Dust Collector "return" sizing question

[Bob Cooper]

Building a workshop -- about 16x40 -- and i have a shop basement below it. I'm going to be putting a 5HP cyclone in the basement and run all the ducting in the floor. So everytime i turn on the dust collector i'm pulling air from the shop and moving it to the basement. There's a set of stairs in the shop going down to the basement but they will be seperated by a door. Also i'm planning on putting in a couple minisplits in the shop for heating and cooling.

Question...i'm going to need a return path for the air to get from the basement to the shop, what's best the way to do this? I'm thinking just 2-3 HVAC (20x20) returns with filters...but i'm just guessing on the sizing and approach. Any thoughts? Any special types of returns for example to accomodate thicker filters. Also i do realize that I'm actually conditioning the basement because of this so the basement's going to get insulation as well. (i feel like i'm building a house again).

[Bob Wingard]

I don't know if this would be of any interest to you, but I think I may have a few pieces of heavy fiberglass grating left over from my own installation. I out mine in the attic ... fed the air into a large box of filters ... then, the cleaned air returned to the shop through a large hole in the ceiling beneath the box. The hole was covered by grating more for appearance than anything else. We used this stuff to cover floor drains at work and drove fork lifts over them without breaking, so I know they are strong enough for home shop use.

If you think you'd be interested, I'll see what I have left and let you know.

[Dan Friedrichs]

You want the area of the return duct to be at least as large as the area of the "supply" (suction) duct. In reality, since return ducting is so easy to do (it doesn't need to be airtight, for example), you want to make the return duct much larger than the supply duct, to ensure that you aren't adding any restriction to the system.

Do you WANT to condition the basement? If not, why not put the cyclone in the basement, but route the cyclone exhaust to upstairs, and have the cyclone filter upstairs? If you're short on space upstairs and want the cyclone's filter in the basement for that reason, just build an tight box around the cyclone filter, and build some ductwork between the box and upstairs.

I would avoid using HVAC filters in the return air duct. They will just add unnecessary restriction. Your cyclone's final filter should be much, much finer than any HVAC filter you can buy.

[ian maybury]

A lot depends on whether your return is the (quite highly pressurised) exhaust from the fan/blower, or air which has been blown out through the filters and is now at room pressure.

If the former, and it's short (not more than a few feet) then a size up on the cyclone inlet header (= 8in diameter) seems to work well. If the latter (which is my situation - my filter cabinet exhausts into a sharpening area/office next door to the shop) then you need lots of open area - maybe something like x10 the cross section area of the duct.

I replaced the solid lower panels in a glazed door with an open mesh - see photo. My system is in the Pentz/Clear Vue format but with separate filters in an out to in flowing cabinet, and is also 5hp. These systems move large amounts of air, there's quite a definite draught through the door.

Watch out for noise levels. Placing the hardware down in the basement sounds good for quietening the shop, but wouldn't be so good if somebody wanted to work down there. It's also worth building in the option to switch from recirculation through the filters to exhausting outside (which also requires a set of fresh air intake louvres or similar somewhere) weather permitting - but be careful of ending up with too much ducting length, too far to get air back down into the basement via the stairs, or too many bends. It's worth running the numbers to figure out a total pressure drop for the system to make sure it's not getting too high.

I've not seen it done on a woodworking system, but some industrial systems (e.g. paint powder coating) have a compressed air blow back facility that is used every few days to push air backwards through the filters at times to clear them. I don't know how effective it'd be to install a by-pass so that the fan can blow back through the filters, but I'm a little sorry I didn't try it....

ian

[Bob Cooper]

I like the idea of *not* conditioning the basement though i probably will insulate it and put a dehumidifier down there, i'd rather not push all my conditioned shop air down there

So how far away can the filter be from the cyclone and what size pipe? Can i just extend the pipe which connects the cyclone to the filter so that it goes through the ceiling then maybe sits above the cabinets. note that i haven't bought the cycle yet so the interconnect is not super clear in my mind right now. i'm also a little concerned w/sound

[Bob Wingard]

It sounds to me like you want to build an upside down version of my system.

If all of you intake ducting runs near or under the floor ... then feeds into the cyclone to spin the debris out of the air stream ... then the cleaned air is run through filters ... the only remaining element is to get the clean air back up into the shop. This is a pretty simple task. Simply build a large plywood box that surrounds your filter(s) and mount it up tight to the ceiling. Provide some form of outlet from the box back into the shop ... then cover the transition with grating. This directs the clean air from the filters back into the shop without ever exposing it to the temperatures & conditions in the basement. In other words, it is a closed, circulating system. It will work to your slight advantage in the winter months, as the impeller puts a lot of "work/energy" into the air and causes a slight increase in the temperature. If you want to learn a little more about this concept, do a search for "temperature rise across a fan" and you should get some hits.

It is best to run the cyclone output into the filter with as short a run as possible, and preferably without any turns ... then, let the diffused air be directed with transitions & covered by grating, if necessary. Once the air has gone through the filtration, it has lost it's velocity and direction ... it simply wants to return to ambient conditions. With that thought in mind, I would keep the filter(s) in the basement ... enclose them in a "bag house" as I mentioned above ... then release the air back into your shop. The hole that makes the transition should be as large as possible to keep down the air velocity and the noise.

[ian maybury]

That makes sense. My recently built system (build photos here: http://thewoodhaven.co.uk/phpBB3/vie...php?f=3&t=2048) has the blower in a loft, with the cyclone dropping down through a hole in the ceiling to a sharpening room/office adjacent to the shop. The filters are likewise cabinet mounted (for outside to inside flow) in this room, the blower exhaust runs about 72in before dropping down into the cabinet.

It's not that noisy, but it is a bit louder than your typical bag filter unit. Seems to be an inevitability with these larger systems. I have the option to box in the cyclone and blower as many do, but haven't decided whether it's needed just yet. One downside of using a filter cabinet like mine is that the 8in dia outlet creates a definite draught in the room when it's running - although a diffuser will sort that.

There's no problem that I know of with piping the blower exhaust some distance to get to the filters - other than that in a similar way to the ducting on the dust collection side more length increases the pressure drop and reduces the cfm/air volume delivered by the system. Moderate length increases have pretty minor effect - see a HVAC design book for numbers but it's around 4.5 in water gauge/100ft at 4000ft/min airspeed/800cfm for 6in ducting, it's bends (each equivalent to say 6 - 12 ft of straight duct depending on the type) and flex hose (x3 the pressure drop/ft run with straight duct) that do the real damage. Going up one duct size to 8in for the exhaust should as mentioned above improve the flow too. In comparison the target total system pressure drop in a system (machine hoods +ducting +cyclone +blower +exhaust +filters) needs to be roughly 6 - 12 in WG, much more and the output of the LP centrifugal fans used on these systems starts to fall away fairly rapidly.

Straight cyclone inlet and blower exhaust duct runs for a decent distance (there's a rule of thumb which I can't remember, but it's possibly about 72in) apparently improve both airflow and cyclone separation efficiency - although many systems have bend on the blower exit which doesn't seem to do any great harm.

Fitting a silencer on the exhaust of the blower is supposedly a good way of reducing noise levels if the blower doesn't exhaust directly into the filters. I have one, but can't confirm this as the system has never run without one...

ian

[Alan Schaffter]

For noise, space, and easy cleaning, I put my DC at garage level- my shop is above the garage. My unit looks a little different because it is configured as a "push-through", but air exits from the top of the cyclone and enters a filter plenum that mounts two filters in parallel, or is vented outside in mild weather.

Filter plenum is above square cut-out in the ceiling:





Filter plenum at shop level:

[Jim Becker]

For noise control, don't make it a straight path--that will eliminate direct transmission of sound. The filtration almost isn't needed....keep it to a minimum and only to add a little more quiet. As to sizing, I built my return at a minimum of twice the area of my 7" Oneida cyclone inlet/8" outlet.