Inside an AHU we will typically find two sets of filters. The first set is known as the primary filters and we also then have the secondary filters. These both have different functions in an air handling unit.
The primary filters are usually panel filters. These are there to catch the dust. They`re typically made with a pleated design which increases the surface area and allows it to catch much more dust and last a bit longer than if it were just a flat sheet. They`re usually made from a synthetic material with a cardboard outer frame and they`re held in shape with a wire mesh. Depending on the size of the AHU, you`ll usually have to buy these filters in pre-made sizes which will be slotted together to create a permeable wall covering the direction of air flow.
The secondary filters are there to catch pollen, bacteria and also some other dust which wasn`t caught initially by the primary filters. These are typically a
bag filter type made from a synthetic material.
These filters generally slide out from the side, alternatively some are held in place with a latch allowing them to be removed in the direction of, or against, airflow.
There are too measurement points for the pressure sensors, the high and low side. The high side reads the pressure at the front face side of the bag filters. The incoming air will come into contact with the face of the
filter and then flow through the filter. This will create a high pressure side as it collides. On the back side we have another filter which is reading the pressure of the air as it leaves the filter. The air has lost pressure due to the filter so this is the low pressure side.
Filters do vary in energy efficiency, set by different standards. The higher efficiency ones will create a lower pressure drop, and that means that your fans can overcome that pressure a lot easier. The fans driving the air in the system are typically going to be around five to 10 kilowatts, so, as they`ll be running maybe 12 to 24 hours a day, there`s potentially a lot of energy you can save just from switching to a more energy efficient filter.
