When comparing tunnel ventilation fans, most people focus on two key specifications: airflow and energy efficiency. While both are important, they do not tell the complete story. A fan may deliver impressive airflow figures and consume relatively little power under laboratory conditions, but how does it perform when the pressure in a livestock building increases? This is where the Air Flow Ratio (AFR) becomes an important performance indicator.
For poultry and pig producers, the AFR provides valuable insight into how well a fan maintains its airflow under real-world operating conditions. As modern livestock houses continue to incorporate more equipment and airflow restrictions, understanding AFR is becoming increasingly important when selecting tunnel ventilation fans.
The Air Flow Ratio shows how well a fan maintains its airflow capacity as static pressure increases. Traditionally, the AFR is calculated by comparing a fan's airflow at a higher static pressure (0.20”) to its airflow at a lower static pressure (0.05”). The higher the ratio, the better the fan is able to maintain airflow as resistance in the ventilation system increases.
Formula Air Flow Ratio = cfm at (0,20” / cfm at (0,05”)
In simple terms, the AFR indicates how resistant a fan is to performance losses caused by increasing static pressure. A fan with a high AFR retains a larger percentage of its airflow as static pressure rises. A fan with a low AFR experiences a greater reduction in airflow under the same conditions.
Tunnel ventilation systems rarely operate under ideal conditions. Over the years, livestock houses have become more sophisticated and often include additional components such as:
Evaporative cooling pads
Light traps
Air inlets
Shutters
While these technologies can provide important benefits, they also increase resistance to airflow. As a result, exhaust fans are required to operate against higher static pressures than in the past. The impact of dust and dirt accumulation should not be underestimated either, as it can further increase airflow resistance.
Static pressure is not only influenced by components inside the building. Wind conditions can also affect fan performance. Strong winds blowing against the discharge side of a fan increase the resistance to airflow, making the fan operate at a higher pressure point on its performance curve.
In these situations, airflow measured at a single pressure point may not accurately predict real-world performance. A fan with a high AFR is better able to maintain airflow when system pressure increases. This helps ensure that the designed ventilation rate and air speed can still be achieved when conditions become more demanding.
Tunnel ventilation is designed to create high speed air movement that results in a windchill effect cooling down the animals beside removing heat, moisture, dust, and gases from the building. When fan performance drops significantly under higher pressure, the resulting reduction in airflow can impact air speed and overall ventilation effectiveness. A fan with a high AFR provides greater confidence that the required airflow will still be available when it is needed most. This is one reason why the AFR can be considered as an important indicator of ventilation reliability. It helps reveal how a fan is likely to perform beyond standard test conditions.
The Air Flow Ratio should not be viewed in isolation. A fan with an extremely high AFR but limited airflow may not be the best choice. Similarly, a fan with excellent energy efficiency but poor pressure performance may struggle in demanding tunnel ventilation applications.
The best tunnel ventilation fans achieve a balance between three important characteristics:
High airflow capacity
Good energy efficiency
High Air Flow Ratio
Considering all three factors provides a more complete picture of fan performance and can help producers, installers, and consultants make better-informed decisions.
The importance of AFR is reflected in independent fan testing results. For example, the Multifan Fiberglass Cone Fan has achieved an Air Flow Ratio of 0.86, making it one of the highest-performing single-phase 230 V tunnel fans tested by BESS Lab. View the research from the University of Georgia here:
The latest Multifan Venturon tunnel fans also achieve AFR values up to 0.85, demonstrating their ability to maintain airflow under increasing static pressure conditions.
These values are significantly higher than those achieved by many conventional tunnel ventilation fans and illustrate the importance of the right motor / impeller combination combined with an aerodynamic fan design when pressure resistance becomes a factor.
As ventilation systems become more advanced and airflow restrictions increase, fan performance under pressure is becoming more important than ever. The Air Flow Ratio helps quantify how well a fan maintains airflow when static pressure rises. A high AFR indicates that a fan is less likely to experience significant performance losses, providing greater confidence that the required ventilation rate can be maintained under real operating conditions. When selecting tunnel ventilation fans, it pays to look beyond airflow and energy consumption alone. Considering Air Flow Ratio alongside these specifications provides a more complete understanding of fan performance and long-term reliability.
Discover how Multifan solutions improve poultry house performance. Visit our poultry webpage.