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PureSubstance
Pure substance is a chemically homogenous substance and invariable in chemical
composition. The chemical composition in a pure substance is in a fixed ratio throughout
the system and does not change during any process. A fixed relationship between the
pressure and temperature of the pure substance can be determined when two phases of a
pure substance are in equilibrium. The pure substance which used in this experiment is
water. Water has a freezing point of 0
o
C and a boiling point of 100
o
C. Besides, it can
exist in three different states which are solid, liquid and gaseous states.
Steam Properties
There are several steam properties and saturation temperature is one of the steam
properties. Saturation is the condition which the mixture of liquid and vapour can exist
together at a given temperature and pressure. Saturation temperature is the equilibrium
temperature during phase change and the temperature where the evaporation or the
condensation process of the pure substance starts to occur in a given system pressure.
Besides, saturation pressure is the equilibrium pressure in a given system temperature.
Property diagram is a diagram which shows the phases of a substance and the
relationships between its properties. There are two common property diagram which are
P-V property diagram and T-V property diagram. Figure 1 below shows the T-V property
diagram which could clearly describe the property of the pure substance during phase
change in this
experiment.
Figure 1: Property Diagram
Based on Figure 1, when the saturation temperature is higher than the liquid
temperature, it is fallen into the sub-cooled liquid region. The liquid in this region is
known as sub-cooled liquid or compressed liquid. Next, when the heat is continually
supplied to the liquid, the temperature of the liquid will rise gradually. The liquid will
reach saturation when it reaches the saturation temperature and it is also known as
saturated liquid. Vaporization occurs when further heat is applied and the phase of the
pure substance is starting to change. In this phase, liquid and vapour exist together and
the temperature is in equilibrium. It is known as saturated wet vapour as the vapour
fraction in the mixture is increasing. When the liquid has evaporated fully into vapour
state and the temperature is still remained at saturation temperature, it is known as dry
saturated vapour. Lastly, when the vapour temperature is higher than the saturation
temperature, it is categorized in superheated region and known as superheated vapour.
Enthalpy is one of the steam properties and it is the total sum of internal energy
plus the product of the pressure and volume as shown in the Equation 1 below.
H = U + P * V (Equation 1)
where,
H = Enthalpy (J)
U = Internal Energy (J)
P = Pressure (Pa)
V = Volume (m
3
)
The formula in Equation 1 can be expressed in term of per unit mass and it is
known as specific enthalpy equation as shown in Equation 2 below.
h = u + P*v (Equation 2)
where,
h = Specific Enthalpy (J/kg)
u = Specific Internal Energy (J/kg)
P = Pressure (Pa)
v = Specific Volume (m
3
/kg)
Steam quality is another steam properties and it can only be determined in
saturated vapour region. Dryness fraction is the ratio of total mass of dry vapour to the
mass of liquid and vapour mixture as shown in Equation 3 below. When the state of pure
substance reaches saturated vapour region, the dryness fraction will change from 0 to 1 in
the condition of the temperature is still at saturation temperature with the corresponding
system pressure. Furthermore, pressure and temperature are not independent properties in
the region .
(Equation 3)
Importance of Subject Investigated
The subject investigated in this experiment is the saturation pressure and
temperature of water. Water is a pure substance which is commonly used in industry
for different purposes such as cooling, generating steam energy, transporting and
cleaning. By investigating and understanding the properties of water, the usage of
water can be maximized in industries. Saturation pressure and temperature will be
very important in generating steam energy for mechanical work purposes as pressure
and temperature has a fixed relationship when two phases of water are in equilibrium.
Unit Conversion
Unit conversion played an important role in exploring the further information of
this experiment. The unit of the measured value in the experiment was different from the
unit of the information in steam table. Hence, the experimental value had to be converted
into the same unit in the steam table. For instance, the pressure obtained from the
experiment was in kPa while the unit for pressure in the steam table was in unit bar. So,
the conversion of pressure in kPa to unit bar is shown in Equation 4 and 5 below.
1 bar = 1 x 10
5
Pa (Equation 4)
0.01 bar = 1 kPa (Equation 5)
Linear Interpolation
The experimental data obtained from the experiment might slightly different from
the tabulated data in steam table. Hence, linear interpolation was used to evaluate the
experimental data from the steam table and obtain the corresponding value respectively.
The formula used in the linear interpolation is shown in Equation 7 below.
Figure 2: Linear Graph
(Equation 7)
From Figure 2, a linear graph is shown and linear interpolation method can be
explained easily from the given graph. There are three points lie on the straight line and
they have the same gradient. Assumed that X1Y1 and X2Y2 are the two different values in
the steam table and XY is the value between the X1Y1 and X2Y2. Hence, the gradient
between XY and X1Y1 is same with the gradient between X1Y1 and X2Y2 as they fall on
the same straight line. Equation 7 is used to find the y value for the data within the range
of two tabulated data from steam table. Besides, Equation 7 is the final form of the
derivation between the two gradient equations and the derivation is shown below.
(Proved)
Dryness Fraction
Steam quality was being determined as it was one of the aims of this
experiment.
Steam quality can also be represented by dryness fraction and can be defined as the ratio
of total mass of vapour to the total mass of the mixture. Specific enthalpy values were
used to calculate the dryness fraction in this experiment. In addition, it can be obtained
from the formula as shown in Equation 8 below.
(Equation 8)
where,
h = Specific steam enthalpy in liquid-vapour phase
hf = Specific saturated liquid enthalpy
hg = Specific saturated vapour enthalpy
x = Dryness fraction
Throttling process
Throttling process is a process to boil the water into vapor in the experiment to measure
the quality of the system. It is a steady flow expansion process whereby the gas flows out
under high velocity due to fall in pressure during expansion. This process is also known
as adiabatic process. There is no change noted in the enthalpy from one state to another
state and no work is done. Hence,
h1=h2, W=0.
Boiler
It was a rigid vessel and a cartridge heater was installed inside the boiler. The
water inside the boiler would be heated until it was completely boiled. Besides, the
cartridge heater was an electric heater with variable power input. There was a sight glass
on the boiler for the purpose of monitoring the water level inside the boiler. The boiler
was connected pipe loop which consisted of a relief valve, filling valve and a discharged
valve.
Relief Valve
During the boiling process, the relief valve was normally turned off and it acted as
a safety device. It released steam to the atmosphere when the boiler pressure exceeded
the maximum pressure of 8 bar in the process. If the steam was not released and the
boiler pressure exceeded 8 bar, explosion might occur.
Filling Valve
It ensured the water level in the boiler remained at a constant water level. When
the water level dropped during an operation, the pure water would be added into the
boiler through the filling valve.
Discharge Valve
When the steam had been produced during the operation, it could be released to
the atmosphere through discharge valve.
Throttle Valve
It played an important role in the experiment as it controlled the pressure in the
boiler.
Analogue Pressure Gauge
It acted as a back-up measurement for pressure as there was a digital pressure
transducer attached to the boiler.
Digital Pressure Transducer
It was attached to the steam boiler provided the gauge pressure. This apparatus
was used to indicate the gauge pressure inside the boiler. The unit measured from this
apparatus was kPa and its accuracy was ±1 kPa.
Platinum Resistance Thermometers (PRT)
There were 2 platinum resistance thermometers with sensors installed on the pipe
loop. It was used to measure the temperature in term of resistance. The first sensor was
PT1 and it was used to measuring the steam temperature at the boiler. Next, the second
sensor was PT2 and it was used to indicating the throttle valve outlet temperature to the
atmosphere. The unit of this apparatus was Ohm, Ω and its accuracy was ±0.1 Ω.
Pipe
It acted as a connection for all the apparatus in the experiment. It connected relief
valve, filling valve and discharge valve with the boiler to complete the pipe loop in this
experiment.
Procedure
The boiler was filled with ¾ of pure water as refer to the mark on the sight glass. It is
important to ensure that the pressure reading is zero and the temperature reading is about
109 Ω. The filling valve was let opened and the discharge valve was closed in which the
water was rapidly heated to reach boiling point. As the filling valve released the steam,
the apparatus was operated for about 1 to 2 minutes to purge any trap air inside the pipe
loop. After that, filling valve was closed and the heater is turned down.
Part1
1. The high heater power was set to raise the steam power quickly and reduced the
heating to achieve stable condition.
2. The steam pressure and the temperature in resistance (PT1) temperature sensor
were recorded.
3. The procedures are repeated by increasing the steam pressure at interval of 50kPa
until it reached 500kPa.
Part2
1. The heater power was switched off after recording the temperature reading in part
1 and the discharge valve was opened to decrease the steam pressure to about
400kPa.
2. The heating power was adjusted to maintain the stable conditions if essential.
3. When the pressure decrease down to 400kPa, the steam pressure and the
temperature in two resistance thermometers (PT1 and PT2) were recorded.
Discussion
From the result obtained in Part 1, it could be observed that the pressure inside the
boiler was increasing when the temperature in term of resistance was increasing as well.
This phenomenon was clearly shown in Figure 4 as the pressure increased with the
temperature. In this experiment, the volume of the water had been kept constant. Hence,
when the pressure inside the boiler increased, the water molecule collided with each other
more vigorously and this had also increased the internal energy of the pure substance. As
the collision between the molecules increasing, the temperature of the water was
increased too.
In this experiment, the value of PT2 increased when the pressure was increased.
However, measured resistance PT2 should be constant throughout the experiment as the
discharge valve had been closed and no steam would be released. This was due to the
heat transferred from a hot end to a cold end. The heat from the steam boiler had been
transferred to the pipe loop and surrounding as well through conduction and convection.
This could affect the temperature reading at PT2.
From Figure 4, it could be seen that the corresponding temperature was lower
than the saturation temperature. However, the difference between them was very small as
the percentage of error was in the range of 0.41% to 2.54%.
There were some sources of error which affected the accuracy of the reading. First,
there was error when taking the reading of measured resistance. The experiment was
conducted as when the pressure reached 50 kPa, the switch was turned quickly to PT1
and PT2 to obtain their readings. However, the pressure was increasing during the
switching action. Hence, the obtained temperature was lower than the saturation
temperature. Besides, the volume of water was decreased during the experiment.
Although the discharge valve had been closed, some steam was still able to escape and
release to the surrounding. Hence, the volume of water during the experiment was not
same as the initial water volume. So, there was some difference between the saturation
temperature and the corresponding temperature.
In part 2 of the experiment, it could be observed that when the pressure decreased,
the temperature in resistance form, PT1 also decreased. Besides, PT2 remained at the
same temperature when the pressure inside the boiler was dropping. PT2 was the
measured resistance at the downstream of the throttle valve. The discharge valve was
opened to release the steam from the boiler to the surrounding through throttle valve.
Hence, the temperature experienced at the downstream of the throttle valve remained
constant.
Throttling process had occurred in this part of the experiment as the enthalpy
value remained constant during the steam expansion. Hence, the steam enthalpy at the
upstream of the throttle valve, h1 was same as the steam enthalpy at the downstream of
the throttle valve, h2. The values of saturated liquid enthalpy, hf and the dry saturated
vapour enthalpy, hg were determined from the saturated water and steam table by using
the absolute pressure in the boiler. Then, the h2 value was determined from the
superheated section of the steam table. As steam enthalpy had been obtained, dryness
fraction, x could be determined by using Equation 8.
When the pressure in the boiler decreased, the dryness fraction had increased.
Besides, when the pressure decreased, the saturation temperature of the water also
decreased and this had caused the conversion from water to vapour became much more
easier. Hence, the dryness fraction had been increased throughout the experiment.
Conclusion
As a conclusion, when the pressure increased, the corresponding temperature
increased as well. This trend can be seen in the graph in Figure 4. There was a slightly
difference between the corresponding temperature and saturation temperature but the
percentage error was very low which not more than 3%. Lastly, the dryness fraction of
the water increased when the pressure inside the boiler decreased. Hence, the water was
easier to change into vapour phase.
Discussion
Based on the result tabulated in Table 7.1 for part 1, the pressure was converted
into absolute pressure and temperature in resistance using resistance correction chart and
conversion of resistance to temperature chart shown in appendix. Later, a graph of
absolute temperature against absolute pressure was plotted as shown in Graph 1. From
the graph, it demonstrates that when the temperature increases, the pressure also increases.
This is because the water heated in the boiler had turned into vapors eventually which
caused the gas to collide more frequently to increase the pressure. The ideal gas law
PV=nRT also proved in this experiment. There is a rapid increase in the start of the curve
shown in graph show that the experimental value had minor difference from the
theoretical value. The difference occurs might be due to several errors which will be
discussed later on.
For part 2, the results of the experiment were tabulated in Table 7.2 and Table 7.3.
The dryness fraction or quality, x of the vapor was calculated with the enthalpy using the
equation shown:
hi = (1-x)hf + xhg
Throttling process was carried out in this part to determine the quality of the vapor. There
is no change noted in the enthalpy from one state to another state as a result h1=h2 and no
work is done, W=0. Hence, it is said that a steady flow expansion was noted and the
liquid eventually change to vapor. Refer to steam table, the quality of steam, x was
determined in Table 7.3 shows that it is approximately to 1. Hence, it can be deduced that
the steam is good in quality.
As for Table 7.2, the temperature of PT1 is lower as compared to PT2 even both
experiences decreasing as pressure decreased. This is due the lost of vapor and heat to the
surrounding when pass through throttle valve. The atmospheric pressure is much lower
that the absolute pressure allows the heat to transfer out of the system. Throttling process
theory stated that the enthalpy is no change through the process, however there is still
some heat transfer due to the large exposed surface of throttling apparatus.
Errors might be occurring during this experiment due to several factors. Graph 1
shows that the experimental values are slightly differ from the theoretical values. This is
because first experimental temperature reading is recorded before the water boils. It
should be taken after the water starts to boil to increase the accuracy of the result.
Furthermore, the pure substances (water) used in this experiment might contain
impurities is also a possible reason in difference of results. This can be improved by
using distilled water or filtering the water for the experiment carried out. Instrumental
error might also occur due to the strength and accuracy of the apparatus used is lost as it
is used repeatedly. Thus, maintenance should be done in order to obtain an accurate result.
After the improvement are done, an ideal throttling process would be achieved in this
experiment.
Conclusion
In conclusion, the main objectives of the experiment were achieved. The relationship
between the temperature and pressure of pure substance during the liquid-to-vapour
phase equilibrium was shown through graph. The curve of the graph show that when the
temperature increased, the pressure increased steadily. Moreover, the experimental results
are also close with the theoretical value from Steam table with a slight difference. The
dryness fraction of the vapour was calculated and it was good quality which was
approximately to 1.