How Do the Pressure-Drop Characteristics of Panel Prefilters and Medium-Efficiency Filters Change over Time?

Common Pressure-Drop Trend over Time

A filter's pressure drop—the pressure loss as air passes through it—is governed mainly by the amount of dust retained in the media. Overall, pressure drop develops in three stages.

1. Initial Stage: Stable Low Pressure Drop

• When a new filter is put into service, the media is clean and its pores are open, so airflow resistance is at its minimum, known as the initial pressure drop. A G4 prefilter, for example, normally starts at approximately 30–80 Pa, while an F8 medium-efficiency filter starts at approximately 80–150 Pa.

• This stage is brief, generally lasting from a few days to one week. The media begins to capture a small quantity of large particles, but they have very little effect on overall resistance, which remains close to the initial value.

2. Middle Stage: Rapid Pressure-Drop Growth

• As operating time increases, dust accumulates on the surface and inside the media. Pores become blocked, the paths through which air can pass become narrower, and pressure drop begins to increase rapidly, either linearly or nonlinearly.

• This is the filter's main working period and normally lasts the longest. A panel prefilter commonly remains in this stage for about 1–2 months, while a medium-efficiency filter remains for about 3–5 months, depending on ambient dust concentration.

• The larger the dust-holding capacity and the lower the ambient dust concentration, the more slowly pressure drop rises.

3. Final Stage: Stabilization or Replacement Threshold

• When the media approaches saturation and most pores are blocked, air becomes difficult to pass. The rate of growth then slows and pressure drop gradually approaches a stable value or fluctuates slightly. This value is called the final pressure drop and is normally two to three times the initial value. Typical final values are approximately 100–200 Pa for a G4 prefilter and 200–300 Pa for an F8 medium-efficiency filter.

• Continued use may cause a small apparent decrease in pressure drop as air bypasses completely blocked regions through the remaining open areas. Filtration efficiency falls substantially under this bypass condition. Final pressure drop is therefore the key indicator for determining when the filter must be replaced.

Differences between Panel Prefilters and Medium-Efficiency Filters

Comparison factor | Panel prefilter | Medium-efficiency filter

Initial pressure drop | Lower, normally 30–80 Pa, with better initial airflow | Higher, normally 80–150 Pa

Rate of pressure-drop growth | Faster in the early period, then gradually slows | Slower initially, then accelerates in the middle period as fine pores become obstructed

Time to reach final pressure drop | Shorter, generally 1–3 months and even less in a dusty environment | Longer, generally 3–6 months and strongly influenced by prefilter protection; better prefiltration extends this period

Final pressure drop | Normally 2–2.5 times the initial value; for example, a G4 filter may rise from 50 Pa to 100–125 Pa | Normally 2.5–3 times the initial value; for example, an F8 filter may rise from 120 Pa to 300–360 Pa

Slope of the pressure-drop curve | Relatively gradual because the pores are large, although limited dust-storage space allows saturation to be reached quickly | Steeper after the middle period because fine particles block the denser media and cause a pronounced rise in resistance

Key Factors Affecting the Trend

• Ambient dust concentration: Higher concentration loads the media more rapidly and accelerates pressure-drop growth. Resistance may rise two to three times faster in an industrial area than in a residential area.

• Media dust-holding capacity: A larger capacity slows pressure-drop growth and delays the final value. For example, a bag-type medium-efficiency filter generally holds more dust than a panel prefilter.

• Protection provided by the prefilter: The more effectively the prefilter removes large particles, the fewer reach the medium-efficiency stage and the more slowly its pressure drop rises. This is the main reason the two stages are used together.

• Airflow stability: Large system-airflow fluctuations, including operation above rated airflow, accelerate dust deposition and the rise in pressure drop.

Conclusion

Both panel prefilters and medium-efficiency filters follow a progression from stable low resistance to rapid growth and finally the replacement threshold. Because a medium-efficiency filter uses denser media and usually offers greater dust-holding capacity, its increase is more delayed—slow at first and faster later—while its final pressure drop and replacement interval are both higher. In practice, a differential-pressure gauge should be used to monitor the trend, and the filter should be replaced before it exceeds its specified final pressure drop to balance filtration performance with system energy consumption.