1. The problem statement, all variables and given/known data I came across this equation denoting the work done by an open system (e.g. turbine or compressor). I wonder how they arrived at such an equation. 2. Relevant equations Differential form of the steady flow energy equation for an infinitesimally small control volume neglecting kinetic and potential energy changes u+pv+dQ-dW-(p+dp).(v+dv)=u+du du+pdv+vdp=dQ-dW ...(neglecting the term dp.dv) 3. The attempt at a solution If we use dQ=du+pdv in the above equation we shall get the required equation for work done. But how can we use this relation of dQ=du+pdv which is valid for close systems having only one kind of work i.e. expansion work. If you feel the above method is incorrect please provide the appropriate method to prove this relation.