Question for the dust collector gurus

[Mike Desch]

Howdy:

Still working on the design of my dust collector system.
I have a small 2 HP cyclone with only a 5" duct inlet.

Main duct thru the shop is 5", and all the drops are 4".

Question is whether I need to always have about 3 sq. inches of open pipe (no matter which drop is being used)
so that the DC is always "seeing" about 12 1/2 sq. inches of air inlet.

Since my collector is CFM challenged, I am trying to get the best performance I can out of what I have.

OK, gurus--pipe up with your comments. (In the past, they have been most helpful.

Thanks.

[Alan Schaffter]

Howdy:

Still working on the design of my dust collector system.
I have a small 2 HP cyclone with only a 5" duct inlet.

Main duct thru the shop is 5", and all the drops are 4".

Question is whether I need to always have about 3 sq. inches of open pipe (no matter which drop is being used)
so that the DC is always "seeing" about 12 1/2 sq. inches of air inlet.

Since my collector is CFM challenged, I am trying to get the best performance I can out of what I have.

OK, gurus--pipe up with your comments. (In the past, they have been most helpful.

Thanks.

I don't understand your question about always needing "3 sq. inches of open pipe (no matter which drop is being used) so that the DC is always "seeing" about 12 1/2 sq. inches of air inlet."

With a 5" diam. opening, your DC has 19.6" sq in. of inlet area, but by reducing to 4" for your drops you have limited it to 12.5 sq. in. Since in our systems air is treated as non-compressible, that (and velocity to some degree) is what will determine what CFM the DC can move (effectively "see"?) Your max CFM at a port is determined by the diam. of the drop/port- you don't get it back by enlarging the duct or DC inlet. Conversely, you will never move more CFM than can pass through the 5" duct or DC inlet, regardless of number of ports open.

Here's the deal. Typically with hobby shop DC setups, there is no reason to reduce the diam. of duct at the drops. The main reasons for doing so are (1) to maintain sufficient velocity so chips don't fall out of the air stream and begin to clog the duct- rarely a problem in our systems unless you have a woefully underpowered DC or extra long duct or one with many bends and high SP, and (2) to help balance the system (think HVAC) if you will regularly have more than one port open at a time- not recommended with a small 2 hp DC. Not all 2 hp DCs are created equal. You didn't say brand or impeller diam.

That being said, you can "collect" from smaller diam. ports but you risk (1) reducing the flow though the cyclone and therefore its separation efficiency, (2) may not have the CFM to properly capture airborne dust at the source (you likely don't have much capability for that anyway). I run 6" everywhere right up to the machine, except at my drill press where I do not create fine dust (no drum sanding) and the dual 2.5" pickups are right at the tip of the bit, and at my router table, the configuration of which also does a great job capturing the dust via a 4" port.

As far as stress on the blower motor, small or even closed ports mean high static pressure and low CFM- the motor/blower will be moving less air and therefore not working as hard.

[Michael W. Clark]

I second most of what Alan said. Not sure I understand the 3 in2 vs 12.5 in2 portion either??

If you are only using one machine at a time, run 5" or 6" all the way to the tools. Enlarging the ports or splitting the 6"(or 5") into multple pick-up points at the tool is also highly recommended.

Mike

[Mike Desch]

Hi guys:

Thanks for the responses.
Sorry for the lousy math.

I see why my post was confusing.

Yes, the 5" duct has a cross-sectional area of 19.625 and the 4" duct has a cross-sectional area of 12.56.

My concern was getting enough CFM thru the 5" pipe to keep things moving instead of settling.

Thought I read somewhere that folks often leave a second Blast partially open to allow the dust collector
to be able to pull the full amount of air, instead of throttling it down to the size of a drop (and losing the CFM throughput). I was hoping that I could make an arrangement at the end of the main duct to supply the makeup air the equivalent of about 7" so the dust collector gets full air flow through the 5" duct.

Where is my screwed up thinking messed up--pulling 12.56 inches from any of the 4" drops, and getting another 7" or so from the end of the main duct, so that the dust collector is always seeing 19.6"???

[Alan Schaffter]

Hi guys:

Thanks for the responses.
Sorry for the lousy math.

I see why my post was confusing.

Yes, the 5" duct has a cross-sectional area of 19.625 and the 4" duct has a cross-sectional area of 12.56.

My concern was getting enough CFM thru the 5" pipe to keep things moving instead of settling.

Thought I read somewhere that folks often leave a second Blast partially open to allow the dust collector
to be able to pull the full amount of air, instead of throttling it down to the size of a drop (and losing the CFM throughput). I was hoping that I could make an arrangement at the end of the main duct to supply the makeup air the equivalent of about 7" so the dust collector gets full air flow through the 5" duct.

Where is my screwed up thinking messed up--pulling 12.56 inches from any of the 4" drops, and getting another 7" or so from the end of the main duct, so that the dust collector is always seeing 19.6"???

What are you trying to do, collect dust, or collect chips? This issue keeps popping up from time to time, but I really can't ever see a need to open a second gate when you don't need to- it cuts your collection (CFM and SP) at the source of dust in half at best. If the open/unused gate is upstream it will be much worse than half. Settling dust in the duct is just not much of a problem in home shops with a 2 hp DC. You never did say the length of your runs. Other than reducing the chance of insufficient separation in a cyclone (which is a function of velocity, not CFM), what possible reason is there to have the "collector seeing 19.6 sq.in." (that is a cross-section anyway, not a volume) it will not improve CFM or SP at any drop. You are never going to see more than the max CFM possible through a 4" line through any of your 4" drops, regardless of the diameter of upstream pipe .

[David Kumm]

. You are never going to see more than the max CFM possible through a 4" line through any of your 4" drops, regardless of the diameter of upstream pipe .[/QUOTE]

Not exactly right. Because CFM drops as SP increases you will get a t least a little and maybe a fair amount of additional cfm through a certain sized port if the main lines are properly larger and the runs are long with some elbows. Cfm can drop pretty quickly with even 1" additional resistance so it pays to be careful in sizing pipe relative to your blower and its capacity to deliver cfm under pressure. A fine line. Dave

[Ole Anderson]

You need to maximize airflow through the hood of the tool in use. To open the duct elsewhere to allow more air to help move chips down the duct will only reduce the efficiency of collection at the tool, which is what dust collection is all about.

You already have your main run sized at 5". Too bad. You should have gone with a 6" main (unless you have a very short main run) even though your DC inlet is 5" so as to reduce friction and subsequent losses. Many 2 hp units optimise with a 7" main duct run.

[Alan Schaffter]

.
You are never going to see more than the max CFM possible through a 4" line through any of your 4" drops, regardless of the diameter of upstream pipe .

Not exactly right. Because CFM drops as SP increases you will get a t least a little and maybe a fair amount of additional cfm through a certain sized port if the main lines are properly larger and the runs are long with some elbows. Cfm can drop pretty quickly with even 1" additional resistance so it pays to be careful in sizing pipe relative to your blower and its capacity to deliver cfm under pressure. A fine line. Dave

True. We don't know his duct length or configuration. Assuming he as a relatively straight shot to his DC and a 5" intake on the DC, he really won't see much of a change. The limiting factors in order are (1) diam. of machine port, (2) diam. of drop, and (3) diam. of the DC intake. Unless the main duct is very long or has multiple bends, increasing the diam. beyond 5" will have little to no effect on CFM at the port. It will result in a decrease in the velocity in the main however.

Here is a good (?) analogy- you have 10' deep filled swimming pool (the pool is a super large duct) with a 5" diam. drain hole at the bottom and a 10' long, 5" diam. pipe (small DC duct) standing on end also filled with water. The weight of the water is the same in both cases - a 10' column of water. For a given amount of time, the amount of water that will drain from the pool through the 5" drain is the same as the amount water that will drain from the pipe.

[David Kumm]

Alan, we are on the same page. In this case there isn't any choice larger than a 5" main and the less 4" the better. Where I see the advantage is where a 5 hp, 16" impeller system is tied to 6" mains. Increasing the main- assuming the impeller inlet is larger to 7 or 8" for some length has a measurable benefit, particularly with the BI blades used in most systems. I'm a little more concerned with pipe size because I run 7-8000 fpm through my pipes and the resistance really goes up with velocity. Dave

[Jim Neeley]

Mike,

Here's my $0.02...

Opinions will differ as to whether or not you should have another blast gate open while you are collecting from an "undersized" port. The basis for this question is whether or not the mains will maintain sufficient air velocity to sweep out all of the wood sucked up or whether you will have some settle to the bottom. I am facing a similar issue on my planned "central shop-vac" setup and I'll share my approach.

I want the maximum high-suction volume I can get when gathering from my sliding compound miter saw, so my plan is to take 2 existing Cra*sman shop-vacs and run them in parallel through 3" central vac mains. This will serve the blade guard collection on my SCMS well but won't work worth a darn when I'm using the 1" hose attachment on my random orbital sander. The velocity in the 1" hose will be great but I"m expecting the low capacity of the 1" hose will let some (or most?) of the sawdust settle in the mains. To accommodate this, here's my plan:

My sanding station will be connected to a wye-branch off of the mains, with a small gate. The straight leg will have a full size gate that I can use either of two ways: (1) I can open it part way when sanding to maintain the air flow or I'm hoping to (2) close the large gate while sanding, opening it up briefly when I'm done to sweep out the mains. My intent is to experiment with #2 and if I don't like the result (due to flow problems or shop-vac overheating) then utilize it explained in #1. Either way, the pieces are in place.

I'm not yet to the step of installing the central vac piping in my shop upgrade but it seems to be the most versatile approach to me.

Again, just my $0.02.. YMMV.


Jim in Alaska

[Ole Anderson]

I see we disagree on the worth of using 6" (or larger) ducts when the DC has a 5" inlet and he has 4" drops. One 4" drop will likely reduce the airflow to 300-400 cfm, easily handled by a 5" run. But if at some point Mike wants to upgrade his TS system to a 5" bottom and 3" top as recommended by BP, he really needs close to 800 cfm to adequately serve the TS. So, lets look at that 800 cfm (which his DC probably will move if there is a well designed duct system). Assume that with bends and straight run he has the equivalent of 50 feet of main run. At 800 cfm just the main run of 5" pipe will choke 5.5" of loss into the system, and starting with probably 6-8" of available suction from his cyclone, leaves little elsewhere. Increase that main run to 6" and the losses decrease to 2.2", increase that run to 7" and you drop to 1.0". Now you have enough left to account for filter, cyclone, drop and inlet losses. Maybe. As stated a 2 hp DC will be marginal, but with a 5" main run, you are dead.

There is a common misconception that the smallest of your pipes will in effect doom your system to that flow. Not so unless that smaller pipe is quite long. To verify this you need to do a pipe flow analysis with a tool such as the one done by BP.

[David Kumm]

Ole, I think we are only in disagreement based on the 2 hp system. You are correct but I have real doubts that you can pull 800 cfm through a small diameter cyclone with filters with a 2hp motor and likely a 12-13" impeller designed to not overamp the small motor. The configuration of the impeller to protect the motor means it won't operate well under high pressure so there may not be any way to juice the cfm enough to get velocity through the larger mains. All speculation with no details but in my limited experience it is pretty hard to design 800+ with less than a 3hp 14" impeller with a cyclone and filters. Dave

[Chris Parks]

By now the OP is totally confused and who can blame him.

[Alan Schaffter]

I see we disagree on the worth of using 6" (or larger) ducts when the DC has a 5" inlet and he has 4" drops. One 4" drop will likely reduce the airflow to 300-400 cfm, easily handled by a 5" run. But if at some point Mike wants to upgrade his TS system to a 5" bottom and 3" top as recommended by BP, he really needs close to 800 cfm to adequately serve the TS. So, lets look at that 800 cfm (which his DC probably will move if there is a well designed duct system). Assume that with bends and straight run he has the equivalent of 50 feet of main run. At 800 cfm just the main run of 5" pipe will choke 5.5" of loss into the system, and starting with probably 6-8" of available suction from his cyclone, leaves little elsewhere. Increase that main run to 6" and the losses decrease to 2.2", increase that run to 7" and you drop to 1.0". Now you have enough left to account for filter, cyclone, drop and inlet losses. Maybe. As stated a 2 hp DC will be marginal, but with a 5" main run, you are dead.

There is a common misconception that the smallest of your pipes will in effect doom your system to that flow. Not so unless that smaller pipe is quite long. To verify this you need to do a pipe flow analysis with a tool such as the one done by BP.

Ole, the problem is that the inlet to his DC is only 5". He would need to open that up to achieve any possible improvement in CFM which is doubtful, especially without over amping and possibly burning up the motor. Due to material handling blower/impeller design, air is considered non-compressible so the max amount of SP (and CFM) it will pull is restricted by the fan curve. Just like my water analogy, he's not going to get any more CFM through his 5" inlet than the fan curve will allow. If blower is already operating in a near vertical area of the fan curve with a 5" inlet, he won't see any improvement. Also, without changing drop/machine port or blower inlet diam. if he enlarges the mains, all he will do is reduce the velocity in there and risk dust precipitating from the air stream and clogging the duct.

Check out the Ducting FAQs (http://billpentz.com/Woodworking/Cyclone/ducting.cfm#FAQs) on BP site.

[Michael W. Clark]

In order to comment intelligently, a layout of the system would be required with lengths, number and location of elbows, tools connected, etc. If only running one machine at a time, then we only need the branch with the most losses.

The only reason to increase the duct size is to reduce the system losses and allow you to operate farther to the right on the fan curve (more CFM). There is a limit to the duct size as you want it small enough to maintain conveying velocity. The maximum duct size depends on this conveying velocity and the CFM of the blower.

The measured open area of the duct and/or bleed has nothing to do with the system losses and how much air the blower can pull. If you had a 6" long piece of 5" duct on the inlet of the blower, you would pull more air than if that duct were 100 feet long, but you have the same open area.

Opening the inlet to the cyclone is not going to make your pressure drop go down necessarily. However, it probably will reduce the efficiency because the inlet velocity will be reduced. The cyclone efficiency is related to the inlet velocity, which is also related directly to the CFM as long as the cyclone inlet area is fixed. Q=VA

Mike

[David Kumm]

I recently did something similar on my system. My blower is located in the attic. The outlet from the cyclone was reduced to 8" with an ell into a 9" blower inlet. The 8" outlet ran through an ell into cartridges. I wanted to eventually upgrade and install a 10" muffler so I changed the pipe from the cyclone to the blower to 10" and increased the outlet pipe from 8" to 10" and then back to 8" at the filters to accomodate the larger muffler. All in all a swapped about 10' of pipe and two ells from 8" to 10". No other piping has been changed yet because I'm lazy. My 7.5 hp system showed an increase in cfm at various ports in the range of 5-10%. I can't get scientific but my velocity definately went up according to my anemometer. Now this is all specific to my big system and impeller but the changes were pretty minimal in the big picture. When I swap more pipe and replace with a larger diamter cyclone I may have to dial the speed down a little to compensate. I'm running a little too much velocity into my cyclone so I would really be interested in how speed affects separation. Dave

[Michael W. Clark]

I'm running a little too much velocity into my cyclone so I would really be interested in how speed affects separation. Dave

The speed is anlogous to pressure drop for a fixed cyclone. The more CFM (and therefore velocity) the higher the pressure drop and more efficiency. However, if you change to a larger cyclone, but keep the inlet speed the same, your pressure drop will only go up slightly (tenths of an inch), but your efficiency will go up significantly. This is because you increased the residence time in the cyclone. Our high efficiency cyclone models use long residence time versus increased pressure drop. There is only so much efficiency to be gained from increased pressure drop before it is not practical. Also, because increased pressure drop means higher velocities, abrasion goes up significantly (with the cube of the velocity change). Abrasion is a major concern in most industrial wood processing facilities and other applications. Probably more than you wanted to know.

[David Kumm]

Michael, are you saying that if I increase my cyclone diameter from 20" to 24" but keep the inlet at 8" vs 10 to keep the velocity in the 6000+ fpm range as long as possible entering the cyclone rather than increasing the inlet pipe I will gain efficiency in fine dust separation? The pressure drop is irrelevant in my case due to my blower capacity. I had planned to change out the pipe nearest the cyclone to slow the velocity. Dave

[Michael W. Clark]

Michael, are you saying that if I increase my cyclone diameter from 20" to 24" but keep the inlet at 8" vs 10 to keep the velocity in the 6000+ fpm range as long as possible entering the cyclone rather than increasing the inlet pipe I will gain efficiency in fine dust separation? The pressure drop is irrelevant in my case due to my blower capacity. I had planned to change out the pipe nearest the cyclone to slow the velocity. Dave

Yes, that is correct. If you can increase the height as well, even better.

[Mike Desch]

Thanks guys:

I didn't mean to start a technical argument among the gurus--but I get the drift was what is being said.

Yeah, I screwed up years ago by getting a too-small cyclone, but for now I've gotta live with it.

I'm still digesting all the info in this thread--and a lot of head scratching before my feeble brain can digest it all.

Thanks for sharing your knowledge.

[David Kumm]

No argument Mike. This is how we all learn. Thanks for the question. Questions generally serve as a platform for tangents which add to all of our knowledge. I'm on my fourth system so I'm familiar with the too small dilemma. Dave

[Anthony Whitesell]

You need to maximize airflow through the hood of the tool in use. To open the duct elsewhere to allow more air to help move chips down the duct will only reduce the efficiency of collection at the tool, which is what dust collection is all about.

You already have your main run sized at 5". Too bad. You should have gone with a 6" main (unless you have a very short main run) even though your DC inlet is 5" so as to reduce friction and subsequent losses. Many 2 hp units optimise with a 7" main duct run.

A while back I asked a question about a similar situation. My thoughts were that the main duct would "self-size". If you had a DC with a 5" inlet and install 10" diameter duct, obviously you don't have the CFM to keep the dust moving but the duct will never fill up smaller than what the DC can handle (in a perfect world, 5" in of open space in your 10" duct). OK so the 10" duct is too much over kill. But I would sumise the same thing would apply to a 6" or 7" main duct. And you would get the benefit of the reduced SP losses due to the larger pipe.

I would still seriously consider, trying to open up the machine ports to 5". There's rarely a good reason to have a 4" port on a tool. Personally, opening my drum sander and bandsaw to 5" would be mroe pain and effort than I want. But for the jointer, table saw, and planer I would love to open to 5" as 4" just doesn't cut it. (no pun intended)

[Mike Desch]

HI again, guys:

What started this entire thread was an article I read sometime ago where a woodworker was using a 4" port to his router table. After noticing that the chips where not being properly extracted (not enough air flow through the cabinet), he tried an experiment where he added a 4" gate to the front on his router table cabinet. He found that changing the degree of opening on the "extra" blast gate made a big difference on the amount of chips being extacted.

My feeble brain thought that the same might be true of a 5" main duct (19.625 sq. in. opening) and 4" drops (12.56 sq. in. opening)--needing extra sq. in. of opening for the 5" duct to work at maximum efficiency.

Guess I am way off in my thinking, but that was the thought that got me started on this thread.

Again, I sure do appreciate the many comments.

[Ole Anderson]

HI again, guys:

What started this entire thread was an article I read sometime ago where a woodworker was using a 4" port to his router table. After noticing that the chips where not being properly extracted (not enough air flow through the cabinet), he tried an experiment where he added a 4" gate to the front on his router table cabinet. He found that changing the degree of opening on the "extra" blast gate made a big difference on the amount of chips being extacted.

I am not sure what he was experiencing with regard to chips being properly extracted. But if all he had allowing air into the cabinet was the opening around the router bit, he would be starving the cabinet for air and he would not be getting enough air into the cabinet and therefore up the duct to move chips. So by introducing more air to the cabinet via a blast gate he now had enough air to properly evacuate chips from the otherwise nearly sealed cabinet. This is somewhat different from opening a blast gate at the end of a duct run to facilitate enough flow to clear the duct as it would not help clear the tool itself. I did something similar by cutting a slot in the bottom of my router cabinet in order to sweep chips to the 4 inch outlet hose.

[Mike Desch]

OK guys:

As Mike Clark asked, here is my longest run (and seeing it, I think I am in trouble).
26'-2" of 5" duct; 1 90 degree long radius sweep, 2 45 degree bends (5-5-5 laterals), and 2 45 degree long radius els.
(Yeah, I cheated already--made the assumption that I will now run 5" all the way to the Dewalt 735 planer.)

To add fuel to the fire, I have a Penn State Tempest cyclone.

OK, I will now duck as the barbs come flying!

[Anthony Whitesell]

HI again, guys:

What started this entire thread was an article I read sometime ago where a woodworker was using a 4" port to his router table. After noticing that the chips where not being properly extracted (not enough air flow through the cabinet), he tried an experiment where he added a 4" gate to the front on his router table cabinet. He found that changing the degree of opening on the "extra" blast gate made a big difference on the amount of chips being extacted.

My feeble brain thought that the same might be true of a 5" main duct (19.625 sq. in. opening) and 4" drops (12.56 sq. in. opening)--needing extra sq. in. of opening for the 5" duct to work at maximum efficiency.

Guess I am way off in my thinking, but that was the thought that got me started on this thread.

Again, I sure do appreciate the many comments.

You're on the right and wrong tracks at the same time. Yes, adding a sacrificial gate to the trunk will increase CFM but may reduce pickup at the tool. The other thread that you were reading is about CFM at the tool. Increasing the opening at the tool allowed the collection at the tool to increase. It's not that you've compared apples and oranges, but rather two types of apples.

[Michael W. Clark]

OK guys:

As Mike Clark asked, here is my longest run (and seeing it, I think I am in trouble).
26'-2" of 5" duct; 1 90 degree long radius sweep, 2 45 degree bends (5-5-5 laterals), and 2 45 degree long radius els.
(Yeah, I cheated already--made the assumption that I will now run 5" all the way to the Dewalt 735 planer.)

To add fuel to the fire, I have a Penn State Tempest cyclone.

OK, I will now duck as the barbs come flying!

I get about 4"wg SP required for the duct system at 550 CFM (SP required at the cyclone inlet). If you add 2" for the filters and 5" for the cyclone (?) and a fan efficiency of 45%, then you need 2.11 BHP.

If you could use the run side of the 5x5x5 wyes, this would give you another 0.57"wg of SP.

Mike

[Michael W. Clark]

You're on the right and wrong tracks at the same time. Yes, adding a sacrificial gate to the trunk will increase CFM but may reduce pickup at the tool. The other thread that you were reading is about CFM at the tool. Increasing the opening at the tool allowed the collection at the tool to increase. It's not that you've compared apples and oranges, but rather two types of apples.

This is accurate. Think of it like connecting your ductwork to a box. If the box is sealed and you turn on the DC, then it will not pull any air, because the box is sealed. It would be the same result as closing a blast gate in the line. However, if you provide openings on the box so air can enter it, your CFM will increase and so will the collection.

Part of the magic in this is where to located the exhaust point and where to locate the inlet air. This comes up quite often when discussing designs for under table dust collection for router tables.

Mike

[Alan Schaffter]

Not to complicate the issue, but high CFM in a TS cabinet is not all that important unless the cabinet has a lot of large openings. If the cabinet floor slopes to the port, then with just a modest amount of CFM, you will collect the dust and it won't escape from the elevation and angle handwheel slots, etc.- what is in the cabinet will stay there until sucked out.

However, no amount of CFM or SP will ever draw dust down through a blade insert or prevent dust caught in the blade gullets from being carried above the table where it can be hurled at the user. That stuff is best handled with an above-table pickup.

The only consideration about make-up air in the TS cabinet is whether you are starving the system and whether there is enough velocity in the ducting to prevent dust and chips from precipitating out of the air stream. If the tap for the over-table pickup is close to the cabinet port, that shouldn't be a problem.

[Rick Potter]

Mike,

I don't know if your were possibly referring to one of my posts saying I have a 2 1/2" hose on the guard of my RAS open all the time. If so, I was mentioning that because some folks have worried about having too much suction and collapsing their DC lines. I have plenty of suction, and even though I don't have any lightweight sections of pipe, this is kinda like my safety valve. Probably don't need it, but it's there.

[Mike Desch]

The plot thickens ...

Thanks, Mike, for your calculations. Now wondering if I gain anything by increasing the main run to 6" (but still have to reduce down to 5" at the DC). I "think" my friction losses will be less, but am concerned about loosing the proper CFM to keep the ducts clear of chips.

Please comment on the following scenario (still only in my head).
Use 6" main run of ducting (apx 22 ft), still need to reduce to 5" into DC, and use 5" vertical tool drops instead of 4".

With this scenario, I am concerned about having enough CFM to keep the chips moving all the way back to the DC, and to also have the ability to "lift" the chips up apx 5' of vertical drop (before entering the main).

Thanks for sharing your knowledge and expertise.

[Michael W. Clark]

The plot thickens ...

Thanks, Mike, for your calculations. Now wondering if I gain anything by increasing the main run to 6" (but still have to reduce down to 5" at the DC). I "think" my friction losses will be less, but am concerned about loosing the proper CFM to keep the ducts clear of chips.

Please comment on the following scenario (still only in my head).
Use 6" main run of ducting (apx 22 ft), still need to reduce to 5" into DC, and use 5" vertical tool drops instead of 4".

With this scenario, I am concerned about having enough CFM to keep the chips moving all the way back to the DC, and to also have the ability to "lift" the chips up apx 5' of vertical drop (before entering the main).

Thanks for sharing your knowledge and expertise.

If you are trying to stay at 4000 FPM in the 6" (800 CFM), the 5" is a killer. If you could run 6" all the way at 800 CFM, then you need about 3.79" at the inlet. Lower SP, but you are moving more air. The power required is 3.0 BHP for the assumptions in the previous post for cyclone and filter DP and fan efficiency. I think you are seeing why 2HP is tough with a cyclone and filter. Remember, I am making assumptions about the cyclone pressure drop and fan efficiency. If these are different, it will definitely affect the power required. Also, you may not have any problems running the 6" at 3000 FPM, but depending on the type of work you do, it could become plugged. (It could become plugged at 4000 FPM too).

[Anthony Whitesell]

The plot thickens ...

Thanks, Mike, for your calculations. Now wondering if I gain anything by increasing the main run to 6" (but still have to reduce down to 5" at the DC). I "think" my friction losses will be less, but am concerned about loosing the proper CFM to keep the ducts clear of chips.

Please comment on the following scenario (still only in my head).
Use 6" main run of ducting (apx 22 ft), still need to reduce to 5" into DC, and use 5" vertical tool drops instead of 4".

With this scenario, I am concerned about having enough CFM to keep the chips moving all the way back to the DC, and to also have the ability to "lift" the chips up apx 5' of vertical drop (before entering the main).

Thanks for sharing your knowledge and expertise.

As I was alluding to, why does it matter if the chips fall out of the stream AS LONG AS the pipe doesn't clog? Worst case the duct will never collect so many chips as to be less than 5" diameter.
What goes out must go in. The CFM out will be the CFM in. The difference will be the FPM. In the 6" section the FPM will be lower than in the 5" section.

[Ole Anderson]

I was having the same thoughts. I am sure the diameter will somewhat self regulate. But keep in mind that the friction factor of a ragged pile of dust and chips in the otherwise smooth duct may have a significant effect on the overall roughness of the duct system and cause undue suction losses in your system. However, better to have your duct too big than too small as you can always open a few upstream blast gates to flush any stuff out of your ducts accumulated while using a single drop that fits your machine but is smaller than the optimum size to maintain velocity in your main duct run.

[Michael W. Clark]

As I was alluding to, why does it matter if the chips fall out of the stream AS LONG AS the pipe doesn't clog? Worst case the duct will never collect so many chips as to be less than 5" diameter.
What goes out must go in. The CFM out will be the CFM in. The difference will be the FPM. In the 6" section the FPM will be lower than in the 5" section.

Anthony,
This is true, but there is no way to tell where the system will eventually balance (what CFM?). As the duct fills, the cross-sectional area goes down, the FPM go up, the losses go up, the CFM goes down, FPM go down, more material drops out, cross-sectional area goes down, etc. It is nearly impossible to predict this point even on engineered systems where all the performance data is known for the equipment involved. The system will eventually balance itself but will the fan be so far up the curve that it is not delivering enough CFM at the tool to do a decent job? The other issue with the duct filling with dust is that it could be a fire hazzard. If a spark were to get into the collection system and drop out in the ductwork with the residual sawdust, it could cause a problem. The conveying velocity of 4000 FPM handles most general cases in woodworking, but there are situations where higher velocities may be desirable and times when lower velocites would do the job just fine.

I think the thing to realize is that a 2HP cyclone is going to be marginal in this situation, depending on the CFM required at the machine. The machine exhaust requirement governs the DC selection.
Mike

[Anthony Whitesell]

I was having the same thoughts. I am sure the diameter will somewhat self regulate. But keep in mind that the friction factor of a ragged pile of dust and chips in the otherwise smooth duct may have a significant effect on the overall roughness of the duct system and cause undue suction losses in your system. However, better to have your duct too big than too small as you can always open a few upstream blast gates to flush any stuff out of your ducts accumulated while using a single drop that fits your machine but is smaller than the optimum size to maintain velocity in your main duct run.

That only works if the main duct and the DC inlet are the same size with the tool ports being too small. If the duct is larger than the DC inlet (as described by the OP) then this is irrelevent. Introducing another 'tool port' to suck from will only serve to decrease the CFM from the actual tool.

[Ole Anderson]

That only works if the main duct and the DC inlet are the same size with the tool ports being too small. If the duct is larger than the DC inlet (as described by the OP) then this is irrelevent. Introducing another 'tool port' to suck from will only serve to decrease the CFM from the actual tool.

As long as the DC is capable of enough cfm to clear the somewhat larger main line, and you have enough blast gates open upstream, the size of the inlet in the DC, within reason, is irrelevant. Stated differently, 7" main down to 6" at the cyclone should be no big deal as long as you have the cfm. And you don't open the extra upstream blast gates except to clean the main line when you are done using the tool.

[Jim Andrew]

I think it might cause you more problems to adapt from 5 to 6 for your main line, and if your system is building up chips, I'd be concerned about it plugging. If I were in your situation, think I would look at the cyclone, and see if you could modify the opening from 5 to 6 before changing the pipe. Maybe there is room where you could just cut the hole bigger and mount a 6" flange in place. Your other option would be to watch craigs list and nextech for a used cyclone that is bigger, and sell your cyclone after making the change. Recently saw a 2 hp cyclone someone bought, found they couldn't use and resold it for about 2/3 of new.

[David Kumm]

I think it might cause you more problems to adapt from 5 to 6 for your main line, and if your system is building up chips, I'd be concerned about it plugging. If I were in your situation, think I would look at the cyclone, and see if you could modify the opening from 5 to 6 before changing the pipe. Maybe there is room where you could just cut the hole bigger and mount a 6" flange in place. Your other option would be to watch craigs list and nextech for a used cyclone that is bigger, and sell your cyclone after making the change. Recently saw a 2 hp cyclone someone bought, found they couldn't use and resold it for about 2/3 of new.

Modifying the cyclone opening usually isn't the way to go. Hobby cyclones with round inlets are poorly designed all ready and enlarging them so the inlet is larger than the distance between the back wall and the neutral vane causes turbulence that screws up the separation. The inlet size is determined by the CFM range the cyclone can separate effectively so if you need more cfm you need a different cyclone and likely a different blower and motor. Modifying part of the system can mess up the other parts. It isn't just about air flow but also separation. Dave

[Ole Anderson]

I think it might cause you more problems to adapt from 5 to 6 for your main line, and if your system is building up chips, I'd be concerned about it plugging.
Has anyone ever seen or heard of a system plugging unless they let their drum and filter fill up? Not likely because as a pile builds up in a duct or fitting, velocity goes up at that spot and the extra velocity will clear enough of the plug to maintain a sufficient opening to keep things moving. Just put your hand in front of an open 4" or 5" gate and you will realize it won't plug, no way.