When you need to start your cooling fan, what’s the most efficient way to do so?
We recently ran into a case where a single cooling tower served modular water-cooled chillers in a typical classroom and office building. The first stage of the control loop sent water over the tower with the fan off. Great, no problem. The second stage started the fan and immediately ramped it up to 100% and then backed it down as the temperature approached setpoint. As you guessed, the setpoint was quickly achieved and the fan cycled off. Is it that important to your operation to achieve setpoint that quickly? The answer is most likely not, if you are talking about serving a chiller that does typical comfort cooling. If your tower serves process loads that drastically swing up and down, maybe this would be the right approach.
However, the right way to do this is to change the logic in the control sequence so the fan starts at minimum speed and stays at minimum for a while, before it is allowed to slowly ramp up. It’s not a drag race to see how fast you can get there. If it’s not making setpoint and needs to eventually ramp up to 100% speed, fine, but give it a chance to meet the setpoint before stomping down on the gas pedal. In all but the worst design conditions the fan will likely never need to go that fast. Remember that fan power increases at the cube of the speed, so keeping your fan horsepower low helps with efficiency.
In addition, think about your electric bill and how its demand charge is structured. This particular client’s electric demand rate structure uses the highest electric demand that occurred during the most recent six months and charges the customer based on that one peak. Therefore, if their cooling tower fan was ramping up to 100% when it didn’t need to and caused a new peak in their demand, they paid for that peak for the next six months. Think about that the next time your control sequence puts the pedal to the metal!