Energy management Water and steam Exergy Exergy by heat transfer - - PowerPoint PPT Presentation

Laboratorij za termoenergetiko

Energy management

Water and steam Exergy Exergy by heat transfer Exergy in the case steam turbine expansion

Properties of water and steam

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Properties of water and steam- density

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water Moist steam Superheated steam

Properties of water and steam - volume

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water Moist steam Saturated steam

Properties of water and steam - entalphy

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water Moist steam Superheated steam

Properties of water and steam – Molliere diagram h-s

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Properties of water and steam - table

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tlak specifični volumen specifična entalpija specifična entropija temperatura sprememba agregatnega stanja voda para

Properties of water and steam - table

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tlak nasičenja specifični volumen specifična entalpija specifična entropija vrela voda nasičena para

v = v' + x(v" - v') h = h' + x(h" - h') s = s' + x(s" - s') suhost pare: x = mpara mpara + mvoda

Exergy

Exergy is the convertible part of energy and can be described in several ways

 it is the energy that can be completely converted into any other form of energy at given surroundings conditions  it is the largest quantity of work that can be produces in a technical device from working media with given starting parameters  it is the smallest required quantity of work to raise working media from surroundings conditions to any other condition provided that heat is brought into the process only from the environment Exergy depends on surroundings conditions which limits the 'usability' of enery carried by working media. If working media is in balance with the surroundings no energy can be extracted from it without using additional source of energy. Anergy is the part of energy that cannot be converted into any other form of energy including exergy. Internal energy of the environment is pure anergy.

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Exergy

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Exergy

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Specific exergy of working media

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ideal (reversible) process:

• internal energy of the working

media, U1

• volume work for inserting the

work media into the system, p1 V1

• internal energy of the working

media, Uamb

• volume work to eject the work

media from the system, pamb Vamb

• heat dissipated,

Qout = Tamb(S1 - Samb)

• acquired work

Wt = Wt,max

U1 + p1 V1 = Uok + pamb Vamb + Qod + Wt,max Wt,max = H1 – Hamb – Tamb(S1 – Samb) wt,max = e = h – hamb – Tok(s – samb)

Examples of defining properties of water and steam

Using the tables of water and water vapor properties, determine the volume of 2.5 kg of water/ steam that has

• a temperature of 60 ° C and a pressure of 1 bar
• temperature 150 ° C and pressure 1 bar
• temperature 150 ° C and pressure 20 bar

Determine the state of 4 kg of water/steam at 160 ° C and

• pressure 10 bar
• volume 0.8 m3
• specific enthalpy of 2780 kJ/kg

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Examples of determining the properties of water and steam

Linear interpolation

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 

p n p n p p

y y x x x x y y     

primer parameter 1 parameter 2 iskano

1

p = 20 bar T = 180 °C v =

2

p = 5 bar T = 306 °C h =

3

p = 8 bar h = 3000 kJ/kg T =

4

T = 50 °C s = 6,75 kJ/kgK h =

5

p = 1,9 bar T = 120 °C s =

6

p = 7 bar x = 0,813 v =

7

p = 105,3 bar v = 0,02 m3/kg T =

Examples of determining the properties of water and steam

Linear interpolation

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 

p n p n p p

y y x x x x y y     

primer parameter 1 parameter 2 iskano

1

p = 20 bar T = 180 °C v = 0,001127 m3/kg

2

p = 5 bar T = 306 °C h = 3077 kJ/kg

3

p = 8 bar h = 3000 kJ/kg T = 273,1 °C

4

T = 50 °C s = 6,75 kJ/kgK h = 2163,15 kJ/kg

5

p = 1,9 bar T = 120 °C s = 7,1517 kJ/kgK

6

p = 7 bar x = 0,813 v = 0,2220 m3/kg

7

p = 105,3 bar v = 0,02 m3/kg T = 341,0 °C

The exergy losses in the case of heat transfer

In case of a water-water heat exchanger find the loss of exergy flow and analize dependence

• f transferred exergy flow and required heat transfer area on inlet temperature of the colder
• water. The warm water enters with 90 °C and exits with 60 °C while mass flow rate is

17 kg/s. Mass flow rate of the cold water is 12,8 kg/s and its inlet and outlet temperatures are 30 °C and 70 °C, respectively.

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The exergy losses in the case of heat transfer

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0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 10 20 30 40 50 relativna površina prenosnika, relativna izguba eksergije vstopna temperatura hladne snovi / °C površina izguba eksergije

Exergy loss in a heat exchanger - mixer

For the case of of mixer heat exchanger shown in the figure calculate the global entropy (system and surroundings) if ambient temperature is 20 °C. Calculate lost exergy due to mixing of both flows?

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Energy and exergy flows in the turbine

team is entering a turbine with a pressure of 110 bar and a temperature of 530 °C and expands to a pressure of 0,06 bar. Steam flow rate is 15 kg/s. Surrounding conditions are 1 bar and 25 °C. Calculate

• turbine power
• inlet and outlet exergy flows and
• energy and exergy balance

for an ideal turbine with thermal efficiency 1 as well as a real turbine where outlet steam dryness is 0,84. Verify the validity of Gouy-Stodola theorem.

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Reduction cooling station

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For the system shown in the figure find missing parameters as well as energy and exergy flows for two operating regimes: a) all steam from the boiler is directed throgh the turbine b) parameters of superheated steam are reduced in reducing and cooling station Construct a Rant chart for both regimes. Surrounding conditions are 1 bar and 20 °C.

Reduction cooling station

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reducirno-hladilna postaja turbina