[ivanhoe]

if you don't mind a noise you could buy a 2nd hand vacuum cleaner with a built-in hepa or water filter (ideally also with adjustable speed to make it less noisy) and just let it run without a hose, sucking the air and pushing it through the filter...

[adrianN]

If you don't mind the noise and wasting a kilowatt on something that can be done with two dozen watts.

[ivanhoe]

That kW will move you more air in 30 minutes than PC ventilator can in a whole day, especially if you're trying to push it through hepa filter which is fairly thick and adds quite a resistance.

[clarry]

Most vacs are adjustable. Also, I think they're trying to maintain a roughly constant speed of the fan, not a constant power. Which would mean that they reach peak power when airflow is obstructed, and idle at lower power when air is passing freely and motor is spinning with little resistance.

[robryk]

The vacuum cleaner would have much larger airflow though. Would it still have higher energy usage per unit volume of air pumped?

If not, then cycling the vacuum on and off would actually Dave energy compared to the small fan setup.