Steve,
Yes, I built a duct to return filtered air from the closet to the shop. It will also allow air to enter the closet for compressor intake, but that amount is minuscule compared to the air moved by the 1700 CFM cyclone!
As well as returning filtered air to the shop, I wanted to avoid pumping noise into the shop. As mentioned earlier, the 5hp ClearVue cyclone is a screamer, a banshee on steroids - I don't dare go into the closet without hearing protection when it's running!
I built a duct from inside the closet and back into the shop, made some turns to avoid straight paths, and added some internal baffles. The duct is 1/2" plywood, glued and screwed, with a thick rubber coating sprayed inside and the outside padded with insulation where possible. I built the closet before putting up the shop ceiling so I made use of the space between the ceiling and the shop roof, between the trusses. The return air duct is in the ceiling in the main shop. I have a Dylos air quality monitor and see no change in the particulate count when pulling fine dust through the cyclone. (The ClearVue seems incredibly efficient.)
This is the sketch I made when designing the duct. It goes up into the closet ceiling then folds back between two trusses, then crosses to the next truss space before venting back into the shop.
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BTW, when designing the shop, I filled a fat notebook with detailed sketches of everything from the dirt up. This took a lot of time but it kept me from making a bunch of false starts and "I sure wish I hadn't done that" moments!
Instead of returning the air to the shop, some recommend venting the dust collector to the outside. This will work well but if the shop is heated or air conditioned the replacement air would have to come in somewhere from the outside. (and a big cyclone moves a LOT of air!) Some people don't even filter the output from the cyclone or other DC - just pump the dust outside into a box or on the ground. The noise from this method might not make any close neighbors happy!
One comment about sound-insulating a closet or wall - I used "staggered stud" construction. My walls are all 6". The staggered stud wall uses 2x4 studs spaced so one supports the plywood on the inside wall and the next supports the outside wall, etc. This prevents direct contact and reduces sound transmission. Fiberglass sound insulation weaves between the two sets of studs. Some people line the closet with sound-absorbing materials but I didn't - and as mentioned it's quiet enough with the cyclone running to hear a whisper just outside the wall.
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For access to empty the DC drum, etc, I put double steel doors on the side of the closet away from the main shop, opening into my "maintenance" room.
Since this is an air compressor thread, I'll make a suggestion for those designing a shop: I ran air lines above the ceiling in three zones - one down one side of the main shop, one down the other and to the outside, and one to the back room to the little machine shop area and to a big air hose on a reel just inside a roll-up door. Each line has a shutoff valve on the wall outside the closet so I can only turn on those zones I need. Also, with the power switch on the outside of the closet, it's easy to turn off the power and close the main air valve without going inside - I shut off the compressor power every time I leave the shop. I know someone who lost his shop and all his equipment and wood when his compressor pressure cutoff switch failed and caused the motor to overheat when he was out of town.
Spaced through the shop are air outlet drops from RapidAir. These come without the airline couplers, Home Depot sells two types of couplers: I only use the type in this photo. It allows one-handed connection by simply pressing in the connector on the air hose instead of having to push back the outer sleeve with one hand while inserting the hose connector with the other! Look for the type with a small gap between the sleeve and the end flange.
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BTW, I didn't do this but if designing a large compressed air delivery installation, industrial experts recommend running air lines in a big loop in the building - better delivery efficiency at any one drop, plus adds a bit of extra capacity.
JKJ