68061178 dryness fraction of steam lab
DESCRIPTION
Steam tableTRANSCRIPT
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University of Technology, Jamaica
MECHANICAL TECHNOLOGY
Dryness Fraction of Steam
Name: Roneil Napier
ID: 9712441
Group: B.ENG 4E (Art.)
Date: July 21st, 2011
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LAB 2 - DRYNESS FRACTION OF STEAM
Objective
To gain familiarity with the main components and operation of a gas fired steam boiler.
To determine the dryness fraction of steam sampled from the pipeline
Theory
A separating calorimeter uses mechanical means to separate the liquid from the vapour in a wet
steam sample. The separated vapour is then passed through a water-cooled condenser, so that it
can be collected as a liquid.
Let mf = mass collected from separator
mg = mass collected from condenser.
Then, apparent dryness fraction of steam sample,
xa =
=
However, the separation process is imperfect, and some amount of liquid is included with mg. If
a throttling calorimeter is placed after the separator, the fall in pressure during the throttling
process carries the separated vapour over into the superheat region. This then allows the enthalpy
of the superheated vapour, h2, to be determined from steam tables, if the pressure and
temperature after throttling is measured.
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LAB 2 - DRYNESS FRACTION OF STEAM
Assuming constant enthalpy for throttling process,
h2 = h1 = hf1 + xhfg1.
Hence, dryness fraction of vapour entering throttling calorimeter,
xb =
Therefore, actual mass of separated vapour = xb (mg).
Actual dryness fraction of steam sample =
= xb*xa.
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LAB 2 - DRYNESS FRACTION OF STEAM
Apparatus
Boiler Tubeless Steam Boiler (Vertical)
Manufacturer HURST
Model 0850823
Serial V86-200-4
Working Pressure 150 psig (~10.3 bar)
Fuel Diesel
Equivalent Evaporation
1035lbs (~469.5kg) from and @ 212oF (100oC)
Combined separating and throttling calorimeter (Norwood Instruments, England). Thermometer Large and small cylindrical collection vessels Triple beam balance Fortin barometer
Figure 1
Apparatus
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LAB 2 - DRYNESS FRACTION OF STEAM
Procedure
The arrangement of the boiler was observed and sketched to show the main accessories and
control components. The water level in the boiler was checked to ascertain whether it was in the
operating range on the sight glass. The air vent at the top of the boiler was then opened to ensure
the expulsion of air from the boiler. The Treatment Chemical dosing line and the feed water line
valve to the boiler were then opened. The lock-off valve on the Diesel Fuel supply line was then
opened and the boiler fired. The startup sequence was then observed. When steam began to emit
from the air vent valve, the valve was closed slowly in order to prevent the water level from
rising in the sight glass. The boiler was then allowed to pressurize.
The masses of the small and large containers were determined by weighing them. When the
boiler had run-up to pressure, valves A and E (located at the top of the steam lagging apparatus)
were opened just wide enough to allow a slow bleed of steam through the pipeline. This heated
the pipeline and evaporated any condensate that was present.
After ensuring that valve D was closed, valve C was opened very slowly. Valve F (on the upper
lagged leg on the Lagging Apparatus) was cracked open to the marked position so as to permit
a slight bleed of steam. This was done in order to minimize condensation on the steam line.
Valve C was then closed after adequate heat was supplied to the apparatus, and the throttled
Steam Thermometer T2 showed a reading that was above 90oC. A container was positioned
below the condenser to collect the condensate. With the use of gloves, valve D was gradually
opened to release liquid from the mechanical separator. Valve D was then reclosed and cooling
water turned on to flow moderately into the condenser. The large container was then placed
below the condenser to collect the condensate. Valve C was then slowly reopened fully to allow
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LAB 2 - DRYNESS FRACTION OF STEAM
the steam to flow through the apparatus. After it was observed that the steam was superheated,
readings of steam line pressure, water manometer, and thermometer readings were taken. When
an adequate volume of water was collected in the large container, valve C was closed. After
sufficient time had passed to allow all condensate to drain from the condenser, the small
container was again placed at the outlet of valve D which was then gradually opened to collect
all the water from the mechanical separator, after which valve D was reclosed. The containers
were then weighed with their contents.
In taken the atmospheric pressure, the cistern adjusting screw at the base of the fortin barometer
was rotated until the surface of the mercury in the cistern just touched the tip of the tapered zero
pointer. The vernier scale was then positioned such that the lower edge of the scale was level
with the top of the mercury meniscus with two triangular spaces visible on each side of the
mercury column, after which its height was recorded. The temperature in the barometer cabinet,
the barometer reading, and the corresponding correction multiplier were then recorded.
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Figure 2
Sketch of Vertical Tubeless Boiler
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Boiler Firing Sequence
The control firing sequence occurs at cold startup or when the steam pressure drops. The
pressure control completes an electric circuit, which starts a timer motor cam turning in the
programmer. The first contact on the timer motor cam closes and starts the burner motor that
rotates the primary air fan. The primary air fan blows air into the furnace to purge any unburned
fuel present in a gaseous condition. This process in called pre-purging the furnace. By pre-
purging the furnace before pilot ignition, the danger of a furnace explosion is reduced.
Depending upon the size of the furnace the purge cycle takes approximately 30 seconds but may
take as long as 60 seconds.
The programmer is still operating and when the second contact closes, the circuit of the ignition
transformer is completed. This causes a spark in front of the gas pilot tube. At the same time, a
solenoid valve is opened in the gas pilot line, allowing gas to flow through the gas pilot tube and
be ignited by the spark. The scanner is located on the front of the boiler and is used to sight the
pilot. Sighting the pilot through the scanner will verify that the pilot is lit. This process is
referred to as proving pilot.
The next step is to close the contact which completes the circuit to the main fuel valve, which
opens only after the scanner has proved pilot. With the main fuel valve open the fuel enters the
furnace and is ignited by the pilot. The scanner is then used to prove the main flame. The
programmer continues to operate for a few more seconds, securing circuits to the ignition
transformer and the gas pilot. After the circuits are secured, the programmer stops. The burner is
now regulated by the pressure control and the modulating pressure control. If the scanner senses
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LAB 2 - DRYNESS FRACTION OF STEAM
a flame failure, the system is purged and secured. The programmer is then manually reset to the
start cycle.
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LAB 2 - DRYNESS FRACTION OF STEAM
Results
ParametersGroup 1 Readings Group 2 Readings
Value Inst. Units Value SI Units
Mass (Small Container)
Empty 1.65 lb 0.748 kg
With Contents 2.10 lb 0.953 kg
Mass of Separated Liquid, Mw 0.45 lb 0.205 kg
Mass (Large Container)
Empty 1.60 lb 0.726 kg
With Contents 10.0 lb 4.536 kg
Mass of Condensed Vapour, Ms 8.4 lb 3.810 kg
Line Steam Pressure, P1 117 MN/m2 (psig) 8.0672 bar
Line Steam Temperature, T1 68 oC 341 K
Throttled Steam Pressure, P2 1.5 Inch (w.g.) 0.1034 bar
Throttled Steam Temperature, T2 102 oC 375 K
Mercury Column Height 746 mm of Hg 0.746 m of Hg
Fortin Barometer Temperature 32 oC 305 K
Height Container Factor 0.00519 Null 0.00519 Null
Gravity = 9.784 ms-2
Figure 3
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LAB 2 - DRYNESS FRACTION OF STEAM
Separating and Throttling Calorimeter
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LAB 2 - DRYNESS FRACTION OF STEAM
Sample Calculations
Conversions:
K = oC + 273
68 oC = (68 + 273) K
= 341 K
1 psi = 6.895 kPa
117 psi = (117 * 6.895) kPa
i.e. P1 = 806.72 kPa
Calculation of Apparent Dryness Fraction of Steam Sample xa:
xa =
=
= 0.949
Absolute pressure, Pabs = Gauge pressure, Pgauge + Atmospheric pressure, Patm
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LAB 2 - DRYNESS FRACTION OF STEAM
Patm = Hggh
Now h = Mercury Column Height (Mercury Column Height Height Container Factor)
= 0.746 (0.746 * 0.00519)
= 0.742 mHg
Patm = 13595.5 * 9.784 * 0.742
= 98,699.63 Pa
= 0.987 bar ~ 1.0 bar
Working pressure, P = P1 + Patm
= 8.067 + 1
= 9.067 bar
Pressure of Throttled Steam, P2 = Manometer pressure, Pm + Atmospheric pressure, Patm
h = 1.5 in
= 0.0381 m
Pm = 1000 * 9.784 * 0.0381
= 373.77 Pa
= 0.00373 bar
P2 = 0.00373 + 1.0
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LAB 2 - DRYNESS FRACTION OF STEAM
= 1.00373 bar
Calculation of Dryness Fraction of Vapour Entering Throttling Calorimeter, xb
For hf at 9.067 bar:
From Steam Tables and by Interpolation:
=
hf = (0.067 * 20) + 743
= 744.34 kJ/kg
For hfg at 9.067 bar:
From Steam Tables and by Interpolation:
=
hfg = (0.067 * -16) + 2031
= 2029.93 kJ/kg
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P (bar)
10.0
9.0
9.067
743 763hfh (kJ/kg)
P (bar)
10.0
9.0
9.067
2031 2015hfgh
fg (kJ/kg)
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LAB 2 - DRYNESS FRACTION OF STEAM
The temperature of the throttled steam is 102 oC which tells us that it is superheated. From the
superheated section of the steam tables and by interpolation:
At 1 bar:
=
h2 = (0.4 * 101) + 2676
= 2716.4 kJ/kg
xb =
=
= 0.922
Actual dryness fraction of steam sample, x:
x = xa * xb
= 0.949 * 0.922
= 0.875
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T (oC)
150
100
102
2676 2777h2
h (kJ/kg)
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LAB 2 - DRYNESS FRACTION OF STEAM
Discussion
The dryness fraction of steam describes how dry steam is, with a value of 1 representing steam
that is 100% dry, and therefore free of entrained moisture. Steam with a dryness fraction of 0.99
consists of 99% steam and 1% water. If we measure the latent heat present in steam that has a
dryness fraction of 0.99 we will find that it possesses 99% of the full quotient of latent heat.
The actual dryness fraction in this experiment was calculated to be 0.875, and was found to be
lower than both xa and xb which are 0.949 and 0.922 respectively. From the analogy in the
paragraph above, we can deduce that the steam under investigation consisted of 87.5% steam and
12.5% water, intrinsically a 7:1 ratio. While this fraction may be acceptable for some processes,
others may require a drier degree of steam or even steam that is less dry.
The amount of water (12.5%) present in the sample of steam under observation can be attributed
to errors that may have occurred during the experiment, leading to the incomplete separation of
water. Possible sources are human errors in the form of parallax or reading accuracy when
measurements were taken, and calculations. There was also evidence of steam escaping into the
room resulting in systematic errors which might have altered room temperature and pressure.
Conclusion
The objectives of this laboratory exercise were realized, thus validating this experiment.
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LAB 2 - DRYNESS FRACTION OF STEAM
References
Van Ness, H. C. (1983). Understanding Thermodynamics. Dover Publications; Dover ed
edition (January 1, 1983). ISBN-13: 978-0486632773
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