Cooling Tower VFD energy savings 1

[Jonkatz]

Can anyone suggest a resource or methodology about how to calculate the energy savings acheived by replacing a constant fan speed cooling tower with a VFD controlled cooling tower ?
THANKS - JK

 

[psighi]

Consider a 2-speed, single winding motor. Manufactures claim at low speed (50% rpm), the tower is 60% of rated capacity. Running the fan at ~80% speed, will yield approximately 50%bhp.

The sacrifice, for those who look at the big pix, is CWS temperature rises. For every degree of CWS temperature increase, efficiency worsens by 1-2%.
At say 100 Tons applied, @ .6kW/T at full speed, if the tower full speed is 8kW, the total electrical demand is 68kW. If by lowering the speed to 80% speed causes an increase in CWS temperature of 5F, potentially the efficiency could rise to .66kW/Ton at 100 Tons. The total electrical demand then would be 66kW+4kW=70kW. In this case, because of the tower selection, it wouldn't make sense.

I've found that tower selection is critical for vfd fan consideration. If one doesn't plan for >15% reserve capacity in the tower, a vfd should not be considered.
Choose a 2 speed fan.

 

[boyceg]

If you go to a variable frequency motor drive, make sure the motor is "Inverter Duty" rated. If it is unreasonable to change the motor, a filter between the drive an motor may help. If the application reliability is critical, an inverter duty motor and filter may be considered.
Boyceg