work in polytropic process...a bit confusing

[mihalj]

Hi,
can anyone explain why boundary work in closed system for compressor is calculated as W = ∫pdV and polytropic process after integration is generally W = -(p2V2 - p1V1)/(1-n)...(numbers behind p and V are subscripts)...
When we talk about control volume, isentropic internally reversible process (isn't above one also reversible???) then work is defined as W = -∫Vdp and for polytropic process for compressor work is defined as W = -n*(p2V2-p1V1)/(n-1).
When I am supposed to use one way of calculation and when other? The results are not same. Aren't they same type of group as control volume compressor should behave during the process of compression as closed system (there is no constant add or removal of mass during compression).
Thanks in advance!

 

[chicopee]

I guess you did not study your thermo. well if you are a graduate ME.

 

[Engdoitbetter]

Work in closed system is -pdv (convention: positive if received by the fluid)

In open systems aka "control volume" you have to add the work needed to permit the mass flow during process: -pdv+d(pv)=vdp.

Regards,

Stefano