I'm looking to build a DIY room air purifier and I need a fan (AC or DC) that has enough static pressure to power a HEPA filter and an active carbon filter. The room is about 700 sq ft.
I want to buy a quality, name brand fan that has high enough pressure and flow rate for my purpose. Ideally one that I can control the rpm of using a microcontroller (via PWM perhaps). But I'm a newbie when it comes to fans so I need some advice when it comes to the type of fans (centrifugal? squirrel cage?) and the supplier where I can get it from. Quietness would also be great.
I wonder if there are large versions of those PC fans that can be controlled via PWM and generates decent air pressure. I would like I need about 500 Pa at least.
Fans generally are specified in mm of water rather than pascals.
1 mm H2O = 9.80665 pascals (Pa)
so you are looking for around 50mm H2O;
You would need to define the flow rate you want.
Typical 12cm computer case fans provide 1 - 5 mm H2O and flow rates of around 40 - 60 cfm.
To achieve higher pressures you would need a fan running at very high speed and probably centrifugal rather than axial. It would NOT be quiet. Think about a leaf bloweer!
Quietness will be a function of the enclosure that you build. Do you have an enclosure or can you show a drawing of what you will build? Remember, all the air that the fan draws on must also be exhausted and fan sound will move the same way.
Can you recommend any good sources to find the type of the fan I need? I think flow rate of 200 cf/m would be enough. Yes, I will need a centrifugal blower (probably AC powered).
The static pressure will depend on the required airflow, the active area of the filter and its resistance to air flow. The cooker hood is quiet (XXX not unbearably loud ) bnecause it uses a large filter with little resistance to air flow.
This is for purifying air in an in-door space, so portability/size is not too important. I'd be happy if I can accomplish this with something the size of a mini fridge.
They are 15'' by 10'' approximately and I can use 2 of them side by side to lower the pressure needed. But I have no data on the HEPA filter or its pressure curve. But these are used in indoor commercial air purifiers and I don't see loud/big fan inside them.
I would want an airflow of maybe 200 cubic feet/min.
I'm just a little lost on finding a suitable fan. AC/DC, brushless, etc. I just know I want it to be squirrel cage.
So basically you are designing with no knowledge of the engineering requirements, but strangely are being very specific on the type of motor, but not the type of fan.
So lets make some sensible assumptions.
First, lets assume your "room " is typical ie around 10 feet high. So you have 7000 cubic feet.
This site
suggests as a minimum 0.35 air chages per hour.
Lets take 1 air change an hour as an appropriate value
then you need an air flow of 7000 cuft/h or 120cf/m
Another site compares domestic air purifiers, and quotes CADR Clean air delivery rate of around 300 - 400 cf/h which equates to a flow rate of around 10cf/m
The metrics assume a closed environment
In your research did you find this site? it didnt take me long.
in our testing, most Smart Air HEPA filters have a pressure drop of between 50-200Pa, depending on the airflow, air duct and how clogged the filter is.
Thanks,
but I did do some initial research. For instance, I did similar calculation as you when it comes to the flow rate required, which is why I mentioned 200 cubic feet/min.
I'm still in the experimental phase. Assuming a clean HEPA filter requires 250 Pa, and a "clogged" one needs double the pressure (I will use manometers in my design), I think a fan of 750 Pa or 3 inches water would suffice for now. I'm currently looking at this fan:
At 100 - 200 cf/m, it seems to meet my pressure needs (the blue curve).
I wonder if it's better to use a bigger/powerful fan because that way you can tune down the rpm which would result in quieter operations? I think these fans can be PWM controlled, not sure. Still learning.
The reason I want to select my fan first is because I need to design the rest of the filter housing/duct around the dimensions of the fan.
Thanks
I feel I probably should get a better idea of the pressure drop of the HEPA filter I want before choosing a fan, otherwise I might end up with way more fan than I need. I'm using a 3M home air purifier now and it's less than 100 Watt, and is quiet. The HEPA filter it uses is the kind that I want. I did disassemble one before and noticed the fan is squirrel cage, but there's nothing on the motor to indicate anything. I feel it's an overkill to get an expensive fan that's bulky and is over 300W for now.
I want to measure the pressure drop of the HEPA filter using manometer. This would entail doing some surgery on my 3M air purifier. Do you know of any easy to work with manometer that I can use with the Arduino? Or should I just buy a commercial handheld one like this:
