Exhausting - filtering from a contaminated space to a sterile space in a single pass.
Recirculating/scrubbing - exhausting the system back into the contaminated space.
In an exhaust setting the aim is to remove as close to 100% of the contaminate as possible with one filter pass. This requires that the contaminant have a longer time in contact with the carbon to fully adsorb odours. If you are using this system to exhaust, DO NOT exceed the exhaust rating for the filter.
In a recirculating system it is not necessary to remove 100% of the odour in a single pass so it is recommended to double the exhaust rating to reduce the contact time but increase the cycle rate. This removes approximately 60% but with twice as many cycles passing through the filter reducing the overall build-up of contamination (odour) in the space.
Recirculating systems can be better for your space when atmospheric adjustments have been made, for example, using air conditioning or CO2 when exhausting outside can be less efficient and more expensive than leaving a window open. Recirculating systems are intended to be used in sealed rooms.
For an exhaust system it is recommend to calculate length x width x height /3.
This will give you the amount of air (cubic feet per minute, CFM) that you will need to move to properly ventilate your space allowing for a three-minute air exchange (all the air in the space is exchanged for fresh air every three minutes). This is the number you will match your filter to.
Example: Room size: 10’x12’x8’ (960 cubic feet) 960 cubic feet / 3 minutes = 320CFM (Minimum recommended CFM to exchange the air every 3 minutes)
For recirculating systems, you can use the samelength x width x heightbut divide by 1.5. This will give you a high cycle rate while still ensuring the removal of 60% of any contaminant with each pass through the filter.
Take that CFM target used to find your filter and match it with the closest fan rating. Allow for a 10% -25% drop in airflow due to the resistance of pulling through the carbon filter. Just remember, if you are running an exhaust system, DO NOT choose a fan that will exceed the exhaust rating of your filter.
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