How this cooling water makeup calculator works
An open cooling water system loses water as evaporation while the dissolved solids stay behind, so the concentration keeps rising. Holding it in place means wasting part of the water as blowdown and replacing the same amount as makeup. This page gets the blowdown and the makeup from the evaporation rate and the cycles of concentration, and also shows the makeup over 24 hours. Flows are entered in US gallons per minute and the results show gpm with m³/h in parentheses.
The relations applied come from a mass balance: blowdown B = E / (CoC - 1) and makeup M = E x CoC / (CoC - 1). When the evaporation is unknown it is obtained first as E = circulation rate x specific heat x temperature drop / latent heat, with the drop (the tower range) in °F, specific heat in Btu/lb·°F and latent heat in Btu/lb; the page converts these to SI internally before applying the relation. Entering a drift loss reduces the blowdown share by that amount and leaves the makeup unchanged. Figures reflect October 2026 and the underlying values come from secondary sources, so checking the originals is advised.
Recommended cycles of concentration and evaporation factors were not confirmed from an authoritative source. They depend on water quality and the treatment program, so you enter your own control value, and the specific heat and latent heat are entered for the same reason. As a rough check only, a rule of thumb widely quoted in US cooling tower practice puts evaporation at about 1% of the circulation rate for every 10 °F of range, which matches a specific heat near 1 Btu/lb·°F and a latent heat near 1,000 Btu/lb. Makeup and blowdown are taken to share the density of the circulating water, so density cancels out of the relation. Cooling tower capacity, boiler blowdown and chemical dosing are not covered. This calculator is for reference and does not replace design. Review by a qualified engineer is required.
Frequently asked questions
It is a control value that depends on the water quality and the treatment program, so no recommended figure is offered here because none was confirmed from an authoritative source. Use the value set by your water treatment supplier or operating instruction.
The makeup stays the same and only the blowdown falls. Drift also carries concentrated water out of the system, so it takes over part of what the blowdown would have removed.
Switch the path and enter the circulation rate, the temperature drop, the specific heat and the latent heat, and the evaporation is worked out first. Specific heat and latent heat vary with operating conditions, so they are entered from your own data.