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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.

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