For HVAC technicians and commissioning/water-treatment techs balancing open recirculating cooling towers on a mechanical spec, who need real bleed and makeup GPM instead of the "just run it at 3 cycles" rule of thumb — the page shows the water balance behind the number, including the 0.00085 empirical evaporation constant that produces the familiar "about 1% of recirculation per 10°F" figure.
Makeup M (Gallons/Day): —
Enter the tower's design recirculation rate in GPM from the pump/piping schedule.
Enter the range ΔT in °F — the difference between entering and leaving condenser water temperature.
Enter the target cycles of concentration from the water-treatment program or the tower's max allowable (commonly 3–5 with good makeup quality).
Read Evaporation, Blowdown, and Makeup GPM live from the three cards; copy the makeup gallons/day figure onto the mechanical/plumbing schedule.
Raise CoC and watch blowdown and makeup drop — that is the water and sewer saving you are trading against chemical treatment cost and scaling risk.
A: Blowdown is evaporation divided by (cycles of concentration − 1): at 1,000 GPM and 10°F ΔT, evaporation is 1000 × 10 × 0.00085 = 8.5 GPM, so at 4 cycles blowdown is 8.5 ÷ 3 = 2.83 GPM and makeup is 8.5 + 2.83 = 11.33 GPM (about 16,320 gallons/day).
A: It is the standard empirical coefficient for water in an open cooling tower: E (GPM) = recirculation GPM × range ΔT (°F) × 0.00085. Because 0.00085 × 10 = 0.0085, it reproduces the field rule of thumb that roughly 1% of recirculation flow evaporates for every 10°F of range.
A: At CoC = 1 the tower would be running once-through, with blowdown growing without limit; the formula B = E / (CoC − 1) divides by zero there, and any CoC below 1 yields a negative (physically meaningless) blowdown, so the tool rejects it. Real towers run 3–5 cycles; pushing to 6+ cuts bleed and makeup further but concentrates silica and hardness, which is a treatment decision, not a math one.
A: Yes for this water balance — M = E + B — which deliberately excludes windage/drift (typically 0.005–0.02% of recirculation) and any overflow.