Estimate the constant specific heats for R-134a from Table

Fundamentals of Thermodynamcs | 8th Edition | ISBN: 9781118131992 | Authors: Claus Borgnakke, Richard E. Sonntag

Problem 129HP Chapter 3

Fundamentals of Thermodynamcs | 8th Edition

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Fundamentals of Thermodynamcs | 8th Edition | ISBN: 9781118131992 | Authors: Claus Borgnakke, Richard E. Sonntag

Fundamentals of Thermodynamcs | 8th Edition

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Problem 129HP

Estimate the constant specific heats for R-134a from Table B.5.2 at 100 kPa and 125°C. Compare this to the specific heats in Table A.5 and explain the difference.

TABLE B.5.2 Superheated R-134a

Temp.

v

u

h

s

v

u

h

s

(°C)

(m3/kg)

(kJ/kg)

(kJ/kg-K)

(kJ/kg-K)

(m3/kg)

(kJ/kg)

(kJ/kg)

(kJ/kg-K)

 

50 kPa (−40.67°C)

 

100 kPa (−26.54°C)

 

Sat.

0.36889

354.61

373.06

1.7629

0.19257

362.73

381.98

1.7456

-20

0.40507

368.57

388.82

1.8279

0.19860

367.36

387.22

1.7665

-10

0.42222

375.53

396.64

1.8582

0.20765

374.51

395.27

1.7978

0

0.43921

382.63

404.59

1.8878

0.21652

381.76

403.41

1.8281

10

0.45608

389.90

412.70

1.9170

0.22527

389.14

411.67

1.8578

20

0.47287

397.32

420.96

1.9456

0.23392

396.66

420.05

1.8869

30

0.48958

404.90

429.38

1.9739

0.24250

404.31

428.56

1.9155

40

0.50623

412.64

Step-by-Step Solution:

Solution 129HP

Step 1 of 3:

We are going to compare the constant specific heats for R-134a from the tables B.5.2 and A.5 for the given temperature and pressure values.

The given pressure P = 100 kPa

The given temperature T = 125°C

The table B.5.2 for the corresponding temperatures is listed below. 

Temp.

v

u

h

s

v

u

h

s

(°C)

(m3/kg)

(kJ/kg)

(kJ/kg-K)

(kJ/kg-K)

(m3/kg)

(kJ/kg)

(kJ/kg)

(kJ/kg-K)

 

50 kPa (−40.67°C)

 

100 kPa (−26.54°C)

 

120

0.63835

480.44

512.36

2.2128

0.31797

480.16

511.95

2.1555

130

0.65479

489.63

522.37

2.2379

0.32626

489.36

521.98

2.1807

The specific enthalpy at T120 (h1) = 511.95 kJ/kg

The specific enthalpy at T130 (h2) = 521.98 kJ/kg

The specific internal energy at T120 (u1) = 480.16 kJ/kg

The specific internal energy at T130 (u2) = 489.36 kJ/kg

The approximate specific heats at the temperature T are given by

The specific heat at constant pressure is expressed as

Putting the values

The specific heat at constant volume is expressed as

Putting the values

Step 2 of 2

Chapter 3, Problem 129HP is Solved
Textbook: Fundamentals of Thermodynamcs
Edition: 8th
Author: Claus Borgnakke, Richard E. Sonntag
ISBN: 9781118131992

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