Cooling Tower Condensor Water Control

[Hileg]

Does anyone have a good way to control/stage 3-600 ton cooling towers?
Should a control sequence be created using wet bulb to stage these 3 towers. The fans do have vfd's.Thanks

 

[quark]

I never heard about controlling a cooling tower based on WBT. It will be useless even if it exists. Better way is to control the towers by leaving water temperature. If all the towers are connected to a common cooling water tank (or sump or receiver)then my option would be to put off the fans one by one with respect to leaving water temperature.

Regards,

 

[krb]

Controlling on wet bulb is common in HVAC applications, see the following link.

http://www.trane.com/commercial/library/vol241/enews24_1.pdf

KRB

 

[25362]

Hileg: Either VSD or two-speed motors are in use. Two-speed drives operate either at full speed (full power) or at half speed with a 1/8 power consumption (proportional to the speed cubed for centrifugal fans). Adjustable-speed operation of motors is cost-effective and reliable.

Measurements of return- or cold-water temperatures are generally employed to generate the control signal for speed to the fan motors.

In very special cases, electronically-controlled reverse operation, to prevent ice buildup on the blades, can be provided.

Noise is also reduced by lowering fan speeds, 20-25 dBA can be decreased by a 3:1 speed reduction. Additional benefits of reduced fan speeds: less water evaporation, less wind-drift losses, less vibration, and less fan-drive maintenance.

Depending on the cooling loads, the way the cooled water is mixed from the three towers in the sump, and the weather conditions, all the three towers could simultaneously be under speed control or staged as you suggest.

As I understand the GPSA Engineering Data Book, Vol. 1,, 10th Ed, 1987, provides examples and a handy nomograph to evaluate cooling tower operation, when varying any one, or more than one, of the following parameters:
Circulation rate, wet bulb (ambient conditions), temperature range, fan horsepower.

I suggest you get hold of Rules of Thumb for Chemical Engineers edited by Carl R. Branan (Gulf) a manual that brings worked out examples.