Why Airflow Is Reduced in Air-Cooled Heat Exchangers?
Understanding the interaction between fan performance, system resistance, mechanical condition and airflow distribution.
Airflow is one of the most important factors determining the thermal performance of an Air-Cooled Heat Exchanger (ACHE). When airflow through the finned-tube bundles decreases, heat transfer is directly affected — potentially resulting in higher process outlet temperatures, reduced cooling capacity and, in some cases, production limitations.
Yet reduced airflow is rarely a problem with just one possible cause.
The fan may be operating. The motor may be running at full speed. The equipment may even appear to be in acceptable condition. But that does not necessarily mean the required airflow is actually passing through the heat exchanger bundle.
Understanding why airflow is reduced requires looking at the complete airside system — from ambient air conditions and fan performance to bundle resistance, mechanical condition and airflow distribution.
The Airflow Balance
An air-cooled heat exchanger relies on its fan system to generate sufficient airflow to overcome the resistance of the complete air path.
In simplified terms, airflow is determined by the interaction between two characteristics:
- Fan performance — the pressure and airflow the fan is capable of generating.
- System resistance — the resistance the air encounters as it moves through the fan inlet, fan ring, plenum and finned-tube bundle.
The actual operating point occurs where these two characteristics meet.
This means airflow can decrease for two fundamentally different reasons:
The fan is producing less aerodynamic performance than expected, or the resistance of the system has increased.
In practice, both can occur simultaneously.
1. Finned-Tube Bundle Fouling
External fouling is one of the most common causes of reduced airflow through an air-cooled heat exchanger.
Over time, contaminants can accumulate between and on the fins, including dust, sand, pollen, fibres, salts and industrial airborne deposits.
As the bundle becomes contaminated, the resistance to airflow increases.
The fan must therefore operate against a higher static pressure differential, potentially moving the operating point toward a lower airflow condition.
Importantly, fouling is not always uniformly distributed.
Certain areas of the bundle may become significantly more restricted than others, creating localised airflow limitations that are difficult to identify from visual inspection alone.
This is why measuring only the average airflow can sometimes hide the true condition of the exchanger.
2. Fan Blade Pitch and Aerodynamic Performance
The airflow delivered by an axial fan depends strongly on blade geometry and blade pitch.
If the actual blade pitch differs from the design or intended setting, the fan may deliver significantly less airflow than expected.
Possible causes include incorrect initial adjustment, inconsistent blade angles between individual blades, changes following maintenance or incorrect reassembly.
Even relatively small deviations can influence aerodynamic performance.
However, increasing blade pitch should not automatically be considered the solution to an airflow problem.
A higher pitch generally increases the aerodynamic load on the fan and may increase motor power consumption. Any adjustment should therefore consider the complete fan operating condition, including motor load, vibration and system resistance.
3. Incorrect Fan Tip Clearance
The clearance between the fan blade tips and the fan ring has an important influence on fan efficiency.
When the tip clearance becomes excessive, air can recirculate around the blade tips instead of contributing effectively to useful airflow through the exchanger.
This reduces aerodynamic efficiency.
The issue can be caused by incorrect installation, fan ring geometry, mechanical deformation or dimensional changes over time.
Fan tip clearance should therefore be evaluated as part of the overall mechanical and aerodynamic condition of the fan system.
4. Fan Ring and Inlet Conditions
Axial fans are sensitive to the way air approaches the fan.
Poor inlet conditions can create turbulence and uneven velocity profiles before the air reaches the fan blades. This can reduce effective fan performance and contribute to unstable airflow.
Potential causes include nearby structural obstructions, poor fan inlet geometry and external wind conditions.
The fan may therefore be mechanically healthy while still operating under aerodynamic conditions that prevent it from achieving its expected performance.
5. Airflow Maldistribution and Bypass
Average airflow across an exchanger can appear acceptable while parts of the bundle receive insufficient cooling.
Airflow maldistribution may result from uneven fan performance, missing or damaged seals, gaps around the bundle, localised fouling, poor plenum geometry, wind effects or one fan being unavailable.
Air follows the path of least resistance.
Where bypass routes are present, part of the airflow generated by the fan may avoid the effective heat-transfer surface.
The total airflow may therefore appear reasonable while the effective airflow through the finned surface is significantly lower.
For performance assessment purposes, airflow distribution can be just as important as total airflow.
6. Hot Air Recirculation
Not every apparent airflow problem is caused by insufficient fan capacity.
In some installations, hot exhaust air leaving the top of the air cooler can be drawn back toward the air inlet.
This phenomenon — known as Hot Air Recirculation (HAR) — increases the temperature of the air entering the exchanger.
The fan may still move the expected volume of air, but the cooling potential of that air has been reduced because the inlet temperature is higher.
Wind direction, adjacent equipment, exchanger layout and interaction between neighbouring air cooler banks can all influence recirculation behaviour.
This distinction is important:
Reduced cooling performance does not automatically mean reduced airflow.
Airflow and inlet air temperature should therefore be evaluated together.
7. Mechanical Condition of the Fan System
The mechanical condition of the drive system can also affect airflow.
Potential issues include incorrect fan speed, belt slippage in belt-driven systems, gearbox performance issues, coupling problems, motor limitations or incorrect rotational speed.
A fan that is rotating does not necessarily mean it is rotating at the required speed or delivering its intended aerodynamic performance.
Actual operating conditions should therefore be verified rather than assumed.
8. Environmental and Operating Conditions
Air-cooled heat exchangers operate in an uncontrolled atmospheric environment.
Their performance can therefore be influenced by ambient temperature, wind speed and direction, surrounding structures and interaction with adjacent equipment.
Strong crosswinds, for example, can disturb fan inlet conditions or contribute to hot air recirculation.
At large installations with multiple ACHE bays, the aerodynamic interaction between units can become particularly important.
The performance of an individual exchanger cannot always be understood by looking at that exchanger in isolation.
Finding the Real Constraint.
When an air-cooled heat exchanger is underperforming, increasing fan speed or adjusting blade pitch may appear to be an obvious solution.
But without understanding the actual constraint, this can treat the symptom rather than the cause.
If bundle resistance has increased due to fouling, the solution may be cleaning.
If airflow is bypassing the bundle, sealing improvements may provide greater benefit.
If the fan is operating inefficiently, mechanical or aerodynamic optimisation may be required.
If hot air is recirculating, increasing airflow alone may provide limited improvement.
The first question should therefore not be:
“How do we increase airflow?”
It should be:
“What is preventing the air cooler from achieving its required airflow and thermal performance?”
Measure. Analyse. Improve.
A proper airside performance assessment should combine field measurements with engineering analysis.
This may include airflow measurements across the bundle, airside pressure-drop measurements, ambient and inlet air temperatures, fan rotational speed, blade pitch, fan tip clearance, motor load, bundle condition and observations of wind and recirculation behaviour.
By combining these measurements, it becomes possible to distinguish between fan limitations, increased system resistance, airflow maldistribution and environmental effects.
Only then can the improvement potential be properly quantified.
At Enthalpex, we believe that air cooler performance should be improved based on measured data and engineering analysis — not assumptions.
Because when airflow is reduced, the real opportunity is not simply to move more air.
It is to understand where the performance is being lost — and recover it.
MEASURE. ANALYSE. IMPROVE.
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