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joken
10-29-2007, 02:07 PM
Whats the most energy efficient way to stage 3 towers? Run them individually to 100% before starting another tower or run two or more at lesser speeds? Thanks, Ken

10-29-2007, 03:37 PM
Common sumps between all three? VFD's on all three fans?

joken
10-29-2007, 04:02 PM
Yes, VFD and common sumps. It seems the answer is obvious lower fan speeds equal lower KW. Ken

Shophound
10-29-2007, 04:05 PM
Generally speaking, the fan laws can give you an idea, speaking from an energy consumption standpoint, whether running one or two towers uses more juice. The brake horsepower requirement for a fan/blower changes with the cube of the CFM change. For example (if I'm crunching the numbers right):

(1200 cfm/800 cfm) ^3 divided by 1 hp/new hp = .296 hp or about 1/3 hp

So...at 1200 cfm the motor was putting out one horsepower to spin the blower for that rate of volume. When the volume (cfm) was dropped to 800, the horsepower requirement from the motor dropped to about 1/3 hp. From there you can figure 1 hp = 746 watts, so ~ 1/3 hp = ~ 246 watts, a considerable difference in power consumption.

While running more than one tower at slower speeds will reduce power consumption, it will also reduce the heat rejection capacity of each tower due to reduced air speed through each tower.

Short of a more complex staging system involving a strategy that compares chiller power consumption to tower power consumption and stages accordingly, a reliable way to stage cooling towers is by wet bulb reset. Find the design approach temperature for your cooling towers for the design conditions in your area. Then, devise a way for the leaving tower temperature to be reset "x" amount of degrees above the entering air wet bulb temperature, according to the tower design approach temperature. For example, say each tower you have are on VFD's, either physically are or have the ability to share sumps via isolation valves, and have a design approach temperature of seven degrees. So...when your outdoor wet bulb temperature is, say, 78 degrees, your towers are outputting 85 degree water to the chillers, a common design entering condenser temp for many chillers. As wet bulb temperatures improve, your tower leaving temperature will also lower, but should be limited on the low end if your chillers do not like running too cold in the condenser (can cause oil level to drop in sumps on certain chiller models). A high reset limit should also be set, so if one or two towers can't hold 85 degrees, the third one stages on and all three do it.

The towers at my plant are on a strategy like this, and on all but the most humid days, two towers at most are needed to handle the chiller's load, and most times both towers are running below 60 hertz on the VFD. At what point it would be more economical to run one tower balls to the wall vs. two at a speed that combined consumes more power than the one tower going balls to the wall would be something a more complex strategy could handle, but there are likely hundreds of towers out there running on no sort of reset strategy at all...just some arbitrary staging that isn't optimum for both the chiller and tower energy consumption/tower efficiency...that could benefit from just the wet bulb reset strategy.