How Does the Resistance of a Separatorless HEPA Filter Change at Different Airflow Rates?

Basic Relationship Between Resistance and Airflow

The resistance produced as air flows through a filter comes mainly from two sources:

Media resistance: pressure loss caused by friction, interception, diffusion, and other effects as air passes through the gaps between fibers.

Structural resistance: additional pressure loss caused by disturbances such as vortices as air moves through the frame and the spaces between pleats.

Separatorless HEPA filters use evenly spaced V-shaped pleats without separator plates, typically with a pleat spacing of 2–5 mm. This design creates a more stable airflow distribution and a lower proportion of structural resistance than separator-type filters. Resistance is therefore dominated by the filter media. Within approximately 50%–150% of the rated airflow, the relationship follows the fluid-dynamic square law: resistance ≈ K × airflow², where K is a constant determined by the media type and filtration area.

Resistance at Different Airflow Rates

The following example uses a common 1220 × 610 × 50 mm separatorless HEPA filter rated for 500 m³/h with a rated resistance of 150 Pa.

Airflow | Percentage of rated airflow | Theoretical resistance | Actual resistance | General trend

250 m³/h | 50% | 150 × (0.5)² = 37.5 Pa | 40–50 Pa | Halving the airflow reduces resistance to about one quarter

375 m³/h | 75% | 150 × (0.75)² = 84.4 Pa | 85–95 Pa | At 75% airflow, resistance is about half the rated value

500 m³/h | 100% | 150 Pa | 140–160 Pa | Rated design point

625 m³/h | 125% | 150 × (1.25)² = 234.4 Pa | 220–240 Pa | A 25% airflow increase raises resistance to about 1.5 times the rated value

750 m³/h | 150% | 150 × (1.5)² = 337.5 Pa | 320–350 Pa | Raising airflow to 1.5 times the rated value increases resistance to about 2.2 times the rated value

Key Conclusions

When airflow is below the rated value, resistance decreases by a greater percentage than airflow. For example, a 50% reduction in airflow produces a resistance reduction of about 75%.

When airflow exceeds the rated value, resistance rises much faster than airflow. For example, a 50% increase in airflow can increase resistance by approximately 125%.

Actual resistance is generally 5%–10% above the theoretical value because airflow disturbances between the pleats add a small amount of structural resistance at high flow rates.

Factors That Affect the Resistance Curve

1. Filter media

Ultrafine glass-fiber media: Because the fibers and pores are very fine, resistance increases relatively slowly at low airflow. Above 150% of rated airflow, however, the resistance curve becomes much steeper.

PTFE membrane media: Its smooth surface produces less airflow disturbance, so its resistance curve is closer to the ideal square-law relationship, usually with a deviation of less than 5%.

2. Filtration area

A larger filtration area—such as a higher pleat density within the same external dimensions—reduces the airflow load per unit area. Resistance is therefore lower at the same total airflow, and it increases more gradually as airflow rises. For example, a 1220 × 610 × 90 mm filter has approximately 30% more media area than a 50 mm-thick model. At 750 m³/h, its actual resistance may be only 280–300 Pa, about 10%–15% lower than that of the thinner filter.

3. Dust loading and service time

As dust accumulates, the pores in the media become blocked and resistance at the same airflow rises. The increase in resistance relative to a new filter is closely related to the accumulated dust load. For example, when the filter reaches about 80% of its dust-holding capacity, resistance at rated airflow may rise from 150 Pa to 250 Pa. At this stage, resistance also becomes more sensitive to airflow changes: a further 25% increase in airflow may push resistance above 400 Pa.

Practical Considerations

Recommended airflow range: A separatorless HEPA filter should generally operate at 70%–120% of its rated airflow. Long-term operation below 50% may cause uneven dust loading because the face velocity is too low. Long-term operation above 150% can sharply increase resistance, raise fan energy use, or damage the media.

Importance of resistance monitoring: Real-time pressure-drop monitoring helps determine whether the filter is becoming clogged. Replacement is normally required when resistance reaches about twice the initial value, preventing excessive pressure loss from disrupting the pressure balance of the clean area.

In summary, the resistance of a separatorless HEPA filter increases nonlinearly with airflow. Understanding this relationship helps engineers match fan pressure correctly, optimize system energy consumption, and determine the appropriate filter replacement time.