HVAC Central Plants, Industrial Water Treatment & Mass Balance
Cooling Tower Cycles of Concentration & Blowdown Formula
Calculate Cooling Tower Evaporation Loss (GPM), Chloride Cycles of Concentration (COC), Controlled Basin Blowdown Rate (GPM), and Total Makeup Water Demand (GPM).
Diminishing water-saving returns above COC = 6—and deduct drift loss from controlled blowdown!
Because pure water vapor leaving an evaporative cooling tower carries zero dissolved minerals while liquid droplets leaving via Blowdown (B) and Drift (D) carry concentrated basin water, steady-state mineral conservation (Makeup * C_makeup = (B + D) * C_basin) dictates Cycles of Concentration COC = C_basin / C_makeup and Total Liquid Purge (B + D) = Evaporation / (COC - 1)! Raising COC from 2 to 4 slashes purge water by 66.7%, whereas pushing COC above 6 yields tiny incremental water savings while inviting severe CaCO3/silica condenser tube scaling. When auditing a cloud collaboration utility workbook—whether in the app or via the Arte Sheets ChatGPT connector 0.1.0 (currently In review / Not published, where read_range inspects up to 200 cells of raw inputs/formulas and revision without recalculating and consumes 0 Arte AI credits under a 120-calls/min account limiter)—always verify controlled blowdown subtracts drift: B = MAX(0, Evap / (COC - 1) - Drift)!
Verify on a 10,000 GPM Data-Center Cooling Tower—Recirculation_Flow = 10,000 GPM (B2), Cooling_Range_DeltaT = 15.0 °F (B3), Evaporation_Factor = 0.0008 per °F (B4 -> Evaporation = 10,000 * 15 * 0.0008 = 120.00 GPM in E2), Makeup_Chlorides = 50 ppm (B5), Max_Basin_Chlorides = 300 ppm (B6 -> COC = 300 / 50 = 6.00 in E3), and Drift_Loss_Fraction = 0.0002 (2.00 GPM drift, B7): Controlled Blowdown equals 120 / (6 - 1) - 2 = 22.00 GPM (E4), and Total Makeup Water is 120 + 22 + 2 = 144.00 GPM (E5)!
| Cooling Tower Water-Balance KPI (10,000 GPM, 15°F Range, 50->300 ppm Cl) | Formula | Verified Output |
|---|---|---|
| Evaporation Loss Rate (GPM) | Recirc_GPM * Range_F * Evap_Factor | 120.00 GPM |
| Cycles of Concentration (COC) | Max_Basin_Chlorides / Makeup_Chlorides | 6.00 (3.00 @ 150 ppm Limit) |
| Controlled Basin Blowdown (GPM) | MAX(0, Evap_GPM / (COC - 1) - Drift_GPM) | 22.00 GPM (58.00 GPM @ COC=3) |
| Total Makeup Water Demand (GPM) | Evap_GPM + Blowdown_GPM + Drift_GPM | 144.00 GPM (180.00 GPM @ COC=3) |
Cooling Tower Mineral-Balance Controlled Blowdown Formula
Validate Recirc_GPM > 0, Range_F > 0, Evap_Factor > 0, 0 < Makeup_ppm < Max_Basin_ppm (so COC > 1!), and Drift_Fraction >= 0!
=LET(rc_g, B2, dt_f, B3, ev_k, B4, cl_m, B5, cl_b, B6, dr_f, B7, IF(OR(rc_g <= 0, dt_f <= 0, ev_k <= 0, cl_m <= 0, cl_b <= cl_m, dr_f < 0), "Invalid Cooling Tower Input", LET(ev_g, rc_g * dt_f * ev_k, co_c, cl_b / cl_m, dr_g, rc_g * dr_f, ROUND(MAX(0, ev_g / (co_c - 1) - dr_g), 2))))Hard-Water Low-COC Penalty Check (Set Max Basin Chlorides = 150 ppm -> COC = 3)
Lower Max_Basin_Chlorides B6 from 300 ppm to 150 ppm (COC E3 = 3.00): Controlled Blowdown E4 jumps from 22.00 GPM to 58.00 GPM, lifting Total Makeup Water E5 from 144.00 GPM to 180.00 GPM (+18.92 million gallons/yr of extra city water)!
=IF(ISTEXT(E4), E4, ROUND(E2 + E4 + B2 * B7, 2))Try the example
Use the original values or change them to test your understanding.