Thread: CFM ratings of portable dust extractors

Originally Posted by JillB

Hello, I have been looking at portable 1hp ( 750W) dust extractors and find that the cfm on thse range wildly from 500cfm to 914 cfm, depending on the manufacturer.

Does anyone know the reason for this wide range of flow rates?

Also, can anyone please recommend a unit with a high cfm flow rate? Are they 30 micron bags or 5 micron?

The size of the motor usually indicates the power of the vacuum blower which can be any combination of negative vacuum pressure and flow (cfm). If you increase the flow with the same sized motor you would have to decrease the vacuum pressure.

Many woodworking systems have high flow with little vacuum lift because they do not have to lift heavy material so if you ran a small hose from one it would not lift water much from a bucket. More powerful vacuums can have drastically smaller flows (cfm), but quite good water lift because they develop a strong negative pressure.

The ones with a stronger vacuum pressure will carry the dust a longer distance and through slightly smaller hoses than the ones with less vacuum pressure and more cfm. If you put your hand over the end of the tube and block it off the pull you feel is the vacuum pressure.

I guess the more outlets you want from your dust extractor the more cfm you need, the longer you want to run your hoses, the more vacuum pressure you need.

Jill, with that much variation on a 1hp unit, I would suspect the different manufactures ( or resellers ) are using different methods of measuring the performance.
eg. the higher rated cfm ones may be testing theirs without a bag fitted, or no restriction on the inlet side.
I have seen magasine articles where they test them on an even playing field, it makes some of the claimed cfm ratings look like a joke.

Sorry I am no help in recomending a brand for you.

Hi Jill,

Being a complete novice when I started I thought a dust extractor is a dust extractor. I probably didn't have the insight or confidence to ask on these forums at that time, so I bought a secondhand 1hp Hare and Forbes machine with a cfm rating of 600. It's performance was poor, especially on the machines that generate a lot of waste e.g. thicknesser. But it was cheap.

Now I am simply a novice, I have investigated and bought a Carbatec 2hp machine at 1200cfm. It has a 5 micron needle felt bag - and this is also a definite plus from the secondhand H&F that I had - the H&F had a cloth bag with no needlefelt bag and the fine dust escaped straight through.

I can easily connect to 2 machines at a time on runs of 2m of hose e.g. table saw and bandsaw, however I still use the thicknesser / jointer alone. There is a tremendous subjective improvement.

In response to one other of your q's: I believe at least the carbatec machines are rated to 5 microns with needle felt bags (both the 1hp FM-230 and the 2hp FM-300). They can both be 'upgraded' to a 1 micron pleated filter (which replaces the top cloth bag) for ~$170 - $180.

Pleated Filter Cartridges & Accessories : CARBA-TEC

My message may therefore be if the budget allows then stretch to a 2hp unit. There is lots of other posts and threads here that cover the technical details of dust extraction, and I am sure lots who might comment that even my dusty isn't ideal. The point is I have found it ideal for me. But as I increasingly found out, even if you were to grab a 1hp unit, in woodworking theres not necessarily a black and white right and wrong - a 1hp unit with short runs of hose and with some research into the motor sizes as mentioned in these previous posts, will let you make an informed decision that suits all your needs and budget.

(by the way - what is the main machines / tasks you'll be using it for?)

Nick

As Fine Woodworking found, there's no standard for measuring the CFM, so it can be measured directly at the intake, or down a 3 inch pipe, or down a 4 inch pipe, or even calculated theoretically from impeller design, or - as I would expect happens more often than any other approach - quoted based on the performance of a similar sized impeller in some other manufacturers catalogue and then fiddled with "our impellor is 10% bigger than that one, so it should have at least 10% more of whatever is needed to sell it!"

Performance of dustys is strongly dependent on the design and tolerances used in the manufacture of the fan. Fans could be made more efficient if closer tolerances are use but unless carefully designed and made (ie more $$) this also makes fans noiser. Most tests are without any ducting or pipe attached, or dust collector bags. I have never seen a test with "Seasoned" bags.

From Bill Pentz site

Seasoning means loading the filters up with the fine dust that ends up getting trapped in the filter pores and adds to filtering efficiency as well as overall resistance. A brand new very fine filter that attached to a unit that moves 800 CFM when new can so smother the airflow it barely moves 300 CFM when full “seasoned”. It takes about three cleaning cycles for a filter to get more than half its eventual maximum filtering and about nine cleaning cycles to reach that maximum “seasoning” which is also its maximum resistance.

This is also my experience of measuring air flows in working DCs. My old 1HP unit was rated at 600 cfm but that was with no bag and no ducting. As soon as I added cleaned seasoned 30 micron bag it dropped to ~350 cfm. Adding a few metres of 4" ducting and after the bag was 1/6th full, the cfm dropped to less than 250 cfm.

The same happened with a 2HP rated at 1500 cfm but to get that I had to remove the bag and the "impeller to 4" ducting adapter". Most people do not realise what a performance killer those adapters and 4" ducting are on such systems. The actual working flow rate with a used seasoned bag and about 6m of 4" ducting was around the 600 cfm rate.

BTW the CFM is only one of 2 inter related specs needed for successful dust removal. The target specs for dust removal are 1000 cfm and 4000 fpm linear air speed - bth have to be achieved under real working conditions. 1 HP DC physically cannot achieve these specs. 2HP also cannot achieve this using 4" ducting although they come close if short run 6" ducting and clean bags are used. A 3HP is better but a 5HP is the generally accepted minimum for workshop length runs of ducting

Originally Posted by BobL

Performance of dustys is strongly dependent on the design and tolerances used in the manufacture of the fan. Fans could be made more efficient if closer tolerances are use but unless carefully designed and made (ie more $$) this also makes fans noiser. Most tests are without any ducting or pipe attached, or dust collector bags. I have never seen a test with "Seasoned" bags.

From Bill Pentz site

Unfortunately Bob is quite correct. (Another very succinct post, Bob) IMHO any company that tests "Dust extractors" without any ducting, piping or dust separators/bags is acting very dishonestly. Without the ducting and dust collector it is not a dust collector, it is a fan. But big numbers sell; marketing triumphs over ethics and integrity.

Recently I struck a similar issue; we are replacing our kitchen and I wondered why Italian range hoods out-performed German and Australian range hoods by such a large margin. Generally the Germans lead on engineering performance.

For one model the Germans gave separate figures for 1.5 and 2.0 metres of 150 and 200 mm ducting and had the stainless filters in place for the testing. The Italian measures were without filters or ducting. Big difference. But many people do not make the basic enquiries.

It really is worth studying Bill Pentz's stuff in depth; a lot of very useful information and could stop one making expensive mistakes.

Cheers

Graeme

Originally Posted by JillB

Hello, I have been looking at portable 1hp ( 750W) dust extractors and find that the cfm on thse range wildly from 500cfm to 914 cfm, depending on the manufacturer.

Does anyone know the reason for this wide range of flow rates?

Also, can anyone please recommend a unit with a high cfm flow rate? Are they 30 micron bags or 5 micron?

Selecting a dust extractor is not a simple exercise.

To do it properly you need to have details of your system i.e. ducting size and length, and the connected machinery resistance to the airflow. Additionally you need the fan curve ( graph plotting the pressure against volume ). Using these you may then see what volume a particular fan can deliver for the resistance on your system.
I have asked Dust extractor ( fan ) suppliers on several occasions to provide details, & they have always failed to do so.

I suspect there is some poetic licence applied to the flow rates which are most likely the air quantity at zero resistance i.e. nothing connect to the fan. When ducting & equipment is connected the air flow will decrease, by how much will depend on the design of the ducting and equipment and also the design of the fan itself. The filters on a fan will add to the resistance and decrease the air flow the finer the filter the greater the reduction of airflow and as a particular filter loads up this will increase the filtration but also reduce the airflow.
To go through the correct selection process assuming you can't get the info & do the mathematics yourself or get a friend to do it for you, will likely cost more than the unit you are looking at buying. Alternatively you could go to someone who specialises in Dust Extraction units who is prepared to advise on your system and recommend a suitable unit.

As pointed out by others Bill Pentz's site has lots of detailed info to assist. However, if you have cost constraints, for a portable unit using flex to connect up to a single machine the most cost effective is likely to be seeking advice from those who have similar setups.

I hope you can get to where you wish to be and don't forget that you are dealing with an issue that will affect your health.


Regards

Originally Posted by Basilg

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However, if you have cost constraints, for a portable unit using flex to connect up to a single machine the most cost effective is likely to be seeking advice from those who have similar setups.

Unfortunately I have to respectfully disagree. Like most things that, "if people cannot see it, it mustn't be there", the majority of DC still users think if they cannot see see any dust it must not be there. Seeking advice from users of; low HP systems, 1 micron filters, vacuum cleaners, DCs and cyclones (without final absolute filters) that vent back into a workshop; who think their system is safe is yet more of blind leading the blind.

Most dust collectors should really be just called chip collectors.

Bob

I have no problem with your disagreement, but you should note that I started the post with " To do it properly .... " & also referenced Bill Pentz's site for info.

The section you have quoted was really aimed at someone who may have very limited finances, & simply cannot afford to have it done correctly, and rather than them do nothing, I think it would be better (but not preferable ) they seek advice from other users, and then sort the wheat from the chaff, before proceeding.
My preference & recommendation would be for people to address the reduction of dust in a workshop properly wherever possible.

I also closed with a caution on Health.

Regards