3. STEAM
• Steam is the gas formed when water
changes from the liquid to the
gaseous state.
• Steam are vapours of water
suspended in air
• It is a very important and commonly
used working fluid for power
generation.
• It is used as a working medium for
steam engines, steam turbines, etc.,
steam has wide applications in
industry like power plants, textile,
chemical, sugar,paper, etc.
4. DRY STEAM / SATURATED STEAM
• The steam having no water
molecules in it is called Dry
Steam.
• Dry steam applies to steam
when all its water molecules
remain in the gaseous state.
• It’s a transparent gas.
• Dryness fraction,
x = 1
5. WET STEAM / UNSATURATED STEAM
• A steam having both water
molecules and steam
• Wet steam applies to steam
when a portion of its water
molecules have given up their
energy (latent heat) and
condense to form tiny water
droplets.
• Dryness Fraction,
x < 1
6. DRYNESS FRACTION
• It is defined as the ratio of mass of dry steam actually present to the mass of wet steam.
• It is denoted by x.
• .
Where, ms = mass of dry steam, kg
• For dry steam, mw= 0 and x = 1
• For wet steam, x < 1
• Thus if in 1 kg of wet steam 0.9 kg is the dry steam and 0.1 kg water particles then x=0.9.
7. METHODS TO DETERMINE DRYNESS FRACTION
• The dryness fraction of steam can be measured experimentally.
• Calorimeters are used for measurement of dryness fraction of
steam.
• There are four methods of determining the dryness fraction of
steam :
1. Bucket / Barrel Calorimeter
2. Throttling Calorimeter
3. Separating Calorimeter
4. Combined Separating & Throttling Calorimeter
8. BARREL / BUCKET CALORIMETER
The barrel calorimeter consists of a copper
calorimeter placed on wooden block and covered
by a wooden cover. The barrel contains a known
quantity of cold water and is surrounded by an
outer vessel with air space in between, which
acts as an insulator. The temperature is
measured by a thermometer which passes
through one of the holes in the wooden cover.
The whole assembly is placed on the platform of
a weighing bridge. The steam from the main
steam pipe enters through the sampling tube via
the control valve, flows into the cold water
through fine exit holes of the ring provided at
the end of the pipe. The condensation of steam
takes place as it comes in contact with cold
water, with the result that the temperature of
water rises. The pressure of entering steam is
indicated by the pressure gauge.
9. SEPARATING CALORIMETER
This calorimeter, consists of two concentric chambers
which communicate with each other through an opening
at the top. The steam to be tested flows from the main
steam pipe through a sampling tube to the calorimeter
pipe via the valve which must be fully open when
testing. The metal basket has a large number of
perforations through which the steam discharges. The
water particles due to their heavier momentum get
separated from the steam and get collected in the inner
chamber. The quantity of water collected is indicated
by the level in the gauge glass and the pointer which
moves on a scale. The dry steam in the inner chamber
moves up and then down again through the annular
space between the two chambers. The gauge fitted has
two scales, the inner one indicating the pressure while
the outer one indicates the rate of discharge of dry
saturated steam per 10 minutes. Then the steam flows
through the calibrated orifice to the bucket calorimeter
which is placed on the platform balance by which the
most of steam discharged can be further confirmed.
10. THROTTLING CALORIMETER
The throttling calorimeter, consists of the
sampling tube through which the steam from the
main steam pipe flows through the throttling
valve and becomes superheated. The steam then
flows into the inner chamber, flows down and
rises up again to enter the annular space. The
loss of heat by radiation from the inner chamber
is minimised by the hot steam around the
outside of the inner chamber. The temperature
of throttled steam is measured by the
thermometer placed in the pocket filled with
cylinder oil. To obtain good results, the steam
issuing from the throttle valve must be super
heated. To ensure this a manometer is attached
to measure the pressure of steam. Corresponding
to this pressure the saturation temperature of
steam must be lower than the temperature
indicated by the thermometer. The steam finally
escapes through the exhaust. This is suitable for
high dryness steam.
11. COMBINED SEPARATING & THROTTLING
CALORIMETER
In this calorimeter, the steam is first
passed through the separating
calorimeter where it losses most of its
moisture and becomes compeatively
driver. It is then passed through the
throttling calorimeter where
superheating takes place without
change of enthalpy. The temperature
and pressure of steam after throttling
are measured by using a thermometer
and pressure gauge respectively.
