In industrial cooling systems, engineers spend months optimizing layouts, pump ratings, fill packs, and drift eliminators—yet often overlook one of the most influential components in the entire airflow equation: the FRP fan stack, also referred to as a fan ring or fan cylinder.
At the heart of every cooling tower or air-cooled system, the fan stack quietly governs a complex balance of aerodynamics, energy efficiency, noise control, and environmental resilience. But there’s a crucial detail that’s frequently underestimated: the surface finish of the fan stack. More than just a cosmetic consideration, surface finish plays a central role in velocity recovery, power savings, system longevity, and real-world performance.
In this article, we’ll unpack the technical implications of surface finishes, explain why assumptions like 100% velocity recovery can be misleading, and guide you through choosing the right finish for your application—drawing on insights from real-world installations, aerodynamic principles, and years of field data.
Understanding the Role of the FRP Fan Stack
A fan stack is the structure surrounding the axial fan in a cooling tower or air-cooled condenser. Typically cylindrical or conical and made from Fiber Reinforced Plastic (FRP), it performs several critical aerodynamic and protective functions:
- Directs airflow to minimize turbulence and recirculation.
- Recovers velocity by converting kinetic energy into static pressure.
- Reduces fan noise through smoother flow transitions.
- Protects the fan system from UV radiation, weather, and corrosion.
While FRP’s inherent properties—lightweight, corrosion-resistant, durable—make it an ideal material, these advantages can be compromised if the internal and external surfaces are not treated properly. Here, surface finish becomes a performance-differentiating factor.
The Myth of 100% Velocity Recovery
One of the most common missteps in fan system design is assuming perfect velocity recovery. Engineers often model systems under the assumption that the fan stack will recover all kinetic energy from the high-velocity air expelled by the fan, converting it into usable static pressure.
But in reality, this is rarely achieved—and surface roughness is often to blame.
Why Surface Finish Matters:
- A rough interior creates air friction and turbulence, leading to drag.
- Turbulence disrupts smooth laminar airflow, reducing velocity recovery by up to 10–15%.
- This gap between design and real-world performance results in increased fan power consumption, reduced efficiency, and in some cases, early wear and tear.
A well-finished surface, on the other hand, acts like a polished air tunnel, helping air maintain momentum while minimizing energy loss.
The Three Key Types of Surface Finishes
Surface finishes typically fall into one of three categories, each offering distinct advantages depending on operational priorities and environmental conditions:
1. Inside Finish – Optimizing for Airflow Efficiency
An inside-finish fan stack features a highly polished, mirror-smooth interior surface. The smoother the internal flow path, the less resistance air faces as it passes through and exits the stack.
Benefits:
- Enhances velocity recovery by up to 10%.
- Lowers fan power requirements.
- Reduces operational noise.
- Improves laminar flow, especially valuable in high-static-pressure systems.
At Maya Fan Air Engineering, even our inside-finish stacks are provided with a UV-protective exterior as a default upgrade, ensuring they remain resilient to environmental exposure without compromising the aerodynamic interior.
Ideal for:
- Indoor or semi-sheltered cooling towers.
- Systems prioritizing energy savings and quiet operation.
- Data centers, HVAC, and controlled-environment installations.
2. Outside Finish – Shielding Against Environmental Damage
This finish focuses on the external durability of the fan stack, with coatings or gel coats that resist UV radiation, acidic gases, salt-laden air, and thermal fluctuations.
Benefits:
- Protects against material degradation, delamination, and structural fatigue.
- Extends the lifespan of the stack, especially in chemically aggressive or outdoor environments.
- Retains dimensional stability despite harsh exposure.
The interior surface in outside-finish stacks is typically left untreated or minimally polished, which may result in slight airflow inefficiencies.
Trade-offs:
- Reduced velocity recovery, potentially by 5–10%, due to unoptimized airflow.
- Slight increase in fan energy consumption over time.
Ideal for:
- Outdoor cooling towers in coastal regions, chemical processing plants, or mining operations.
- Applications where structural integrity trumps minor efficiency gains.
3. Both-Side Finish – The Premium, Performance-First Solution
A both-side finish combines the aerodynamic interior of the inside finish with the weather-hardened exterior of the outside finish. This is the ultimate configuration for mission-critical operations where system failure or inefficiency is not an option.
Benefits:
- Delivers maximum velocity recovery, approaching theoretical 100% recovery values.
- Maintains durability in the most aggressive environments.
- Reduces noise, power draw, and maintenance needs.
Trade-offs:
- Higher manufacturing cost due to additional processing steps.
- Longer production timelines due to the dual-finish application and curing process.
Ideal for:
- Power generation plants.
- Petrochemical complexes.
- Infrastructure with tight uptime and performance KPIs.
The Science: How Surface Finish Shapes Velocity Recovery
To understand the physics behind these benefits, consider Bernoulli’s principle applied to axial fans: as air exits the fan at high velocity, its kinetic energy can be partially converted to static pressure if the flow is decelerated smoothly.
A conical or bell-mouth shaped stack helps this transition, but surface smoothness ensures minimal energy loss during deceleration. The rougher the surface, the more air becomes turbulent—wasting the opportunity to reclaim energy.
In practical terms:
- A 10% drop in velocity recovery from turbulence could result in a 6–8% increase in fan motor load.
- Over the life of the equipment, this results in significant energy costs, especially in continuous-duty industrial environments.
Computational Fluid Dynamics (CFD) simulations—such as those employed at Maya Fan—demonstrate this relationship clearly, showing how minor imperfections in surface smoothness create turbulence zones that amplify inefficiencies.
Real-World Application: A Case Study from Industry
At a large petrochemical facility in Western India, two nearly identical cooling towers showed dramatically different performance results over a three-year span.
- Tower A: Installed with standard FRP fan stacks (basic outside finish), exposed to corrosive vapors and UV. By year two, exterior cracks and interior buildup began disrupting airflow, reducing efficiency by over 8%.
- Tower B: Fitted with a both-side finish FRP stack from the outset. Velocity recovery held steady within 2% of design projections, and maintenance frequency was halved.
Following a plant-wide audit, Tower A was retrofitted with dual-finish stacks, resulting in:
- A 5% reduction in fan power consumption.
- Lower noise levels, improving workplace comfort.
- ROI on the retrofit in under 18 months through energy savings alone.
Selecting the Right Finish: A Strategic Framework
Choosing a surface finish isn’t just a materials decision—it’s a strategic engineering call. Here’s how to align finish types with your system’s demands:
| Priority | Recommended Finish |
| Maximize energy efficiency | Inside Finish or Both-Side Finish |
| Handle harsh environmental conditions | Outside Finish or Both-Side Finish |
| Balance performance and longevity | Both-Side Finish |
| Budget-sensitive upgrades | Inside Finish with UV-protection |
| Mission-critical uptime | Both-Side Finish (premium) |
Also consider:
- System design: High-velocity fans benefit more from conical stacks with smooth interiors.
- Maintenance cycles: Ease of cleaning is enhanced with smoother surfaces.
- Installation environment: Corrosive gases or particulate matter may demand reinforced finishes.
Beyond Finish: Additional Factors That Matter
Surface finish, though vital, is just one part of optimal fan stack design. Others include:
- Stack Geometry: Conical vs cylindrical profiles affect pressure recovery and recirculation patterns.
- Material Quality: High-grade FRP with tightly controlled resin ratios enhances structural integrity.
- Joint Sealing and Mounting: Poor installation can negate surface finish advantages.
- Inspection & Maintenance: Even polished interiors degrade with dust, algae, or scaling buildup—periodic cleaning is essential.
Final Thoughts: Engineering for Real-World Performance
In the pursuit of cooling tower efficiency, assumptions are the enemy of reliability. Assuming 100% velocity recovery without validating surface conditions leads to overestimated performance and underestimated energy costs.
Surface finish, though rarely discussed outside technical circles, remains a high-leverage design parameter. A modest investment in finishing quality often yields disproportionately large returns in efficiency, durability, and operational savings.
