Marcet Boiler

Mechanical Engineering Laboratory 2

1

Marcet Boiler

Candidate’s Name: Manimendra Shimal Damian De Silva

Student ID: SCM: 029710

Lecturer/ Supervisor: Dr. Lai

Date of Experiment: 25/03/2015

Date of Submission: 1/04/2015

1. Abstract

The aim of this experiment is to investigate the relationship between the pressure and the

temperature of saturated steam in equilibrium and also to demonstrate the vapor pressure

curve. The apparatus used is a market boiler. When the pressure increases the temperature

also increases. Thus the relationship between pressure and temperature is directly

proportional. Based on the data collected and with the help of Clausius-Clapeyron’s

equation the gradient was calculated. The measured slope dT/dP were slightly different to

the calculated slope Tvg/hfg. This maybe occurred due to few errors while conducting the

experiment.

2. Objective

To demonstrate the relationship between the pressure and temperature of saturated

steam in equilibrium with water.

To determine the vapor pressure curve.

3. Introduction/ Theoretical Background

Thermodynamics is a branch of physics that deals with temperature and heat and their

relation to energy and work in a system. Properties of fluids is also part of

thermodynamics. Most thermodynamics substances are known as P-V-T (pressure-

volume-temperature) substances. (Khan Academy, 2013)

An ideal gas is defined as one in which all collisions between atoms or molecules are

perfectly elastic and in which there are no intermolecular attractive forces. In such a gas

2

all the internal energy is in the form of kinetic energy and any change in internal energy

is accompanied by a change in temperature. (Cengel, 1999)

An ideal gas obeys the equation of state that relates the pressure, specific volume or

density, and absolute temperature with mass of molecule and the gas constant, R.

PV = nRT

Where,

P = Absolute pressure

V = Volume

n = Amount of substance (moles)

R = Ideal gas constant

T = Absolute temperature (K)

However, real gas does not absolutely obey the equation of state. A few changes on the

ideal gas equation of state allow its application in the properties of real gas.”

Furthermore, “when energy increases within water, the increasing of activities among the

molecules enables the increase in the number of molecule escape from the surface until

an equilibrium state is reached. The state of equilibrium depends on the pressure between

the water surface and steam. At lower pressure, the molecules become easier leaving the

water surface while less energy required in achieving the state of equilibrium (boiling

point). The temperature where equilibrium occurs at a given pressure level is called

saturated temperature.

The Marcet Boiler is used to investigate the relationship between the pressure and

temperature of saturated steam in equilibrium with water at all temperature levels

3

between the atmospheric pressure and 10 bars. The experimental slope (dT/dP)SAT

obtained is compared to the theoretical value determined through calculation from the

steam table.

Clausius-Clapeyron states:

( dTdP )

SAT=

Tv fg

h fg

( dTdP )

SAT=

T (v g−v f )hg−hf

Where,

h fg=hg−h f

( dTdP )

SAT=

T (v g−v f )hfg

=Tv fg

h fg as vg >> vf

In which,

vf = specific volume of saturated liquid

vg = specific volume of saturated vapor

hf = enthalpy of saturated liquid

hg = enthalpy of saturated vapor

hfg = latent heat of vaporization

4

4. Apparatus

HE169 Marcet boiler

a) Pressure transducer

b) Temperature controller/indicator

c) Pressure indicator

d) Control panel

e) Bench

f) Bourdon tube pressure gauge

g) Temperature sensor

h) Pressure relief valve

i) Water inlet port & valve

j) Heater

5

5. Experimental Method

1. The power supply was switched on.

2. The boiler was initially filled with water, to check the water level the valves at the left side tube was opened and then closed.

3. The temperature controller was set to 185ᴼC, which was slightly above the expected boiling point of water at 10 bar.

4. The feed port valve was opened and the heater was switched on. (It was made sure that the valve at the level sight tube was closed

before the heater was switched on because the sight tube was not designed to withstand high pressure and temperature.)

5. The steam temperature rise was observed as the water boiled.

6. The valve was closed after 30 seconds, to ensure that the air in the boiler was out because the accuracy of the experimental results would get affected when air is present.

7. The steam temperature and pressure was recorded till the steam pressure reached 10bar.

8. The heater was turned off and the steam temperature and the pressure began to drop. The boiler was left to cool down to room temperature.

9. The steam temperatures were recorded at different pressure readings when the boiler was heating and cooling.

10. The heater was switched off and was left for the temperature to drop.

Warnings

Never open valves when the boiler is heated as pressurized steam can cause severe injury.

Do not open the valve at the water inlet port as it is highly pressurized at high temperature.

6

6. Results and Analysis

Atmospheric pressure : 1.01325 bar

Atmospheric temperature: 27 (0C)

Data collected and calculated results

Average Average

T avc (0C) T avc (K)

1 0 0 100 96.8 98.4 371.4 0.2794222721.5 0.345 0.483 110.2 110 110.1 383.1 0.234 0.1999617372 0.965 1.034 119.7 119.4 119.55 392.55 0.189 0.158054969

2.5 1.379 1.448 127.3 126.9 127.1 400.1 0.151 0.1317477313 1.999 2.068 133.6 133.3 133.45 406.45 0.127 0.11363872

3.5 2.413 2.482 138.9 138.7 138.8 411.8 0.107 0.1002705464 2.827 2.965 143.6 143.5 143.55 416.55 0.095 0.089988568

4.5 3.447 3.516 147.9 147.9 147.9 420.9 0.087 0.0819670145 3.861 3.999 151.8 151.9 151.85 424.85 0.079 0.075331133

5.5 4.482 4.482 155.5 155.6 155.55 428.55 0.074 0.0698709636 4.826 4.964 158.8 159 158.9 431.9 0.067 0.065174418

6.5 5.309 5.516 162 162.2 162.1 435.1 0.064 0.0611455077 5.861 5.929 164.9 165.2 165.05 438.05 0.059 0.057619845

7.5 6.412 6.481 167.7 167.9 167.8 440.8 0.055 0.0546049148 6.895 6.895 170.4 170.4 170.4 443.4 0.052 0.051719536

Measured Slope, dT/dP

Calculated Slope,

Tvfg/hfg

Pressure, P (bar) Temperature, T

Gauge (Increase)absolute Gauge (Decrease)Increase

( ᴼC) Decrease

( ᴼC)

Sample Calculation

Formulas used

7

h fg=hg−h f

( dTdP )

SAT=

Tv fg

h fg

Calculations

For calculated slope

When Pressure = 2bar

Temperature = 120.2ᴼC = 393.2 K

hf = 504.5 kJ kg-1

hg = 2706 kJ kg-1

hfg = 2706 – 504.5

= 2201.5 kJ kg-1

v f = 1.061x10-3 m3 kg-1

vg = 0.886 m3 kg-1

v fg = 0.886 – 1.061x10-3

= 0.884939 m3 kg-1

Calculated slope =

Tv fg

h fg

8

= 393.2 X 0.884939

2201.5

= 0.1580549692 K kpa-1

Note: The values used in the above calculations were taken from the “Little book of Thermofluids” which

is shown in the Appendix 1.

For measured slope

( dTdP )

SAT = 392.55−383.1

200−150

= 0.189 K kpa-1

Same calculations were used for the others

Graphs

9

7. Discussion

It is necessary to remove air from the boiler because if the air is not expelled the correct

equilibrium measurements between the steam and boiling water will not be obtained. Due

to the partial pressure of air a lower water temperature will be required to raise the

pressure.

Based on the graph the experiment and calculated graphs are similar and they overlay

each other at certain points so it can be assumed that the results obtained are marginally

accurate. The reason why the two graphs doesn’t overlay at few points might be due to

some errors.

Sources of errors are measurements or reading accuracy, room temperature, pressure, the

stability of materials and calculations.

10

When the liquid absorbs enough heat energy, it will change to vapor form. As this is not

allowed to exit, it will cause an increase in pressure and thus causing the temperature to

increase. Marcet boiler can be used as a water boiler, pressure cooker, steam generated

power plants and many more.

8. Conclusion

Marcet boiler is the device which we use to study the relation in between pressure and temperature for a water at saturated liquid phase. As we did in the lab investigation, we started heating water with constant pressure until it reached boiling point. Then, closing the valve which created a constant volume system. Forcing the pressure to increase as the temperature rises. And thus studying the direct relation between pressure and temperature for water at that point. We notice that it is essential to close the valve as we reach boiling point to make sure we are now in a constant volume process, otherwise pressure would have never increased. After tabulating the results and plotting the graph we found out that the relationship between pressure and temperature is directly proportional. The difference between the theoretical values and the actual values is caused by errors with certain calculation mistakes.

9. References

Khan Academy, 2013, Thermodynamics, [ONLINE] Available at: https://www.khanacademy.org/science/physics/thermodynamics, [Accessed 27 March 15].

Cengel and Boles, Thermodynamics, An engineering Approach, 5th Edition, McGraw Hill, 1999.

S.B.M. Beck and S.G. Blakey, The Little Book of Thermofluids, 3rd Edition, University of Sheffield, 2006

10. Appendix 1

11

12