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Illustrated guide · Industrial water systems

Why does a cooling tower make circulating water saltier?

Follow heat-driven evaporation through makeup, drift, blowdown and leakage balances, then connect cycles of concentration with conductivity, scale, corrosion and aerosol control.

Direct answer

Direct answer

An open cooling tower rejects heat by evaporating a small fraction of its recirculating water. The vapor is mainly H₂O; calcium, magnesium, chloride, sulfate, silica and most other dissolved constituents do not leave in the same proportion. Level control replaces evaporated water with makeup that brings a new mineral load. Water therefore leaves while most salt remains and more salt enters, so circulating-water conductivity and conservative-ion concentration rise. Controlled blowdown removes concentrated liquid and makeup restores volume. Drift, leaks and overflow also remove liquid and salt, but they are losses to minimize rather than substitutes for controlled blowdown. At steady state M=E+B+D+L (makeup equals evaporation, blowdown, drift and other liquid loss). For a suitable nonvolatile, nonreactive tracer, cycles of concentration CoC≈Ccirculating/Cmakeup. If drift and leakage are small, CoC≈M/B and B≈E/(CoC−1). These are balance models, not universal chemistry: conductivity changes with temperature, treatment chemicals and makeup, while carbonate or silica may precipitate and cease to be conservative. Safe cycles therefore require ion chemistry, pH/alkalinity, temperature, treatment and equipment condition. Higher cycles can save water but raise scale, corrosion and biological-control difficulty; low cycles waste makeup, chemicals and discharge capacity. A visible white plume is mostly condensed water vapor, not proof of salt mist. Liquid drift droplets can carry dissolved chemicals and microorganisms and must be limited with drift eliminators, maintenance and a water-management program.

Four facts explain the rising salinity

Evaporation drives concentration, but heat load, makeup chemistry, liquid losses and control establish the final level.

Evaporation removes water while most dissolved salts stay

Warm water forms films over fill and contacts air. A fraction changes phase and removes latent heat. Common ions remain in the liquid; volatile species, gas exchange and chemical reactions need separate balances.

Every unit of makeup brings another mineral load

Level control maintains basin inventory; it does not know whether cycles are safe. A change in makeup hardness, alkalinity, chloride, sulfate, silica or conductivity requires review of blowdown and treatment limits.

Salt leaves with liquid, solids or chemical transformation

Blowdown is the controllable outlet. Drift, leaks and overflow carry salt but should be minimized. Scale only moves material from water onto equipment and adds thermal resistance.

The target is an optimized limit, not the highest cycle

Water savings must fit saturation, wall temperature, residence time, materials, biofilm control, treatment chemistry and discharge permits.

1

An open tower transfers process heat to air while evaporating a small water fraction

Distribution and fill expose hot water to air; the fan exhausts warm humid air, cooled water falls to the basin and pumps return it to the heat load. Makeup and blowdown close the basin balance.

An open tower transfers process heat to air while evaporating a small water fraction：Fan and humid exhaust plume、Air inlet, fill and falling-water zone、Circulation pumps and supply/return headers、Basin, makeup and blowdown boundary1234

What to identify

  1. 1Fan and humid exhaust plume
  2. 2Air inlet, fill and falling-water zone
  3. 3Circulation pumps and supply/return headers
  4. 4Basin, makeup and blowdown boundary

What this proves

The fan does not blow salt into the loop. Evaporation selectively removes water; the pump moves inventory, the level valve replaces loss and the blowdown valve provides the controlled salt outlet.

Field check

Trend makeup M, blowdown B, circulation, basin level, tower inlet/outlet temperature and wet bulb together. Account separately for leaks and overflow before assigning all makeup to evaporation.

2

Matched makeup, circulating and blowdown samples reveal concentration; color does not

Beakers and conductivity probes illustrate point sampling. Location, time, temperature compensation and calibration must be consistent; clarity is not salinity.

Matched makeup, circulating and blowdown samples reveal concentration; color does not：Makeup baseline and conductivity、Lower-cycle sample、Higher-cycle sample、Blowdown sample and laboratory ions1234

What to identify

  1. 1Makeup baseline and conductivity
  2. 2Lower-cycle sample
  3. 3Higher-cycle sample
  4. 4Blowdown sample and laboratory ions

What this proves

Calculate cycles from a suitable tracer in samples from the same period. Conductivity is useful for continuous control, but it is not proof that every ion has concentrated equally.

Field check

Calibrate online and portable conductivity, verify temperature compensation and sample flow, clean fouled probes, and cross-check periodically with chloride or another conservative ion and a complete analysis.

3

A transparent rig puts spray, fill, airflow, basin and balance measurements in one system

Hot water sprays over fill, air crosses the wet area, cooled water reaches the basin, and makeup/blowdown meters and sample points close E, B, M and concentration balances.

A transparent rig puts spray, fill, airflow, basin and balance measurements in one system：Hot-water spray distribution、Wet fill heat and mass transfer、Fan airflow and drift eliminator、Cold basin, makeup/blowdown and instruments1234

What to identify

  1. 1Hot-water spray distribution
  2. 2Wet fill heat and mass transfer
  3. 3Fan airflow and drift eliminator
  4. 4Cold basin, makeup/blowdown and instruments

What this proves

Evaporation primarily follows heat load and ambient air state. Plume size alone cannot measure E; use heat or water balances and verify with the salt balance.

Field check

Record temperature range, circulation, fan state, wet bulb, makeup and blowdown over at least a day, correcting for basin inventory, overflow, side-stream filter backwash and sampling.

4

Matched-load tests show that lower blowdown saves water until chemistry turns concentration into scale

Left and center rigs represent controlled cycles; the right coil and basin show deposit accumulation. Only equal heat load, makeup, treatment and duration make the comparison causal.

Matched-load tests show that lower blowdown saves water until chemistry turns concentration into scale：Lower CoC: more blowdown, lower salts、Optimized CoC: water-risk balance、Excess/uncontrolled CoC: coil scale、Matched outlet and deposit evidence1234

What to identify

  1. 1Lower CoC: more blowdown, lower salts
  2. 2Optimized CoC: water-risk balance
  3. 3Excess/uncontrolled CoC: coil scale
  4. 4Matched outlet and deposit evidence

What this proves

Higher ion activity raises saturation risk, but scale also depends on pH, alkalinity, temperature, inhibitors, nucleation surfaces and residence time. High CoC alone is not a complete diagnosis.

Field check

At matched load compare cycles, pH, alkalinity, hardness, silica, chloride/sulfate, inhibitor residual, corrosion probes, exchanger approach temperature and deposit chemistry.

5

Field sampling, probe checks and deposit inspection connect salinity to equipment outcomes

Portable readings, bottle samples, visible mineral deposits and an opened bundle provide instrument, laboratory, location and equipment evidence.

Field sampling, probe checks and deposit inspection connect salinity to equipment outcomes：Portable conductivity/pH cross-check、Makeup, circulating and blowdown samples、Basin/fill mineral deposits、Opened bundle and localized corrosion evidence1234

What to identify

  1. 1Portable conductivity/pH cross-check
  2. 2Makeup, circulating and blowdown samples
  3. 3Basin/fill mineral deposits
  4. 4Opened bundle and localized corrosion evidence

What this proves

A controller at setpoint proves only that one ionic signal is being controlled. It does not prove that surfaces are scale-free, materials are uncorroded or microorganisms are controlled.

Field check

Use fixed sampling points and frequencies; record makeup source, chemical lot, load, temperature and blowdown actions. Sample deposits before cleaning and apply isolation, LOTO, confined-space and exposure controls.

Seven steps: heat leaves while salts remain

Place energy, water and salts on one path so makeup, blowdown and plume are not confused.

  1. 1 Pick up heat

    Cool water → exchanger/condenser → hot water

    Carry process heat without materially changing salt inventory.

  2. 2 Distribute

    Hot water → nozzles/troughs → wet fill

    Create even area for air-water contact.

  3. 3 Evaporate

    Small liquid fraction → vapor + air

    Reject latent heat while most dissolved salts stay.

  4. 4 Remove drift

    Humid air → eliminator → exhaust

    Return entrained droplets and distinguish vapor from salty drift.

  5. 5 Reuse cold water

    Fill drainage → basin → pump

    Return cooled but more concentrated water to the load.

  6. 6 Add makeup

    Fresh water M → basin

    Replace E+B+D+L and introduce new dissolved minerals.

  7. 7 Blow down

    Concentrated water B → discharge/reuse treatment

    Remove salts within chemistry and permit limits.

Four subsystems manage heat, inventory, salts and risk

Changing one fan, level valve or blowdown valve does not prove the system is controlled.

Process exchanger and circulation

Normal duty
Carry heat to the tower at required flow and range
Typical mismatch
Process leak contaminates the loop, off-design flow/load, bypass or local high wall temperature
Evidence
Circulation Q, supply/return T/P, process tracers, exchanger approach and leak testing

Tower, distribution, fill, fan/eliminator

Normal duty
Provide uniform contact and retain drift droplets
Typical mismatch
Plugged nozzles, scaled/biofouled fill, low air, recirculation, damaged eliminator
Evidence
Range/approach, distribution, fan data, fill pressure/condition, drift and nearby deposits

Basin, makeup and level

Normal duty
Replace all loss and preserve pump submergence
Typical mismatch
Stuck valve, overflow, hidden leak, source change or meter drift
Evidence
Makeup total, level, valve, overflow/leak survey, complete makeup analysis and calibration

Blowdown, treatment and monitoring

Normal duty
Control scale, corrosion and microbes within safe cycles
Typical mismatch
Fouled probe, wrong temperature compensation/setpoint, blocked valve, failed feed or dead legs
Evidence
Actual B, conductivity/conservative ion, pH/alkalinity/hardness/silica, treatment residual, corrosion and microbial trends

The ideal steady balance is M=E+B+D+L. If D and L are small and the tracer is conservative, CoC≈M/B and B≈E/(CoC−1). Real audits must include drift, leaks, overflow, filter backwash, sampling, precipitation and basin inventory change.

Keep three evidence groups on the same timeline

Water and heat

Makeup/blowdown totals, basin level, leaks/overflow, circulation, supply/return temperature, wet bulb, fan/pump state and process load.

Makeup–circulating–blowdown chemistry

Temperature-compensated conductivity, pH, alkalinity, hardness, chloride/sulfate, silica, TDS, treatment residuals and required microbiological indicators.

Equipment and environmental results

Exchanger approach/pressure drop, corrosion coupons or probes, fill/nozzle/eliminator condition, deposit analysis, drift/nearby deposits, cleaning frequency and water-management actions.

Diagnose with M–B–conductivity–equipment results

Signal
Circulating conductivity rises, basin level is normal and measured blowdown is near zero
Suspect first
Probe/control never triggers, blowdown valve/line is blocked or isolated, or the setpoint is too high
Next step
Cross-check conductivity and temperature compensation, stroke the valve and measure real B; inspect logic, interlocks and discharge backpressure
Signal
Conductivity cycles look acceptable but exchanger scale or approach temperature worsens
Suspect first
Local heating, pH/alkalinity/hardness/silica saturation, low inhibitor, or a non-mineral deposit
Next step
Analyze full chemistry, residual and deposit; inspect location, velocity and wall temperature rather than changing one conductivity number blindly
Signal
Makeup rises and cycles fall without a matching heat-load increase
Suspect first
Excess blowdown, basin/piping leak, overflow, hunting level valve, filter backwash or unmetered use
Next step
Close a 24-hour M/B/level/side-loss balance, verify meters and find the water destination before raising cycles
Signal
A large white plume or nearby wet/salt spots appear
Suspect first
The plume may be vapor condensation; wet or salt deposits point more strongly to liquid drift and eliminator/distribution problems
Next step
Do not infer salt loss from visibility; inspect eliminators, fan and distribution, assess drift/aerosol exposure and implement water-management controls

Four common misconceptions

The white plume is salt fog

It is mainly condensed vapor. Salts travel in entrained liquid drift droplets.

The makeup valve controls cycles

It controls level. Blowdown, monitoring and chemistry limits control cycles.

Conductivity ratio is every ion's true cycle

It is a useful proxy affected by temperature, treatment, precipitation, gas exchange and makeup variation.

The highest cycles are always the best

Water savings diminish while scale, corrosion, microbial and discharge risks rise. The optimum is site-specific.

Keep learning

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