Cycles of Concentration (COC)

In plain terms

An evaporative cooling tower loses pure water as vapour, so the salts left behind concentrate in the circulating loop. To stop them reaching harmful levels, operators bleed off some concentrated water as blowdown and replace it with fresh make-up water.

Cycles of concentration measures how far the loop is allowed to concentrate. The textbook mass balance, ignoring drift, is:

COC = C_circulating / C_make-up
Blowdown = Evaporation / (COC − 1)
Make-up = Evaporation + Blowdown

These relations follow the US DOE cooling tower guidance (DOE FEMP, 2011). Higher COC means less blowdown and less make-up for the same heat load.

Typical range

The highest workable COC is limited by scaling and corrosion in the loop (DOE FEMP, 2011). Both depend on the chemistry of the make-up water and on the water treatment programme, so the limit is set site by site rather than as one general range.

Why it moves LCOH

In a hydrogen plant with evaporative cooling, cooling make-up is often the largest water stream. Raising COC from a low value cuts blowdown sharply, which reduces water purchase, treatment and discharge. The gains shrink as COC rises further, while scaling and corrosion risk grows. At water-stressed sites, COC can decide between a viable and an unviable water balance.

Common mistake