01 Basics of Thermodynamics

8/8/2019 01 Basics of Thermodynamics

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Power Plant (Combined Cycle) Operating Principles - Basic

Pg 101 Basics of Thermodynamic

Basics of Thermodynamics

August 2007Lumut Power


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ACTSYS Process Management Consultants

Time schedule


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First law of thermodynamics dU = dQ - dW In a closed system, entire heat input

can not be converted completely towork.

Defines property Internal Energy (U) Constant Volume Process dW = 0.

Internal energy (U) Depends on

Molecular mass Temperature Atomic motion

High temperature means high internalenergy

Water and Steam - Properties of steam

E = mv2

/2

Molecules are in constant random motion frequently collide with each other and withwalls of contact surface. Internal energy is the total kinetic energy of movingmolecules and the potential energy of vibrating and electrical energy of molecules.

Heating of liquid or vapor increases the internal kinetic energy and therebyincreases the internal energy.

At Saturation phase of steam, when water is converted to vapor, the specificvolume increases. This causes high atomic motion and hence increase ininternal energy.


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Thermodynamic system

Basic steady flow energyequation m1 (U + Pv +V

2/2 + Zg)1 + Q = m2(U+ Pv +V2/2 + Zg)2 + W

U = Internal energy

Pv = Pressure x Volume(termed as Flow work)

V2 /2 = Kinetic energy

Zg = Potential energy

W = Work done by the system

Q = Heat input to the system

U + Pv = Enthalpy (H)

Enthalpy is term formed bycombining the internal energy andflow work together - Analysis of anysystem boundary involves these twoforms of energy and hence this iscombined to produce a meaningfulterm.

Application Steam Flow Energy Equation

Steam Boiler (Heat Exchanger)

Work (w) = 0;

Zg and (V2/2) are very small compared to other components in theequation and can be neglected;

Q = H2 H1

Steam Turbine

Heat Loss (Q) is small compared to other components and hence can

be neglected.Zg = 0 as in general the turbine is mounted in horizontal plane.

(V2/2) is not significant for most of the turbine stages.

W = H2 H1


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Application of steady flow energy equation

0

2

2

2

1

21

12

=

=

=

W

negligibleVV

neglibilegZgZ

TCQ

hhQ

p

Heat Exchanger

Enthalpy Drop across the heat

exchanger contributes for the heattransfer of the fluid in the heat

exchanger.

m1 * (U + Pv +V2/2 + Zg) + Q = m2 * (U+ Pv +V2/2 + Zg) + W


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0

2

2

2

1

21

12

=

=

Q

NegligibleVV

neglibilegZgZ

hhW

m1 * (U + Pv +V2/2 + Zg) + Q = m2 * (U+ Pv +V2/2 + Zg)+ W

Steam Turbine

Enthalpy Drop across the Steam

Turbine is the energy converted

into work


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0

0

2

2

2

1

21

12

=

=

=

W

Q

NegligibleVV

neglibilegZgZ

hh

m1 * (U + Pv +V2/2 + Zg) + Q = m2 * (U+ Pv +V2/2 + Zg) + W

Pressure Reducing Process / Throttling

Enthalpy at inlet of the pressure

reducing valve is same as the at

outlet . No heat loss across thepressure reducing valve


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A system with negligible change in kinetic energy andpotential energy and for closed system,

U2 + W= Q + U1 Simplified first law of thermodynamics U = Internal energy W = Work done by the system 1

2 Pdv Q = Heat input to the system

Heat Input = Mass x Specific heat x Temperature For Constant Pressure Process,

W = P (V2 V1) Q = m Cp (T2 T1) = (U2 + PV2) (U1 + PV1) = H2 H1

For Constant Volume Process, W = 0 Q = m Cv (T2 T1) = (U2) (U1)

Enthalpy

Change in enthalpy of fluid can be expressed as Mass x Specific Heat Capacity (atConst Pr.) x Temperature Diff for the constant pressure process.

For air and combustion gas, the zero enthalpy is referred to 27 Deg C and 1.01325Bar as per United States Boiler Industry.


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Entropy (S)

Defines the irreversibility of the process Measure of disorderliness of the system Defines a practical boundary of possible work and heat transfer in a process

Change in entropy (system and surrounding) of the irreversible process is alwayspositive

Reduction in entropy in a process is impossible S2 S1 = Cv ln (T2/T1) + R ln (V2/V1)

= Cp ln (T2/T1) + R ln (P2/P1)

=2

1T

QS rev

Reversible Process

Reversible thermodynamic process exist only in theory.

Reversible thermodynamic process concept serve as a limiting case for theheat flow and work flow in any process.


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Steam table (T-S chart)


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Steam Table (Mollier chart)

Enthalpy

(Btu/lb)

Entropy (Btu/lbF)

Entropy (Btu/lbF)

Enthalpy

(Btu/lb)


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Sample Steam table

Properties of Superheated Steam (Pressure)

Interpolation of the steam parameter for dryness fraction X in the saturation phase:

Enthalpy (H) = Hf + X * (Hfg)

Entropy (S) =Sf + X * (Sfg)

Internal Energy (U) = Uf + X * (Ufg)

Volume (v) = vf + X * vfg

Interpolation of the steam parameter for the temperature T and Pressure P in theSuperheated Region when parameter at T1, P and T2, P are known:

Enthalpy (HP,T) = HP,T1 + (HP,T2 - HP,T1)/(T2- T1)

Entropy (SP,T) = SP,T1 + (SP,T2 - SP,T1)/(T2- T1)

Internal Energy (UP,T) = UP,T1 + (UP,T2 - UP,T1)/(T2- T1)

Volume (vP,T) = vP,T1 + (vP,T2 - vP,T1)/(T2- T1)


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Thermodynamic cycle - Carnot Cycle

1 2

34

1 2

34

Isothermal

heat addition

IsentropicExpansion

Isothermalheat rejection

IsentropicCompression

Entropy

Temperature

Practically impossible to construct aengine that works on Carnot cycle.

All process are reversible process.

Defines the maximum possibleefficiency of engine that operates betweenthe same source and sink temperature.

carnot = (T1 T2 )/ T1

T1

T2

Isothermal heat addition of single phase fluid could not be achieved practically.

Isentropic compression of two phase fluid would have problem in wet compression.


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Thermodynamic cycle - Brayton Cycle


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Thermodynamic cycle - Rankine Cycle

Process: b c : Heat input

c d : Expansion work

d a : Condensing (heatrejection) a b : Pump Work

Simple ranking cycle is a fully condensing cycle.

Cycle efficiency is lower with higher heat rejection in the condenser. Condenserloss is a major loss in the Rankine cycle.

Higher temperature and pressure at turbine inlet enable more power generation forthe same heat input and improves cycle efficiency.

Lower the condenser pressure higher the power generation.

Closed cycle can have better vacuum than the open cycle condenser vacuum.

Source temperature (T1) in the above cycle is the mean temperature of the processb-a and the Sink temperature (T

2) is the mean temperature of the process d-a.

Maximum achievable efficiency = (T1-T2)/T1


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Thermodynamic cycle - Combined Cycle

01 Basics of Thermodynamics

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