dryness fraction of steam lab

University of Technology, Jamaica

MECHANICAL TECHNOLOGY

Dryness Fraction of Steam

Name: Roneil Napier

ID: 9712441

Group: B.ENG 4E (Art.)

Date: July 21st, 2011

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

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of the superheated vapour, h2, to be determined from steam tables, if the pressure and

temperature after throttling is measured.

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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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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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

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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

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

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


a flame failure, the system is purged and secured. The programmer is then manually reset to the

start cycle.

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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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Separating and Throttling Calorimeter

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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

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Absolute pressure, Pabs = Gauge pressure, Pgauge + Atmospheric pressure, Patm

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

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= 0.00373 bar

P2 = 0.00373 + 1.0

= 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 763hf

h (kJ/kg)

P (bar)

10.0

9.0

9.067

2031 2015hfg

hfg (kJ/kg)


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)


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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References

Van Ness, H. C. (1983). Understanding Thermodynamics. Dover Publications; Dover ed

edition (January 1, 1983). ISBN-13: 978-0486632773

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