Which Filter to use

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 Phresh Filters 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 1500 filter we recommend you use a 1500
cubic meters per hour fan. But keep in mind that fans that builld 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 to use 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 bit bigger than you originally planned with the right size ducting can reduce the noise of your system

Fan Size

The Phresh Filter model numbers represent the recommended fan capacity.

Filter size table

-Please note that the filter models correspond to the recommended fan size. Always use a fan with the correct flange size.

The basics

To understand which filter is right for you you need to understand a few basics about airflow 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 it's 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 of air weighs more than 1 kilogram. When you move for example 1000 cubic meters of air per hour you move more than 1000 kg 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 influence the air flow. One of the most important aspects is the diameter of your ducting. The larger the diameter, the higher the capacity. So there is a relationship between the diameter of your ducting and the air flow. The bigger your ducting, the easier it is to move air.

Pressure, pressure drop

A fan can have a certain capacity. This is the amount of air it can move when there is no resistance, so in a free-running situation. The moment you add ducting (or for example a carbon filter) the air will meet resistance ont 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 1000 cubic meters per hour. In a free running situation both will move 1000 cubic meters per hours. 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 most effective. Any resistance, be it ducting, a filter, a silencer or even a bend or corner in your ducting will cause the airflow 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, as pressure, it 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 this example you see a fan rated 750 cubic meters per hour. At a pressure drop of 212 Pa the volume the fan can move less than 500 cubic meters per hour! 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.