Question

Steam is heated from 10 kPa and 50°C to 550 kPa and 500°C. Calculate the change in entropy:
a) If the steam is considered an ideal gas.

b) If the steam is not considered an ideal gas.

c) Discuss if it is appropriate to consider steam an ideal gas for this process.

Answer 1

Given,

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(a)

Assuming steam to be an ideal gas, using 'Table A-2' ;

Change in entropy will be;

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(b)

Consdering steam to be non-ideal gas, by using steam table 'Table A-6' ;

At and ;

and

At and ;

Change in entropy will be;

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(c)

As we can see from results (a) and (b), there is a significant difference between the entropy change. Hence, we cannot use the ideal gas approximation. This huge difference arises due to the fact that, the specific heat () for steam is not constant and it changes significantly at higher temperature. Hence, due to change in the specific heat, the change in entropy will be different.

Hence it is not appropriate to consider steam to be an ideal gas.