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Cooling tower VFD question

Cooling tower VFD question

Cooling tower VFD question

(OP)
O HVAC gurus,
I'm trying to calculate energy savings for a cooling tower VFD proposal.  There are four cells in the tower, each with a 60 hp fan.  Presently, the sump temperature is controlled at two points by cycling two different fans on/off.  (Fan #1 and fan #4).  Fan #2 comes on when fan #1 can't keep up, and fan #3 turns on when fan #4 can't keep up.

Let's assume during a spring month, that 2 fans each run 50% of the time.  If I set it up so that 2 fans run at 50% speed all of the time, I see that I will use half the energy: (P2/P1)=(v2/v1)^2
= (50%)^2 (times 2 for full duty cycle) = 50%

Is there a limit to how slowly you can drive an AC motor?  In the example above, if I run 4 fans at 25% speed all of the time, theoretically, I could cut my energy use in half again.  Seems too good to be true.

Also, is it safe to substitute speed for duty cycle when modeling cooling tower heat transfer?

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