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Calculating CFM and Static Pressure.... Confused

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Forum topic by mrramsey posted 10-29-2016 01:34 PM 2785 views 1 time favorited 18 replies Add to Favorites Watch
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mrramsey

33 posts in 434 days


10-29-2016 01:34 PM

OK – I am trying to find a DC that will deliver ~700-800 CFM at the tool. I will most likely just have a mobile unit with a 10’ hose.

Most all of my machines will have 4” but I will probably add secondary 2.5” for dust collection say on the TS guard etc. So a machine I was looking at stated 880CFM @ 1.9”. When I am reading the Flow Chart for static pressure is the CFM going to be the same regardless of the size of pipe / hose being used?? Is the SP the governing rule?

The DC has a 6” port. Would it be best to drop to 4” for 10’ or use 5 or 6 10’ hose all the way to the tool and reduce at the tool?

-- Mike ~~~ Experience is what you get right after you needed it. ~~~


18 replies so far

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GR8HUNTER

2966 posts in 551 days


#1 posted 10-29-2016 01:48 PM

the pressure generated by fans in ductwork is typically very small l…....... an accurate measurement of static presure is critical to proper fan selection however…...... static pressure in fan systems is typically less than 2’’ SP, or 0.072 Psi…......the amount of static pressure that the fan must overcome depends on the air velocity in the ductwork, the number of duct turns and other resistive elements and the duct length…........so in your case with only a 10 foot hose I am pretty sure you will have no worries

-- Tony Reinholds,Pa. REMEMBER TO ALWAYS HAVE FUN

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AZWoody

1138 posts in 1062 days


#2 posted 10-29-2016 02:31 PM

Most machines overstate the cfm they can generate.

Also at 4” you’re more than likely going to peak at 400 cfm or thereabouts unless you have an oversized motor/impeller combo. I’d stick to 6”. That is over double the overall capacity of a 4” hose and you might be able to get the cfm you want.

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mrramsey

33 posts in 434 days


#3 posted 10-29-2016 02:45 PM



Most machines overstate the cfm they can generate.

Also at 4” you re more than likely going to peak at 400 cfm or thereabouts unless you have an oversized motor/impeller combo. I d stick to 6”. That is over double the overall capacity of a 4” hose and you might be able to get the cfm you want.

- AZWoody

The machine I was considering was the grizzly G0777. Specifically due to the longer cone design of the cyclone. If I go by the flow chart in the manual here is what I come up with… tell me if I am doing this correctly.

10’ of 6” flex to the machine (I would consider this as main line) is 0.136 per foot loss – 1.36”SP. Since I would reduce to 4” typically I will add a foot of 4” loss @ 0.075” and another 1” loss for a seasoned filter. Total SP I come up with would be 2.435”. 3.0 SP on the chard shows approximately 750 CFM which sounds adequate for my purposes.

-- Mike ~~~ Experience is what you get right after you needed it. ~~~

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BobAnderton

239 posts in 2629 days


#4 posted 10-29-2016 02:51 PM

Short answer AZWoody is right, 6” duct and tool ports will be necessary to pull 700-800 cfm at the tool through 10 feet of flex duct.

Download the staticcalc Excel spreadsheet from Bill Pentz’s site here. The FAQ for it is here.

Plug 800 cfm into the “CFM Required” cell and you can do all kinds of “what-ifs” by changing inputs. What I see is at 800 cfm there is 10 inches of static pressure drop across 10 feet of 4” flex duct vs only 1.3 inches of static pressure drop across 10 feet of 6 inch duct at that flow rate. You can also explore the impact of 4” tool ports vs 6” tool ports.

—Edit—MrRamsey, you posted while I was typing I guess. The numbers you’re getting from the manual sound like they agree with what I got using the spreadsheet pretty well. Just wanted to explain that I hadn’t seen your post when I posted the above.

-- Bob Anderton - Austin, TX - Nova 3000 lathe, Alaskan Mark III mill, Husqavarna Saw

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mrramsey

33 posts in 434 days


#5 posted 10-29-2016 03:16 PM



Short answer AZWoody is right, 6” duct and tool ports will be necessary to pull 700-800 cfm at the tool through 10 feet of flex duct.

Download the staticcalc Excel spreadsheet from Bill Pentz s site here. The FAQ for it is here.

Plug 800 cfm into the “CFM Required” cell and you can do all kinds of “what-ifs” by changing inputs. What I see is at 800 cfm there is 10 inches of static pressure drop across 10 feet of 4” flex duct vs only 1.3 inches of static pressure drop across 10 feet of 6 inch duct at that flow rate. You can also explore the impact of 4” tool ports vs 6” tool ports.

- BobAnderton

Thaks Bob, that sheet is helpful. I do know that 6” tool ports would be a benefit for sure and I will likely convert them at some point however trying to size things for what is in place now.

-- Mike ~~~ Experience is what you get right after you needed it. ~~~

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JBrow

1274 posts in 759 days


#6 posted 10-29-2016 04:39 PM

mrramsey,

I would think that using a smooth walled 5” flex hose from the dust collector to table saw where the split and reduction occurs using a 6” x 4” D/C Adapter would give you the best performance. I base my conclusion on the following reasoning.

The larger the pipe the lower the static pressure, but over a distance of 10’, the added static pressure of a 4” pipe over a 5” pipe is not much. However, reducing the diameter of the pipe will reduce the volume of air flowing through the pipe. Therefore matching the cross sectional area of the mainline pipe to the cross sectional areas of the drops added together leads me to believe that this is the way to go to maintain performance. However, whatever the mainline pipe size, the velocity of air moving through the main pipe needs to kept at 3500 feet per minute. If the pipe is too large the air velocity can slow too much and can allow dust and debris settle in the pipe even when air is moving.

By my calculations I figure that the cross sectional area of the 2.5” diameter inlet at the saw guard is 4.9 square inches. I assume the table saw has a lower dust port that is 4” in diameter, which is 12.6 square inches. When these are added together, the total cross sectional area of both inlets is 17.5 square inches. The cross sectional area of a 5” pipe is 19.6 square inches.

Therefore it would seem that a mainline that has a 5” diameter would promote better performance than a 4” diameter mainline pipe. I agree with AZWoody’s skepticism of the manufacturer’s stated 880 cfm as it relates to air flow in your mainline. Therefore I am not sure that a 6” flex hose would work as well as a 5” flex hose. A volume of air equal to or greater than 700 cubic feet per minute is required for a 6” main line pipe and 490 cubic feet per minute for a 5” main line pipe to maintain 3500 feet per minute in the main line. If I got my calculations correct, the 5” pipe is a safer alternative to the 6” main line pipe. If the dust collector pulls less than 700 cubic feet per minute through the 6” pipe, then the potential exists for debris to settle out of the air stream.

Whether you elect to use a 5”or 6” flex hose as your mainline, I think the manner in which the transition is made from the main flex hose to the table saw is important. If, for example, the flex hose is transitioned with a 4” x 4” x 4” wye fitting where one leg is further reduced to 2.5” reduced air flow is likely to occur in the main flex line since both legs from the table saw must pass through the main 4” section at the flex hose. This reduction could slow the flow of air in the main line. An alternative is to transition from the main line with a 6” x 4” D/C Adapter and then reduce one 4” leg to 2.5”. It seems to me that this method would maximize air flow in the main line since the available cross section area of the 6” x 4” D/C Adapter is 25.2 square inches (2 X 12.6 square inches), rather than 12.6 square inches of the 4” x 4”x 4’wye.

http://www.grizzly.com/products/6-x-4-D-C-Adapter/D4240

Another performance consideration is the type of flex hose. Deeply corrugated inexpensive flex hose can increase static pressure in the mainline by as much as 50% over smooth walled pipe and thus degrade performance. More expensive smooth walled flex hose is available and would add less to the static pressure of the system, although I am not sure by how much.

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ArtMann

688 posts in 655 days


#7 posted 10-30-2016 05:30 AM

The quoted cfm ratings for dust collectors is a near worthless number because it refers to a dust collector with little or nothing attached to it. Different machines may behave very differently as the flow restriction increases. Even 10 feet of flex hose will make a substantial difference in the CFM. You need the fan curve at different SP values and an estimate of the total SP of your system to make any conclusions.

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Redoak49

2904 posts in 1827 days


#8 posted 10-30-2016 11:30 AM

ArtMann is so correct about fan curves. So many manufacturers put out worthless numbers. It is about like the 6.5 hp vacuums.

There are a few who put out good data and tell how they tested them. There are also some relatively good magazines who have tested them.

It would be great if people would also do some measurements of their systems and post the info to help others. Too many people report how great their dust collector is and have no idea what the actual cfm really is.

End Rant….

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Manitario

2565 posts in 2721 days


#9 posted 10-30-2016 12:43 PM

A lot of helpful comments and advice above. Dust collection has been made into an (often) hugely complicated issue, partially by misinformation from manufacturers and partially by a lot of inaccurate stuff online.
Figuring out what DC you need and/or what sort of set-up can be made a lot simpler;
1)Find a good static pressure caluculator eg. as BobAnderton mentioned, the Bill Pentz one is the simplest and easiest to use of all that I’ve seen (the one in the Grizzly manual is horribly complicated to figure out).
2)Figure out the max static pressure for your system, ie. the longest run with the smallest diameter pipe.
3)Plug this number into your fan curve to determine what CFM you’re going to get from your DC.

This will give you a pretty good approximation of what you’re going to get from your setup.

As for your specific DC; it has a massive impeller, which is great for CFM; but is a small motor which means it won’t tolerate much static pressure in your system without huge CFM drop-off.

-- Sometimes the creative process requires foul language. -- Charles Neil

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mrramsey

33 posts in 434 days


#10 posted 10-30-2016 03:40 PM



A lot of helpful comments and advice above. Dust collection has been made into an (often) hugely complicated issue, partially by misinformation from manufacturers and partially by a lot of inaccurate stuff online.
Figuring out what DC you need and/or what sort of set-up can be made a lot simpler;
1)Find a good static pressure caluculator eg. as BobAnderton mentioned, the Bill Pentz one is the simplest and easiest to use of all that I ve seen (the one in the Grizzly manual is horribly complicated to figure out).
2)Figure out the max static pressure for your system, ie. the longest run with the smallest diameter pipe.
3)Plug this number into your fan curve to determine what CFM you re going to get from your DC.

This will give you a pretty good approximation of what you re going to get from your setup.

As for your specific DC; it has a massive impeller, which is great for CFM; but is a small motor which means it won t tolerate much static pressure in your system without huge CFM drop-off.

- Manitario

Yes I agree that it can get overly complicated. I had a similar exercise when I built my saltwater reef aquarium. I had a lot of plumbing on drain lines ad return lines, calculating the flow rates based on pipe diameters etc then matching up to the actual pump curve for head loss.

I have figured out my SP’s based on my longest run and worst run. They were very similar. To achieve 800 cfm, if I run 100% 6” rigid pipe and a short piece of 6” flex to the machine I calculated 4.87”on one leg and 4.46” on the other. That included the additional 2” loss for a seasoned filter. Using the fan curve from grizzly as an example do I just look at the curve and plot the calculated SP for the approximate CFM? How do you apply this logic to say the Laguna CFlex 1.5 or 2HP collector where there is not a fan curve listed (that I could find anyway)? They just give a nominal cfm and a real cfm.

-- Mike ~~~ Experience is what you get right after you needed it. ~~~

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Manitario

2565 posts in 2721 days


#11 posted 10-30-2016 06:08 PM


Using the fan curve from grizzly as an example do I just look at the curve and plot the calculated SP for the approximate CFM? How do you apply this logic to say the Laguna CFlex 1.5 or 2HP collector where there is not a fan curve listed (that I could find anyway)? They just give a nominal cfm and a real cfm.

- mrramsey


Yep, that’s exactly how you do it. Frustratingly, a lot of companies don’t provide fan curves. Despite the hype that different companies put on their own systems, I think probably most of the systems have around the same performance for similar sized motors and impellers. The main difference is in cyclone design ie. more/less efficient at particle separation and in filter size; I find most companies have very small filter sizes; larger filter area = less back pressure and better DC performance.

-- Sometimes the creative process requires foul language. -- Charles Neil

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Mosquito

9113 posts in 2131 days


#12 posted 10-30-2016 06:25 PM

When I am reading the Flow Chart for static pressure is the CFM going to be the same regardless of the size of pipe / hose being used?? Is the SP the governing rule?

- mrramsey

Similar to what ArtMann mentioned briefly, what a manufacturer states as the maximum CFM, is at 0 SP, meaning no restrictions in the system at all. Think fan with no ducting at all. In exactly the same way, the SP rating is at 100% restriction. Meaning if you cap off the port completely, that’s your peak SP.

The way you would plot the curve, is having SP as the Y axis and CFM on the X axis (or flip them, doesn’t really matter). As you move further to higher CFM, then the lower the static pressure is. Similarly if you increase the static pressure (reduce the diameter of your piping, as an example), you will get reduced CFM as a result.

This assumes that it doesn’t behave like an electronic speed control on a router, where it ramps up the power to keep a certain RPM. Though that would be an interesting concept for a dust collector to impliment too… holding a certain SP or CFM regardless (to a certain extent) of the restriction… hmmmm.

I’ve done a fair bit of testing of PC watercooling components measuring flow rates, static pressure, and plotting out performance curves for pumps, radiators, and waterblocks. Finding real world performance curves are interesting stuff if you’re into that sort of experimenting and discovery lol.

-- Mos - Twin Cities, MN - http://www.youtube.com/MosquitoMods - http://www.TheModsquito.com

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mrramsey

33 posts in 434 days


#13 posted 10-31-2016 05:00 PM



Similar to what ArtMann mentioned briefly, what a manufacturer states as the maximum CFM, is at 0 SP, meaning no restrictions in the system at all. Think fan with no ducting at all. In exactly the same way, the SP rating is at 100% restriction. Meaning if you cap off the port completely, that s your peak SP.

OK I think I have figured it out. My design target is 800 CFM at the machine. I have laid out a duct plan that has 2 branches ending in 2 hose ports at each branch. Running all 6” metal duct and reducing to 5” at the blast gate. Using the Static Pressure spread sheet from Bill Pentz I came up with 5.34” w/c and 5.84” w/c on each branch. Looking at the fan curve on the Grizzly G0443 1.5 HP DC I would see about 870ish CFM

-- Mike ~~~ Experience is what you get right after you needed it. ~~~

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Redoak49

2904 posts in 1827 days


#14 posted 10-31-2016 07:29 PM

IMHO the Grizzly fan curve for a 1.5 hp cyclone is overly optimistic. I base this on test results that Wood magazine did testing smaller cyclones. I think the article was in December 2003 and titled Twister Tests. To get the results you are looking for I think t

My experience is that companies typically are optimistic with the performance curves. With the cyclone I purchased I found the Oneida data to be 10-15% higher than what I measured. I think you will have much less biased information from magazines which do testing like Wood Magazine. Other sources are people who do actual testing with good instruments.

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mrramsey

33 posts in 434 days


#15 posted 10-31-2016 08:27 PM


IMHO the Grizzly fan curve for a 1.5 hp cyclone is overly optimistic. I base this on test results that Wood magazine did testing smaller cyclones. I think the article was in December 2003 and titled Twister Tests. To get the results you are looking for I think t

My experience is that companies typically are optimistic with the performance curves. With the cyclone I purchased I found the Oneida data to be 10-15% higher than what I measured. I think you will have much less biased information from magazines which do testing like Wood Magazine. Other sources are people who do actual testing with good instruments.

- Redoak49

I looked at that article in Wood Magazine from 2003 – Not the same collector. If I take off 15% still puts me around 750 CFM with a seasoned filter. I could go with their 2HP system.

Here is my current design for the ducting. Very short runs and all about equal as far as SP is concerned. All 6” pipe to 5” blast gates. Just for giggles I got a quote on Nordfab QF duct. Just for the right side of the layout it was $2417. LOL thats crazy expensive stuff… maybe when I get my forever shop.
!

!Click here for Full Size Image!

-- Mike ~~~ Experience is what you get right after you needed it. ~~~

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