(c) The exergy destruction in each component of the cycle is determined as follows Compressor:

Soru 1) Refrigerant-134a enters the compressor of a refrigerator as superheated vapor at 0.20 MPa and -5°C at a rate of 0.07 kg/s, and it leaves at 1.2 MPa and 70°C. The refrigerant is cooled in the condenser to 44°C and 1.15 MPa, and it is throttled to 0.21 MPa. Neglect any heat transfer and pressure drops in the connecting lines between the components,

𝑇₀= 300 𝐾; 𝑇_𝐿= 273 𝐾;

ℎ@ 𝑃=0.2 𝑀𝑃𝑎 𝑎𝑛𝑑 𝑇=−5°𝐶 = 248.82 𝑘𝐽/𝑘𝑔 𝑠@ 𝑃=0.2 𝑀𝑃𝑎 𝑎𝑛𝑑 𝑇=−5°𝐶 = 0.95414 𝑘𝐽/𝑘𝑔𝐾 ℎ@ 𝑃=1.2 𝑀𝑃𝑎 𝑎𝑛𝑑 𝑇=70°𝐶 = 300.63 𝑘𝐽/𝑘𝑔

ℎ@ 𝑃=1.2 𝑀𝑃𝑎 𝑎𝑛𝑑 𝑠=0.95414𝑘𝐽/𝑘𝑔𝐾= 287.23 𝑘𝐽/𝑘𝑔 ℎ@ 𝑃=1.15 𝑀𝑃𝑎 𝑎𝑛𝑑 𝑇=44°𝐶= 114.3 𝑘𝐽/𝑘𝑔

a) Write all mass, energy, entropy and exergy balance equations for each device.

b) Find the rate of heat removal from the refrigerated space and the work input to the compressor.

c) Find the energetic COP and exergetic COP of the refrigerator.

Soğutucu akışkan-134a soğutucunun kompresörüne 0,20 MPa basınç ve -5°C sıcaklıkta 0,07 kg/s debiyle kızgın buhar olarak girmekte; 1,2 MPa basınç ve 70°C sıcaklıkta çıkmaktadır.

Soğutucu akışkan yoğuşturucuda 44°C sıcaklık ve 1,15 MPa basınca soğutulmakta ve daha sonra 0,21 MPa basınca kısılmaktadır. Bileşenler arası bağlantı borularında olan ısı aktarımını ve basınç düşmelerini ihmal ederek,

𝑇₀= 300 𝐾; 𝑇_𝐿= 273 𝐾;

ℎ@ 𝑃=0,2 𝑀𝑃𝑎 𝑣𝑒 𝑇=−5°𝐶 = 248,82 𝑘𝐽/𝑘𝑔 𝑠@ 𝑃=0,2 𝑀𝑃𝑎 𝑣𝑒 𝑇=−5°𝐶 = 0,95414 𝑘𝐽/𝑘𝑔𝐾 ℎ@ 𝑃=1,2 𝑀𝑃𝑎 𝑣𝑒 𝑇=70°𝐶= 300,63 𝑘𝐽/𝑘𝑔

ℎ@ 𝑃=1,2 𝑀𝑃𝑎 𝑣𝑒 𝑠=0,95414𝑘𝐽/𝑘𝑔𝐾= 287,23 𝑘𝐽/𝑘𝑔 ℎ@ 𝑃=1,15 𝑀𝑃𝑎 𝑣𝑒 𝑇=44°𝐶= 114,3 𝑘𝐽/𝑘𝑔

a) Tüm kütle, enerji, entropi ve ekserji denge denklemlerini her bir bileşen için yazınız.

b) Soğutulan ortamdan çekilen ısıyı ve kompresör için gerekli iş gücünü bulunuz.

c) Enerjetik ve ekserjetik COP değerlerini hesaplayınız.

Soru 2) An air-conditioner operates on the vapor-compression refrigeration cycle with refrigerant-134a as the refrigerant. The air conditioner is used to keep a space at 21 C while rejecting the waste heat to the ambient air at 37 C. The refrigerant enters the compressor at 180 kPa superheated by 2.7 C at a rate of 0.06 kg/s and leaves the compressor at 1200 kPa and 60 C.

R-134a is subcooled by 6.3 C at the exit of the condenser. Determine, a) The rate of cooling provided to the space and the COP

b) The isentropic efficiency and the exergy efficiency of the compressor

c) The exergy destruction in each component of the cycle and the total exergy exergy destruction in the cycle

d) The minimum power input and the second law efficiency of the cycle.

Bir klima cihazı buhar sıkıştırmalı çevrim ile ve iş akışkanı R134-a ile çalışmaktadır. Klimanın bulunduğu mahali 21 C de tutması istenirken atık ısıyı da 37 C deki çevre havasına

vermektedir. Soğutucu akışkan kompresöre 180 kPa basınçta 2.7 C kızgınlık derecesiyle, 0.06 kg/s kütlesel debi ile girip kompresörden 1200 kPa basınç ve 60 C de çıkmaktadır. R-134a kondenserin çıkışında 6.3 C aşırı soğuma derecesiyle çıkmaktadır. Bu verilere göre,

a) Mahali soğutmak için gerekli ısıyı ve COP değerini hesaplayınız b) Kompresörün izantropik ve ekserjetik (2. Yasa) verimini hesaplayınız

c) Her komponentteki ekserji yıkımını ve çevrimin toplam ekserji yıkımını bulunuz d) Çevrim için gerekli minimum güç girişini ve çevrimin 2. Yasa verimini bulunuz.

An air conditioner operates on the vapor-compression refrigeration cycle. The rate of cooling provided to the space, the COP, the isentropic efficiency and the exergetic efficiency of the compressor, the exergy destruction in each component of the cycle, the total exergy destruction, the minimum power input, and the second-law efficiency of the cycle are to be determined.

Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible.

Analysis (a) The properties of R-134a are (Tables A-11 through A-13)

K kJ/kg 4229 . kJ/kg 0 26 . 108

kPa 180

kJ/kg 27 . 108

K kJ/kg 3949 . 0

kJ/kg 27 . 108 C

40 3 . 6 3 . 46

kPa 1200

C 3 . 46

K kJ/kg 9615 . 0

kJ/kg 66 . 289 C

60 kPa 1200

kJ/kg 34 . kPa 285

1200

K kJ/kg 9484 . 0

kJ/kg 15 . 245 C

10 7 . 2 7 . 12

kPa 180

C 7 . 12

4 4

4 3 4

C

@40 3

C

@40 3 3

3

kPa sat@1200

2 2 2

2

2 1

1 2

1 1 1

1

kPa sat@180



 























































 







































h s P

h h

s s

h h T

P T

s h T

P s h s P

s h T

P T

f f s

The cooling load and the COP are









m(h1 h4) (0.06kg/s)(245.15 108.27)kJ/kg 8.213kW QL 

kW 88 . 10 kJ/kg ) 27 . 108 66 . 289 kg/s)(

06 . 0 ( )

( ₂ ₃   

m h h Q_H 

kW 670 . 2 kJ/kg ) 15 . 245 66 kg/s)(289.

06 . 0 ( ) ( _{2 1}

inmh h   

W 

3.076





 2.670kW kW 213 . COP 8

Win

Q_L





(b) The isentropic efficiency of the compressor is

90.3%



 

 



  0.903

15 . 245 66 . 289

15 . 245 34 . 285

1 2

1 2

h h

h h_s

C

The reversible power and the exergy efficiency for the compressor are

 

 

kW 428 . 2

K kJ/kg ) 9484 . 0 K)(0.9615 310

( kJ/kg ) 15 . 245 (289.66 kg/s) 06 . 0 (

) ( )

( _{2 1 0 2 1}



















m h h T s s W_rev 

90.9%







 0909

kW 670 . 2

kW 428 . 2

in rev

, .

W W

C

ex 

 

(c) The exergy destruction in each component of the cycle is determined as follows Compressor:

kW/K 0007827 .

0 K kJ/kg ) 9484 . 0 9615 . 0 kg/s)(

06 . 0 ( ) ( _{2 1}

2

gen,1_ m s s    

S 

kW 0.2426





 _{0 gen,1-2} (310K)(0.0007827kW/K)

2 - dest,1 T S x

E 

Q_H

Q_L 180 kPa

1 2s

3

4

s T

·

·

2

W·_in 1.2 MPa

Condenser:

kW/K 001114 . K 0 310

kW 88 . K 10 kJ/kg ) 9615 . 0 3949 . 0 ( kg/s) 06 . 0 ( )

( _{3 2}

3

gen,2_        

H H

T s Q s m S

 



kW 0.3452







 _{0 gen,2-3} (310K)(0.001114kJ/kg K)

3 - dest,2 T S x

E 

Expansion valve:

kW/K 001678 . 0 K kJ/kg ) 3949 . 00 4229 . 0 kg/s)(

06 . 0 ( ) ( _{4 3}

4

gen,3_ ms s    

S 

kJ/kg 0.5202







 _{0 gen,3-4} (310K)(0.001678kJ/kg K)

4 - dest,3 T S x

E 

Evaporator:

kW/K 003597 . K 0 294

kW 213 . K 8 kJ/kg ) 4229 . 0 9484 . 0 ( kg/s) 06 . 0 ( )

( _{1 4}

1

gen,4_        

L L

T s Q s m S

 



kW 1.115







 _{0 gen,4-1} (310K)(0.003597kJ/kg K)

1 - dest,4 T S x

E 

The total exergy destruction can be determined by adding exergy destructions in each component:

kW 2.223



















115 . 1 5202 . 0 3452 . 0 2426 . 0

1 - dest,4 4

- dest,3 3

- dest,2 2

- dest,1 total

dest, Ex Ex Ex Ex

x

E    

(d) The exergy of the heat transferred from the low-temperature medium is

kW 4470 . 294 0 1 310 kW) 213 . 8 (

1 ⁰ 



 

 







 



 





L Q L

T Q T x E L



 

This is the minimum power input to the cycle:

kW 0.4470





QL

x E W  

min in,

The second-law efficiency of the cycle is

16.7%







 0.167

670 . 2

4470 . 0

in min in,

II W

W



 

The total exergy destruction in the cycle can also be determined from kW 223 . 2 4470 . 0 670 .

in 2

total

dest,     

QL

Ex W x

E  

The result is the same as expected.