Originally Posted by
Janis Stipins
OK, I confess that I'm a little baffled here, because it seems like some of you have a vested interest in the CV1800 having poor performance. I don't have an opinion one way or the other, I'm just measuring what's happening. It's especially strange to me that this is causing such a discussion, because the numbers I'm measuring are exactly what anyone should expect based on the physics.
Let's do a back-of-the-envelope calculation to see what I mean.
The impeller on the CV1800 is 15" diameter. The impeller is shaped like a circle; I'll skip the calculus, but it turns out that the average distance of points in a circle from the center is 2/3 the radius. So if the impeller were uniformly filled with air, the average distance of the air from the center of the impeller would be (2/3)*(7.5") = 5". Now, in reality, the air is more heavily distributed toward the outside of the circle, and there is a vacuum in the middle; but we can say that 5" is a lower bound on the average distance of the air from the center. The actual average distance is higher than 5".
Now the impeller spins at 3450 RPM. The average distance the air travels as it goes around one revolution is 2*pi*5", which we can round off to 30", or 2.5 feet. The air, on average, travels (3450 RPM)*(2.5 ft) = 8625 FPM as it passes through the impeller. And remember, this is a LOWER bound, based on a significant UNDER estimate of the average distance of the air from the center of the impeller.
Next let's note that the cross-sectional area of the side of the impeller is very closely matched to the cross-sectional area of the intake duct. This means that I would expect the FPM at the intake duct to be roughly the same as what we've estimated for the FPM in the impeller.
So now, when I tell you that I put an anemometer up to this system, with no muffler, brand new filters, and essentially no static pressure loss due to ducting --- just a straight piece of pipe long enough to actually SMOOTH OUT the airflow going into the cyclone --- and I tell you that I measure something in the vicinity of 9000 FPM... why is that hard to believe? It's very much in line what what we should expect. It's not like I'm claiming my cyclone can cure cancer or travel through time or anything. :-)
I really don't have a horse in this race, so to speak. If the performance of this thing were poor, I'd tell you all that. But it's not poor; it's right in line with what I would expect from a 15" impeller spinning at 3450 RPM. That shouldn't be controversial at all.
If the Wood Magazine article is causing any of you some stress, let me offer the obvious and almost certainly correct explanation:
The 'engineers' involved screwed up, possibly due to disinterest, but most likely due to incompetence, in setting up the system. Then they screwed up again, possibly due to disinterest, but most likely due to incompetence, by accepting the bad data that their common sense should have told them was way off. The editors involved looked at the result, very possibly noticed that one of their major sponsors came out way on top, and did not feel compelled to ask the engineers to double check their work. End of story.
I do hope I'm not hurting anyone's feelings or coming off as TOO much of a jerk when I observe that most people screw most things up most of the time. Engineers, even elite ones who graduate from elite schools, are not exceptions. And the engineers who have time to do odd jobs for Wood Magazine are DEFINITELY not exceptions.
The laws of physics apply in my shop just like they do everywhere else in the universe. There's nothing mystical about a 15" impeller spinning at 3450 RPM and moving air at roughly 9000 FPM.
I note with some interest that the fan curve for the Super Dust Gorilla does not start at zero static pressure loss. Rather it starts at 2.5" of static pressure loss, which implies that that is the internal loss due to the cyclone and filter. That also is very much in line with the internal loss of the CV1800, which is not surprising, since they're very similar.
I don't know the impeller size of the Super Dust Gorilla, or its RPM (which I think might actually be variable based on load), and I don't know how the cross-sectional area of the impeller path compares to the 8" intake port. But I do know that somebody who DOES know those three things can do an estimate just like the one I did here, and I can tell you that what they come up with is going to be in the vicinity of 5000 FPM at the intake port. That won't be mystical either.
-Janis
EDIT: OK, I just did some Googling, and according to Oneida engineers, apparently the 5HP Super Dust Gorilla was basically a minor modification to the 3HP model which has a 6" intake duct; according to them, they just beefed up the motor, increased the fan size, and put a bigger intake port on it. I'm going to guess that the impeller size on their 3HP model was roughly 13", since the 3HP motor is going to struggle a bit with anything much bigger. By the same method we used above, that would result in an airspeed of 7590 CFM in the impeller. Assuming the impeller path was roughly matched to the 6" duct, that would have resulted in roughly 1500 CFM at the intake duct under next to no load. Hey, that matches the fan curve for their 3HP model (which also doesn't start at zero loss).
Now let's do what they did, and increase the fan size --- I'm assuming to 15", to compete directly with the CV1800 --- which would result in 9000 FPM in the impeller, like we estimated above. (The HP of the motor will affect how well the motor can overcome the resistance of the ducting, but we can assume that both the 3HP and 5HP are able to spin at 3450 RPM with next to no load.) Finally let's increase the intake duct to 8", which makes it bigger than the impeller path by a factor of 16/9. Correspondingly we should expect an airspeed of (9/16)*9000 FPM = 5062 FPM at the intake. Magic! No, wait; I said it wouldn't be mystical. ;-)
What this all means is that if I want, I can make my CV1800 into a much uglier Super Dust Gorilla by putting a 8x6 reducer on the intake port. :-D
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I never did manage to stop in to try the garlic ice cream. One of my regrets before moving back east from CA.
