Bag filters are a common component in commercial, industrial and comfort air filtration systems. These filters help to protect equipment, improve indoor air quality and ensure compliance with air handling standards. Correctly measuring and installing bag filters is essential for system performance, energy efficiency and filter lifespan. This guide explains what bag filters are, how to measure them accurately, how to install them correctly and how to choose the right grade for your application.
What Are Bag Filters?
Bag filters, also known as pocket filters, are air filters made up of multiple ultrasonically welded pockets attached to a rigid header frame. These pockets significantly increase the filter’s surface area, allowing for high dust-holding capacity, low pressure drop and long service life.
Unlike panel filters, bag filters are generally designed to handle higher air volumes while maintaining consistent airflow and filtration efficiency. This makes them ideal for use in HVAC systems, air handling units (AHUs) and ventilation systems where performance and reliability matter.
Common Uses of Bag Filters in HVAC and Ventilation Systems
Bag and pocket filters are widely used across commercial, industrial and public-sector environments where air quality control is essential.
Typical Applications for Bag Filters
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Hospitals and healthcare facilities
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Office buildings and retail spaces
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Manufacturing plants and clean production areas
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Schools, universities and public buildings
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Data centres and technical environments
They are often used as pre-filtration or final filtration, protecting expensive components or filters and extending overall system life.
How to Measure Bag Filters Correctly
Accurate measurement is critical when replacing bag filters. Even small errors can cause air bypass, increased resistance or improper fit.
Measuring Bag Filter Width
The width is measured across the face of the header frame, from left to right perpendicular to the pockets.
Tip: Always measure the actual filter frame, not the housing opening.
Measuring Bag Filter Height
The height is measured vertically across the face of the filter, from top to bottom of the header frame, parallel to the pockets.
Measuring Bag Filter Depth
The depth refers to the length of the pockets from the header frame to the bottom of the bags. Common depths include 300mm, 380mm, 500mm and 600mm.
Deeper pockets provide:
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Greater dust-holding capacity
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Longer service life
Measuring Header Depth
The header depth is the thickness of the rigid frame itself (often 20mm or 25mm). This ensures compatibility with your filter housing or retaining system.
Counting the Number of Pockets
The number of pockets directly affects airflow, pressure drop, energy efficiency and dust holding capacity. Always count the individual pockets visible from the face of the filter.
How to Install Bag Filters Correctly
Correct installation of bag filters is just as important as correct measurement. Bag filters should be installed so the pockets sit perpendicular to the floor. Improper installation can lead to higher energy costs, poor air quality and premature filter failure.
Bag Filter Grades Explained: M5 to F9
Bag filters are available in a range of EN779 filtration grades, each suited to different air quality requirements.
| M5 Bag Filters – 55% Efficiency | Colour: | White or Tan |
| Efficiency: | 55% | |
| M6 Bag Filters – 65% Efficiency | Colour: | Green |
| Efficiency: | 65% | |
| F7 Bag Filters – 85% Efficiency | Colour: | Pink |
| Efficiency: | 85% | |
| F8 Bag Filters – 95% Efficiency | Colour: | Yellow |
| Efficiency: | 95% | |
| F9 Bag Filters – >95% Efficiency | Colour: | White |
| Efficiency: | Greater than 95% |
Why Correct Sized Bag Filters Matter for Energy Efficiency
Choosing the correct bag filter grade and size can significantly reduce:
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Fan energy consumption
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System strain
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Maintenance frequency
A properly sized and correctly graded bag filter maintains optimal airflow while delivering the required air quality, saving money over time.
UK Manufactured Bag Filters from Filters Direct
At FiltersDirect.uk, all of our bag filters are manufactured in the UK at our own factory. We are proud members of Made in Britain®, supporting local jobs, British manufacturing and consistent product quality.
Why Choose UK-Made Bag Filters?
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Faster lead times
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British manufacturing quality
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Reduced carbon footprint
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Supporting the British Economy and Jobs
When you buy from Filters Direct, you’re not just buying a filter — you’re supporting the British industry.
For more information, advice or to place an order contact us today by email or phone.
