McCormick93
Mechanical
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?
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?
