Makeup water versus cycles of concentration. Currently 17.00 gallons per minute at 4.0 cycles. The target is 15.30 gallons per minute at 6.0 cycles. The evaporation floor is 12.83 gallons per minute.
Makeup gpm41200
246810
Cycles of concentration
Reference
How the cooling tower model works
A cooling tower rejects heat by evaporating a slice of the recirculating water. Evaporation leaves dissolved solids behind, so the circulating water concentrates, and blowdown discharges some of it to hold the solids at a target. Cycles of concentration compare the circulating water against the makeup, and they are the most important operating variable in the room.
The instrument above runs the full water balance live. The same four relationships are written out below.
Evaporation represents latent heat rejection. Each pound of water that evaporates from the recirculating loop carries roughly 1000 BTU of heat with it. This is how a cooling tower actually cools.
Recirc
Recirculating water flow (gpm)
ΔT
Temperature range across the tower (°F)
Evap Factor
Evaporation factor, 0.80 to 0.92, default 0.85 (user input)
Assumes
10°F range across the tower at design
Evaporation factor is a user input, 0.80 to 0.92, default 0.85
Drift
Drift=Recirc×0.005%
Drift is water carried out of the tower as liquid droplets in the air stream. It leaves with the same dissolved solids concentration as the circulating water.
Recirc
Recirculating water flow (gpm)
Assumes
Drift fixed at 0.005% (0.00005) of recirculating flow, typical for modern drift eliminators.
Blowdown
Blowdown=max(0,Cycles−1Evaporation−Drift)
Blowdown is concentrated water intentionally discharged so dissolved solids do not climb beyond the target cycles.
Evaporation
Water leaving the loop as vapor (gpm)
Cycles
Cycles of concentration
Drift
Water entrained as liquid droplets (gpm)
Assumes
Cycles are greater than 1.
Blowdown cannot be negative.
Makeup
Makeup=Evaporation+Drift+Blowdown
Makeup is the water added to replace every pathway leaving the cooling tower loop.
Evaporation
Water leaving the loop as vapor (gpm)
Drift
Water entrained as liquid droplets (gpm)
Blowdown
Concentrated water intentionally discharged (gpm)
Assumes
Evaporation, drift, and blowdown are the governed losses.
Worked example
Take the room's default case: 1,500 gpm of recirculation, a 10°F range, 4 cycles of concentration, and an evaporation factor of 0.85.
Evaporation
12.75 gpm
Drift
0.08 gpm
Blowdown
4.18 gpm
Makeup
17.00 gpm
Annual makeup water
5,361,120 gallons
Annual water cost
$32,167 per year
Raise the cycles in the instrument and watch blowdown shrink by less and less. That diminishing return is the story of the whole room.
Common questions
What are cycles of concentration?
Cycles compare the dissolved solids in the recirculating water against the makeup. At 4 cycles the circulating water is four times as concentrated as the water feeding it. Cycles set the blowdown requirement, which makes them the operating variable that matters most.
How is cooling tower makeup calculated?
Makeup replaces the three governed losses: evaporation, drift, and blowdown. Evaporation follows the heat load, drift is a fixed fraction of recirculation, and blowdown discharges concentrated water so the solids hold at the target cycles.
Why does raising cycles save less and less water?
Blowdown falls in proportion to one over cycles minus one. The step from 2 to 4 cycles removes most of the blowdown; the step from 6 to 8 removes far less. The savings readout in the instrument shows the flattening directly.