Actual rankine cycle - Work done by pump

[Jameseyboy]

Hi,

In what cases do I neglect the work done by the pump in an actual rankine cycle?

I am reading that h5-h4 is negligible but in what circumstances do I accept this.

Is this only for ideal rankine cycles?

As I understand thermal efficiency is (work out + work in)/Heat in

Thanks

 

[sanka]

Unless you are doing a very specific analysis which requires a good degree of accuracy, you can almost always neglect the work done by the pump. If you are unsure why, I'd recommend that you calculate the work term and compare it to the heat added and rejected by the boiler and condenser, respectively, and the work out from the turbine. You will see notice that the work done by the pump is very insignificant compared to the other processes. The reason the work done by the pump is so small is because it is pumping liquid condensate as opposed to a gas (or vapour) which would require a compressor and a much larger energy input.