What is secondary filtration?

What is Secondary Filtration?

Secondary filtration (also known as intermediate or fine filtration) removes large to medium dust particles (≥0.4 micrometers) and is implemented at the middle stage of an air filtration system, between coarse filtration and HEPA filtration.

Characteristics of Secondary Filtration

• The filter material of the secondary filter is made from synthetic PE fiber, glass fiber, or other materials.
• The frame of the secondary filter is made from galvanized steel, aluminum, or stainless steel.
• The filter medium is designed according to the EN 779 standard.

Filtration Efficiency of Secondary Filters

Similar to coarse filtration, secondary air filtration is also designed according to the EN 779:2012 standard. Secondary filtration can remove 0.4-micrometer dust particles with an average efficiency ranging from 40% to 95% or higher, corresponding to filtration levels M5, M6, F7, F8, and F9.

More specifically, secondary filters with levels M5 and M6 (EN 779) are referred to as intermediate or medium filters, while those with levels F7, F8, and F9 (EN 779) are known as fine filters.

Common Types of Secondary Air Purifiers

The two most popular types of secondary filters on the market are plate filters and bag filters.

Bag-Type Secondary Filter

• The frame is made of materials such as aluminum, galvanized steel, or stainless steel.
• The air filter bag is made from synthetic materials such as glass fiber or PE fiber.

Size (mm)	Rated air flow (m3/h)	Head pressure difference (Pa)
		M6	F7	F8	F9
592x592x380 x6 bags	2400	60	100	120	140
287x592x380 x3 bags	1200
592x592x480 x6 bags	3000
287x592x480 x3 bags	1500
592x592x560 x6 bags	3400
287x592x560 x3 bags	1700
592x592x560 x8 bags	4500
287x592x560 x4 bags	2250
592x592x760 x8 bags	5000
592x592x940 x8 bags	6000
492x492x560 x6 bags	2500

Plate and Box Secondary Filters

Plate and box secondary filters are also widely used in industrial and residential applications.

The structure of plate and box secondary filters consists of two main components: the frame and the filter membrane.

• Frame: Can be made of aluminum, stainless steel, galvanized steel, or other durable materials.
• Filter membrane: Can be made from materials such as glass fiber (for high heat resistance), synthetic fiber (for abrasion resistance), activated carbon (for odor removal), or metal mesh (for oil and water filtration).

Some common sizes of secondary air filter in the form of plates and boxes:

Size (mm)	Rated air flow (m3/h)	Head pressure difference (Pa)
		M5	M6	F7	F8	F9
592x592x46	3400	60	60	100	120	140
287x592x46	1700
492x492x46	2200

Use of Secondary Filter - Medium Filter

Secondary filtration is applied in residential and public spaces (hospitals, schools, commercial centers, office buildings) as well as in industries (medical equipment, pharmaceutical clean rooms, food production, electronics, painting, and interior manufacturing).

The secondary filter is installed in air filtration equipment such as FCUs, AHU air handling systems, and home HVAC systems.

When is the Secondary Filter replacement time?

Secondary filtration is the second stage in the air filtration system, following coarse filtration, where it primarily captures fine dust particles, as large dust particles are removed during the coarse filtration stage.

According to estimates, secondary filters have a lifespan of 6-9 months. Customers can extend this lifespan by regularly cleaning the coarse filter to reduce its dust load.

The lifespan mentioned above is an estimate and varies significantly based on environmental conditions and usage frequency.

In addition to the above estimate, customers can determine the replacement time for the secondary filter by monitoring the final pressure difference, which is the recommended replacement point.

Each filter is designed with an initial pressure difference and a recommended final pressure difference. For secondary filters, the recommended replacement pressure difference is 450 Pa. A pressure differential meter with a measuring range of 0 to 500 Pa can be used to monitor the pressure difference of the secondary filter.

Filter Installation and Maintenance Instructions

Plate-type Secondary Filter

Installation Instructions:

• Gently open the packaging and handle the filter by its frame, avoiding contact with the filter membrane.
• Filters should always be stored in their packaging to prevent contamination or damage.
• Install the filter plate according to the airflow direction indicated by the arrow.
• Use gaskets to seal any gaps between the filter frame and the air filtration device.

Maintenance Instructions:

• Some filter plates are single-use and must be replaced after use.
• For reusable filter plates, maintain them by gently cleaning or vacuuming.
• Do not wash with water or expose them to strong air pressure or harsh chemicals.

Bag-Type Secondary Filter

Installation Instructions:

• Carefully open the filter housing, handle the filter by its frame, and avoid contact with the filter material.
• Ensure the filter is installed with the filter bag positioned vertically.
• Avoid contact between the filter bag and any sharp edges of the device.
• Verify that the filter bags are intact and not stuck together.
• Attach gaskets, and apply sealant to gaps if necessary.

Maintenance Instructions:

• Most bag-type secondary filters are disposable and need to be replaced after use.

Summary

Secondary filtration plays an essential role in the air filtration system, filtering large dust particles and extending the lifespan of HEPA filters. Secondary filters typically come in two types: plate and bag. Bag-type secondary filters are particularly common in clean rooms and air handling systems such as AHUs and FCUs. Select the appropriate secondary filter based on the specific requirements of your project.