Drift Loss Estimation

Understanding Drift Loss in Cooling Towers

Drift loss is the loss of liquid water droplets carried away by the air stream exiting a cooling tower. Unlike evaporation (which is water vapor), drift consists of small liquid droplets that escape the tower. While drift is typically a small percentage of total water flow, it can have significant environmental and economic impacts over time, especially when it carries water treatment chemicals.

Understanding and minimizing drift loss is important for water conservation, environmental protection, and cost management. Modern cooling towers use drift eliminators to minimize drift, achieving rates as low as 0.005% of flow. Older towers without drift eliminators may have drift rates 10-20 times higher.

Drift vs. Evaporation

It's important to distinguish drift from evaporation:

• Evaporation: Water changes to vapor and is absorbed into the air. This is the primary cooling mechanism and is intentional.
• Drift: Liquid water droplets are carried away by air. This is unintentional and should be minimized.

Evaporation is typically 1-2% of flow (depending on range), while drift is typically 0.01-0.1% of flow. Evaporation is much larger than drift, but drift has different implications because it carries liquid water and treatment chemicals.

Drift Loss Calculation

Drift loss is calculated as:

For example, 1,000 GPM flow with 0.01% drift:

Typical Drift Rates

Modern Mechanical Draft Towers

Well-designed modern towers with efficient drift eliminators:

• Excellent: <0.005% (less than 0.005% of flow)
• Good: 0.005-0.01%
• Typical: 0.01-0.02%

Natural Draft Towers

Natural draft towers typically have slightly higher drift rates:

• Typical: 0.01-0.02%
• May be higher depending on design and drift eliminator efficiency

Older Towers

Older towers without drift eliminators or with poor eliminators:

• Without eliminators: 0.1-0.2% of flow
• With poor eliminators: 0.05-0.1%

These rates are 10-20 times higher than modern towers, representing significant water and chemical loss.

Drift Eliminators

Drift eliminators are devices installed at the air outlet of cooling towers to capture water droplets before they exit. They work by:

• Creating multiple direction changes in the air path
• Using surface tension to capture droplets
• Draining captured water back into the tower

Modern drift eliminators can achieve 99.99% efficiency, reducing drift to less than 0.01% of flow. They're essential components of modern cooling towers.

Types: Various designs exist including chevron-type, wave-type, and cellular eliminators. Selection depends on tower design, airflow, and performance requirements.

Environmental and Economic Impacts

Water Conservation

While drift is small percentage-wise, it adds up over time. A 1,000 GPM tower with 0.1% drift loses 52,560 gallons per year. Reducing drift to 0.01% saves 47,304 gallons annually.

Chemical Loss

Drift carries water treatment chemicals (biocides, scale inhibitors, corrosion inhibitors) out of the system. This represents:

• Chemical waste and cost
• Environmental discharge of treatment chemicals
• Need for additional chemical feed to maintain treatment levels

Environmental Regulations

Many jurisdictions regulate drift discharge, especially when it contains treatment chemicals. Minimizing drift helps ensure compliance and reduces environmental impact.

Practical Applications

Water Balance Calculations

Drift is one component of the cooling tower water balance: Makeup = Evaporation + Blowdown + Drift. Accurate drift estimation is needed for proper water balance analysis.

Chemical Feed Calculations

Chemical feed rates must account for drift loss. Chemicals lost to drift must be replaced to maintain treatment levels.

Environmental Reporting

Facilities may need to report drift discharge, especially when it contains regulated chemicals. Accurate drift estimation supports compliance reporting.

Upgrade Planning

Estimating drift helps evaluate the benefits of upgrading drift eliminators. Water and chemical savings can justify upgrade costs.

Real-World Examples

Example 1: Modern Tower

1,000 GPM flow, 0.01% drift (modern tower with good eliminators):

Example 2: Older Tower

Same 1,000 GPM flow, 0.1% drift (older tower without eliminators):

Example 3: Large System

5,000 GPM flow, 0.01% drift:

Minimizing Drift

Drift Eliminators

Install and maintain efficient drift eliminators. Modern eliminators can achieve <0.01% drift. Regular inspection and cleaning maintain performance.

Tower Design

Proper tower design, including airflow patterns and water distribution, minimizes drift. Work with tower manufacturers to optimize design.

Airflow Control

Excessive airflow can increase drift. Proper fan control and operation maintain performance while minimizing drift.

Maintenance

Regular maintenance of drift eliminators ensures they function properly. Fouled or damaged eliminators have reduced effectiveness.

Important Considerations

Measurement Difficulty

Direct measurement of drift is difficult. It's typically estimated from water balance calculations or manufacturer data. Some methods use collection devices, but they're not always practical.

Variability

Drift rates vary with operating conditions: airflow, water flow, wind conditions, and eliminator condition. Use typical or worst-case values for planning.

Manufacturer Data

Tower manufacturers provide drift rates for their equipment. Use manufacturer data when available, as it's more accurate than general estimates.

Water Balance

In water balance calculations, drift is often the smallest component (after evaporation and blowdown). However, it's still important for accurate analysis.

Tips for Using This Calculator

• Enter circulating flow rate (not makeup flow)
• Select tower type for typical drift rate, or enter custom percentage
• Modern towers with good eliminators: 0.005-0.01%
• Older towers without eliminators: 0.1-0.2%
• Results show drift rate, hourly, daily, and annual losses
• Use for water balance calculations and chemical feed planning
• Consider upgrading drift eliminators if drift is high
• Remember: drift is separate from evaporation
• Always verify critical calculations independently, especially for environmental reporting

Common Pitfalls

• Using legacy drift assumptions. Cooling towers built before ~1990 had drift rates of 0.1–0.2% of circulating flow. Modern high-efficiency mist eliminators (cellular PVC drift eliminators) deliver 0.001–0.005%. Applying a 0.1% assumption to a new tower overstates water loss and Legionella risk by 20×. Check manufacturer certification (CTI STD-140 or equivalent).
• Forgetting drift in dissolved-solids mass balance. Drift droplets contain the same dissolved solids as the tower bulk water, so drift removes TDS just like blowdown. For towers with high drift (old or damaged mist eliminators), effective blowdown is higher than measured — cycles of concentration get capped even when you reduce mechanical blowdown.
• Treating drift as an "aerosol" problem only. Drift droplets are large (100+ microns) and fall out locally — they aren't the same as the respirable aerosols that carry Legionella. Legionella risk is tied to fine mist that escapes past the drift eliminator. ASHRAE Standard 188 focuses on biological control and building water management, not just drift percentage.
• Natural-draft towers ignored because "no fan." Hyperbolic power-plant-style natural-draft towers still produce drift (often more than mechanical-draft because of the buoyancy-driven airflow). Their drift just ends up farther downwind. Include drift in water balance for natural-draft towers too.
• Wind drift when siting near sensitive areas. Modern mist eliminators handle design airflow, but high winds past the tower can entrain droplets past the eliminator. Siting a tower upwind of parking (salt/chemical deposits on cars) or air intakes (humidity load, freeze-thaw on the building) requires extra attention to drift rates and wind direction.

Frequently Asked Questions

What is drift in a cooling tower? Drift is the fine water droplets that escape out of the tower airstream without evaporating. It's distinct from evaporation (water changing to vapor) and blowdown (bulk water discharged). Drift carries dissolved solids and chemical treatment out with the droplets.

What's a typical drift rate? Modern towers with high-efficiency mist eliminators: 0.001–0.005% of circulating flow. Older towers or those with degraded mist eliminators: 0.02–0.2%. California Title 24 and ASHRAE 90.1 effectively require ≤0.002% for new installations.

Is drift the same as plume? No. Plume is the visible vapor cloud above a tower — condensed water vapor formed when warm humid exhaust meets cooler ambient air. Plume is pure water (no dissolved solids), while drift is droplets from the basin water (with TDS and treatment chemicals). Plume abatement and drift control are separate design concerns.

Does drift carry Legionella risk? Yes — any aerosolized cooling tower water can carry Legionella bacteria if the system isn't properly treated. Drift contributes, but fine aerosols generated in the fill are the main respirable vector. Modern drift eliminators plus ASHRAE 188 water management practically eliminate the airborne risk.

How much water does drift waste? For a 500-ton tower running 1500 GPM: at 0.001% drift = 0.015 GPM = ~7900 gal/yr. At 0.1% drift = 1.5 GPM = ~790,000 gal/yr. The difference between old and new drift eliminators easily justifies a retrofit in water-constrained regions.

Related Calculators

Drift loss is one component of total cooling tower water balance:

• Cooling Tower Approach & Range — tower performance drives evaporation, which scales with heat rejection.
• Blowdown Rate Calculator — blowdown + drift together control TDS concentration and cycles of concentration.
• Chilled Water Flow Calculator — condenser-side flow sets the tower circulation rate and therefore absolute drift GPM.
• Cooling Capacity Converter — convert heat rejection ratings between units when comparing tower specs.
• Psychrometric Calculator — understand plume formation and ambient wet-bulb effects on tower operation.
• Refrigeration Cycle Calculator — chiller heat rejection (what the tower dissipates) = evaporator load + compressor power.

Disclaimer

This calculator is provided for educational and informational purposes only. While we strive for accuracy, users should verify all calculations independently, especially for critical applications. Drift rates vary with tower design, condition, and operating conditions. Use manufacturer data when available for more accurate estimates. We are not responsible for any errors, omissions, or damages arising from the use of this calculator.