Thread: Real DC flow versus Manufacturers specifications

Originally Posted by BobL

Recently I saw the details of the air flow tests performed by a manufacturers of generic DCs.

Good Morning Bob

Are the above tests in the public domain ?


Fair Winds

Graeme

Originally Posted by GraemeCook

Good Morning Bob
Are the above tests in the public domain ?
Fair Winds

Graeme

Do you mean the test "procedure" or the "results"?

If its the former then there is not much more to it than I describe in the original post.
Basically;
Remove impeller from the DC, and remove all secondary inlets and outlet flanges from the impeller.
Add a test duct of appropriate length and diam to suit the impeller inlet.
Insert pitot tube in middle of test duct
Measure pressure differential using pitot tube and convert to a velocity , and then flow rate using cross sectional are of test duct.'

If you mean the results then they are are the ones listed on websites and in brochures etc.

Sorry, Bob. I was just being very cautious in case you were refering to propriety information that you could not reveal in detail.

You have basically answered my concerns, I think. What I was looking for was some insights into how fast that efficiency factor drops off from the !00% manufacturers measurement to your realistic 50%. Obviously that drop off factor would be much more for long lengths of rough, narrow pipe with many bends, rather than smooth, strait, large bore pipes.

In some set-ups I have seen, I suspect that your 50% would be optimistic.


Fair Winds
Graeme

Originally Posted by GraemeCook

Sorry, Bob. I was just being very cautious in case you were refering to propriety information that you could not reveal in detail.

The tests for the impeller are fairly standard and are also what Bill Pentz does except Bill does allow for the fact that air in a pipe moves significantly slower at the edge of the pipe and that is also where the largest cross sectional area is.

You have basically answered my concerns, I think. What I was looking for was some insights into how fast that efficiency factor drops off from the !00% manufacturers measurement to your realistic 50%. Obviously that drop off factor would be much more for long lengths of rough, narrow pipe with many bends, rather than smooth, strait, large bore pipes. In some set-ups I have seen, I suspect that your 50% would be optimistic.

Correct. Factors of 6 of more between the manufacturers spec and reality are not unusual. A typical shed set up I see is a generic 2 HP DC with 10 m of 4" ducting and flex and maybe a chip catcher connected to a 4" port on a machine that can't really breathe. The manufacturers spec is 1200 cfm, so the hobbist thinks "I'm good to go" but the real performance can be less than 200 cfm. In a similar vein I have seen several 1HP DC (manufacturers spec of 650 cfm) with say 5 m of 4" and 2" ducting/flexy connected to a belt sander and a filthy/choked filter bag - real flow <100 cfm

Thanks Bob

As the V8 enthusiasts say ... "There ain't no substitute for cubic inches." ...................... big motor, big impellor, big pipes.....

Fair Winds

Graeme

No real surprises for me there, Bob. I've always mentally deducted about 50% of claimed ratings from any mfr., whether it be flow, HP or whatever, so have rarely been disappointed in performance.

(Besides, if something needs to perform at X rating it's much better to buy something at a 2X rating. To allow for future productivity... or if the machine only performs half as well as advertised, it should at least cope with what is needed now ratjher than being a total waste of coin. Make sense? )

I guess manufacturers see 'cfm' differently to the International Standards Board... much like the way Hard Drive manufacturers have invented 'decimal Terabytes' for their advertising promo's, rather than 'real TBs.'

It almost restores my faith in human nature... 'cept I'm in a good mood and don't feel like being depressed.

Originally Posted by Skew ChiDAMN!!

No real surprises for me there, Bob. I've always mentally deducted about 50% of claimed ratings from any mfr., whether it be flow, HP or whatever, so have rarely been disappointed in performance.

(Besides, if something needs to perform at X rating it's much better to buy something at a 2X rating. )

Hi Skew

My reading was that Bob was saying that you need at least 2x minimum, and possibly as much as 6x. Design is really critical.

Fair Winds
Graeme

Originally Posted by Skew ChiDAMN!!

No real surprises for me there, Bob. I've always mentally deducted about 50% of claimed ratings from any mfr., whether it be flow, HP or whatever, so have rarely been disappointed in performance.

I agree - no surprises but once again some disappointment.

Because current varies with the load, the HP rating on most machines is always a nominal value. For example my 3HP rated TS motor the free running HP is less than 2HP, and by pushing it hard it's not too difficult to get it to generate 4HP for a short period. I would think the HP ratings on motors would be for some sort of a "normal maximum load" which is what I try to put DC motors under when I measure their HP. In general when I have assessed the power rating of machine motors I have not been disappointed with the measured HP as they are usually within 10% of the rating. The motor on the DC I am currently playing with in this thread has been one that is well below the specified rating, but certainly not the lowest I have measured.

It's interesting how we seem to accept some specs but not others. For example how many people would accept a 1/4" drill if they went to buy a 1/2" drill, or 1.2 m long piece of wood if they needed a 3.6m long board? The bottom line criteria is usually "does it do the job". If a drill says it is 750 W but it only 500W but it still does the job nobody seems to care too much.

If CFM was a performance spec like HP then it would be neither here nor there, i.e. who really cares if drill takes a little longer to drill the hole. The difference with CFM is now that someone has finally determined that 1000 cfm is needed to capture dust at source, some folks out there are trying to meet that criteria by purchasing a commercially available DC. The issue would not be that significant if a 1200 cfm rated DC could deliver a real say 900 or even 800 cfm, but 567 cfm before it is even connected to a machine and ducting is IMHO just going a bit far. The problem is compounded by the fact that 567 CFM will do a nice job visually ie chip collecting and few folks would realize that it is not capturing the finer dust at source

Even Aussie industry is also way behind on this whole issue. They (like most of the rest of the world) are still operating at the mg/m^3 level which makes no allowance for particle sizes.