Cycles of Concentration (CoC)
Determine the cycles of concentration in a cooling tower system using flow rates or dissolved ion ratios.
Method A (Flow-Based):
CoC = QMU / QBDMethod B (Ion Ratio):
CoC = Csystem / CmakeupWhere C is concentration of a conservative tracer (Cl⁻, conductivity, or SiO₂). All three tracers should agree within 5%; divergence indicates leak, unaccounted feed, or analytical error.
Reference: ASHRAE Guideline 12-2020; EPRI Cooling Tower Chemistry Manual.
Blowdown & Makeup Flow Estimator
Estimate required blowdown, makeup, evaporation, and drift flows from system operating parameters.
E (GPM) = Qcirc × ΔT × 0.00085 (ΔT method)E (GPM) = QMMBTU/hr × 1.905 (duty method)D (GPM) = Qcirc × (DE% / 100)BD (GPM) = E / (CoC − 1) − DMU (GPM) = E + BD + DAnnual makeup = MU (GPM) × 60 × 8,760 gallons/yr. Acre-ft = gal/yr ÷ 325,851. Metric: 1 GPM = 0.2271 m³/hr.
Evaporation Rate Estimator
Estimate cooling tower evaporation from temperature rise or cooling duty, with annual water loss projection.
E (GPM) = Qcirc × ΔT × 0.00085E (GPM) = QMMBTU/hr × 1.905Annual evaporation = E × 60 × operating_hours (gallons)The 0.00085 factor derives from latent heat of vaporization (~1,000 BTU/lb) and water density. The 1.905 factor converts MMBTU/hr to GPM.
Drift Loss Estimator
Quantify entrained droplet losses and associated TDS discharge from drift eliminators.
D (GPM) = Qcirc × (DE% / 100)TDS load (lb/day) = D (GPM) × TDS (mg/L) × 0.0001 × 1440Factor 0.0001 converts mg/L × GPM to lb/min; × 1440 = lb/day. CTI standard ATC-128 governs drift eliminator testing. Drift rates >0.002% elevate Legionella risk (ASHRAE 188).
LSI / RSI Scaling Index
Calculate the Langelier Saturation Index and Ryznar Stability Index to assess CaCO₃ scaling or dissolution tendency at bulk water conditions.
TK = (°F − 32) / 1.8 + 273.15A = (log₁₀(TDS) − 1) / 10B = −13.12 × log₁₀(TK) + 34.55C = log₁₀(Ca hardness as CaCO₃) − 0.4D = log₁₀(Alkalinity as CaCO₃)pHs = (9.3 + A + B) − (C + D)LSI = pH − pHsRSI = 2 × pHs − pHImportant: LSI indicates CaCO₃ saturation tendency only, not general corrosivity. Negative LSI means the water is undersaturated in CaCO₃ (will tend to dissolve CaCO₃ deposits). Reference: Langelier (1936), AWWA Water Quality Handbook.
Chemical Dosing Rate
Calculate inhibitor or biocide feed rates to achieve and maintain a target system residual concentration.
QMU (L/min) = GPM × 3.785Cprod (mg/mL) = C% × SG × 10Feed rate (mL/min) = [QMU × Ctarget] / [CoC × Cprod]GPH = mL/min × 0.01585oz/day = mL/min × 1440 / 29.574lb/day active = GPM × 1440 × 8.34 × (Ctarget / CoC) / 1,000,000lb/day product = lb/day active / (C% / 100)Annual gal = GPH × 8,760Retention Time (RT50, RT75, RT90, RT95)
Calculate system hydraulic retention time and solute decay milestones — useful for biocide contact time and chemical exchange planning.
Qloss = QBD + QdriftHRT (min) = Vsys / QlossHRT (hr) = HRT (min) / 60RT50 = HRT × ln(2) = HRT × 0.693RT75 = HRT × ln(4) = HRT × 1.386RT90 = HRT × ln(10) = HRT × 2.303RT95 = HRT × ln(20) = HRT × 2.996C/C₀ at time t = exp(−t / HRT)Assumes a well-mixed (CSTR) system. Reference: EPRI CT-6787, ASHRAE Guideline 12.