Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Chapter 8 | 439

P 1 = 0.14 MPa

T 1 = –10°C

T 2 = 50°C

P 2 = 0.8 MPa

T 0 = 20°C

COMPRESSOR

FIGURE 8–25

Schematic for Example 8–8.

The exergy content of the compressed air can be determined from

We note that

Therefore,

and

Discussion The work potential of the system is 281 MJ, and thus a maxi-
mum of 281 MJ of useful work can be obtained from the compressed air
stored in the tank in the specified environment.

X 1 
m 1 f 1 
 1 2323 kg 21 120.76 kJ>kg 2 
280,525 kJ281 MJ

120.76 kJ>kg

1 0.287 kJ>kg#K 21 300 K2aln

1000 kPa

100 kPa



100 kPa

1000 kPa

 1 b

f 1 
RT 0 a

P 0

P 1

 1 bRT 0 ln

P 1

P 0

RT 0 aln

P 1

P 0



P 0

P 1

 1 b

T 01 s 1 s 02 
T 0 acp ln

T 1

T 0

R ln

P 1

P 0

b
RT 0 ln

P 1

P 0

¬¬ 1 since T 1
T 02

P 01 v 1 v 02 
P 0 a

RT 1

P 1



RT 0

P 0

b
RT 0 a

P 0

P 1

 1 b¬¬ 1 since T 1
T 02

m 3 P 01 v 1 v 02 T 01 s 1 s 024

mc1u 1 u 02 Q

0

P 01 v 1 v 02 T 01 s 1 s 02 

V^21

2

Q^0

gz 1 Q

0

d

X 1 
mf 1

EXAMPLE 8–8 Exergy Change during a Compression Process

Refrigerant-134a is to be compressed from 0.14 MPa and 10°C to 0.8
MPa and 50°C steadily by a compressor. Taking the environment conditions
to be 20°C and 95 kPa, determine the exergy change of the refrigerant dur-
ing this process and the minimum work input that needs to be supplied to
the compressor per unit mass of the refrigerant.

Solution Refrigerant-134a is being compressed from a specified inlet state
to a specified exit state. The exergy change of the refrigerant and the mini-
mum compression work per unit mass are to be determined.
Assumptions 1 Steady operating conditions exist. 2 The kinetic and poten-
tial energies are negligible.
Analysis We take the compressoras the system (Fig. 8–25). This is a con-
trol volumesince mass crosses the system boundary during the process.
Here the question is the exergy change of a fluid stream, which is the
change in the flow exergy c.