Industrial Cooling Tower Selection + Water Treatment: Types, Range/Approach, and Scaling/Corrosion/Legionella Control per CTI/ASHRAE 188 for Thai Plants

A cooling tower installed and commissioned, but the leaving-water temperature never reaches target — the chiller draws far more electricity than expected — fill and nozzles foul within a year — white scale deposits appear throughout the system — or a nearby facility has suffered a Legionella outbreak. These are almost never a "bad tower." In nearly every case, the root cause is a cooling tower that was mis-specified from the start or one that has no real water treatment and Legionella management programme: wrong type selected, thermal duty calculated at the wrong wet-bulb, no structured water treatment, or no written Legionella Water Management Plan.

A cooling tower is a core plant utility that directly affects chiller COP and employee health. This article is for anyone deciding to purchase or upgrade an industrial cooling tower: select the right type → calculate Range/Approach from Thailand's actual wet-bulb → build a water treatment programme from Scaling/Corrosion through to Legionella per CTI/ASHRAE 188

This article focuses on cooling towers (open/closed) for chiller and process cooling systems. For selecting a direct refrigeration cold-room system, see Industrial Cold Room: Chiller vs Freezer vs Blast Freezer. For choosing between Glycol Chiller and DX cooling, read Glycol Chiller vs DX System for Food Processing Plants.

1. Three Main Cooling Tower Types — Wrong Choice Affects the Entire Project

Open-Circuit vs Closed-Circuit

Counterflow vs Crossflow

• Counterflow: water falls from the top while air rises upward against the flow. Highest heat-transfer efficiency, smaller footprint, but the tower is taller and slightly more difficult to access for maintenance.
• Crossflow: water falls from the top while air flows horizontally through the sides. Easier to install and better suited to height-restricted sites, but the same thermal duty requires a larger plan area than counterflow.

Induced-Draft vs Forced-Draft

• Induced-Draft (fan on top): fan draws air out through the top. More even air distribution, less hot-air recirculation — the standard configuration in the Thai market.
• Forced-Draft (fan at bottom): fan pushes air upward from the bottom. Motor is in a dry location and easier to service, but carries a higher risk of hot-air recirculation and less uniform air distribution.
• Natural Draft: no mechanical fan; relies on the buoyancy of hot air inside the tower. Used only in very large power-station cooling towers (hyperboloid structures) and is not applicable to typical Thai industrial plants.

2. Calculating Range and Approach — The Foundation of Sizing

Before selecting a type and size you must clearly define these two parameters; they govern every downstream design decision.

Range (°C) = T_hot_in − T_cold_out

Approach (°C) = T_cold_out − WBT_design

where WBT_design = Design Wet-Bulb Temperature at the installation site.

Key rules about Approach:

• The smaller the Approach, the closer the leaving-water must get to the wet-bulb limit — this requires a larger tower, stronger fans, or more fill — and cost rises steeply.
• An Approach below ~2.8°C is rarely economical: tower size increases dramatically as Approach decreases toward that threshold.
• A wider Range (more cooling per unit of circulation flow) allows a smaller tower at the same Approach, but may require deeper fill sections.

The most common and costly mistake: specifying from a European catalog at WBT 20–22°C instead of Thailand's 27–28°C

A tower rated to achieve a 4°C Approach at 22°C wet-bulb will only achieve perhaps an 8–10°C Approach at 28°C wet-bulb — meaning the leaving-water is 4–6°C warmer than targeted. The downstream chiller's COP drops measurably and electricity bills climb accordingly.

Worked comparison:

Suppose the design requires leaving-water at 32°C at a site wet-bulb of 28°C → required Approach = 32 − 28 = 4°C. A tower specified only at a European wet-bulb cannot confirm it achieves 4°C Approach in Thailand. Always demand a thermal performance curve from the supplier validated at WBT = 28°C.

3. Decision Map: Select Type + Verify Water Quality

flowchart TD
    A["Define Heat Duty (kW)
+ T_hot_in / T_cold_out target
+ WBT design 28°C (Thailand)"] --> B{"Must cooling water remain
clean and uncontaminated?"}
    B -->|"Yes — hydraulic oil
flammable fluid
special process"| C["Closed-Circuit
Fluid Cooler
sealed coil separates process fluid"]
    B -->|"No — chiller condenser
HVAC / general process"| D["Open-Circuit
Evaporative Tower"]
    D --> E{"Is installation height
restricted?"}
    E -->|"Height not a constraint"| F["Counterflow Induced-Draft
— highest efficiency, small footprint
Thai industrial standard"]
    E -->|"Height restricted"| G["Crossflow Induced-Draft
— needs more plan area
but lower overall height"]
    F --> H{"Makeup water quality:
high TSS or biological load?"}
    G --> H
    H -->|"Good makeup water
low TDS"| I["Film Fill
highest thermal efficiency
requires controlled WTP / LSI"]
    H -->|"Turbid water
high SS"| J["Splash Fill
fouling-tolerant
easier to clean"]
    I --> K["Develop Water Management Plan
per ASHRAE 188
+ CTI ATC-105 acceptance test"]
    J --> K
    C --> K

4. Evaporation, Makeup, and Blowdown — Calculating Water Consumption

Quantifying water losses from the cooling tower is fundamental both for supply planning and for designing the water treatment system.

Core equations:

Evaporation (E) ≈ 1% of circulation flow rate per ~5.5–7°C of Range

Drift (D) ≈ 0.001–0.005% of circulation flow (with modern drift eliminators)

Blowdown (B) = E ÷ (COC − 1)

Makeup (M) = E + D + B

COC (Cycles of Concentration) ≈ circulating conductivity ÷ makeup conductivity = M ÷ B

Worked numerical example:

Assume a 1,000 kW heat-rejection cooling tower, hot water 37°C in / 32°C out (Range = 5°C), circulation flow rate ≈ 172 m³/hr.

• Evaporation: ≈ 1% × 172 m³/hr × (5 ÷ 5.5) ≈ ~1.6 m³/hr
• Drift: ≈ 0.002% × 172 ≈ ~0.003 m³/hr (negligible with modern drift eliminator)
• Assume COC = 4: Blowdown = 1.6 ÷ (4−1) ≈ 0.53 m³/hr
• Makeup = E + D + B ≈ 1.6 + 0.003 + 0.53 ≈ 2.13 m³/hr (≈ 51 m³/day at 24-hour operation)

These figures are an illustrative estimate — real calculations require site psychrometric data, operating hours, and a COC target derived from makeup water analysis.

Effect of COC on water consumption and scaling risk:

5. Water Treatment — Guarding Against the Three Mechanisms That Destroy Towers

flowchart LR
    A["Circulating water
in Cooling Tower"] --> B["Scaling
CaCO3 / MgSO4 deposits
clog fill and piping"]
    A --> C["Corrosion
attacks basin, fill, pipework
causes leaks and short life"]
    A --> D["Biofouling & Legionella
bacteria and algae
fouling + health risk"]
    B --> E["Control with:
LSI monitoring
Acid dosing / Antiscalant
Blowdown / COC control"]
    C --> F["Control with:
pH in correct range (7.0–8.5)
Corrosion inhibitor
Corrosion-resistant materials"]
    D --> G["Control with:
Oxidizing biocide (Cl/Br)
Non-oxidizing biocide
Drift eliminator
ASHRAE 188 WMP"]

5.1 Scaling — The Langelier Saturation Index (LSI)

LSI quantifies whether water tends to scale or corrode:

LSI = pH − pHs

where pHs is calculated from TDS, calcium hardness (Ca²⁺), total alkalinity, and temperature.

Scaling control measures:

• Blowdown: the most basic tool — discharge a controlled fraction of circulating water and replace with makeup, reducing COC.
• Acid dosing: H₂SO₄ or HCl reduces pH and alkalinity, driving pHs lower and thus lowering LSI. Requires a dosing pump with automatic pH controller.
• Antiscalant: synthetic polymers (phosphonates, polyacrylates) prevent CaCO3 crystal growth on fill surfaces and heat-exchanger tubes, enabling operation at higher COC without scale.

5.2 Corrosion — pH Balance and Inhibitors

• Maintain circulating-water pH in the 7.0–8.5 range (optimal for both scale and corrosion management).
• Use a corrosion inhibitor matched to the system's metallurgy: molybdate, silicate, zinc-based, or azole types for copper.
• Prevent chloride ion accumulation (aggressive toward steel and stainless) — manage COC and blowdown accordingly.
• Install corrosion coupons and inspect every 3 months to measure actual corrosion rates and validate inhibitor effectiveness.

5.3 Biofouling and Legionella — ASHRAE Standard 188

This is a life-safety issue, not merely a performance issue.

Why Legionella is dangerous in cooling towers:

• Circulating water at 25–45°C is the ideal growth range for Legionella pneumophila.
• Drift from the tower creates fine aerosols that can travel hundreds of metres and be inhaled directly into the lungs.
• Legionnaires' disease (severe pneumonia) carries a case-fatality rate of 5–30% in at-risk populations.
• Risk amplifiers include dead legs and stagnation in pipework, biofilm accumulation on fouled fill, organic loading in the water, and fluctuating temperatures.

ASHRAE Standard 188 requirements:

Biocides used for Legionella control:

Effective biocide strategy: alternate oxidizing and non-oxidizing biocides to prevent resistance and destroy biofilm together. Monitor chlorine residual continuously. Perform shock disinfection (hyperchlorination) before startup after a shutdown or when Legionella counts exceed threshold.

6. Fill Media — Select by Water Quality, Not Efficiency Alone

Fill is the medium that creates air-water contact area. It is central to thermal performance but also the primary site of fouling and Legionella colonisation.

For Thai plants using tap or borehole makeup water: film fill works well if a proper water treatment programme is maintained, but fill should be inspected every 6–12 months for biofilm and scale. If makeup water Total Suspended Solids (TSS) exceed 25 mg/L, consider pre-filtration or splash fill.

7. Materials — Match to Operating Environment

8. Acceptance Testing and CTI Certification

CTI (Cooling Technology Institute) is the global reference body for cooling-tower standards:

• CTI ATC-105: Acceptance Test Code defining the standard method for measuring thermal performance in the field — inlet/outlet temperature, wet-bulb temperature, flow rate, and heat rejection under controlled conditions.
• CTI STD-201: Certification standard for pre-delivery thermal performance — manufacturers with CTI-certified products publish reliable performance data that can be independently verified.

Best practices:

• Specify a thermal performance test per CTI ATC-105 on-site within 30–90 days of commissioning in the purchase contract.
• Require CTI-certified performance curves showing range/approach at WBT 28°C — not just a European rated condition.
• Include penalty provisions if the tower fails to meet contracted thermal performance.

9. Checklist to Ask Your Contractor / Supplier Before Signing

10. What Buyers Most Often Overlook

Test the wet-bulb at the actual installation point: do not assume WBT from regional tables alone. Some industrial sites — particularly those with nearby evaporative sources or enclosed courtyards — may experience local wet-bulb values higher than the area average. Verify with an on-site sling psychrometer or electronic WBT instrument before specifying.

Drift eliminators are a safety investment, not a cost: cutting costs on drift eliminator quality is never justified. Drift carrying Legionella that reaches people in the vicinity creates both a public-health risk and direct plant liability. A modern high-efficiency drift eliminator (≤ 0.002%) is inexpensive relative to the tower capital cost and the consequences of an outbreak.

Water treatment is an investment, not a cost: annual chemical treatment typically costs 0.5–2% of tower capital per year. Without a proper programme, fill fouls within 2–3 years, basin corrodes prematurely, and the entire tower may need replacement well before its design life — far more expensive than the treatment programme it replaced.

Cathodic protection for metallic components in aggressive water: cooling towers with hot-dip galvanised steel or carbon-steel basins in high-chloride or aggressive-water environments should consider cathodic protection for structural metallic components. See Cathodic Protection for Cooling Towers and Data Centers for a detailed treatment.

Test for Legionella before and after commissioning: do not wait for an outbreak. Establish a baseline Legionella test before the system enters service, then monitor on the schedule defined in the WMP per ASHRAE 188. Startup after a prolonged shutdown is a period of particularly elevated risk and warrants shock disinfection followed by confirmatory testing.

Basin drainage design matters: basins with dead zones or incomplete drainage allow sediment to accumulate and become a reservoir for Legionella colonisation. Proper basin slope toward a drain and correct drain-valve sizing reduce risk significantly and should be reviewed at the design stage.

Performance degrades as fill fouls: thermal performance drops noticeably when fill is as little as 20–30% blocked, and chiller COP follows. Inspect fill every 6 months, clean when fouled, and schedule fill replacement as part of the life-cycle maintenance plan.

Consult the Engineering Team

Correctly specifying a cooling tower starts from the actual heat duty and the real wet-bulb temperature in Thailand — not a European catalog — together with a water treatment programme that covers everything from scaling and corrosion through Legionella management. Send us the heat duty (kW), target T_hot_in / T_cold_out, makeup water analysis results, and any compliance requirements, and the engineering team will help select the type and size, design the water treatment programme, and review the contractor's specification before you sign.

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