How Does Water Quality Affect Evaporative Cooling Performance?

Evaporative coolers are deceptively simple machines: a fan, a pump, a wetted medium, and a sump working together to drive sensible heat from air into liquid water that then evaporates. But behind that simplicity lies a complex chemistry set. The composition of the water circulating through the pads—the evaporative cooling water quality—governs not only immediate cooling performance (approach to wet-bulb, saturation efficiency, airflow) but also the longevity of media and metallic components, the frequency of service, and the total cost of ownership.

Key Parameters of Evaporative Cooling Water Quality

Water in an evaporative cooler does not behave like water in a once-through system. Because evaporation selectively removes pure H₂O, dissolved and suspended solids concentrate in the recirculating loop. As cycles of concentration (CoC) increase, the risk of scaling, fouling, and corrosion rises non-linearly. Understanding the core water quality parameters—and how they interact—is foundational.

Makeup water chemistry and cycles of concentration (CoC).

• TDS / Conductivity (µS/cm): A proxy for total dissolved solids; as evaporation proceeds, conductivity climbs. Conductivity control (via bleed/blowdown) is the primary lever for limiting CoC.
• Hardness (mg/L as CaCO₃): Primarily calcium and magnesium. Hardness plus alkalinity under elevated pH is the classic recipe for calcium carbonate (CaCO₃) scale on pads, distribution trays, and sump surfaces.
• Alkalinity (mg/L as CaCO₃) and pH: Alkalinity buffers pH and, with CO₂ off-gassing during evaporation, tends to push pH higher. High pH favors carbonate scale; low pH increases corrosivity.
• Silica (mg/L as SiO₂): Concentrates with evaporation; above solubility limits (influenced by pH and temperature) forms very tenacious glassy scale that rapidly degrades media performance.
• Chloride and sulfate (mg/L): Non-scaling anions that concentrate and drive pitting/stress-corrosion risks for certain alloys if allowed to accumulate.
• Iron, manganese, and particulates: Cause staining and deposit formation; particulates seed scale and obstruct nozzles.
• Microbiological activity (heterotrophic plate count, ATP, biofilm): Slime layers reduce wetting efficiency, increase pressure drop, and can undercut hygiene if not controlled.

Indices and control targets.
For evaporative systems, practitioners often use Langelier Saturation Index (LSI), Ryznar Stability Index (RSI), or Puckorius Scaling Index (PSI) to estimate scaling tendency. In broad terms:

• LSI > 0: Scale-forming tendency increases with magnitude.
• LSI ≈ 0: Saturation equilibrium—often a practical aim for mineral control in open recirculating systems.
• LSI < 0: Corrosive tendency rises (especially toward carbon steel and copper alloys).

Because rigid evaporative media (cellulosic or synthetic) are sensitive to both scale deposition and aggressive chemistry, many OEMs and water treatment providers target moderate CoC with conductivity control, keep pH roughly neutral to mildly alkaline, and limit free halogen residuals to avoid media degradation. Always refer to the media manufacturer’s specific tolerances, as formulations vary.

Monitoring hardness and scaling in evaporative cooling water quality

Scaling mechanics.
When calcium hardness and alkalinity concentrate, and CO₂ is stripped in the media, the local pH in the film rises. If the ionic product of Ca²⁺ and CO₃²⁻ exceeds the solubility product of CaCO₃, crystals nucleate on the most convenient surfaces—pad fibers, distribution trays, and sump walls. Scale:

• Reduces evaporative surface area by occluding media pores.
• Impairs wicking and wetting, creating dry channels that bypass heat/mass transfer.
• Increases pressure drop across the pad and can reduce airflow at a given fan curve.
• Shifts approach to wet-bulb (poorer cooling performance).

What and how to measure.

• Hardness and alkalinity (mg/L as CaCO₃) on makeup and recirculating water; track CoC = (Recirc Conductivity / Makeup Conductivity) or using hardness ratios.
• Conductivity (continuous sensor) tied to an automatic bleed valve.
• pH (in the loop and the sump), especially during warm, dry conditions when evaporation rates are highest.
• Silica if makeup water exceeds ~20–30 mg/L; silica scale is harder to remove than carbonate scale.
• Deposit characterization (if scale occurs): basic acid fizz test (carbonate) vs. non-reactive (silica/iron oxides).

Control levers.

• Conductivity setpoint to cap CoC and keep LSI near zero to mildly positive.
• Softening or partial softening of makeup water to reduce Ca²⁺ load (particularly effective when alkalinity is moderate).
• Dealkalization or reverse osmosis (RO) if alkalinity/silica are high and CoC cannot be pushed without scaling.
• Threshold inhibitors/antiscalants for CaCO₃ and CaSO₄ where compatible with pad media.

Corrosion risks from low pH in evaporative cooling water quality

Corrosion mechanics.
Corrosion is electrochemical. Low pH increases hydrogen ion availability, accelerating metal dissolution—especially of carbon steel and copper alloys. Meanwhile, chloride promotes pitting and crevice corrosion (concern for stainless steels and aluminum components), and differential aeration across wetted/dry regions sets up galvanic microcells.

Risk factors in evaporative coolers.

• Low pH (< ~6.5) from overdosing acids or from aggressive low-alkalinity water.
• High chlorides due to over-concentration (Clark numbers rising with CoC).
• Mixed metallurgy in pumps, fasteners, and housings generating galvanic couples.
• Oxidizing biocides at high residuals attacking copper and aluminum; also potential degradation of cellulosic media binders.

Monitoring and mitigation.

• pH continuous or frequent spot checks; maintain in a neutral to mildly alkaline range unless OEM otherwise specifies.
• Chloride and sulfate tracking as CoC rises; adjust bleed setpoint to limit their accumulation.
• Corrosion coupons or probes (linear polarization resistance) in side-stream loops for quantitative corrosion rates (mpy).
• Material selection: robust stainless housings, non-corroding distributors, and compatible fasteners minimize risk when water chemistry deviates.
• Passivation and inhibitors compatible with pad media; avoid chemistries known to embrittle or soften cellulose.

Effects of Poor Evaporative Cooling Water Quality on Equipment

1) Degraded cooling performance (higher approach to wet-bulb).
Scale and biofilm both thicken the mass-transfer boundary layer across pad surfaces. Even thin scale deposits can reduce effective surface area and disrupt capillary wetting, causing channeling. The result is lower saturation efficiency (η), higher discharge air temperature, and more fan energy required to achieve the same sensible cooling rate.

2) Reduced media lifespan.
Rigid media fail in several predictable modes when water quality is uncontrolled:

• Scale loading increases pad weight and makes the matrix brittle; cleaning becomes ineffective once silica or carbonate scale penetrates fiber interfaces.
• pH extremes (either low or very high) can attack binders in cellulosic media, leading to softening, fiber loss, and pad “shedding.”
• Excess oxidants (chlorine, bromine, ozone at high dose) discolor and weaken organic media; oxidative embrittlement manifests as crumbling edges and dusting.
• Biofouling blocks channels and creates odor complaints; mucilaginous biofilm can be stubborn, requiring mechanical cleaning that abrades fibers.

3) Pump and distribution problems.

• Nozzle and orifice clogging from particulate load, corrosion scale, or biofilm tails leads to poor water distribution, dry spots on pads, and localized thermal performance loss.
• Abrasive solids can erode pump impellers and seals, raising hydraulic losses and increasing NPSH requirements.
• Foaming from organics/surfactants causes uneven flow and can carry solids into the air stream.

4) Corrosion and leakage.

• Carbon steel and aluminum components exposed to low pH or high chlorides pit and thin over time, risking leaks and unplanned downtime.
• Galvanic couples between dissimilar metals in wet environments accelerate localized attack (e.g., copper tube to steel fitting).
• While stainless steel housings dramatically improve resilience, persistent high-chloride conditions or crevice conditions can still initiate pitting without proper control—another reason to monitor and limit CoC.

5) Increased maintenance frequency and cost.

• Frequent pad replacements, acid cleaning (if applicable and safe for media), and sump cleanouts raise labor and chemical spend.
• Emergency service (clogged sprays, seized pumps) interrupts operations during peak-load periods when cooling is most needed.

6) Indoor air quality and hygiene concerns.
Though evaporative coolers are not cooling towers, they still generate aerosolized moisture. Biofilm and algae in the sump or on pads increase microbial counts downstream. Uncontrolled growth can produce earthy odors, discolor runoff, and erode occupant confidence. Maintaining oxidant residuals within media-safe limits and ensuring periodic bleed are central to hygiene.

Water Treatment Strategies for Evaporative Cooling Water Quality

Effective treatment begins with data. A baseline makeup water analysis (hardness, alkalinity, TDS/conductivity, pH, chloride, sulfate, silica, iron/manganese) predicts which limits will be reached first as CoC rises. From there, combine mechanical and chemical controls to maintain water quality inside the “safe operating envelope” for both metals and media.

1) Control cycles with conductivity-based bleed.
Install a conductivity sensor in a representative recirculating line and control an automatic bleed valve. As evaporation raises conductivity, the bleed opens to maintain a setpoint tied to target CoC. This is the single most important control for avoiding runaway concentration of chlorides, sulfates, and silica.

2) Filtration—side-stream and full-flow options.
Suspended solids seed scale and clog orifices. A coherent filtration strategy will pay for itself in fewer cleanouts and longer pad life:

• Side-stream sand or multimedia filters (5–20% of recirculation flow): Continuous removal of fines keeps the sump clean and reduces deposition on pads.
• Hydrocyclones/centrifugal separators: Effective for heavier grit and cutting debris in industrial settings; low maintenance, no media to replace.
• Bag or cartridge filters: Good polishing step upstream of nozzles; choose micron rating based on nozzle orifice size to avoid plugging.
• Automatic self-cleaning strainers: For large flows where manual element cleaning is impractical.
• Sump screens and trash traps: Prevent pad fragments and large debris from entering the pump suction.

3) Hardness and alkalinity management.

• Softening (ion exchange): Replaces Ca²⁺/Mg²⁺ with Na⁺, dramatically lowering carbonate scale potential. Particularly useful when alkalinity is moderate; note that softened water may raise sodium levels and total TDS—conductivity control remains essential.
• Dealkalization: Anion exchange or weak acid cation exchange to lower alkalinity when bicarbonate is the limiting factor.
• Reverse osmosis (RO): Reduces TDS, hardness, silica, and chloride simultaneously, enabling higher CoC with minimal scaling—ideal for high-silica or brackish makeup water. RO adds capital and operating cost but can stabilize performance where conventional treatment falls short.

4) Silica control.
Silica is notoriously difficult; once deposited, it is resistant to acid cleaning. Options include:

• Limiting CoC via conductivity control to keep silica below solubility limits at system pH.
• RO or specific silica scavenging in makeup (less common).
• Threshold inhibitors that raise the supersaturation tolerance (use only if compatible with media).

5) Microbiological control.

• Oxidizing biocides (free chlorine, bromine) at low, controlled residuals can manage planktonic counts, but many rigid cellulosic media have upper limits for free halogen to prevent degradation—verify OEM limits.
• Non-oxidizing biocides (e.g., isothiazolinones) can be used intermittently to disrupt biofilms while being gentler on media; follow label and compatibility guidance.
• Physical methods such as UV or ozone (with controlled residual) can assist; ozone is a strong oxidant and must be dosed carefully relative to media tolerance.
• Housekeeping: Regular sump cleaning, eliminating dead legs, and maintaining even water distribution reduces stagnation.

6) pH management and corrosion inhibition.

• If LSI is highly positive, a controlled acid feed (often sulfuric) can reduce pH to tame carbonate scale—but take care: pushing pH too low invites corrosion and media damage.
• Inhibitors (phosphates, molybdates, azoles for copper) can form protective films; select products compatible with pad media and environmental requirements.
• Material upgrades—stainless housings, non-metallic distributors, and corrosion-resistant fasteners—reduce sensitivity to chemistry excursions.

7) Automation and data logging.

• Conductivity, pH, and ORP (if using oxidants) under a small PLC or industrial controller allow closed-loop control and alarms.
• Turbidity or differential pressure across filters triggers maintenance before flow is compromised.
• Data logging enables trend analysis and proactive changes to setpoints as seasons shift.

Filtration and chemical dosing for optimal evaporative cooling water quality

Bringing filtration and dosing together with instrumentation makes the program coherent:

Integrated skid concept.

• Side-stream takeoff from the recirculation line feeds a multimedia filter sized at ~10% of system recirculation. Filter effluent returns to the sump.
• Upstream of the filter, a Y-strainer or self-cleaning screen protects the filter bed; downstream, a cartridge polisher (10–50 µm) prepares water for chemical injection.
• Chemical dosing panel with metering pumps for biocide, inhibitor, and acid/base (if used), all interlocked to flow switches and level sensors to prevent feed on no-flow.
• Instrumentation: conductivity cell controls the bleed valve; pH probe governs acid/base feed; ORP monitors oxidant activity; pressure gauges monitor filter loading.
• Safety: containment for chemical drums, backflow prevention on makeup, and clearly labeled isolation valves.

Operating setpoints and practices (illustrative, verify with OEM/media guidance):

• Conductivity: establish a setpoint that yields your target CoC without pushing LSI beyond slightly positive; seasonally adjust for changing wet-bulb and evaporation rates.
• pH: neutral to mildly alkaline (e.g., ~7.0–8.3) unless the media specifies otherwise.
• Oxidant residual: as low as practical for control (often tenths of ppm), avoiding pad damage; consider non-oxidizing biocides as adjuncts.
• Filter maintenance: base backwash or element change on dP rise or turbidity, not just calendar days.
• Bleed strategy: continuous small bleeds are gentler on chemistry than large intermittent dumps; however, conductivity-triggered intermittent bleed is common and effective when tuned.

Commissioning checklist.

1. Obtain a full water analysis for makeup and set initial CoC and bleed setpoint.
2. Verify sensor calibration (pH and conductivity) against standards.
3. Start with conservative CoC; record LSI/RSI during the first warm week; adjust gradually.
4. Inspect pads for uniform wetting; adjust distribution weirs/nozzles and correct pump flow if needed.
5. Implement a weekly test log (makeup and recirc hardness, alkalinity, conductivity, pH, silica if relevant; biocide residuals if used).
6. After 2–4 weeks, examine a pad face for early scale crystals or biofilm; fine-tune setpoints.

Monitoring hardness and scaling in evaporative cooling water quality

While addressed earlier, monitoring deserves its own operational lens:

• Field test kits for hardness, alkalinity, and drop-count titration are inexpensive and accurate enough for daily rounds.
• Portable conductivity meters validate installed probes and help troubleshoot odd readings (e.g., air bubbles, oil film).
• Silica colorimetric tests are critical in high-silica regions; treat rising trends as an early warning to lower CoC.
• Deposit mapping: Photograph pad faces monthly at identical locations and angles; scale and biofilm patterns reveal distribution problems before performance drops.
• Use indices judiciously: LSI is temperature-dependent; measure at actual sump temperature. Don’t “chase” a single number—corroborate with visual inspection.

Corrosion risks from low pH in evaporative cooling water quality

Operationally, corrosion control in evaporative coolers must balance metal protection with media safety:

• Avoid aggressive acid cleanings on installed pads; if descaling is required, remove pads or use pad-safe cleaners per OEM guidance.
• If copper is present, maintain azole inhibitor residuals if compatible with media and microbiological program.
• Watch for chloride creep: in arid climates, make-up chloride can already be high; with high CoC, the chloride level may push stainless toward pitting thresholds. Lower CoC or consider partial RO if this trend appears.
• Check for stray DC or grounding issues on packaged units; floating potentials in wet environments amplify corrosion.

Filtration and chemical dosing for optimal evaporative cooling water quality

Execution specifics ensure theory becomes reliable practice:

• Sizing filters: aim for side-stream turnover of the entire sump volume 4–6 times per hour in dirty environments; otherwise, ~1–2 turnovers per hour is often adequate.
• Micron rating: select based on the smallest critical orifice in the distribution system; keep a 2–3× margin (e.g., 30 µm filter for 90 µm orifices).
• Dosing sequence: inject corrosion/scaling inhibitors upstream of oxidants to prevent oxidative degradation; maintain a minimum contact time before ORP control points.
• Interlocks: tie chemical feed to pump VFD run status and flow switches; alarm on low day-tank levels to avoid dry-running metering pumps.
• Documentation: maintain a site-specific water treatment SOP and a control philosophy narrative so operators understand why each setpoint exists.

Bringing It All Together

At a system level, think in terms of mass and heat balance and chemistry balance. Evaporation removes water but not minerals; bleed removes minerals but at the cost of water and sewer. Filtration removes particulates that catalyze fouling; biocides reduce biomass that robs efficiency. Material choices (e.g., stainless housings) buy resilience and widen the safe operating window. The optimal program is the one that holds water chemistry inside the narrow band where scale is suppressed, corrosion is minimized, media remains intact, and microbiology stays quiet, all while using the least water and chemicals for the duty.

Practical signs that you’re in that window include:

• Stable conductivity with modest bleed fractions during hot, dry days.
• Even pad wetting with no dry streaks or chalky crystals at the air inlet face.
• Low differential pressure across pads at a given airflow setpoint.
• Quiet sumps—no excessive foam, no algae stringers.
• Steady discharge air temperature approaching the psychrometric ideal for the ambient wet-bulb.

When any of these drift, measure first, then adjust. The path back is usually a combination of slightly lower CoC, filter service, and calibrated chemical tweaks rather than a single drastic move.

Effects of Poor Evaporative Cooling Water Quality on Equipment (Summary Highlights)

• Efficiency loss: Scale/biofilm increase approach to wet-bulb, forcing longer runtimes and higher energy use.
• Media damage: Chemistry extremes and oxidants degrade rigid pad structure and binders.
• Hydraulic issues: Solids and slime clog distribution and erode pump components.
• Corrosion: Low pH and high chlorides attack metals; mixed metallurgy accelerates galvanic problems.
• Cost escalation: Frequent pad replacements, emergency service, and water/chemical waste.

Water Treatment Strategies for Evaporative Cooling Water Quality (Summary Highlights)

• Analyze makeup; set target CoC; automate bleed via conductivity.
• Filter continuously to keep solids low; size side-stream filters appropriately.
• Manage hardness/alkalinity with softening, dealkalization, or RO depending on local water.
• Control microbes with media-compatible biocide programs and good housekeeping.
• Keep pH near neutral; use inhibitors judiciously and compatible with pad media.
• Instrument, interlock, and log for stable, repeatable control.

Contact Us Today

If you’re designing, upgrading, or troubleshooting an evaporative cooling system, the right water strategy is the difference between a clean, efficient, low-touch system and a maintenance headache. Premier Industries, Inc. has been manufacturing evaporative coolers in Phoenix, AZ, for over 30 years, with lifetime stainless steel housings and high-efficiency rigid-type cooling media with up to 98 percent cooling efficiency. Our engineering and design team can help you specify the correct water treatment approach, specify compatible filtration and chemical programs, and integrate controls so your unit performs to its psychrometric potential—all while protecting media and metals.

Whether you need a custom skid, replacement equipment, or a ground-up engineered solution, we can design and build equipment to meet virtually any evaporative cooling, filtration, or air-handling need. If you’re looking for an efficient, reasonably priced, and innovative way to cool your home or commercial space, look no further than Premier Industries, Inc. Contact our team today to discuss your application, request a specification review, or schedule a site assessment. Let’s optimize your evaporative cooling water quality—and your system’s performance—together.

Categorised in: Evaporative Coolers

This post was written by Mike Nicolini