Affect of Exhaust Vaccum on Steam Turbine Efficiency.?

We're having trouble finding a solution for a problem that I have with my Project. We are doing research to see if installing VFDs onto a local combined cycle power plant's cooling tower fans to maintain a constant cooling water temperature we be economically beneficial. In our research we have found... show more We're having trouble finding a solution for a problem that I have with my Project. We are doing research to see if installing VFDs onto a local combined cycle power plant's cooling tower fans to maintain a constant cooling water temperature we be economically beneficial.

In our research we have found out that Vacuum pressure on the exhaust side of the steam turbine has an effect on the efficiency. The Steam Turbine has a
BHP = 170 065
Steam Consump = 515732 lbs/hr
Inlet Pres HP = 12.51 mPa
Inlet Temp HP = 565
Exhaust Vacuum = -88 kPa

The performance condition of the cooling tower are:
72,500 GPM, HW 28.6 C, CW 20.5 C, WB 16.55 C, and BHP of 250.

The plant has told us that 17 C is their ideal cooling water temp, and over 20 C they become inefficient. 13 C their cold water alarm comes on.

The question is how do we calculate out how much efficiency does the plant lose when running at 20 C and why is it bad for the plant to go below 13 C?
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