By Chad Edmondson
We like to vary flow through our hydronic systems, don’t we? After all, less flow equals less pump energy. But when it comes to cooling tower performance and longevity, just be careful not to overflow or underflow the tower. Either scenario can lead to some pretty costly problems.
What Happens When You Over-flow a Cooling Tower?
Flow rates through cooling towers are critical for ensuring maximum distribution throughout the cooling tower channels. If flow rates exceed the maximum limitations of a given cooling tower, problems will occur. Most notably, the hot water basin or hot deck of the tower will flood, resulting in the loss of water and costly chemical treatments, not to mention waste of fan and pump energy. If outdoor temperatures are low enough, the cooling tower water can also freeze.
What Happens When You Underflow a Cooling Tower?
Under-flowing a cooling tower can severely impair its efficiency and can even shorten its life. Keep in mind that cooling tower fan speed is controlled based on the supply temperature to the compressor. If the supply temperature starts to exceed set point, the cooling tower responds by speeding up the fans in an effort to increase evaporation. But if the cooling tower media is not sufficiently wet, the fill areas will dry. Like water, air follows the path of least resistance, so as this situation worsens, more and more air bypasses the fill areas containing water and flows through the unrestricted dry spaces.
This is indicative of what is known as “dry air disease”. The evidence will invariably show up as large amounts of scale on the heat transfer surfaces. The problem escalates as the fans work even harder to maintain supply temperatures. The scaleis caused by wet areas of the fill drying out leaving behind mineral deposits. The more frequently this drying occurs on the surface areas, the more scale that builds up. Meanwhile, the fans use excessive amounts of energy while the dirty heat transfer surfaces continue to impair the overall efficiency of the tower.
Just how bad can it get? At JMP, we’ve seen some pretty extreme situations, including one facility where the cooling tower underflow situation was so severe that it took three cooling towers operating just to maintain the condenser supply water temperature to just one small chiller—during winter operation! Imagine the associated energy waste, all caused by providing too little flow to the chiller.
Cooling Tower Turndown
If you are an engineer designing a chilled water system, one way you can help your client avoid under-flowing a tower is to specify that all cooling towers have a 50% turndown. This allows for substantially more flow reduction without endangering the cooling tower. Consider a 500-Ton cooling tower with a design flow of 1500 GPM. With a 50% turndown, the cooling tower need only maintain a minimum flow of 750 GPM. Compare that to an identical cooling tower with a 20% turndown. The minimum flow would be 1200 GPM – significantly more.
ASHRAE actually passed an addendum to Standard 90.1-2010 requiring that all new cooling tower installations have a 50% turndown. For existing installations with lower turndowns, Weir dams should be installed to maintain minimum flow. Weir dams are in the hot water basin and keep the water flowing where it should to ensure thoroughly wetted surfaces. During low flow operation, these dams direct water to the outer areas of the hot water basin where it is most needed.
Varying Flow Through Condensers
It’s not only important to abide by the flow requirements of the cooling tower, but the chiller as well. Here again, under-flowing the equipment can lead to scale build-up, as higher water velocities through the condenser tubes help keep them clean. Also, most chiller manufacturers still advocate constant flow through the chillers to assure more stable and efficient operation.
For more information, please view our video series on Cooling tower and Condenser Water Piping Design.