Effect of the filtering surface of a filter

The important thing for a filter’s proper operation is that the speed of the air flow (usually measured in m/s) through the filtration material is correct. The speed of the air is affected by the filter surface. The more efficient the ventilation system, the more surface is required. Also, the level of filtering fundamentally affects the surface. The higher the level of filtering, the higher the pressure loss in the filter material.  Choice of the correct surface has a direct impact on:

• The useful life of the filter
• The filtering values
• The energy consumption of the ventilation device

Calculation of the correct area

The surface area of the filtering part of a filter is calculated from the air volume of the ventilation device and the selected filtering class.  The formula for calculating the correct area is:

Ventilation device air flow rate (m³/s)
______________________________ = required surface area (m²)
Air flow through material (m/s)

Example

A suitable filtering class for an office space is F7, and the air flow rate of the ventilation device is 5.4 m³/s. The device can accommodate six standard-sized filters.  In the selected class, the desired air flow rate through the filter canvas is 0.1 m/s. To obtain the correct air flow rate through the material, the effective area of the filter must be sufficiently large.

1) Required area:

5.4 m³/s
______________________________ = 54 m²
0.1 m/s

As the device can accommodate six filters, each filter needs to have a surface area of 9 m² (54 m²/6).

2) Select a filter of the right size. The surface area is at least 9 m².

Typical problems

If the surface area is insufficient for the air flow rate, the filter may, in the worst case, be damaged because of the excess strain generated by the pressure.  It is also possible for the pressure applied to the insufficient filtering area to release particles from the filter into the duct system and subsequently to the indoor air. The surface area of the filter is directly proportional to its useful life and operation efficiency.