Technical note

Water Savings from Higher Cycles

Evaporation is not saveable; blowdown is. Add water, sewer and chemical charges to get the real benefit.

“Raising cycles of concentration from 3 to 5 — how much water can I save in a year?” Three formulas settle it: evaporation, blowdown, make-up. Do the arithmetic before discussing money, or a slogan like “40% water saving” will lead you astray.

1. Calculate evaporation first

Evaporation is approximately circulating flow multiplied by the temperature drop, related through latent heat and specific heat. Engineering practice uses an empirical coefficient: evaporation is a few parts per thousand of circulating flow, varying with the temperature difference. It is set by heat load and is independent of the treatment method — the water that evaporates cannot be saved.

2. Then calculate blowdown

At steady state, blowdown equals evaporation divided by (cycles of concentration minus one). This is the most useful step: raising cycles from 3 to 5 cuts blowdown from half of evaporation to a quarter. In other words, the saving comes mainly from blowdown, not evaporation. Make-up equals evaporation plus blowdown and falls with the cycles as well.

3. Add all three money items

Water is only the first item. The second is sewer charges: less blowdown means less discharge, and this charge is often higher than the water charge. The third is chemicals: less blowdown means less chemical lost with it. Only the sum of the three is the real annual benefit. Raising cycles also has a cost: scaling and corrosion risk rise, so treatment and monitoring must be upgraded, and that cost belongs in the denominator.

4. One boundary to keep in mind

Cycles cannot rise indefinitely. They are limited by make-up water quality, system materials, temperature and residence time. Beyond a certain point, the water saving no longer covers the energy and cleaning cost caused by scaling. That turning point must be calculated for the specific water — other plants' numbers cannot be borrowed. Never push a system to the scaling edge just to show a water saving.

5. We can run the numbers

Send the four make-up water numbers, circulating flow, temperature difference, current and target cycles, and local water and sewer charges. We will calculate annual make-up, blowdown and the cost difference, stating which inputs are measured and which are estimated. That is more dependable than any “percentage saved” figure.