Marcet Boiler1. AbstractThis experiment was carried out to determine the relationship between the pressure and the temperature of saturated steam in equilibrium. Besides that this experiment was also done to demonstrate the vapor pressure curve. The market Boiler was used for this experiment. When the pressure increases, the temperature also increases. Therefore, the relationship of pressure and temperature is directly proportional. The derived formulae and the data were used to calculate the slope. The dT/dP measured was compared with the data in the steam table. Theoretically, the values measured should be almost the same with the predicted values. However, at certain points, the values are not the same. This may because of the errors made in the experiment.

2. Introduction Thermodynamics study about the interchange of heat and work between a system and the surroundings which occurs when the system undergoes a process. Thermodynamic also concerned about the changes in the properties of fluid. Most thermodynamics substances such as gases and vapours are often referred as P-V-T substances.

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. However, real gas does not absolutely obey the equation of state. A few modifications on the ideal gas equation of state allow its application in the properties of real gas. When energy is added within water, the increasing of activities among the molecules caused 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. The lower the pressure, the easier the molecules leaving the water surface and thus less energy is required to achieve the state of equilibrium (boiling point). The temperature where equilibrium occurs at a given pressure level is called saturated temperature. The pressure where equilibrium occurs at a given temperature is called saturated pressure.

Marcet Boiler is used to investigate the relationship between the pressure and temperature of saturated steam in equilibrium with water at all temperature levels between the atmospheric pressure and 10 bars. The measured value of the slope of the graph (dT/dP) obtained from the experiment results can be compared to the theoretical value determined through calculation from the steam table. Clausius-Clapeyron states:

fg

fg

SAT h

Tv

dPdT =

gf

fg

SAT hh

vvT

dP

dT

−−

=

)(

hfg = hg - hf

fg

g

fg

gf

SAT h

Tv

h

vvT

dPdT =

−=

)(

3. Experiment Methods and Materials

The apparatus used was a unit of Marcet Boiler and water. The unit of Marcet Boiler consists of a stainless steel pressure vessel fitted with high pressure immersion electrical heater, control panel, a safety relief valve, water inlet port & valve, Bourdon tube pressure gauge, temperature and pressure measuring devices.

Before starting the experiment a quick inspection was performed to ensure the unit is in proper operating condition. The unit of Marcet Boiler was later connected to the nearest power supply. Valve at the feed port and the level sight tube were opened. After that, the boiler was filled with distilled water through the feed port and the water level was made sure to be at about half of the boiler’s height. Then, the valves at the level sight tube were closed and the power supply is turned on.

The temperature controller was set at 185.0 °C which is slightly above the expected boiling point of the water at 10.0 bar(abs). The valve at feed port was opened and the heater was turned on. The valve at the level sight tube are made sure to be closed before turning on the heater as the sight tube is not designed to withstand high pressure and temperature. The rise of steam temperature was observed as the water boils. The steam from the valve was allowed to come out for 30 seconds before the valve was closed. This step is important to remove air from the boiler as the accuracy of the experimental results will be significantly affected if air is present. The steam temperature, gauge pressure at 1.00 bar (abs) and time taken for the steam to reach 1.00 bar (abs) were recorded. This step was repeated by increase the interval of pressure with 0.5 bar (abs) until the steam pressure reaches 10.0 bar (abs). After that, the heater was allowed to cool down to room temperature. All the results were recorded and tabulated under the table below. Graph of temperature against absolute pressure was plotted. Calculation of the slope of the graph

was made. Graph of SATdP

dT

versus P and fg

fg

h

Tv versus P was also plotted.

4. Data AnalysisThe specific result from this experiment allows us to determine the relationship between the pressure and temperature of saturated steam in equilibrium.Experimental Results

Pressure, P (bar) Temperature, T Time, t (S) Measure Slope, dT/dP

Calculated Slope, Tvg/hfgGauge Absolute 0C K

0.00 1.00 99.4 372.55 0 0.037 0.27930.50 1.50 111.3 384.45 375 0.238 0.19991.00 2.00 120.3 393.45 697 0.180 0.15801.50 2.50 127.5 400.65 979 0.144 0.13182.00 3.00 133.7 406.85 1228 0.124 0.11372.50 3.50 139.0 412.15 1447 0.106 0.10043.00 4.00 143.8 416.95 1645 0.096 0.09013.50 4.50 148.2 421.35 1803 0.088 0.08204.00 5.00 152.3 425.45 2015 0.082 0.07545.00 6.00 155.9 429.05 2183 0.036 0.06475.50 6.50 159.2 432.35 2337 0.066 0.06076.00 7.00 162.4 435.55 2481 0.064 0.0573

6.50 7.50 165.3 438.45 2620 0.058 0.05436.90 7.90 168.0 441.15 2707 0.068 0.05197.50 8.50 170.6 443.75 2871 0.043 0.04928.00 9.00 173.2 446.35 2997 0.052 0.04708.50 9.50 175.5 448.65 3113 0.046 0.04509.00 10.00 177.8 450.95 3229 0.046 0.04339.50 10.50 179.9 453.05 3338 0.042 0.041610.00 11.00 181.9 455.05 3434 0.040 0.0401

0

50

100

150

200

250

300

350

400

450

500

1 1.5 2 2.5 3 3.5 4 4.5 5 6 6.5 7 7.5 7.9 8.5 9 9.5 10 10.5 11

Temperature (K)

Pressure (abs bar)

Graph of Temperature versus Pressure

0

0.05

0.1

0.15

0.2

0.25

1 1.5 2 2.5 3 3.5 4 4.5 5 6 6.5 7 7.5 7.9 8.5 9 9.5 10 10.5

Slope

Pressure (abs bar)

Graph of dT/dP versus P and Tvfg/hfgversus P

dT/dP

Tvfg/hfg

5. DiscussionIt is necessary to remove air form the boiler as this could affect the accuracy of the experimental results. Based on the graphs, the relationship between pressure and temperature is directly proportional while there is a small deviation between the experimental slope and the theoretical slope. The percentage of error ranged form 0.3% to 44%. Sources of errors are measurements or reading accuracy, room temperature and pressure, the stability of materials and calculations. When the liquid absorbed enough heat energy, it will change to vapour form. As the steam is not allowed to exit, it will cause an increase in pressure and thus causing the temperature to increase. Applications of Marcet Boiler include water boiler, gasifier, power plant and cooking utilities.

6. ConclusionIn this experiment, the relationship between pressure and temperature is found to be directly proportional. When compared to the theoretical slope, the experimental slope shows a small deviation between them because of certain errors. The percentage of error ranged form 0.3% to 44% which can validate this experiment.