chapter 9 power and refrigeration systems with ... -...

Fundamentals of Thermodynamics

Chapter 9

Power and Refrigeration Systems

With Phase Change

Thermal Engineering Lab. 2

9.1 Introduction to power systems

Chapter 9. Power and Refrigeration Systems – With Phase Change

Shaft Work: Steady-state Process-

2 2

21 2

1 2 1 21 2 2

l

V Vw vdp gz gz w

l genw T s

. , ,rev process negligible KE PE

w vdp

- Boundary-movement Work2

121

w pdv



-cycle 에대해서는

2 2

1 212 12 1 1 2 20

2 2

V Vq w h gz h gz

Tds dh vdp

2 2

2 1 121 1

lTds h h vdp q w 2 2

121 1

gen gen

qds s Tds q T s

T

lw

pdvvdp


9.2 The Rankine cycle


1- 2 : Reversible adiabatic pumping process

2 -3 : P= const, reversible heat transfer (addition)

3-4 : Reversible adiabatic expansion

4 -1 : P= const, reversible heat transfer (rejection)



,

,

1 1 1L avgL L

th

H H avgH

Tds T Sq

q T STds

,

,

1L avg

H avg

T

T

area 1-2-2'-3-4-1

area a-2-2'-3-b-a

net H Lth

H H

w q q

q q



average temperature

2 1

avg

Tds QT

S S S

avgT 고온측에서는

avgT 저온측에서는

avgT


Ex. 9.1 Determine the efficiency of a Rankine cycle using steam as the working

fluid in which the condenser pressure is 10 kPa. The boiler pressure is 2

MPa. The steam leaves the boiler as saturated vapor.


2

2 11

2 1 2 1

Assume isentropic process, Integration

For incompressible substance

Tds dh vdp

h h vdP

h h v P P


9.3 Effect of pressure and temperature on the Rankine cycle


i) Exhaust pressure ↓

th

:x Corrosion

, :H avgT slighly

, : L avgT

,

,

1L avg

th

H avg

T

T



ii) Superheating

th

x

, : H avgT

,L avgT const

,

,

1L avg

th

H avg

T

T



iii) Boiler pressure ↑

th

x

, : H avgT

,L avgT const

,

,

1L avg

th

H avg

T

T


Ex. 9.2 In a Rankine cycle, steam leaves the boiler and enters the turbine at 4

MPa and 400℃. The condenser pressure is 10 kPa. Determine the cycle

efficiency.



9.4 The Reheat cycle


: th 큰변화없음 :x


Ex. 9.3 Consider a reheat cycle utilizing steam. Steam leaves the boiler and

enters the turbine at 4 MPa, 400℃. After expansion in the turbine to 400

kPa, the steam is reheated to 400℃ and then expanded in the low-

pressure turbine to 10 kPa. Determine the cycle efficiency.



9.5 The Regenerative cycle and feedwater heaters


2-2'

Rankine cycle

.

의평균 온도가 2'-3보다

낮기때문에 의효율은

Carnot cycle 보다낮음



:

not practical

heat transfer impossible

Ideal Regenerative cycle Carnot cycle 과동일한열효율

• Ideal regenerative cycle – Carnot cycle과 동일한 열효율



• Regenerative cycle with open feedwater heater


Ex. 9.4 Consider a regenerative cycle using steam as the working fluid. Steam

leaves the boiler and enters the turbine at 4 MPa, 400℃. After expansion

to 400 kPa, some of the steam is extracted from the turbine to heat the

feedwater in an open FWH. The pressure in the FWH is 400 kPa, and the

water leaving it is saturated liquid at 400 kPa. The steam not extracted

expands to 10 kPa. Determine the cycle efficiency.




• Regenerative cycle with closed feedwater heater



• Actual power plant utilizing regenerative feedwater heaters


9.6 Deviation of actual cycles from ideal cycles


Turbine Losses

Pump Losses

21 :actual

21 :ideal

s

43 :actual

43 :ideal

s



Piping Losses

a b : Pressure Loss

b c : Heat Transfer


Ex. 9.5 A steam power plant operates on a cycle with pressures and temperatures

as designated in Fig. 9.17. The efficiency of the turbine is 86%, and the

efficiency of the pump is 80%. Determine the thermal efficiency of this

cycle.



9.7 Combined heat and power: other configurations


• Cogeneration system (열병합 발전) : Electricity & Heat


9.8 Introduction to refrigeration systems



9.9 The vapor-compression refrigeration cycle


: refrigerator

: heat pump

L

c

H

c

qCOP

w

q

w

4 - 1 : P= const, evaporation

3 - 4 : isenthalpic expansion

2 - 3 : P= const, condensation

1 - 2 : isentropic compression

COP: Coefficient of Performance

T

S

h

ln P

3

4

2

1

23

4 1

S const

2 1s s

4 3h h

H L cq q w



1 : wet compression 문제점

4 : isentropic expansion

문제점

구성의어려움


Ex. 9.6 Consider a refrigeration cycle that uses R-134a as the working fluid. The

temperature of the refrigerant in the evaporator is -20℃, and in the

condenser it is 40℃. The refrigerant is circulated at the rate of 0.03 kg/s.

Determine the COP and the capacity of the plant in rate of refrigeration.



9.10 Working fluids for vapor-compression refrigeration systems


3R -12, R - 22, R -11, NH

134 , 22 407 , 410R -12 R a R R c R a

2

3

2

Natural Working Fluid:

CO

NH

H O

Propane+Butane


9.11 Deviation of the actual vapor-compression refrigeration cycle from the ideal cycle


h

ln P

24

7 8

5

6 1

3


Ex. 9.7 A refrigeration cycle utilizes R-134a as the working fluid. The following

are the properties at various points of the cycle designated in Fig. 9.24:


P1 = 125 kPa

P2 = 1.2 MPa

P3 = 1.19 MPa,

P4 = 1.16 MPa,

P5 = 1.15 MPa,

P6 = P7 = 140 kPa,

P8 = 130 kPa

T1 = -10℃

T2 = 100℃

T3 = 80℃

T4 = 45℃

T5 = 40℃

x6 = x7

T8 = -20℃

The heat transfer from R-134a during the compression process is 4 kJ/kg.

Determine the COP of this cycle.



9.12 Refrigeration cycle configurations



3 2 Cascade System

(Netsle)

NH CO



9.13 The ammonia-absorption refrigeration cycle

L

H

qCOP

q

chapter 9 power and refrigeration systems with ... -...

Documents