I have looked at duct design and layout, which is one of the major friction losses in an air conditioning system, particularly the ducted and ventilation systems.
One of the other contributors of friction losses is the air filters. As we all know, without filtration the air conditioning principle is incomplete, as nature does not provide clean air at all times.
Figure 1 above shows the air movement across a filter. It is obvious that this filter will cause some air restrictions when air flows across it. This restriction will contribute to the pressure drop of the system and it must be added to the total pressure before selecting a fan. The air downstream will be clean when compared to the air upstream.
Methods of Filtration
There are various mechanical methods of filtration, depending on the nature of the system:
• There is the straining method.
• The impingement method.
• The interception method.
• The diffusion method.
• The electrostatic method.
Types of Filters and Performance
The filtered, clean air offers the following: protection of equipment, protection of internal surfaces of the building, improved quality of manufactured items, improves health and also recovers valuable airborne material. The choice of filter-type is important, but that goes with pressure drop across the filter.
• Panel filters’ rated face velocity is 2.5m/s. It has an initial resistance of 40Pa and a final resistance of 125Pa. The average arrestance is between 65 to 85 percent.
• Extended surface filters (pleated filters) have a recommended final resistance is 250Pa.
• Roll filters have a 125Pa final resistance.
•Electrostatic filters (low pressure devices) of 35Pa to 65Pa, have a face velocity of 2.5m/s
• It can be seen that dirty filters can be detrimental to a system.
• Dirty filters create a high-resistance, which the fan may not handle. The fan bearings may fail.
Literature:  RACA, ASHRAE.