I should first point out that the system I'm referring to will be a self build DIY project. Fan and Filters being the only items purchased complete, And I'm still in the design phase but I'm not a HVAC expert at all. USA has forced air more commonly than here in the UK so I'm hoping you can help me out.
This is a room air filter, It will go under the stairs where I can hopefully sound proof it.
2 or 3 stage filter setup depending on pressure drop to clean the air for a 45m3 room, intending to get 4 air changes per hour.
Fan I have selected thus far is an ERK 250 which will blow into the filters.
Each filter has an initial Pa drop of lets say 50Pa new with a replacement final pressure drop of say 200Pa. My intention is to design a circuit that will have a differential pressure sensor to measure the pressure before and after the filter to calculate the pressure drop caused by that one filter thereby allowing me to calibrate it to indicate when said filter should be changed. The same for the other filters that follow.
Will the first filter effect the reading for the 2nd filter? I can't think of a reason why it would but then air flow dynamics is a black art to me.
Will it work? Are there any commercial filter change sensors out there and how do they work?
Best Answer
You are quite correct to note that pressure drop across an element within a dynamic fluid system is related to volumetric flow…this has certain implications.
To return to your original question, “Can I sense filter replacement need based on pressure drop?” the short answer is, not really. This is for the reason you state, that such a differential pressure value is dependent upon the volumetric flow, and if the flow is indeterminate or unknown, the value of the pressure differential does not reveal anything quantitatively as to the state of the filter element.
By way of example, a brand new filter element could indicate need of replacement if the fan speed were increased, resulting in a pressure differential increase to whatever level deemed to constitute the service limit of the filter.
The volumetric ‘nameplate’ capacity of the fan or blower purchased cannot be assumed to prevail within the target system, as the impeller speed and volumetric flow will depend upon many variables, not least those of the filters employed.
In commercial and industrial scale systems incorporating multiple elements, pressure differential measurements prevailing across individual elements may be monitored in addition to volumetric flow, which is controlled at specified process levels, as well motor energy required to sustain process levels, and these parameters and other cost factors integrated in calculations to arrive at maintenance schedules yielding least cost operation.
In a domestic DIY situation, the first and second filters could be changed alternately to determine which one, or whether both require replacement to restore desired system performance.
‘Filter capacity’ has been reached when the filter(s) in whatever combination do no longer yield the desired performance (basically CFM/min) which can be quantitatively determined. I hope this explanation will be of assistance.