A steam engine receives steam at 3 MPa and 300 degrees Celcius and expands isentropically and reversibly to 1 MPa. Determine the work done if the cylinder volume at the start of the expansion is 3.2 x 10^(-3) cubic metre.

# Question on finding work done

Ans is 6.46 KJ

You should apply the first law for a process, i.e. Q=U+W.

Q=0.

So -Delta U=W

W=-m(u2-u1)

Use steam tables to find values of internal energy.

This answer is incorrect

This is a closed system. you can use the first law of thermodynamics.

Q = 0 as the process is reversible and isentropic, which means it is adiabatic.

use the steam tables to find the value of internal energies, entropy and specific volume at state 1.

Total volume is given, so from specific volume at state 1, we can find the mass of steam in the engine =

As the process is isentropic, so

Great work!

understood sir. thank you.

What u have done is correct. There is no problem in applying this equation. The steam engine cannot be a closed system. It is considered as closed system only during compression and power stroke and in the remaining 2strokes there is mass flow to the system and from the system.

So first law of thermodynamics can be applied for power stroke only.