We must apply the 1st Law to the compressor.We can get H1 from the R-134a tables or the NIST Webbook, but we do not know H2.The key to solving
this problem is that a process that requires the minimum shaft work is an isentropic process.Knowing that S2 = S1 gives us the value of
a 2nd intensive variable for state 2.This allows us to use the R-134a tables or NIST Webbook to detemine H2.We can then plug H2 into the 1st Law to determine the work requirement per kg of R-134a.
The compressor is isentropic.
The compressor operates at steady-state.
Changes in kinetic and potential energies are negligible.
work and flow work are the only types of work that cross the system boundary.
/ Data / Solve:
Apply the 1st Law to the compressor to determine the shaft work requirement.
For a steady-state, single-inlet, single outlet system with no heat
transfer and negligible kinetic
and potential energy changes, the 1st Law is:
We can get H1 from the R-134a tables or the NIST Webbook because we know the temperature and we know it is a saturated
The compressor is isentropic, so S2 = S1 and we can get S1 from the R-134a tables or the NIST Webbook.
Now, we know the
values of two intensive properties at state 2, so we can use the R-134a tables or the NIST Webbook to evaluate any other properties by interpolation.Here, we are
interested in H2.
= 700 kPa:
Now, we can plug
values back into Eqn 1 :
The assumptions made
in the solution of this problem cannot be verified with the given