Cooling Tower Water Balance: Evaporation, Drift, Blowdown and Cycles of Concentration

A cooling tower consumes water in three different ways, and they behave completely differently. Separating them is what makes water management tractable: one is unavoidable and proportional to the heat load, one is a design property of the drift eliminator, and one is a deliberate control action that trades water against chemistry.

The balance

At steady state, what goes in equals what goes out:

Makeup = Evaporation + Drift + Blowdown

Every plant number about water — litres per hour, cost per year, cycles of concentration, chemical dosing rate — comes out of that equation plus the concentration of dissolved solids carried by each stream. The rest of this article takes each term in turn.

Evaporation: the one you cannot avoid

Nearly all the heat a cooling tower rejects leaves as latent heat, carried away by water that evaporated inside the fill. The rest is sensible heat given up to the air. That is why evaporation tracks the heat load so directly:

mE ≈ Q / hfg, where Q = mcp,w (Th − Tc)

The latent heat hfg is about 2400 kJ/kg in the normal cooling-tower range, so the arithmetic is simple. For a tower circulating 200 kg/s with a 10 K range:

Q = 200 × 4.18 × 10 ≈ 8360 kW

mE ≈ 8360 ÷ 2400 ≈ 3.48 kg/s

As a fraction of circulation that is about 1.74%, which lands neatly in the familiar rule of thumb that a tower evaporates roughly 1% of its circulating flow per 5.5 K of range. Note what the range dependence implies: the evaporation fraction is not a property of the tower, it is a property of how hard the tower is being worked. Run a wider range and you evaporate proportionally more.

Illustrative example — the figures above are illustrative, not measured.

Drift: small in water terms, large in deposition terms

Drift is the liquid water carried out of the tower as droplets, and it is quoted as parts per million of the circulating flow. The design approximation the industry handbooks carry is about 0.02% of circulation, and the drift mass loss is simply:

mD = drift fraction × mcirculation

At 200 kg/s and 0.02%, mD = 0.04 kg/s. That is roughly 1% of the evaporation, which is why drift is rarely the reason a tower’s water bill is high — and why it is always the right answer to a different question. Drift carries dissolved solids with it, so it is the mechanism behind salt deposition on nearby structures, the visible plume, and corrosion on surfaces downwind of the tower. A drift eliminator is selected for where the droplets land, not for the water they contain.

Blowdown and cycles of concentration

Water leaves the tower by evaporation as pure vapour; dissolved solids stay behind. Unless water is deliberately drained, their concentration in the circulating water rises until scaling and corrosion follow. That drain is blowdown, and it is what fixes the cycles of concentration — the concentration in the circulating water divided by the concentration in the makeup water.

A dissolved-solids balance gives a compact relationship. Both blowdown and drift leave the tower at circulating-water concentration, while all solids enter through the makeup, so:

Makeup = CoC × (Blowdown + Drift)

Combined with the mass balance above, and with drift as part of the loss, blowdown follows directly:

Blowdown = Evaporation ÷ (CoC − 1) − Drift

Illustrative example

The following numbers are an illustrative example, not a measured result.

Using the tower from the previous section — evaporation 3.48 kg/s, drift 0.04 kg/s, operated at 5 cycles of concentration:

Blowdown = 3.48 ÷ (5 − 1) − 0.04 = 0.87 − 0.04 = 0.83 kg/s

Makeup = 3.48 + 0.83 + 0.04 = 4.35 kg/s

Check the concentration balance: Makeup ÷ CoC = 4.35 ÷ 5 = 0.87 kg/s, and Blowdown + Drift = 0.83 + 0.04 = 0.87 kg/s. The two agree, which is the balance being satisfied.

Note what the makeup-to-blowdown ratio turned out to be — 4.35 ÷ 0.83 = 5.24, not 5. Drift leaves the tower carrying concentrated water too, so the ratio of makeup to blowdown sits slightly above the cycles of concentration. A useful check, and a number that is easy to get wrong when drift is neglected.

The trade: cycles up, water down, risk up

Raising cycles of concentration is the standard lever for cutting makeup water and chemical consumption. At 5 cycles this example needs 4.35 kg/s of makeup; at 3 cycles it would need 3.48 ÷ 2 − 0.04 ≈ 1.70 kg/s of blowdown and about 5.22 kg/s of makeup — roughly 20% more water for the same heat load.

That saving is not free. Higher cycles mean higher concentration of everything the water carries: calcium and magnesium hardness that scale the fill, chlorides that attack concrete and steel, and dissolved solids that raise the fouling potential of every wetted surface. The practical limit is set by three things working together:

• the makeup water’s own chemistry, and what the treatment programme can hold in solution;
• the fill’s water-quality envelope — the manufacturer’s limits on the loading and chemistry the fill will tolerate without losing thermal performance or mechanical life;
• the tower’s materials of construction and the corrosion allowances designed into them.

Pushing cycles past those limits converts a water saving into a fill replacement, which is a poor trade. That is why the water balance and the water treatment programme have to be read together rather than managed by two separate departments.

The balance as a diagnostic

Because all three terms are calculable from the heat load and a few design values, the water balance doubles as a fault-finding tool. Measure makeup water over a period with a metered supply, calculate what the balance says it should be, and look at the gap:

• Makeup higher than predicted points to a leak in the basin or piping, an overflowing basin from a stuck level control, drift beyond the eliminator’s rating through damaged or displaced panels, or a circulating flow higher than the assumed value.
• Makeup lower than predicted usually means the assumed evaporation is too high — a range or flow that is not what the design documents say — or that the conductivity control is running the tower at higher cycles than intended, which is a scaling risk rather than a saving.

Either direction is worth investigating, because both mean the tower is not behaving the way its documentation says it does.

Read more

• Overhaul and maintenance service
• Cooling tower water too warm? 7 checks before a major repair
• Cooling tower glossary

Sources

• Hensley, J.C. (ed.) — Cooling Tower Fundamentals, 2nd edition (SPX Cooling Technologies, 2009) — the water balance this article works through — makeup as evaporation plus drift plus blowdown, the concentration-ratio relation, and the evaporation and drift approximations, with drift taken at 0.02% of the circulating flow.