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

Chapter 3, Problem 129HP

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QUESTION:

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.

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QUESTION:

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.

ANSWER:

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

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