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Steam Propulsion System
Theory
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To understand the working
principles of steam generationand power plants
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Objectives Principle of steam generation
Wet, saturated and superheated steam Dryness fraction
Enthalpy, entropy and internal energy
Principle of Rankine Cycle
Cyclic work output
Thermodynamic and mechanical efficiencies
Schematic Block Diagram of Steam Power
Plant
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Principle of steam generation Steam - excellent working agent due to:
Carry large quantities of stored energy. Produced from water cheap.
Multipurpose.
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Formation of Steam
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Formation of Steam Stage 1 Water temperature increases to saturation temperature.
Energy required liquid enthalpy, hf.
Stage 2 Takes place at constant temperature - water converted into steam.
Begins at saturation temperature, ends at dry saturated steam
Still in wet steam dryness fraction Energy required enthalpy of evaporation, hfg.
Stage 3 Begins when all dry saturated steam formed at saturation temp.
Further heat up produces superheated steam Further rise in temperature
Amount of energy added superheat enthalpy, h.
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Dryness fraction Degree of wetness for steam
Represent by symbolx. Can be represent by following ratio:
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Internal Energy (u) Store or static energy results from atom & molecules random
motion
In any state, atoms / molecules have a degree of random motion
In a pure substance, degree of random motion will be same, eachtime the substance returns to that state thus this become a property
Internal energy function of degree of random motion,
However, internal energy increase not always result in temperature
increase When phase is changed (water into steam), temperature remains
constant
Thus, internal energy increases at constant temp necessary to carry
out degree of molecules separation Thus, rate of internal energy depend on substances state
In fluid condition, atoms and molecules have greater motions thansolids
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Enthalpy (h) Combination of internal energy (u), pressure (P)
and volume h = u + Pv
Since they are forms of properties, enthalpy also
is a properties
h is a specific enthalpy
Enthalpy of any mass other than unity is given by
H
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Entropy During transformation of substance, an amount of
heat transferred to surrounding or more useful asa work done
When energy / heat transferred, temperature
difference will occur
When substance is at abs 0 temp, energy content
is also zero
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Rankine cycle
Important modification from Carnot cycle to allow a completeThus, easily dealt in feed pump before raise to boiler pressure
Cycle becomes more practical
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Steam power plant
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Steam plant Steam is generated in boiler passes into steam main Steam feed into a turbine or pass into other plant (heaters etc)
If plant works on a 'dead-loss' system exhaust steam lost toatmosphere
Not efficient no steam recovery plant due to limited space
In steam recovery plant exhaust steam passes into condenser,condensed to water (condensate)
Condensate is extracted by condensate pump, passes as feed waterinto feed water main and back to boiler
Feed pump is use to increase the feed water pressure normallyboiler works at higher pressure
Thus, a steam recovery plant circulates the same water all the time
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Steam plant (cont/) However, losses still occur back up by make-up
water supply Advantages of steam recovery plant:
Pressure in condenser can be operated well below
atmospheric pressure greater steam expansion, thus
more work done
Feed water can be chemically treated reduce scaleformation
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