A cooling tower rejects heat by evaporating water, and the minerals in that water stay behind. In the words of the U.S. Department of Energy, “as more water evaporates, the concentration of dissolved solids increases.” A conductivity controller measures the tower water and discharges blowdown “only when the conductivity set point is exceeded” (FEMP Best Management Practice 10). Blowdown is how the tower gets rid of the minerals that arrive with its makeup water.
A recovery system takes over part of that job. It treats the tower water, returns the cleaner water to the tower, and sends the minerals out in a much smaller concentrate stream. Sizing it comes down to one question: can it remove minerals as fast as the makeup water brings them in?
Three ways the size can land
- Undersized. The system removes minerals, but fewer than arrive. Conductivity still climbs to the setpoint, only more slowly, so the controller still opens the blowdown valve. Blowdown happens less often but does not stop.
- Matched. The system removes minerals as fast as they arrive. Conductivity stays in the target range and routine blowdown stops.
- Oversized. The system can remove more than arrives. It holds conductivity below the setpoint and keeps a margin for months with higher load.
To stop routine blowdown, size for the matched case or slightly above it. An undersized system still saves water, but the tower keeps draining.
What sets the load
The mineral load depends on how much makeup water the tower takes in and how much dissolved solids that water carries. Both can change through the year: a tower evaporates more water when it rejects more heat, so the months with the highest cooling load bring the most makeup water and the most minerals.
Before sizing, collect:
- Makeup and blowdown meter totals by month for at least a year. The Department of Energy notes that cycles of concentration are “approximately equal to the ratio of volume of make-up to blowdown water”, so these two meters also tell you your current cycles.
- The conductivity of your makeup water, and the conductivity setpoint on the tower controller
- The months and hours when the tower carries its peak load
- Your water chemistry report, including hardness and silica
Size to the peak months, not the annual average. A system sized to the average will be undersized in the peak months, which is when the tower drains the most.
How RECOVER is sized
Nona RECOVER is modular. Nona’s technical sheet gives each unit a nominal feed rate of 10 gallons per minute, and a site adds units until they match its tower’s load. The savings calculator gives a first estimate from your tower size and local water rates. Our technical team then checks it against your meter data and water chemistry before recommending a number of units. Our guide to choosing a recovery system covers what else to ask any provider.
Sources
- U.S. Department of Energy, Federal Energy Management Program, Best Management Practice 10: Cooling Tower Management