CFM loss when reducing hose sizes.

[Clarence Martinn]

Got 2 650 CFM Dust Collectors. One is the Jet the other is the pretty much the same model but it is a Sears Craftsman. Never used the Sears Dust Collector . It's just been sitting in the corner of the shop. Now, I have that old Sears Craftsman Whole Shop Dust Collection System , that Sears sold 20 years or so ago. It's got the 2 1/2 " clear pipe with blast gates, etc. If I run all 2 1/2 Inch pipe around the shop, and then connect that 1/2 pipe system to 4 inch flexible dust collector hose; what might the reduction in CFM be ? This is assuming that I would be running 1 machine at a time. Would I need to run Both Dust Collectors , or would 1 be good enough ? Shop is 25 x 30

[Jim Becker]

The answer is simple: you lose a lot. You can only fit a fraction of the air volume in a 2.5" pipe that you can fit in a 4". Dust collection is about moving volumes of air which in turn move the dust and chips. 4" pipe can only support about 400-450 CFM max as it is. The small diameter pipe you have is fine for a shop vac which works on different principles, but it is not going to aork well at all with a dust collector.

[Derek Cohen]

Got 2 650 CFM Dust Collectors. One is the Jet the other is the pretty much the same model but it is a Sears Craftsman. Never used the Sears Dust Collector . It's just been sitting in the corner of the shop. Now, I have that old Sears Craftsman Whole Shop Dust Collection System , that Sears sold 20 years or so ago. It's got the 2 1/2 " clear pipe with blast gates, etc. If I run all 2 1/2 Inch pipe around the shop, and then connect that 1/2 pipe system to 4 inch flexible dust collector hose; what might the reduction in CFM be ? This is assuming that I would be running 1 machine at a time. Would I need to run Both Dust Collectors , or would 1 be good enough ? Shop is 25 x 30

My guess would be around ... oh .... 7 1/2 cfm. Just rough guessing, but ...

The 4" hose is throttled down to 2-1/2". Now 2-1/2" pipe runs to about 1/4 the cfm of 4" hose. 4" hose is 1/2 that of 5" hose, which is 1/2 that of 6" hose, which is half that of 8" hose. An 25 feet of 8' hose at the end of a 650 cfm machine probably creates no better than 250 cfm. You do the math.

Regards from Perth

Derek

[glenn bradley]

I agree that mixing apples and oranges (dust collection with vacuuming) will yield you a lackluster result after all the work to install the DC and 2-1/2" duct. Beware of folks who claim fabulous results with 4" plastic pipe running 30 feet around their shop. When you've only used a shop vac even a 650CFM DC using a 4" pipe can seem "fabulous" but, the math won't support it. I'm not saying it doesn't work. I'm just saying it has a bit of that "I built these speakers myself and they sound better than anything out there" syndrome.

I am also a sinner. I run a 6" main when I should be running a 7" main. I could up my performance for a bit of cash and effort but, we all make compromises in a shop. I try to make smart ones when I can. I run a couple of shop vac systems with Dust Deputy separators. One of these handles general shop cleanup and powered hand tools. The other handles cleanup in another area of the shop and ties into the 2-1/2" vacuum system I use for my drill press, smaller bandsaw and spindle/disc sander.

These smaller fixed and mobile/handheld tools work better with a shop vac than with a reduced port on a DC. We all do the best we can with what we have but, I think your time and effort to hook a "air mover" type DC to a high static pressure vacuum type duct system will not end up making you happy. I would use the 2-1/2" system as designed and spend my larger dust collection effort elsewhere.

[Eric Arnsdorff]

A quick calculation of air in a 30' pipe comparing the 4" to the 2.5" pipe with the same pressure drop yields a drop from 650 cfm to 200 cfm. This value would be much lower when additional restrictions (bends, blast gates, etc.) are taken into account. This also isn't accounting for the final operating point based on the fan performance. As the others have indicated - it will lose a lot of flow in a 2.5" pipe compared to a 4" pipe. Most likely well under 100 cfm.

[Jim Becker]

Eric, you can't fit 650 CFM in a 4" duct anyway...so the numbers are actually less. And the 650 CFM rating for a mass market DC is, um...not reliable...in real life.

[Eric Arnsdorff]

Eric, you can't fit 650 CFM in a 4" duct anyway...so the numbers are actually less. And the 650 CFM rating for a mass market DC is, um...not reliable...in real life.

I was simply doing a quick flow comparison of what would happen with the same pressure drop when the diameter alone is reduced.
My statement about the fan (or blower as it is) operating point is in reference the flow being dependent on the duct work. The actual flow is dependent on all these factors.

I was agreeing with the you and The other comments. I was just putting some numbers on it for comparison.

[Frank Pratt]

Real life example: I have a 5 HP collector with an 8" trunk. Off that trunk, there is a 2.5" flex about 4' long that drops to my table saw guard. Even with all the other gates closed, there isn't a lot of flow through that flex. It's enough for the guard, but that's not much. I'd have far more flow hooking it up to my shop vac. I think 100 CFM is probably optimistic.

[Jim Becker]

I was simply doing a quick flow comparison of what would happen with the same pressure drop when the diameter alone is reduced.
My statement about the fan (or blower as it is) operating point is in reference the flow being dependent on the duct work. The actual flow is dependent on all these factors.

I was agreeing with the you and The other comments. I was just putting some numbers on it for comparison.

Got it! I tend to head to details too fast sometimes...my bad.

[Larry Frank]

It is not correct to say you can not you can get 650 cfm thru a 4" pipe. I did extensive measurement on my Oneida Super Dust Gorilla 5 hp and got numbers that high. I presented the information on this forum but the posts got somewhat messed up.

I did the measurements using a hot wire anemometer doing a traverse across the duct and digital manometer to develop performance curve and check performance at various points in my system.

[David L Morse]

It is not correct to say you can not you can get 650 cfm thru a 4" pipe....

Yes, Jim's statements about maximum airflow for 4" duct bothered me too. I assume he just left out some of the context, i.e., "For a typical hobbyist dust collector and a shop full of 4" duct....". Unfortunately, leaving out that context can give people the impression that there's some physics involved that makes 400-450 CFM a hard limit. As you know, if you have enough fan pressure available, numbers much higher than even 650 CFM are achievable.

[Derek Cohen]

It is not correct to say you can not you can get 650 cfm thru a 4" pipe. I did extensive measurement on my Oneida Super Dust Gorilla 5 hp and got numbers that high. I presented the information on this forum but the posts got somewhat messed up.

I did the measurements using a hot wire anemometer doing a traverse across the duct and digital manometer to develop performance curve and check performance at various points in my system.

Using 5 hp and 15" impeller ... perhaps. but 1 hp and <13" impeller?

To get higher cfm, somehow enough air needs to be pulled through the 4" pipe. A lot more power is needed.

Regards from Perth

Derek

[Larry Frank]

Of course you need a bigger DC to do this. One needs to be careful with some generalities concerning DC. Unfortunately, there are too many false claims about dust collectors capabilities.

[Curt Harms]

Eric, you can't fit 650 CFM in a 4" duct anyway...so the numbers are actually less. And the 650 CFM rating for a mass market DC is, um...not reliable...in real life.

So coming up with shop vac and dust collector performance numbers is what political campaign staffers do on off years then?

[Jim Becker]

Velocity would need to increase to be able to "fit" 650 CFM through a 4" duct...so we are all correct in the end as in this case, it isn't going to happen based on the available hardware to move the air.