a steam propulsion system theory ok

8/3/2019 A Steam Propulsion System Theory OK

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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

a steam propulsion system theory ok

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