Marq wrote:You don't compress the air so what you displace you replace.
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Fact 1. Air is a compressible fluid. Consequently, there will be a small degree of pressure development when using a feed fan arrangement... It (pressure) is very small, but it is there UNLESS you have BOTH a feed fan AND an exhaust fan. The pressure may exist only between the fan and filter (if the fan is in front of the filter), but it will be there. The only way there would be no pressure in the booth is if there is a stronger exhaust fan or the open area for exhaust air is so large that the pressure drop through the open area is virtually non-existent.
Fact 2. In the two fan scenario described above (both feed and exhaust fans), you will still have a slight amount of pressure unless the fans are perfectly matched. This scenario's pressure development can be either positive or negative, depending upon which fan is the strongest and how much pressure loss there is across the openings and the filters.
Verification of pressure development... What is your booth cover doing (assuming it is plastic, tarp, or something similar)? Is it billowing out, hanging perfectly flat, billowing around the bottom edges, or getting pulled into the booth space? Billowing outwards proves there is a positive pressure in the booth, and getting pulled into the booth space proves a negative pressure, or vacuum, inside the booth.
Obviously, if the booth is made from all hard surface materials, the proof lies in whether or not air is leaking into or out of the booth joints and seams while the fan(s) is(are) running.
It's just plain and simple physics!