (11) that are located near the bottom. A sight glass (10) on the front
of the boiler allows the internal processes to be observed, namely boiling patterns at
the surface of the water while heating or reducing the system pressure and cessation
of boiling/condensation during cooling. The sight glass also allows the water level in
the boiler to be monitored. Saturated steam leaving the top of the boiler passes
around the pipe loop before condensing and returning to the base of the boiler for reheating. The operating range of the boiler and loop is 0 to 7 bar gauge.
The top limb of the pipe loop incorporates a PRT temperature sensor (3) and an
electronic pressure sensor (9) to measure the properties of the saturated steam. A
filling point (38) on the top limb allows the loop to be filled with pure water and allows
all air to be vented safely before sealing the loop for pressurised measurements
using the filling valve (4).
A vapour offtake, with isolating valve (6), allows steam from within the loop to be
passed through a Throttling Calorimeter (7), the purpose of which is to demonstrate
how the dryness fraction of the saturated steam in the loop can be determined.
The steam expands to atmospheric pressure as it passes along a labyrinth (13) and
a second PRT temperature sensor (14) installed inside the calorimeter is used to
measure the temperature of the steam after it has expanded to atmospheric
pressure. A container (15) below the calorimeter collects condensing vapour and
allows it to be drained safely from the apparatus.
In normal use the power control on the electrical console is adjusted to produce the
required rate of climb in steam temperature for saturation pressure measurements or
to produce a steady stream of steam to the Throttling Calorimeter at the required
system pressure.
The apparatus is designed for safe operation with the following safety features:
A pressure relief valve (5) is incorporated at the outlet from the boiler. The relief valve
is designed to operate if the pressure rises above the above 8 bar working pressure
in the boiler. The outlet from the relief valve is vented through a pipe (41) into the
container below the calorimeter to prevent injury to the operator.
The apparatus is pressure tested prior to despatch.
A Bourdon gauge (8), marked with the maximum working pressure, gives an
approximate indication of the steam pressure in the loop and remains operational
when power is disconnected from the electrical console.
The boiler and pipe loop are mounted in a support frame (1) that incorporates clear
plastic shields (12) at the front and the back to provide protection against inadvertent
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Armfield Instruction Manual
contact with the exposed hot surfaces on the apparatus. These shields must be fitted
when the equipment is in use.
All power supplies, signal conditioning circuitry etc are contained in a simple
electrical console (20) with appropriate current protection devices and an RCD (21)
for operator protection. The electrical console is designed to stand alongside the pipe
loop on the bench top.
All circuits inside the console are protected against excessive current by miniature
circuit breakers as follows:
CONT (22) This breaker protects the power supply and circuits inside the
console.
HEAT (23) This breaker protects the heaters inside the boiler.
O/P (24) This breaker protects the electrical output marked OUTPUT (35) at
the rear of the console. The socket is used to power the IFD interface used for
data logging.
Readings from the sensors are displayed on a common digital meter (26) on the
electrical console. A selector switch (27) and all corresponding signals are connected
to an I/O port (28) for connection to a PC using an optional parallel interface with
educational software package. Alternatively, the signals can be connected to a user
supplied chart recorder if required.
Before use, the boiler vessel and pipe loop must be filled with pure water
(demineralised or distilled) via the filling point on the top of the pipe loop. To obtain
accurate results when operating the equipment, it is essential to boil the water
thoroughly before closing the filling valve to expel all traces of air from the system.
Full details are given in Commissioning and the Operation section.
Note: An accurate Barometer (not supplied) will be required to determine the
absolute pressure.
10
Installation
Advisory
Before operating the equipment, it must be unpacked, assembled and installed as
described in the steps that follow. Safe use of the equipment depends on following
the correct installation procedure.
Where necessary, refer to the drawings in the Equipment Diagrams section.
Electrical Supply
Refer to Figure 3 in the Equipment Diagrams.
Initially fill with pure water (3 litres) and replenish as consumed.
Before connecting the TH3 to the mains electrical supply ensure that the apparatus
has been assembled as described in the Assembly section of this instruction manual.
Before connecting the appropriate electrical supply check the following:
Ensure that the mains on/off switch (30) on the front of the console is in the
OFF position.
Electrical Supply for Version TH3-A
The equipment requires connection to a single phase, fused electrical supply. The
standard electrical supply for this equipment is 220/240V, 50Hz. Check that the
voltage and frequency of the electrical supply agree with the label attached to the
supply cable on the equipment. Connection should be made as follows:
GREEN/YELLOW
-
EARTH
BROWN
-
LIVE (HOT)
BLUE
-
NEUTRAL
Fuse rating
-
5 AMP
Electrical Supply for Version TH3-B
The equipment requires connection to a single phase, fused electrical supply. The
standard electrical supply for this equipment is 120V, 60Hz. Check that the voltage
and frequency of the electrical supply agree with the label attached to the supply
cable on the equipment. Connection should be made as follows:
GREEN/YELLOW
-
EARTH
BROWN
-
LIVE (HOT)
BLUE
-
NEUTRAL
Fuse rating
-
10 AMP
Electrical Supply for Version TH3-G
The equipment requires connection to a single phase, fused electrical supply. The
standard electrical supply for this equipment is 220-240V, 60Hz. Check that the
11
Armfield Instruction Manual
voltage and frequency of the electrical supply agree with the label attached to the
supply cable on the equipment. Connection should be made as follows:
GREEN/YELLOW
-
EARTH
BROWN
-
LIVE (HOT)
BLUE
-
NEUTRAL
Fuse rating
-
5 AMP
Installing the optional PC software
If it is required to operate TH3 using the optional software supplied with TH306IFD
then it will be necessary to install the software from the CD-ROM supplied with
TH306IFD onto an appropriate PC (PC not supplied).
For instructions on how to install and run the software insert the CD-ROM into the
optical drive on the PC (PC not supplied) then choose ‘Help’ from the menu.
After installing and running the software on the PC, instructions on how to operate
the software can be obtained by choosing the ‘Help’ tab in the top right hand corner
of the screen as shown below:
Note that when operating the software for the first time it will be necessary to enable
the USB virtual COM port by choosing the Red telephone icon (Start COM session).
Full instructions about enabling the port are included in the Help menus.
Installing the Equipment
Remove the TH3 apparatus and console (20) from the box, and remove all
packaging, taking care not to discard any components.
Place the TH3 apparatus on a solid work surface away from flammable substances,
with the Bourdon pressure gauge dial (8) facing towards the operator. Place the
console to the right of the apparatus.
12
Installation
Plug the mains cable (29) into the socket marked INPUT (25) on the rear of the
console, and connect to the mains electrical supply. Do not switch on the mains
supply at this stage.
Plug the heater cables (19) into the HEATER sockets (31) on the rear of the console.
Plug the pressure transducer cable (18) into the PRESSURE socket (32) on the rear
of the console.
Plug the cable from the platinum resistance thermometer T1 (16) into the PT100(1)
socket on the rear of the console (34).
Plug the cable from the platinum resistance thermometer T2 (17) into the PT100(2)
socket on the rear of the console (33).
Attach a length of tubing/place a cup (not provided) beneath the container (15), to
catch any water dripping from it.
Commissioning
The following procedure should be followed to verify the operation of the TH3.
Check that the RCD (21) and three circuit breakers (22, 23 and 24) on the rear of the
console are all in the up position (ON).
Check that he mains switch (30) on the front of the console is switched off. Check
that the heater switch (36) is off (0) and the heater power control (37) is set to
minimum.
Check that the drain valve (39) is closed using the tool supplied.
Check that the calorimeter isolating valve (6) is closed – lever vertical.
Open the filler valve (38) on the top of the apparatus using the tool supplied.
Slowly fill the system with pure water (preferably deionised or demineralised) until the
level reaches ¾ of the way up the sight glass (10) on the front of the boiler.
Approximately 1.75 litres of water will be required. It may be helpful to use a funnel
(not supplied) when filling the apparatus. Do not close the filler valve at this point.
Switch on the electrical supply to the console then check the operation of the RCD by
pressing the TEST button on the RCD. The RCD must trip when the button is
pressed. If the RCD does not trip or it trips before pressing the test button then it
must be checked by a competent electrician before the equipment is used.
Switch on the mains power switch (30) on the console. The digital display should be
illuminated.
Use the selector switch to check that initial readings for PT100(1), PT100(2) and
Pressure P are sensible. P should read approximately 0 kN/m2. The actual values for
PT100(1) and PT100(2) will depend on the ambient temperature but a typical reading
will be 109 Ohms at 20ºC (Refer to Data Sheet 1 for corrections to the resistance
bridge, then to Data Sheet 2 for a full table of temperature vs. resistance values).
Close the isolating valve (6) to the calorimeter. Switch on the heater switch (36) then
set the power control (37) to maximum and allow the water to heat. When the water
is boiling, with steam escaping from the filler valve, gradually reduce the heater
13
Armfield Instruction Manual
power to maintain a trickle of steam. This will allow all air to be removed from the
system without excessive loss of water or rise in system pressure.
Confirm that the pressure in the system remains at approximately 0 kN/m2. Wait until
the reading from PT100(1) is approximately 138.0 Ohms indicated, 138.5 Ohms
corrected resistance, corresponding to the boiling point of water at a temperature of
100ºC (Actual readings will depend on the actual atmospheric pressure).
When all air has been expelled from the system and the reading from temperature
sensor PT100(1) is sensible close the filler valve using the tool supplied, taking care
to avoid the escaping steam. Increase the power control to maximum and allow the
pressure/temperature in the system to rise. Observe that the readings for pressure P
and temperature corresponding to resistance reading PT100(1) both increase
together. When the pressure indicated is approximately 4 bar gauge (400 kN/m2)
check that the reading from PT100(1) is approximately 154 Ohms indicated, 158
Ohms corrected corresponding to a temperature of 152ºC. Open the isolating valve
(6) to the throttling calorimeter. Wait for the reading from PT100(2) to stabilise then
confirm that the reading is typically 140-142 Ohms, the actual reading depending on
the actual quality of the steam and the atmospheric pressure. Close the calorimeter
isolating valve and continue heating to confirm that the relief valve operates at no
more than 8 bar gauge (800 kN/m2).
Switch off the heater power, open the calorimeter isolating valve and allow the
system to cool to atmospheric pressure/temperature with steam gradually escaping
into the container below the throttling calorimeter.
DO NOT attempt to open the filler valve or drain valve until the system pressure has
reduced to atmospheric pressure (the reading on the Bourdon gauge must be zero
before opening the filler or drain valve).
Commissioning of the TH3 Saturation Pressure Apparatus is complete and the
equipment is ready for performing the Laboratory Teaching Exercises.
Electrical Wiring Diagram
Click on the relevant link to invoke the Wiring Diagram:
Wiring Diagram CDM27764H
Printed Versions of this Instruction Manual
Please note, all wiring diagrams are appended at the rear of this manual. If viewing
this Instruction Manual via Help Text in Armfield Software refer to the printed version
of the manual for these diagrams.
14
Operation
Where necessary, refer to the drawings in the Equipment Diagrams section.
Operating the optional PC Software
Details about operating the optional software can be obtained by choosing the ‘Help’
tab in the top right hand corner of the screen as shown below:
Operating the Equipment
The following procedure should be followed to fill the TH3 apparatus
before use
Check that the mains switch (30) on the front of the console is switched off.
Check that the drain valve (39) is fully closed using the tool supplied.
Close the calorimeter isolating valve (6) - lever on valve vertical.
Open the filler valve (38) on the top of the apparatus, using the tool supplied.
Slowly fill the system with pure water (preferably deionised or demineralised) until the
level reaches ¾ of the way up the sight glass (10) in the side of the boiler (2). This
requires approximately 1¾ litres of water. It may be helpful to use a funnel (not
supplied) during this procedure.
Do not close the filler valve at this point.
The following procedure should be followed to expel air from the TH3
apparatus before use
Air remaining in the pipework or expelled from the water when boiled for the first time
will affect the accuracy of the measurements taken and can cause significant errors.
To avoid this, the following procedure should be carried out:
Switch on the electrical console, switch on the heater (36) and set the power control
(37) to maximum. Allow the water to heat until it is boiling.
When the water is boiling, with steam escaping from the filler valve, gradually reduce
the heater power to maintain a trickle of steam. This will allow all air to be removed
from the system without excessive loss of water or rise in system pressure.
Confirm that the pressure in the system remains at approximately 0 kN/m2. Wait until
the reading from PT100(1) is approximately 138.0 Ohms indicated, 138.5 Ohms
corrected resistance, corresponding to the boiling point of water at a temperature of
100ºC (Actual readings will depend on the actual atmospheric pressure).
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Armfield Instruction Manual
When all air has been expelled from the system and the reading from temperature
sensor PT100(1) is sensible close the filler valve (38) using the tool supplied, taking
care to avoid the escaping steam. Increase the power control to maximum and allow
the pressure/temperature in the system to rise as required for the exercise being
performed.
The following procedure should be followed to drain the TH3 apparatus
after use
IMPORTANT: Allow the apparatus to cool to room temperature before attempting to
drain the water.
Open the calorimeter isolating valve (6) and check that the internal and external
pressures have equalised (the internal pressure should be approximately 0 kN/m2 or
0 bar gauge). The Bourdon gauge (8) provides a convenient visual indication of the
internal pressure and must read zero before opening the drain valve.
Attach a length of tubing (not provided) to the drain valve near the base of the boiler
(39), and place the other end of the tube in a suitable container or drain.
Open the filler valve (38) using the tool supplied.
Open the drain valve (39) using the tool supplied, and allow the water to drain from
the system.
NOTE: If deionised or demineralised water has been used for filling, the apparatus
need not be drained except for storage.
16
Equipment Specifications
USB Channel Numbers
The channel numbers for the USB port are listed below for information:
Pin No
Channel No
Signal Function
Analog Outputs (0-5 V dc exported from socket):
1
Ch 0 Signal
2
Ch 0 Return
3
Ch 1 Signal
4
Ch 1 Return
5
Ch 2 Signal
6
Ch 2 Return
7
Ch 3 Signal
8
Ch 3 Return
9
Ch 4 Signal
10
Ch 4 Return
11
Ch 5 Signal
12
Ch 5 Return
13
Ch 6 Signal
14
Ch 6 Return
15
Ch 7 Signal
16
Ch 7 return
17-21
Not used
PRT100(1) (0V = 0 Ohms, 5V = 164.8 Ohms)
Not used on TH3
Not used on TH3
Not used on TH3
Not used on TH3
Pressure 0V = 0 Kn/m2, 5V = 1378 Kn/m2
PRT100(2) (0V = 0 Ohms, 5V = 164.8 Ohms)
Not used on TH3
Analog Inputs (0-5V dc input from socket):
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Armfield Instruction Manual
22-25
Not used
Digital Outputs (0-5V dc):
26-37
Not Used
Digital Inputs (0-5V dc):
38-50
Not used
Environmental Conditions
This equipment has been designed for operation in the following environmental
conditions. Operation outside of these conditions may result reduced performance,
damage to the equipment or hazard to the operator.
a. Indoor use;
b. Altitude up to 2000m;
c. Temperature 5°C to 40°C;
d. Maximum relative humidity 80% for temperatures up to 31°C, decreasing
linearly to 50% relative humidity at 40°C;
e. Mains supply voltage fluctuations up to ±10% of the nominal voltage;
f.
Transient over-voltages typically present on the MAINS supply;
Note: The normal level of transient over-voltages is impulse withstand (overvoltage) category II of IEC 60364-4-443;
g. Pollution degree 2.
Normally only nonconductive pollution occurs.
Temporary conductivity caused by condensation is to be expected.
Typical of an office or laboratory environment
18
Routine Maintenance
Responsibility
To preserve the life and efficient operation of the equipment it is important that the
equipment is properly maintained. Regular maintenance of the equipment is the
responsibility of the end user and must be performed by qualified personnel who
understand the operation of the equipment.
General
In addition to regular maintenance the following notes should be observed:
1. The TH3 should be disconnected from the electrical supply when not in use.
2. Water should be drained from the apparatus after use to minimise fouling.
Note: If it is necessary to change the pressure sensor then any offset in the
output from the sensor should be eliminated by adjusting VR14 on the PCB
inside the console to give a reading of 0 kN/m² on the display with the system
open to atmosphere.
If recalibration of the PT100 bridge circuits is necessary then calibration should be
carried out with the system boiling at atmospheric pressure. Adjust VR10 for
PT100(1) or VR11 for PT100(2) until the display reads the correct resistance- use
Data Sheet 7, Data Sheet 2 and then Data Sheet 1 to obtain the resistance
corresponding to the boiling point of water at the local atmospheric pressure (a
barometer will be required).
19
Laboratory Teaching Exercises
Index to Exercises
Exercise A - Characteristic behaviour of a two phase fluid
Exercise B - Principles of saturation pressure measurement
Exercise C - Concept of a saturation line
Exercise D - Steam tables
Exercise E - Use of the steady flow energy equation
Nomenclature
The following nomenclature has been used for the theory and calculations presented
in this manual:
Name
Symbol Unit
Measured
resistance
R m1
Measured
resistance
Corrected
resistance
Corrected
resistance
Type
Definition

Recorded
Indicated resistance of the platinum
resistance thermometer within the
pipe loop.
R m2

Recorded
Indicated resistance of the platinum
resistance thermometer in the steam
offtake pipe.
R c1

Referenced
Actual resistance of the platinum
resistance thermometer in pipe loop
after correction for bridge circuit.
Referenced
Actual resistance of the platinum
resistance thermometer in steam
offtake pipe after correction for bridge
circuit.
Referenced
Temperature corresponding to
indicated resistance of the platinum
resistance thermometer within the
pipe loop.
Referenced
Temperature corresponding to
indicated resistance of the platinum
resistance thermometer in the steam
offtake pipe.
R c1

Temperature T 1
o
Temperature T 2
o
Pressure
kN/m² Recorded
Pressure reading from electronic
pressure sensor.
s
Time elapsed since the test run was
started.
P1
Elapsed time t
C
C
Recorded
20
Laboratory Teaching Exercises
Absolute
temperature
T abs
K
Absolute temperature at the point of
measurement, as found from the
Referenced or platinum resistance thermometer
reading. May be referenced directly
calculated
from the table provided, or calculated
as:
T abs = T (1 or 2) + 273.15
Actual
temperature
T act
Atmospheric
P atm
pressure
Absolute
pressure
P abs
K
Referenced
True absolute temperature at the
point of measurement, referenced
from the graph of the vapour point of
water using the absolute pressure
P abs .
kN/m² Recorded
Ambient pressure of the surroundings
of the apparatus.
kN/m² Calculated
Absolute pressure corresponding to
the pressure indicated by the
electronic pressure sensor.
Calculated as:
P abs = P 1 + P atm
Enthalpy of
fluid before
throttling
Enthalpy of
fluid after
throttling
h1
h2
kJ/kg Referenced
Enthalpy of steam in the pipe loop.
Referenced using the absolute
temperature corresponding to T 1 .
kJ/kg Referenced
Enthalpy of steam after passing
through the throttling calorimeter.
Referenced using the absolute
temperature corresponding to T 2 .
21
Armfield Instruction Manual
Data Sheet 1
Resistance Bridge Correction Chart
The bridge will be balanced when the PT100 has a resistance of 100. At any other
value of resistance there will be an imbalance in the bridge resulting in an error in the
reading. This chart can be used to correct for this error.
Use of the chart
Take a reading for the resistance of the PT100 from the display on the front of the
console.
Find the closest value to this reading from the ‘Measured resistance’ column of the
table, then read the corresponding ‘Corrected resistance’.
Values may be interpolated for improved accuracy.
22
Laboratory Teaching Exercises
Data Sheet 2
PT100 Platinum Resistance Thermometer Reference Chart
23
Armfield Instruction Manual
PT100 Platinum Resistance Thermometer Reference Chart (cont.)
Use of the chart
Take a reading for the resistance of the PT100 from the display on the front of the
console.
Find the corrected value for the resistance from the Resistance Bridge Correction
Chart (Data Sheet 1).
Find the closest value to the corrected resistance from the ‘Measured Resistance’
column of the above table.
Read along the column to find the reading in degrees Celsius and the corresponding
reading in degrees Kelvin.
24
Laboratory Teaching Exercises
Data Sheet 3
Vapour Point of Saturated Water
25
Armfield Instruction Manual
Data Sheet 4
Enthalpy of Saturated Water
26
Laboratory Teaching Exercises
Data Sheet 5
Enthalpy of superheated water vapour
The properties of superheated water are normally given in standard text books in a
similar manner to the example given below. The enthalpy of superheated water
vapour may be determined by finding the table for the pressure closest to the
measured pressure, and then cross-referencing the appropriate column with the
temperature closest to the measured temperature. For greater accuracy,
intermediate values may be interpolated from the given data.
For use with this equipment, the required information from these tables has been
extracted, and is given in the following table. The enthalpy of superheated water
vapour may be found by cross-referencing the values closest to the measured
pressure and temperature.
27
Armfield Instruction Manual
Data Sheet 6
Relative and absolute scale values
As with the measurement of any physical property, the measurement of pressure and
temperature rely upon comparison with some fixed reference point. In the
measurement of pressure, an obvious reference point is that of the ambient pressure
of the surroundings. Pressure scales have been based around a zero point of the
pressure of the atmosphere at sea level. Pressures lower than atmospheric pressure
are assigned negative values; pressures higher than atmospheric pressure have
positive values.
Gauges for measuring pressure give readings relative to this zero point, by
comparing the pressure of interest to the pressure of the surrounding air. Pressures
measured using such a gauge are given relative to a fixed value, and are sometimes
termed gauge pressure. These measure pressure difference between the pressure to
28
Laboratory Teaching Exercises
be measured and the barometric (ambient) pressure. This may then need adjusting,
to take into account any difference between barometric pressure and the pressure at
sea level.
Many calculations using equations derived from fundamental physical laws require
absolute pressure values. Absolute pressure is the pressure relative to a total
absence of pressure (ie. a total vacuum). On an absolute pressure scale, all
pressures have a positive value. The following chart illustrates the difference
between gauge pressure, barometric pressure, and absolute pressure.
As for pressure, temperature scales have also been based around a comparison with
fixed reference points. For example, the ice point of water is assigned a value of 0°
on the Celsius scale. Temperatures measured on the Celsius scale are therefore
given relative to a fixed value. Absolute temperature scales take as a zero point the
theoretical temperature at which an ideal gas has a zero volume and no internal
energy, equal to –273.15°C. One such scale is the Kelvin scale, which has the same
scale interval as the Celsius scale.
If two different scales have the same scale interval, then the number of intervals of
difference between two temperatures will be identical. For example, the difference
between 25°C (298.15 K) and 27°C (300.15 K) is 2 for both the Celsius and Kelvin
scales. When performing calculations involving temperature difference, it is good
practice to first convert all values into the same scale required for the answer. This
avoids possible confusion.
Most industrial pressure instruments read gauge pressure, especially where the
working pressure of a vessel is being monitored. These must be converted to
absolute pressure before the experimental pressure data is used for calculations.
This means that there must be some sensor, such as a barometer, to measure the
ambient pressure of the location.
In the case of the TH3, the pressure sensors give a reading relative to ambient
pressure. Because of the level of accuracy of the other readings being taken and the
magnitude of the experimental errors involved, it is only necessary to determine
absolute pressure to ± 0.01 bar. This means that atmospheric pressure also only
needs to be determined to the same accuracy. This can be achieved with a simple
mercury barometer (not supplied).
29
Armfield Instruction Manual
The temperature sensors included with the equipment are of the platinum resistance
type. The resistance of the sensors may be read from the display on the console.
The value can then be compared to the graph provided to determine the
temperature. This potentially provides better accuracy than transponder circuitry to
convert the values into degrees electronically, as it eliminates zero errors. It also
allows a direct conversion from sensor resistance into an absolute temperature scale.
Data Sheet 7
Boiling Point of Water (kPa-°C)
30
Laboratory Teaching Exercises
Boiling Point of Water (mm Hg-°C)
31
Armfield Instruction Manual
Boiling Point of Water (Atmospheres-°C)
32
Laboratory Teaching Exercises
Boiling Point of Water (bar-°C)
33
Exercise A - Characteristic behaviour of a two phase fluid
Objective
To study the behaviour of water during the transition between liquid and vapour
phases.
Method
To investigate the behaviour of water, at temperatures around the vapour point, over
a range of pressures. To study the change in vapour point with increasing pressures,
and to watch the fluid behaviour using a sight glass set into a pressure vessel.
Theory
Boiling regimes
When a body of water is heated at constant volume by means of a hot surface, such
as a heater element, various different stages may be observed in the heating
process:
At very low heat flux between the heater and the fluid, no boiling occurs. Heat
transfer between the element and the fluid is by conduction, and fluid motion occurs
through free convection. Phase change occurs only as evaporation at the free
surface.
During this stage, slight swirling of the water surface may be seen.
34
Exercise A
At increased heat flux, phase change will occur at the heater surface, with small
bubbles of vapour forming as the layer of fluid surrounding the heater reaches
saturation temperature. These rise out of the hot boundary layer between the heater
and the main volume of the water, until they reach cooler fluid, where they condense.
Final phase change occurs as evaporation at the free surface.
During this stage, small bubbles may occasionally be seen condensing on the sight
glass.
As heat flux rises, heat is transferred through the fluid mainly by free convection, until
most of the volume reaches saturation temperature or higher. Many bubbles of
vapour form on the heater surface, and rise through the fluid to the surface. The
bubble movement agitates the fluid, producing increased mixing and consequently
better heat transfer from the heater to the fluid. This stage is sometimes termed
nucleate boiling.
During this stage, vigorous bubbling may be seen through the sight glass.
As heating within the vessel is at constant volume, the internal pressure of the
system increases during the heating process. As pressure increases, the saturation
temperature of water also increases, and the liquid in the system becomes
superheated (it remains liquid at a temperature above the boiling point at
atmospheric pressure).
35
Armfield Instruction Manual
If the pressure is now reduced without a corresponding reduction in temperature, for
example by bleeding off steam from the system, then the saturation temperature is
reduced. The superheated liquid vaporises as the vapour point falls, producing
violent frothing.
The pressure – volume – temperature relationship
The relationship between pressure, specific volume and temperature can be found in
most standard thermodynamics textbooks. This exercise investigates the pressuretemperature relationship at constant volume. A graph summarising this relationship is
given below; the exercise will cover temperatures and pressures occurring along the
L-V line. The relationship is covered in greater detail in later exercises.
Equipment Set Up
Check that the calorimeter valve and the drain valve at the base of the boiler are both
closed.
Check that the mains power to the console is switched off before filling the boiler.
Open the filling point at the top using the key provided. Fill the equipment using
purified or de-ionised water, until the water level is halfway up the sight-glass at the
front of the boiler. Do not seal the filling point until instructed later.
Switch on the mains power to the console, and switch on the console itself.
Procedure
Switch on the heater, and turn the heater power control to maximum.
36
Exercise A
Observe the appearance of the fluid in the boiler through the sight glass as the
temperature increases. Allow the water to reach boiling point, indicated by intense
movement at the surface and steam escaping from the filling point. Reduce the
heater power slightly to maintain a steady but not excessive stream of steam. Wait
until the resistance reading (R m1 ) becomes steady, meaning that all air has been
expelled. Note the pressure inside the vessel, as indicated by the pressure sensor
P 1 , and the resistance indicated by the platinum resistance thermometer R m1 . The
resistance may be converted into temperature using the tables provided in Data
Sheet 1 and Data Sheet 2. Close the filler valve then return the heater to maximum
power.
At intervals of approximately five minutes, note the readings for P 1 and R m1 . Note the
approximate temperature and pressure at which significant changes occur in the
appearance of the fluid.
When the system reaches maximum working pressure (7 bar), fully open the
calorimeter valve and switch off the heater power. After thirty seconds take readings
for pressure (P 1 ) and resistance (R m1 ), and continue to do so at thirty-second
intervals. Note any changes in fluid appearance as the pressure drops.
LEAVE THE CALORIMETER ISOLATING VALVE OPEN AFTER THE DATA HAS
BEEN TAKEN. Leaving the valve closed, after the pressure reaches atmospheric
pressure, may result in partial vacuum inside the apparatus as it cools to ambient
temperature. This could permanently damage the apparatus.
Results
Tabulate your results under the following headings:
Plot a graph of pressure against temperature.
Conclusion
Describe the behaviour observed as the fluid was heated and then the pressure
reduced. Did the fluid show sudden changes in behaviour, or were the transitions
gradual?
Comment on the pressure and temperature graph obtained. Does it look similar to
the theoretical graph provided?
37
Exercise B - Principles of saturation pressure
measurement
Objective
To obtain an understanding of the principles of saturation pressure measurement.
Method
To measure the saturation pressure of water using a pressurised vessel. To examine
the effect of unsteady conditions on measurement accuracy.
Theory
It is recommended that students read Data Sheet 6, Relative and absolute scale
values, before beginning this experiment.
The properties of water at constant volume can be represented as a function of
pressure and temperature as shown in the diagram below.
The saturation point of water is the condition at which a phase change occurs from
liquid to vapour, or vapour to liquid. It occurs at a very precise set of conditions,
which form a line when plotted on a graph such as that shown above (marked L V).
Measurement of the saturation point therefore requires accurate measurement of
absolute pressure and absolute temperature. Selection of suitable measuring devices
must take into account several factors:
38
Exercise B

The range of temperatures expected – devices must be able to operate over
the full temperature range of the apparatus.

The range of pressures expected – devices must be able to operate over the
full pressure range of the apparatus.

The type of sensor output required – automated monitoring will generally
require an electrical output, for example.

The response times required – which will partly depend upon the rate of
heating and consequently the rate of temperature change.

The size of sensor required, including any output transducer and display.

Hot, wet conditions – combination of heat, air and water produces a highly
corrosive atmosphere.
The TH3 apparatus uses sensors that are fairly typical of those used in a similar
industrial situation:

Platinum resistance thermometers are used to measure temperature, giving
an electrical output in Ohms. These have a wide temperature range and give
excellent accuracy. They can operate under a wide range of pressures, but
require a protective shield in liquid and corrosive atmospheres. Such
shielding does increase the response time of the sensor and the size of the
sensor probe.

An electronic pressure sensor of semiconductor type has been used. In this
sensor, one side of a diaphragm is exposed to the pressure to be measured,
while the other side is open to atmosphere. The resulting deflection forces a
rod into a metallic strip with semiconductor resistance gauges bonded to the
surface. The resulting tension or compression in these gauges produces a
measurable change in the semiconductor resistance, which can then be
converted to a pressure reading by a suitable conditioning circuit.
A Bourdon-type gauge has been included to give a visual indication of the pressure
inside the equipment. This is intended as an extra safety measure, indicating when
the system is pressurised even if the water is not visibly active. Bourdon gauges can
be constructed to cover a wide pressure range, but due to the nature of the display
the accuracy of such gauges decreases as the total range of the scale increases.
39
Armfield Instruction Manual
Temperature changes will affect the accuracy of the sensor, but where temperature
variation is pressure-dependent the sensor may be calibrated to compensate. The
gauge is relatively bulky and robust, and output purely mechanical.
The accuracy of the measurements taken will be affected by the properties of the
sensors chosen. This experiment will investigate the effect of one property, that of
response time or thermal lag, on the accuracy of the results.
Equipment Set Up
Check that the calorimeter valve and the drain valve at the base of the boiler are both
closed.
Check that the mains power to the console is switched off before filling the boiler.
Open the filling point at the top using the key provided. Fill the equipment using
purified or de-ionised water, until the water level is halfway up the sight-glass at the
front of the boiler. Do not seal the filling point until instructed later.
Switch on the mains power to the console, and switch on the console itself.
Procedure
Switch on the heater and turn the heater control to maximum.
Allow the water to reach boiling point, indicated by intense movement at the surface
and steam escaping from the filling point. Reduce the heater power slightly to
maintain a steady but not excessive stream of steam. Wait until the resistance
reading (R m1 ) becomes steady, meaning that all air has been expelled. Note the
pressure inside the vessel, as indicated by the pressure sensor P 1 , and the
resistance indicated by the platinum resistance thermometer R m1 . The resistance
may be converted into temperature using the tables provided in Data Sheet 1 and
Data Sheet 2. Close the filler valve then return the heater to maximum power.
At intervals of two minutes, record the thermometer output and the reading from the
electronic pressure sensor.
When the pressure reaches maximum working pressure (7 bar), turn off the heater.
At intervals of five minutes, record the thermometer output and the reading from the
electronic pressure sensor.
Continue recording until the readings stabilise, or for as long as possible if the
readings have not stabilised within the available time.
The first part of the experiment can be repeated with the heater at lower power
settings, to investigate the effect of different heating rates. The cooling rate should
remain unchanged, so need not be repeated.
LEAVE THE CALORIMETER ISOLATING VALVE OPEN AFTER THE DATA HAS
BEEN TAKEN. Leaving the valve closed, after the pressure reaches atmospheric
pressure, may result in partial vacuum inside the apparatus as it cools to ambient
temperature. This could permanently damage the apparatus.
Results
Tabulate your results under the following headings:
40
Exercise B
Barometric pressure: kN/m²
Use the reference tables in Data Sheet 1 and Data Sheet 2 to find the absolute
temperature indicated by the Platinum Resistance Thermometer, from the
thermometer output in Ohms.
Add the barometric pressure to the readings from the electronic pressure sensor, to
give an absolute pressure reading.
Assuming that the measured temperature of the steam is equal to the vapour point of
water, the actual temperature may be found from the absolute pressure, using the
graph in Data Sheet 3.
Plot a graph of indicated absolute temperature and actual absolute temperature
against time.
Conclusions
Comment on the shape of the graphs obtained. Discuss any differences between the
actual temperature and the temperature indicated by the platinum resistance
thermometer output.
Estimate the thermal lag of the thermometer. If results are available, compare the
thermal lag at different rates of heating.
What is the significance of these results when measuring saturation pressure? How
could the effect of thermal lag be reduced?
41
Exercise C - Concept of a saturation line
Objective
To study the relationship between pressure and temperature of vaporisation of a
fluid.
Method
To heat water contained in a closed system of constant volume, and to measure the
resulting changes in temperature and pressure.
Theory
It is commonly understood that the temperature at which water undergoes a liquid-tovapour phase change varies with pressure. For example, water boils at a lower
temperature when at high altitudes, such as encountered on mountains. This
relationship between pressure and temperature at which the liquid-to-vapour phase
change occurs may be plotted on a graph. The resulting line is termed the saturation
line. Saturation lines may be obtained for any fluid, although in this experiment water
will be used.
When plotted on a graph of absolute pressure P abs against absolute temperature T abs ,
the result is a smooth curve. The curve does not have a simple describing equation,
but over a limited range of pressure it is possible to obtain a good fit using:
Equation (1)
This equation is not derived from any theory or underlying physical laws. It only
describes behaviour. For any particular range of pressures, there are particular
values of the coefficients a and p 0 which minimise the differences between the
measured points and the curve given by the equation. These differences arise both
through experimental errors (random, scale and zero errors) and because the real
behaviour does not perfectly match the describing equation.
Obtaining best-fit values of the coefficients a and p 0 may only be obtained by
linearising the curve. The usual method for doing this is to take logarithms of
Equation (1):
42
Exercise C
Equation (2)
Therefore a and ln p 0 are respectively, the gradient and the intercept of a graph of ln
P abs vs. (1/T).
At an elementary level, the coefficients may be obtained by drawing a best-fit line to
a plot of experimental results. The value of a may be obtained as
. To
obtain a good spread of data, the zero of the x-axis will have been suppressed, so
the value of p 0 cannot be read from the graph and must be obtained by substituting
the co-ordinates of a point on the line into equation (2).
At a more advanced level, the values of a and p 0 may be obtained by applying linear
regression, using either a calculator or a spreadsheet.
Equipment Set Up
Check that the calorimeter valve and the drain valve at the base of the boiler are both
closed.
Check that the mains power to the console is switched off before filling the boiler.
Open the filling point at the top using the key provided. Fill the equipment using
purified or de-ionised water, until the water level is halfway up the sight-glass at the
front of the boiler. Do not seal the filling point until instructed later.
Switch on the mains power to the console and switch on the console itself.
Procedure
Allow the water to reach boiling point, indicated by intense movement at the surface
and steam escaping from the filling point. Reduce the heater power slightly to
maintain a steady but not excessive stream of steam. Wait until the resistance
reading (R m1 ) becomes steady, meaning that all air has been expelled. Note the
pressure inside the vessel, as indicated by the pressure sensor P 1 , and the
resistance indicated by the platinum resistance thermometer R m1 . The resistance
may be converted into temperature using the tables provided in Data Sheet 1 and
Data Sheet 2.
When heating fluid in the boiler, it takes time heat to conduct through the apparatus
to the pipework and the temperature probe. The platinum resistance thermometer
must reach the same temperature as the fluid before it will provide an accurate
reading. This time delay between the fluid reaching a given temperature and the
sensor reaching the same value is known as thermal lag.
Close the filler valve then return the heater to maximum power.
Allow the water to heat for two minutes, then switch off the heater power. Watch the
platinum resistance thermometer output R m1 , and wait until the value stabilises. This
allows heat to conduct from the fluid to the rest of the apparatus, and thus reduces
the effect of thermal lag.
Take a second set of readings from the platinum resistance thermometer and the
electronic pressure sensor.
43
Armfield Instruction Manual
Heat at maximum power for another two minutes then turn the heater off, wait for the
temperature reading to stabilise and repeat the sensor readings
Continue in the same way until the pressure reaches 7 bar. Leave the heater power
switched off after taking the final set of readings.
Open the isolating valve to the calorimeter, and allow steam to bleed off. This will
reduce both the pressure and temperature. Close the valve again after thirty
seconds.
Take a second set of data as the system cools, opening the isolating valve for thirty
seconds to allow steam to escape, then closing the valve and allowing time for the
sensors to stabilise before recording the sensor outputs.
LEAVE THE CALORIMETER ISOLATING VALVE OPEN AFTER THE DATA HAS
BEEN TAKEN. Leaving the valve closed, after the pressure reaches atmospheric
pressure, may result in partial vacuum inside the apparatus as it cools to ambient
temperature. This could permanently damage the apparatus.
Results
Tabulate your results under the following headings:
Plot a graph of ln P abs vs. 1/T, and use the graph to obtain approximate values of a
and p 0 .
On the same axes as the previous graph, plot a graph of ln P abs calculated using the
equation
and the values for a and p 0 obtained from the graph of ln P abs vs. 1/T.
Conclusions
Compare the two graphs and comment on the accuracy of the describing equation.
It is suggested that students begin Exercise D on completion of this exercise.
44
Exercise D - Steam tables
Objective
To investigate the accuracy of saturation data obtained using basic equipment.
Method
To compare laboratory data with published steam tables. It is recommended that
Exercise C, ‘Concept of a saturation line’, is completed before commencing this
experiment. The data from Exercise C may then be used for this exercise.
Theory
Published graphs and tables of the liquid-to-vapour phase change temperature over
a range of pressures are available for reference. These may be found in relevant
textbooks and may be provided by the manufacturers of equipment involving the use
of high-pressure steam. These curves are obtained from very accurate experiments,
which eliminate experimental error.
In Exercise C, it was seen that a reasonable fit to the saturation line might be
obtained using the equation
However, the range of values for temperature and pressure obtainable without the
use of highly specialised equipment is limited. Estimation of ln p 0 is by extrapolation
over a logarithmic scale, and small differences in a can lead to a wide range of
values of p 0 .
The limitations of the method used to obtain the relationship between vapour point
and pressure may be seen by comparing experimental values obtained during the
laboratory exercise with values taken from published steam tables.
Equipment Set Up and Procedure
If the results from Exercise C are available, these may be used and no equipment is
required. If there are no previous results available, follow the equipment set up and
procedure instructions for Exercise C.
Results
Tabulate your results under the following headings:
Plot a graph of ln P abs vs. 1/T, and use this to obtain approximate values of a and p 0 .
(This may be omitted if the results from Exercise C are available).
45
Armfield Instruction Manual
Plot a graph of P abs vs. T from the experimental data.
Plot a graph of P abs vs. T, calculating P abs using the equation.
and the values for a and p 0 obtained earlier. The axes should cover the temperature
range 0° - 200° Celsius.
Compare the graphs of P abs vs. T with the graph provided in Data Sheet 3. This
graph is taken from steam tables typical of those published in standard
thermodynamics texts.
Conclusions
How well does the experimental data compare to the standard graph over the
temperature and pressure range covered by the experiment?
How well does the describing equation obtained from the experimental data fit the
standard graph?
What are the implications of these results for the design of an automatic monitoring
system for liquid-vapour transitions?
46
Exercise E - Use of the steady flow energy equation
Objective
To determine the quality of steam exiting a pressurised vessel.
Method
To make use of a throttling calorimeter in conjunction with the Steady Flow Energy
Equation, in order to calculate the enthalpy of the escaping steam. To determine the
quality of the steam using standard reference tables.
Theory
After reaching the saturated liquid stage, heated fluid in the system will form a twophase liquid-vapour mixture. The ratio of the mass of vapour to the total mass of the
mixture is referred to as its quality, x. The quality varies from x = 0 (saturated liquid
state) to x = 1 (saturated vapour).
From the thermodynamic laws of specific internal energy and enthalpy, it is possible
to derive an equation for the specific enthalpy of a two-phase fluid given in terms of
the quality:
Equation (1)
where h f = Enthalpy of vapour
h g = Enthalpy of liquid
The increase in enthalpy during vaporisation is sometimes termed h fg . Values for the
enthalpy of saturated water are provided in Data Sheet 3. A detailed explanation of
enthalpy is beyond the scope of this manual.
The thermodynamic laws for conservation of energy in a steady flow process give
rise to the Steady Flow Energy Equation:
Equation (2)
47
Armfield Instruction Manual
= Rate of change of energy of system
where
= Rate of doing work by system
= Rate of mass change within system
h 1 = Enthalpy of fluid before throttling
h 2 = Enthalpy of fluid after throttling
c
= Velocity of fluid
g
= Gravitational constant
z
= Potential energy of fluid
Applying this to the throttling process (where heat transfer and changes in potential
and kinetic energies are negligible and the system does no work) indicates that the
specific enthalpy of the fluid is the same before and after throttling:
Equation (3)
This means that the specific enthalpy of a sample of steam is the same as that of the
supply. Substituting back into equation (1) then produces the equation:
Equation (4)
The Two Property Rule states that the thermodynamic state of a fluid is defined by
any two independent properties. Published reference tables are commonly available
giving the thermodynamic state of water at given conditions, obtained from accurate
experimental data.
The only two intensive properties that can be directly measured in this experiment
are temperature and pressure. The sample must be in a state where these two
values are independent, ie. superheated steam. This condition is ensured by forcing
the sample through a greatly restricted pipe section, producing a large pressure drop,
and causing the steam sample to enter the superheated region. In this condition, the
measured values of pressure and temperature for the sample may be used in
combination with the reference table provided in Data Sheet 4, to determine the
steam quality.
Equipment Set Up
Check that the calorimeter valve and the drain valve at the base of the boiler are both
closed.
Check that the sensors are correctly connected to the console.
Check that the mains power to the console is switched off before filling the boiler.
Open the filling point at the top using the key provided. Fill the equipment using
purified or de-ionised water, until the water level is halfway up the sight-glass at the
front of the boiler. Do not seal the filling point until instructed later.
48
Exercise E
Switch on the mains power to the console and switch on the console itself.
Procedure
Allow the water to reach boiling point, indicated by intense movement at the surface
and steam escaping from the filling point. Reduce the heater power slightly to
maintain a steady but not excessive stream of steam. Wait until the resistance
reading (R m1 ) becomes steady, meaning that all air has been expelled. Note the
pressure inside the vessel, as indicated by the pressure sensor P 1 , and the
resistance indicated by the platinum resistance thermometer R m1 . The resistance
may be converted into temperature using the tables provided in Data Sheet 1 and
Data Sheet 2.
Close the filling point then set the heater to maximum power. Allow the pressure to
rise to 1 bar above atmospheric. Turn the heater control to minimum, and allow the
sensors to stabilise.
Take readings for the pressure and temperature inside the boiler, using the reference
charts in Data Sheet 1 and Data Sheet 2 to find the temperature corresponding to the
reading from the platinum resistance thermometer.
Open the calorimeter valve to bleed off a sample of steam. Wait until the reading for
PRT100(2) stabilises then record the pressure and temperature values for the
sample, and close the calorimeter valve again.
Increase the pressure and temperature inside the boiler in increments of one bar, to
a maximum of seven bar. At each stage, turn the heater control to a minimum and
wait for the sensors to stabilise, then take a set of readings.
Switch off the heater.
At maximum pressure, open the calorimeter valve, and allow steam to escape
continuously. Take readings in one bar increments as the pressure decreases.
LEAVE THE CALORIMETER ISOLATING VALVE OPEN AFTER THE DATA HAS
BEEN TAKEN. Leaving the valve closed, after the pressure reaches atmospheric
pressure, may result in partial vacuum inside the apparatus as it cools to ambient
temperature. This could permanently damage the apparatus.
Results
Tabulate your results under the following headings:
49
Armfield Instruction Manual
50
Exercise E
Conclusions
Explain the variation in steam quality as pressure and temperature change.
What effect might steam quality have on design considerations for a plant producing
or working with superheated water vapour?
51
Contact Details for Further Information
Main Office:
Armfield Limited
Bridge House
West Street
Ringwood
Hampshire
England BH24 1DY
Tel: +44 (0)1425 478781
Fax: +44 (0)1425 470916
Email: sales@armfield.co.uk
support@armfield.co.uk
Web: http://www.armfield.co.uk
US Office:
Armfield Inc.
436 West Commodore Blvd (#2)
Jackson, NJ 08527
Tel: (732) 928 3332
Fax: (732) 928 3542
Email: info@armfieldinc.com
52
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