DC/CFM #'s confusing me

[Greg Scott]

I'm doing some research on a new dc, getting tired of connecting the Delta 1HP unit to one machine at a time. I don't have the ceiling height for a cyclone so I need to stick to a 2 stage.

Anyway I stumbled across this chart on Oneida's site showing a review done by American Woodworker. The first number after the inlet dia is CFM max and the second is CFM under load. It shows that the Penn State DC2-5 has more CFM under load than any other dc except the Oneida cyclone. They say the Penn State has 860 Max and 610 UL the next closest is the Jet DC-1200 with 1025 Max and 555 under load.

How can a 1-1/2 hp unit with an 11" impeller have more CFM under load that a 2HP unit with a 12" impeller???

Model | Price | Inlet | Max CFM | CFM Under Load

[John Preston]

A couple of things:

First, I am no expert.

1. I just cut a hole in my ceiling for my cyclone motor to stick up through. Worked like a champ.

2. A guy named Jim Halbert swore up and down on his method of using a 1200 CFM DC to suck up the dust, then blew the exhaust into the cyclene. This reduces the height of the cyclone considerably. He actually used two 1200s in series with his because he just didn't like any dust in his shop. I was impressed by the cleanliness of his shop, he could have had carpet in there. White carpet.

3. It looks like Jet got those huge free air (no load) CFMs by using a bigger inlet diameter than their competitors. At that flow, the motor is probably real close to overloaded. That could give them the high no load number, or they could have an impeller design (vane shape and angle) that naturally gives more flow at low static pressures, and the one that has the high flow at high static pressure is optimized for higher flow at higher static pressure.

4. It was once posted by someone who claimed to know more than me that here's the way fans work:
Assume all fans are rotating the same speed
a. Larger impeller diameter gives higher static pressure
b. Depth of impeller gives higer flow
Now obviously those are not going to work for all fans, because changes in static suction or back pressure are going to change the effects of depth or diameter.

4a. Also, the angle of the vanes makes a difference. On pumps, vanes swept back produce higher pressure than straight vanes. Can't remember about reverse vanes, not common on pumps.

5. Sometimes companies report misleading values for numbers like horsepower. Just look at any 6 hp shop vac.

Does that make things nice an muddy?

[Dennis Peacock]

John has posted some really good info there and is on the dot with the accuracy of his info.

When looking for DC setup.....pay NO Attention to HP.....A 50HP DC turning a 12" impeller will create the same CFM as a 3HP DC turning a 12" impeller. So disregard HP....at least for now.........

No load numbers? Provide a LARGE opening for the DC and turn it on and you will get fairly high numbers. Put the front back on the DC and you start cutting your CFM rating greatly.
Terry Hatfield came down and tested my cyclone setup with his test equipment at my tablesaw....75 foot run of piping. 4" opening the CFM was around 725CFM.....changed it to a 6" opening and it went up to 1,048 CFM....I left it at the 6" opening.

You want at least 600CFM at the machine...and should prefer 800CFM at the machines that produce chips rather than dust...planer and jointer for example.

Unfortunately....most DC stuff is highly opinionated as there are a LOT of variables to plunk into the equation for each person setting up a DC system.

All of this and $1 will get you a cup of coffee at the local McDonalds.!!