Air Handling Units Air Leaks and Filters

It is desirable that the air escape in the power plants is naturally as low as possible. The main cause of air leaks is the pressure difference between the inside and outside of the power plant.In case of positive pressure in the power plant, the air in the power plant leaks to the outside, while in the case of negative pressure, the air leaks from the outside into the power plant. The air leak rate may be different at equal positive or negative pressure values. For this reason, in the EN 1886 standard, the air leakage class is defined at different rates for positive and negative pressure conditions.

Depending on the construction of the air handling unit and the nominal working pressure, air leakage is measured under the following test conditions:

• If only negative pressure is present in the device, all cells are exposed to 400 Pa negative pressure
• If the next working pressure is greater than 250 Pa, the positive pressure cells are above 700 Pa.

If the working pressure is above 700 Pa, positive pressure cells are tested in the working pressure. The remaining parts are tested at 400 Pa negative pressure. The permitted air in the relevant body section is associated with the fugitive filter class. At the end of the article, air leakage classes are given together with the relevant filter classes.

Air leakage class according to EN 1886 – 2007

Fugitive class 2007 (1998)Maximum leakage at -400 Pa L / cm2Maximum Leakage L / cm2 at +700 PaMaximum filter class according to EN779Quality
L1 (-)0.150.22Good from F9++
L2 (B)0.440.63F8-F9+
L3 (A)1.321.90G1-F7


The task of filters in HVAC systems is not only to protect ventilated rooms from pollution, but also to protect the HVAC system itself. This can be achieved by the use of F5 and F9 filter classes according to EN 779. When producing filters, the elements and materials that cause bacteria formation should not be used. General requirements for sealing, resistance and bypass leakage are specified in EN 1886. The door should be placed on the service side of the filter compartment.The width and height of the service door must be larger than the filter element to be removed and replaced. There should be enough free space on the service door side and on the front of the filters for removing and replacing filters. A pressure gauge should be added to see the pressure loss in the filter compartment. In cold climates, the inlet air must be slightly preheated to allow the frost to accumulate. The filter classes that should be used according to the particle size of the tablet are given below. We can collect the filters used in the air handling unit in six main groups.

1. Pre-filters (Low Efficiency)
2. Precision Filters (Medium / High Efficiency)
3. Rigid Filters
4. HEPA and ULPA Filters (Very High Efficiency)
5. Active Carbon Filters
6. Nuclear Filters

Relationship between particle size and filter class

Particle size (μm)Filter class
30 – 10 μmG1-G3
10-15 μmG4
5-3 μmF5
3-1 μmF6-F8
1-0.5 μmF9
0.5-0.01 μmH13

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