Dueling Dust Collectors?

[glenn bradley]

I saw a post where someone is going to add another dust collector to an existing system. It seems like running two DC's at the same time would be OK; or would they potentially cause a problem by fighting against each other.

Now that I type that question out it looks kinda dumb but, as long as I've gone this far. . .

What are your thoughts? Could you "bolt on" a 1 HP DC to an existing 1 - 3 HP system and run them simultaneously to get more air flow? I'm assuming an addition of bags/filters that would handle it.

Hmmmmmm.

[Chris Barton]

I think this would be like bolting on a propeller engine to a jet to see if it would go faster.

[Mike Weaver]

I seem to recall Bill Pentz addressing this on his site somewhere I think...

http://billpentz.com/

-Mike

[Rob Russell]

It seems to me that the additional DC should be additive in terms of CFM, provided you hooked it in to a Y fitting right before the 1st DC. It wouldn't make sense to me to have the 2 DC's hooked in to the system in different places.

[Matt Meiser]

I seem to remember that they were supposed to be in series, but then again I could be remembering it in the wrong way.

[Jeffrey Makiel]

Fans in series:
If you install two fans in series, you theoretically almost double the static suction pressure. However, this will only create only about 42% more flow maximum. It's sort of like the 'law of diminishing returns'. A little more flow takes a lot more pressure (to double flow, the pressure must increase by 4 times). Also, the increased flow will be the same thru each fan, but the 42% increase can overload the fan's motors if they are not large enough to handle that flow. Amp draw is a direct relationship to flow, not pressure.

When I designed hvac systems in another life, we would sometimes install two fans in series to help squeeze more air thru long runs of small duct. We simply needed the pressure. However, we used vaneaxial fans (similar to tube fans) that have a minimum turbulence at the discharge side. Conversely, using centrifugal fans can be messy because the flow profile is very upset after leaving the first fan, which then enters the second fan. An upset flow entering a fan is a bad thing and will immediately reduce the fan’s performance by 25% or more. Therefore, a fair amount of separation between centrifugal fans would be very important whereas the second fan may be thought as a ‘booster fan’. However, this is impractical for a small shop and is a very inefficient way to accomplish the same goal by either purchasing a larger dust collector fan, or increasing the size of the duct.

Fans in parallel:
If you install the fans in parallel, as with a wye fitting, you will theoretically double the flow, but the suction pressure will be the same as just having one fan. And that’s the catch! In reality, the attempt to increase system capacity from both fans will be fought by the backpressure exerted by the undersized duct (again, the law of diminishing returns). In the end, flow will increase, but it will probably be much less than 25%.

Parallel fans are a good solution when you need to increase flow (that is, volume), on a system with little or no ductwork, like force draft blowers on a boiler. However, the fans won't fight one another, as some may believe. Instead, they will both equally fight the pressure exerted back by the undersized duct while unsuccessfully trying to squeeze twice the volume of air thru a duct that was meant for a single fan’s flow. In the end, they will both produce about 60% of their rated flow each, and that mean less stress on the motors.


cheers, Jeff