Air Ventilator & Filter for Classrooms - a Low Cost HEPA Alternative

While waiting for schools to reopen physically post Covid19, I started with a project to have adequate air ventilation and filtration for the indoor spaces of my learning community at ActonKL.

The inspiration for this project is the Corsi air filter which is MERV13 grade. This prototype should give an airflow rate of ~1,200 cfm. For a room that's 1,900 sqft with 13ft ceilings, I need two units of this air filter to achieve 5-6 air changes per hour.

Calculation Sheet - The calculation sheet I used. My reference is the US/Harvard standard for 5 to 6 air changes per hour (ACH) and Malaysian DOSH of 10 L/s per room occupant. Tests of this filter gave a noise sound of ~40db, which is about the level of a whisper.

Air Filters Panels

• US MERV13 or Euro ENN779 class F7Pleated filter panels
• Nominal size (20"x20"x2")
• Quantity: 4 units (to have better air flow)

16" Fan I used an exhaust fan

• Outer frame size of 48x48cm
• Airflow = 1,380 cfm
• I was told the larger the blade size, the less noisy it is.

Corrugated plastic mounting board

• This is for the base

Tape

• I used duct tape (white), lots of it.

Activated carbon fabric [Optional]

• The grills of the exhaust fan had large gaps, and needed to use this in close proximity to young children.
• Acts to suppress the noise a little.
• And hopefully absorbs bad odour

The tricky bit for me is sourcing a suitable fan. I wanted a fan with long blades as it is supposed to reduce noise. I ended with the 16" fan.

The filter panels I used had a nominal size of 20"x20"x4". Do check the actual sizes before ordering. I found a supplier who will make-to-order the air filter panels with the MERV13 filter grade I need.

Now comes the taping up. Tape the filter panels according to the image in Step 1.

Apply the tape on the outside. The negative pressure when the fan is blowing will keep the tape in place. When I made this, I taped the corrugated plastic mounting board to the base at this stage. [Based on your build, you might want to leave this to the last step]

Important: Make sure the tape seals the panels together tightly.

The exhaust fan I got fits nicely at the top. I had to tape the mounting holes to ensure all airflow goes through the filter panels.

Wire the fan up as required by your fan and power plugs in your country.

Important: Make sure the tape seals the fan to the panels tightly.

After this step, the air filter is done but if you wanted extra mile...

With many young children around, I needed to secure the fan air outlet from prying fingers The gaps of the fan grill were rather large.

I placed the activated carbon fabric between the fan blades and the grill. The size of my fabric was about 50x50cm.

If you choose to do this step, I suggest not to close the base in Step 2 but to close it only after completing this step. I had difficulty accessing the screws for the grill and would not have that problem if the base was open.

A reader had made this filter and I thought he had an elegant way to prevent cheeky monkeys from poking and loosing their fingers.

Using an old fan cover (in this case, a 16 inch fan), just place it at the top and zip tie it down.

Thanks Mr Cheah for sharing the idea and the photo.