Steam Cycle Theory

Dr. K.C. Yadav, AVP & Head,

Noida Technical Training Centre

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Learning Agenda H2O availability status Energy potential Power generation applications Thermodynamic

Properties, Processes & Cycles

Steam temperature and pressure management

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H2O Energy Potential Potential Energy Kinetic Energy Pressure Energy Flow Energy

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ThermodynamicProperties

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Thermodynamic Processes

Non Flow Processes

P=C, W=mp(v2-v1), Q=H2-H1, U=mCvdT

V=C W=0 Q=U2-U1, U=mCvdT

T=C W=mpV1ln(v2/v1), Q=W, U=0

Poly W=m(p1v1-p2v2)/(n-1), Q=(r-n)W/n-1 U=mCvdT

Isent W=m(p1v1-p2v2)/(r-1), Q=0 U=mCvdT

H=C Free Expansion & Throttling (W, Q & U = 0)

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Thermodynamic Processes

Flow Processes

P=C, Ws=0 Q=H2-H1 U=mCvdT

V=C W=-vdP Q=U2-U1 U=mCvdT

T=C W=RTln(p2/p1) Q=W U=0

Poly W=nm(p1v1-p2v2)/(n-1) Q=(r-n)W/n-1 U=mCvdT

Isent W=rm(p1v1-p2v2)/(r-1) Q=0 U=mCvdT

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Steady Flow Energy Equation

q+hi+ci**2/2+gzi = w+he+ce**2/2+gze

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Applications of Steady Flow Energy Equation

Nozzles; C2 = sq root of [2(h1-h2)+C1**2] Diffuser; C2 = sq root of [2(h1-h2)+C1**2] Centri. Pump; p2v2 - p1v1 + (C2**2 – C1**2)/2 + g(z2-z1) Turbine; W = h2-h1 : Compressor; W = h2-h1 Condenser; q = h2-h1 : Boiler; h2-h1 Throttling; h2 – h1 = 0 : Free Expansion; h2 – h1 = 0

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Uniform State Uniform Flow Process

Qcv + Sum[mi(hi + Ci**2/2 + gzi)]

= Wcv + Sum[me(he + Ce**2/2 + gze)]

+ [m2(u2+C2**2/2+gzi)-m1(u1+C1**2/2+gzi)]

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H2O Phase Cycles

Ice – Water – Ice Cycle Water – Steam – Water Cycle Steam – Ice – Steam Cycle Water – Steam – Ice – Water Cycle

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Two Phase Cycles

Ice Water

Steam

IceSteam

Water

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Steam Cycle (Natural)Three Phase Cycles

Water

Steam

Ice

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Water/Steam Cycles

Natural Cycle

Carnot Cycle

Rankine Cycle (Thermal Cycle)

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Carnot Cycle Hypothetical Carnot Equipments

IsentropicPressure Reducing

Device

Isothermal Heat

AdditionDevice

IsentropicPressure RaisingDevice

Isothermal Heat

RejectionDevice

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Carnot Cycle Temperature V/S Entropy

Entropy

Temp

1

2 3

4

η = 1 – T1/T2 = 1- TR/TA

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Carnot Difficulties & Rankine Solution T-S diagram of Possible Processes

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Rankine Cycle Four Equipment Rankine Cycle

Boiler FeedPump

Boiler

Condenser

Turbine

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Why Rankine Cycle for a Coal Fired Thermal Power Plant?

Does not it related to:

Coal combustion problems at a desired high pressure?

High erosion rate of the prime mover due to highly erosive

impurities in the products of coal combustion?

Metallurgical impossibility?

Techno-economic feasibility?

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Rankine Cycle (Thermal Cycle)

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Rankine Cycle (Thermal Cycle) T-S diagram of simple Rankine Cycle

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Rankine Efficiency Comparison

Work done – work consumed

Heat addedThermal Cycle Efficiency =

= He – Hf – Wp

He – hb =

He – Hf – Wp

He-ha –(hb-ha)=

He – Hf – Wp

He – ha – Wp

=He – Hf

He – ha =

fun(Ta) – fun(Tr)

fun(Ta) – fun(Tr)

Cycle Efficiency is function of heat addition and rejection temperatures (Ta & Tr)

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Thermal Cycle Efficiency

Ratio of isentropic heat drop across the turbine to

the heat supplied to the water in converting it into

steam.

It is directly proportional to the average heat

addition temperature and inversely proportional to

the heat rejection temperature

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Thermal Cycle Efficiency

Average Heat Addition and Rejection temperature can be

suitably changed by

High boiler working pressure

High steam temperature at boiler outlet

High condenser vacuum

Reheating cycle

Regenerative feed heating Cycle

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High Boiler Working Pressure

Variation in water/steam properties (S, L, Cp & Cv) at higher parameters improve Cycle Efficiency

Thermal CycleEfficiency

=Turbine output

Heat added to steam=

Function of (Cp, Cv)

Function of (S, L, Cp)

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High steam temperatureW = F x d

=(F/A) x (A x d)

=P x V Volume of steam is directly proportional to its temperature

and hence increases the turbine output and in turn Cycle Efficiency

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High Condenser Vacuum

Reduces the corresponding saturation temperature

at which heat is rejected. Increase the turbine

output and thermal cycle efficiency

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Reheating cycle

High pressure steam cannot be heated beyond the

metallurgical limits and hence reheated after temperature

reduction in some of the high pressure stages. Thus the

average heat addition temperature increases and in turn

increases the cycle efficiency

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Regenerative feed heating Cycle

High Energy and Less Energy Steam is utilized in preheating the boiler feed water, otherwise the energy would have rejected in the condenser

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Thermal Cycle 250 MW Specific

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Thermal Cycle Processes

Two stage water pressure raising processes (a-b & c-d) in

condensate extraction pump and boiler feed pump are

represented by very small vertical lines at the left of TS

diagram

Two curved lines above each water pressure raising lines

(b-c & d-e), represent sensible heat addition in Drain

Cooler, Gland Steam Condenser, Low Pressure Heaters,

Deaerator, High Pressure Hearters and Economizer

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Thermal Cycle Processes

Two Horizontal lines (e-f & j-a) represent heat addition in

the evaporator and heat rejection in the condenser

Two curved lines (f-g & h-i) before the expansion stages,

represent sensible heat addition to steam (i.e.

Superheating) in Super Heaters and Re Heater

Two stage steam expansion processes in High Pressure

Turbine and Intermediate Pressure / Low Pressure

Turbines are represented by two vertical lines (g-h & i-j) at

the right of TS diagram

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Properties of H2O

Density

Relative density

Specific gravity

Specific heat

Sensible heat

Latent heat

Freezing/melting temperature

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Properties of H2O

Boiling/condensing/saturation temperature Critical temperatures Triple point temperature Vapour pressure Saturation pressure Critical pressure Triple point pressure Viscosity Electrical conductivity Thermal conductivity

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Properties of H2O

Physical Stability Chemical Reactivity

Non toxic

Non corrosive)

Behavior in terms absorption, adsorption and solution Cohesive and adhesive forces Surface tension Internal energy Enthalpy Entropy

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Variation in H2O PropertiesNo Ts Ps Vf Vg Hf Hfg Hg

deg C bar cubic meteter per Kg KJ/Kg KJ/Kg KJ/Kg1 0.1 0.0061 0.001 206.31 0 2501.6 2501.62 4 0.0081 0.001 157.27 16.8 2492.1 2508.93 15 0.017 0.001001 77.978 62.9 2466.1 25294 46 0.1008 0.00101 14.557 188.4 2394.9 2583.35 100 1.0133 0.001044 1.675 419.1 2256.9 26766 165 7.0077 0.001108 2724 697.2 2064.8 27627 200 15.549 0.001156 0.1272 852.4 1938.5 2790.98 235 30.632 0.001219 0.0652 1013.8 1788.5 2802.39 250 39.776 0.001251 0.05 1085.8 1714.6 2800.4

10 300 85.927 0.001404 0.0216 1345 1406 275111 350 165.35 0.001741 0.0087 1671.9 895.8 2567.712 355 175.77 0.001809 0.008 1716.6 813.8 2530.413 360 186.75 0.001896 0.0072 1764.2 721.2 2485.414 365 198.33 0.002016 0.006 1818 610 242815 370 210.54 0.002214 0.005 1890.2 452.6 2342.816 374.15 221.2 0.00317 0.0032 2107.4 0 2107.4

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Steam Generation

Heating Surface Phenomenon

Water Surface Phenomenon

Due to occurrence of vapour pressure

Due to occurrence of low relative humidity

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Steam Quality Parameters

Dry Saturated Steam

Either saturation temperature or saturation Pressure

Wet Steam

Either saturation temperature or saturation Pressure

dryness fraction (DF) = Ms/M(s+w)

Super Heated Steam

Either saturation temperature or saturation Pressure

Degree of superheat (DS) = T – Ts

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Thank you

4th October, 2008