Water balance and cycles of concentration

Makeup, evaporation, blowdown and drift, and what cycles of concentration cost.

Makeup, evaporation, blowdown and drift at the reference circulation and range, and what blowdown and makeup do as the cycles of concentration fall.

Reference conditions: 200 kg/s circulation, 10 K range, 5 cycles of concentration, drift at 0.02 % of circulation — evaporation 3.48 kg/s, drift 0.04 kg/s, blowdown 0.83 kg/s, makeup 4.35 kg/s; at 3 cycles the same heat load needs 5.22 kg/s, 20 % more water.

Formula: Makeup = Evaporation + Drift + Blowdown, Blowdown = Evaporation ÷ (CoC − 1) − Drift, mE ≈ Q / hfg.

Cooling-tower water balance: makeup 4.35, evaporation 3.48, blowdown 0.83 and drift 0.04 kg/s at 5 cycles of concentration for a 200 kg/s circulation over a 10 K range, with blowdown and makeup rising as cycles fall (illustrative)Illustrative cooling-tower water balance for a 200 kg/s circulation over a 10 K range. Heat load Q = 200 × 4.18 × 10 = 8360 kW, so evaporation is about 8360 ÷ 2400 = 3.48 kg/s, or 1.74 % of circulation — the largest term. Drift at 0.02 % is 0.04 kg/s. The balance Makeup = Evaporation + Drift + Blowdown with Blowdown = Evaporation ÷ (CoC − 1) − Drift gives at 5 cycles: blowdown 0.83 kg/s and makeup 4.35 kg/s; at 3 cycles: blowdown 1.70 kg/s and makeup 5.22 kg/s, 20 % more water for the same heat load. Values are computed from the formulas, not measured. (chart-water-balance)Water balance — Illustrative — not a measurementFlows, blowdown and makeup012345Flow (kg/s)makeup4.35evaporation3.48blowdown0.83drift0.04234567801234Cycles of concentration (CoC)Share of circulating flow (%)makeupblowdown4.350.83Reference: 200 kg/s circulation · range 10 K · cp,w 4.18 kJ/kg·KBlowdown = evaporation ÷ (CoC − 1) − driftdrift = 0.02 % of circulation · makeup = evaporation + drift + blowdowncirculation (kg/s)200evaporation at 10 K range (kg/s)3.48drift at 0.02 % (kg/s)0.04blowdown at 5 CoC (kg/s)0.83makeup at 5 CoC (kg/s)4.35makeup at 3 CoC (kg/s)5.22Computed from the formula — not field data.
Illustrative, computed from the balance above — not a measurement. The bars are the four terms of the balance at the reference conditions; the two curves are what blowdown and makeup do as the cycle count changes.

Sources

Measurement

How far the number can be trusted once it is read off an instrument.