Phresh Filters and Pressure Drop

Author: phresh filters  


Phresh Filters and Pressure Drop

Carbon filters cause a pressure drop across the fan. An unrestricted fan working on full power will give the manufacturer’s specified air flow but when a carbon filter is connected to the fan it will reduce the air flow. Phresh Filters have the lowest air flow drop across any fan due to the carbon quality and superior design.

The triple activated carbon RC4/8 is perfectly sized to give you the best air flow and the optimum contact time for absorption of organic particles. You may have two different fans which are rated at 1000m3/hr but the fans will react completely differently under pressure. Always use a quality fan and check against the manufacturer’s pressure drop graphs as this will indicate how the fan performs.

Pressure Drop: The basics

To understand which filter is right for you, you need to understand a few basics about air flow and pressure drop. In any extraction system there are a few basic parameters:

  • Duct size (diameter of your ducting)
  • Resistance (any obstacle the air may encounter on its way)
  • Extraction fan capacity (the volume of air a fan can move per hour)
  • Extraction fan maximum pressure (the pressure a fan can build to overcome resistance on its way)


One cubic meter (1m3) of air weighs more than 1kg. When you move, for example, 1000 m3/hr (cubic meters of air per hour), you actually move more than 1000kg of air! Keep this in mind when designing your extraction system. When you move so much air you want the air to move through your system as smoothly as possible, without too many obstacles that could impede the air flow. One of the most important aspects is the diameter of your ducting. The larger the diameter, the higher the capacity for air movement.

Pressure/Pressure Drop

A fan has a certain capacity. This is the amount of air it can move when there is no resistance. The moment you add ducting or a carbon filter, the air will meet resistance on its path. This will cause the capacity to drop considerably. How much your capacity will drop depends on two factors:

  • The maximum pressure your fan can build
  • The resistance the air will meet on its way

You can have two different fans, both rated 1000m3/hr. In a free running situation both will move 1000m3/hr. But if you add resistance in their path, the fan that can build the highest pressure will be able to move the most air through your system and will be the most effective. Any resistance, be it ducting, a filter, a silencer or even a bend or corner in your ducting will cause the air flow to drop considerably. How much the volume of air will drop depends on the pressure your fan is able to build. Resistance is also called pressure drop and is expressed in Pascals (Pa). So in order to know how much capacity will be effectively left in your system you need to know the volume a fan can move at a certain pressure drop. Fan manufacturers publish diagrams which show the capacity at a certain pressure drop. These are called the pressure/volume diagrams. In the example on this page you see a fan rated 950m3/hr. At a pressure drop of 180 Pa the volume the fan can move is less than 800m3/hr! So, in any system, you will move much less air than the maximum rating of the fan because you always have resistance in your ducting and through your filter.

Fan Selection

So now you understand that if you use a carbon filter you will lose pressure and capacity. How much capacity is left depends on the pressure your fan can build and the pressure drop in your system. We at Direct Garden Supplies understand that you are not a HVAC specialist so we have calculated which fan would be suitable for which filter. The average recommended fan capacity is incorporated in the filter model name. So for a Phresh 200 x 600 filter we recommend you use a 200 x 600 fan with a maximum air flow of 1000 m3/h. But keep in mind that fans that build higher pressure will move more air through your filter than fans with a lower pressure rating. For this rated capacity the flange already has the right diameter. NEVER use ducting that is smaller in diameter than the flange of your filter or fan as this will cause pressure drop.


Fans make noise. But most noise is created by the air that moves. A high air speed will create more air noise. We recommend using air speeds up to 7 m/s. If you use a wider ducting the air speed will reduce and so will the noise. Choosing a filter that is a slightly bigger than you originally planned with the right size ducting can reduce the noise of your system considerably.