Cooling Water Makeup (Cycles of Concentration)
Evaporation, blowdown, drift, and total makeup flow from recirculation, range, and cycles of concentration.
Example
You enter
- Recirculation flow (GPM) 1000
- Cooling range delta-T (°F) 10
- Cycles of concentration 4
- Drift fraction (e.g. 0.002) 0.002
You get
- Evaporation (GPM) 10
- Blowdown (GPM) 1.3333
- Drift (GPM) 2
- Total makeup 13.33 GPM
Checked against published worked examples from: CTI / ASHRAE.
Details, formula, and sources
ScopeEvaporation, blowdown, drift, and total makeup flow from recirculation, range, and cycles of concentration, with scaling and drift flags. Per CTI and ASHRAE Systems and Equipment 2020 Ch. 40.
Formula or tableEvaporation (GPM) = recirculation x delta-T / 1000. Drift (GPM) = recirculation x drift fraction. Blowdown (GPM) = evaporation / (COC - 1) - drift, not less than 0 (blowdown and drift together carry the concentrated water, so makeup = evaporation x COC / (COC - 1)). Makeup (GPM) = evaporation + blowdown + drift.
Source and editionCooling Technology Institute (CTI) publications; ASHRAE Systems and Equipment 2020 Chapter 40 (cooling towers).
Free accesscti.org and ashrae.org for the TOCs.
What governsEstimate. Operator of record and primacy agency govern.
API field namesrecirculation_gpm, delta_T_F, coc, drift_fraction, evaporation_gpm, blowdown_gpm, drift_gpm, makeup_gpm
- Evaporation rule evaporation = recirculation x delta-T / 1000CTI / ASHRAE rule of thumb
- Drift default 0.002 (0.2%) default; modern high-efficiency eliminators hold drift near 0.001-0.005% of recirculationCTI drift-eliminator practice
- COC scaling flag > 10 cycles flagged as a scaling riskcooling-water treatment practice