Ever since I picked up my HF 2HP DC a few months back for $140 using the 20% coupon I have had many questions as to its real performance. As you all know the CFM claims on the website are very high and I couldn't even verify the motor's actual amp rating due to conflicting data from the the placards on the machine, the website, the manual, etc. I have spent quite a bit of time on Bill Pentz's website reading about the recommended CFM range for adequate dust collection and have been itching to find out where this DC stands (http://billpentz.com/woodworking/cyclone/DCBasics.cfm#CFMRequirementsTable).
As luck would have it a good friend of mine just bought a Fluke 922 Airflow Meter for some HVAC work that he was doing and dropped by to see what we could learn. The only down side to this meter is that it is basically a pitot tube and operates on positive airflow and pressure. This meant that I could not measure velocity at the inlet point but could assume that CFM in = CFM out so I poked a hole in my plastic DC lower bag allowing me to insert the probe into the air stream inside the 5" duct as it enters the fitler / bag ring.
I don't have a cyclone so the filter loads up pretty quickly. I didn't initially clean the filer (it was cleaned fairly recently) to simulate a realistic weekend in the shop and not a theoretical maximum. The only changes I made to this DC system were the addition of a Wynn filter and the removal of the "grate" blocking chips and hands from entering the impeller housing.
The Data:
-No DC inlet hose: 490 CFM
-10' x 5" flex hose: 445 CFM
-10' x 5" flex hose with 4" reducer at the tool: 402 CFM
-10' x 4" flex hose: 320 CFM
-free air-flow (no DC hose and no filter...theoretical max): 880 CFM
-No DC hose but a CLEAN Wynn filter: 840 CFM
The big surprise for me came with the impact of a clean filter vs. theo max but now that I think about it isn't that surprising. The filter that I bought has 274 sqft of surface area, which means that with that huge area only 3 CFM are being pushed through each sqft of fliter media which mean very low resistance.
Unfortunately we ran out of time and the meter had to go home before I could repeat all the measurements with the clean filter but we can make some assumptions based on "air-flow noise".
With a clean filter and a 5" CD hose (same size as the inlet) I expect a similar efficiency to the clogged filter so I would assume that actual volume would be between 700-750 CFM. It gets tricky to make assumptions from here since this DC does not operate on static pressure and we have shown that the air does not simply speed up when duct size is reduced as it would in Bernoulli's principle. So until I get the meter back I have to rely on my "calibrated ear" attempting to correlate the CFM measured earlier to the noise the air made as it entered the CD bag.
Here are my assumptions:
-10' x 5" flex hose: 735 CFM
-10' x 5" flex hose with 4" reducer at the tool: 650 CFM
-10' x 4" flex hose: 465 CFM
Lessons:
-don't use 4" flex if you can help it, run a full-size hose then reduce at the machine if necessary (or increase the CD fitting at the machine to match the larger hose)
-add a cyclone assuming that the efficiency lost by adding the "load" of a cyclone be less than the efficiency lost by having a partially clogged filer
-for $240 total (including the filter) I have a system that is almost good enough to live up to Pentz' recommendations when the filter is clean; most of the time however it will fall short so I must continue to wear my P95 respirator
As luck would have it a good friend of mine just bought a Fluke 922 Airflow Meter for some HVAC work that he was doing and dropped by to see what we could learn. The only down side to this meter is that it is basically a pitot tube and operates on positive airflow and pressure. This meant that I could not measure velocity at the inlet point but could assume that CFM in = CFM out so I poked a hole in my plastic DC lower bag allowing me to insert the probe into the air stream inside the 5" duct as it enters the fitler / bag ring.
I don't have a cyclone so the filter loads up pretty quickly. I didn't initially clean the filer (it was cleaned fairly recently) to simulate a realistic weekend in the shop and not a theoretical maximum. The only changes I made to this DC system were the addition of a Wynn filter and the removal of the "grate" blocking chips and hands from entering the impeller housing.
The Data:
-No DC inlet hose: 490 CFM
-10' x 5" flex hose: 445 CFM
-10' x 5" flex hose with 4" reducer at the tool: 402 CFM
-10' x 4" flex hose: 320 CFM
-free air-flow (no DC hose and no filter...theoretical max): 880 CFM
-No DC hose but a CLEAN Wynn filter: 840 CFM
The big surprise for me came with the impact of a clean filter vs. theo max but now that I think about it isn't that surprising. The filter that I bought has 274 sqft of surface area, which means that with that huge area only 3 CFM are being pushed through each sqft of fliter media which mean very low resistance.
Unfortunately we ran out of time and the meter had to go home before I could repeat all the measurements with the clean filter but we can make some assumptions based on "air-flow noise".
With a clean filter and a 5" CD hose (same size as the inlet) I expect a similar efficiency to the clogged filter so I would assume that actual volume would be between 700-750 CFM. It gets tricky to make assumptions from here since this DC does not operate on static pressure and we have shown that the air does not simply speed up when duct size is reduced as it would in Bernoulli's principle. So until I get the meter back I have to rely on my "calibrated ear" attempting to correlate the CFM measured earlier to the noise the air made as it entered the CD bag.
Here are my assumptions:
-10' x 5" flex hose: 735 CFM
-10' x 5" flex hose with 4" reducer at the tool: 650 CFM
-10' x 4" flex hose: 465 CFM
Lessons:
-don't use 4" flex if you can help it, run a full-size hose then reduce at the machine if necessary (or increase the CD fitting at the machine to match the larger hose)
-add a cyclone assuming that the efficiency lost by adding the "load" of a cyclone be less than the efficiency lost by having a partially clogged filer
-for $240 total (including the filter) I have a system that is almost good enough to live up to Pentz' recommendations when the filter is clean; most of the time however it will fall short so I must continue to wear my P95 respirator
