Maybe this year will be the one that is the birth of the positive pressure paint booth. Have a large dual squirrel cage fan unit with it's own mounting frame work, a 5hp single phase motor (no 3ph in the country), 54 forty eight inch daylight LED lights, heavy air duct (20g) sheet metal, and tubing and angle to graft the whole works to the existing ancient DeVilbiss paint booth. Using an old booth because the construction is much heavier duty than new ones. Will have both exhaust and pressure fans that have to be in balance, flow wise, to work properly--HELLO Bill Hanlon. With near zero trash being introduced into the booth the guys will need to use full coverage suits and masks so they are trash free. Am getting a few calls on booth rental for high dollar paint jobs but want to get it dialed in using wife's old 64 Ford fishing truck and my very needy 37 Buick as test beds. Maybe I should make this post relevant to the Forum by painting an AD first. The first couple of paint jobs will be very expensive if it doesn't work out.
Electric exhaust fan and electric pressure fans have to be set so about 0.5#'s of pressure are maintained. Guess who is a retired EE that gets called on for advice? I've gleaned info that shows a rough setting if to have the fans set so they push the closed entrance door open and then increase the exhaust or decrease the pressure till door just stays shut. This involves rheostats or motor speed control so your name came to mind.
Let's try .05 instead of 0.5. That will still be too much as I would guess the pull needed to trip the latch open or closed is probably around 40#'s. The old DeVilbiss booths used a spring loaded roller similar to cooler/locker doors. I thought the switch from a draw through to a pressurized would be a week end project but it has turned into a step forward and a half step back endeavor. Sure hope the sucker turns out clean paint jobs. Have a friend in instruments and controls that has signed on and he has both a water and mercury manometer. I'm sure the mercury one won't be needed. Keep any ideas/thoughts coming.
In terms of a paint booth this is way over my head. This is just an observation but if you have already applied equal pressure of intake and exhaust air with filtration on both ends wouldn't you have what you are looking for?
The idea behind a positive pressure booth is any trash is being pushed OUT of any seam or crack while on a regular exhaust fan type booth the unwanted stuff is being drawn IN and onto your work. The pressure fans are in a sealed box with both the incoming air to the fans and the outgoing air to the booth being filtered which is twice the filtration of an exhaust type booth. My friend that is in the controls business has come up with a waste gate for the duct from the fans into the booth. If the people door is opened the the gate closes some to maintain pressure and when everything is closed up the gate opens to bleed off excess pressure. Even if there is a moment of negative pressure the incoming air drawn backward through the waste gate has to pass through the filter at the end of the duct. Untried but sound in theory I hope.
