By Eamonn Ryan
Explore how modern EC fan arrays are transforming cooling tower retrofits in this technical white paper from Ziehl-Abegg, featured on HVACInformed. com. Learn how innovative multi-fan installations deliver enhanced energy efficiency, improved safety and operational redundancy, while providing rapid return on investment – all without the need for costly full tower replacements. A detailed case study from Raleigh, North Carolina, illustrates the real-world impact of this cutting-edge technology.
Executive summary
Cooling towers are critical infrastructure in large-scale HVAC systems, yet the fans that drive their performance often fail long before the towers themselves. Traditional retrofits involve replacing large single fans, which can be costly, time- intensive and unsafe. A new approach – retrofitting with arrays of electronically commutated (EC) fans – offers significant advantages in energy efficiency, redundancy, safety and cost- effectiveness.
Background and challenges
Cooling tower structures are designed for durability, with lifespans of 30–50 years if corrosion is controlled. However, fans typically last only 5–10 years, creating a recurring point of failure.
Traditionally, replacing a failed large fan requires:
- Long lead times for replacement motors and gearboxes
- Significant safety risks in handling heavy impellers (up to 6m in diameter)
- High labour demands, often requiring large teams and cranes
These challenges are amplified in critical facilities such as government campuses, hospitals and data centres, where downtime is unacceptable.
Technology overview: EC fan arrays
Ziehl-Abegg’s EC fan technology introduces a modular retrofit solution that replaces a single large AC fan with multiple smaller EC-driven axial fans.
Key features of EC fans:
- High efficiency: EC motors supply only the precise energy required for rotation
- Variable speed control: Integrated electronics eliminate the need for external VFDs
- Modular installation: Lightweight fans (about 34kg) can be installed by one or two workers
- Safety and redundancy: Failure of one fan does not halt cooling tower operation
- Scalability: Arrays can be sized to meet airflow and static pressure requirements
This approach, already standardised in air handling units (AUs) for over a decade, is now gaining traction in cooling towers.
Case study
In April 2025, a central cooling tower fan failed at a 2-hectare government office campus in Raleigh, NC. The site depended on two 300-ton cooling tower cells; the failure immediately cut cooling capacity by 50% ahead of the peak summer season.
Traditionally, replacing a failed cooling tower fan can present major challenges. In this case, the government facility faced a 10-week lead time to source a replacement motor, while the installation itself carried significant safety risks due to the size and weight of the large fan. To avoid extended downtime, the facility chose an alternative approach – a retrofit with electronically commutated (EC) fans.
Instead of reinstalling a single 75-horsepower AC fan, the solution deployed 21 Ziehl-Abegg ZAplus axial EC fans, each measuring 910mm in diameter. The retrofit was completed in just four weeks, less than half the time of the traditional replacement path. Installation was further streamlined by the use of modular kits, which included pre-engineered wiring harnesses and control panels, reducing complexity and labour requirements.
Results and benefits
The retrofit delivered immediate and measurable advantages across multiple performance areas.
Energy efficiency was one of the most striking outcomes. During peak summer demand, the EC fan array demonstrated a 29% reduction in energy use compared to the twin cooling tower cell that still operated with a conventional large fan. Even greater savings are expected during moderate spring and autumn conditions, when the variable-speed capability of EC fans allows them to run at lower, more efficient speeds.
Under these conditions, total energy savings could reach as high as 50%. At full operation, the 21 Ziehl-Abegg ZAplus fans achieved an airflow of 300 000 CFM at 1.9cm of static pressure, proving the system’s ability to meet demanding cooling requirements.
Reliability and redundancy were also greatly enhanced. Unlike a single large fan that creates a single point of failure, a multi-fan array ensures continuous operation even if one unit goes offline. Remaining fans can automatically compensate by increasing speed, maintaining stable airflow without disrupting HVAC performance. This built-in resilience makes EC fan arrays particularly valuable in mission-critical environments such as hospitals, government facilities and data centres.
Safety in installation and maintenance was another clear benefit. The smaller, lightweight fans – each weighing only a fraction of a traditional fan – significantly reduced the risks associated with lifting and handling heavy equipment. Installation required fewer workers, minimised downtime and created safer working conditions. Over the long term, these improvements help protect skilled labour resources and reduce the likelihood of workplace injuries.
Cost-effectiveness and ROI further strengthened the case for retrofitting. The retrofit cost represented only about 10% of what a full cooling tower replacement would require. Based on energy savings and reduced downtime, the return on investment is projected at three to four years, closely mirroring the two- to three-year ROI already proven in mature AHU retrofits.
Market potential and future outlook
The Raleigh project is just one example of a much larger opportunity. In NewYork City alone, the cooling tower retrofit market is estimated at USD140-million, with nationwide potential extending across nearly every large facility that relies on cooling towers. Growing electricity costs, tightening energy efficiency regulations and an ever-increasing demand for reliable system performance are all driving adoption of this new approach.
Retrofitting cooling towers with EC fan arrays provides a viable alternative to replacing single large AC fans. The Raleigh case study demonstrates measurable energy efficiency gains, safer installations, enhanced redundancy and rapid ROI.
As building operators face mounting pressure to reduce energy consumption and improve reliability, EC fan arrays are poised to become a new standard in cooling tower modernisation. RACA
Source: HVACInformed.com
