Why not use 4" and 5" DC downdrops?

[Bill Pentz]

I just finished getting raked over the coals pretty good for saying that setting up a shop with 6" ducting mains for dust collection then using 4" and 5" down drops was not thinking things through carefully.

I know that air engineers shoot for about 4000 feet per minute (FPM) airspeed to keep our pipes from plugging or clogging.

You can easily calculate FPM by dividing the CFM by the size of the ducting in square feet. With 6" ducting that computes to 4047 FPM which is near perfect. Likewise, the flow into a 4" pipe fed by that 6" duct also calculates to well over the desired 4000 FPM engineering target.

The problem is what happens when that 400 CFM stream hits our 6" diameter main?

I contend that makes for a huge problem unless you open ducting down stream.

Thoughts?

bill

[Steve Jenkins]

I don't know the math for dust collection well enough to teach a class in it but the way I set mine up is to run the main the same size as the input to the fan and have open enough drops of whatever size so their total area is not much bigger than the main. Seems top have always worked well for me. I like to stick to 4-5" drops for most things. I have mine set up now with two 8" mains with gates near the blower to shut off one or the other. One main goes to each side of my shop. Steve

[Todd Burch]

Bill, I was on your web site the other night, after going through Terry Hatfield's side. I read quite a bit, but did not exhaust either site.

I'm fairly new to dust collection. I set up my Onieda cyclone just this last Thanksgiving. It's a 1.5 HP, and from Oneida's support line, they tell me it's undersized for the layout that I installed (not their layout - I moved shops between the time I order it and installed it).

I have 6" runs to 5" drops for the most part. Even as it is, I don't feel it is as strong as it needs to be. (Hence, the suggestion for a 3HP).

Personally, I condsider my 4" line to my tablesaw a godsend. If a 6" line would pick up more dust, I would be very inclined to install 6".

So someone raked you voer the coals. Humm. If your's is working, and it's picking up the dust, and it's moving debris, and it's not clogging... let them rake you over the coals.

Todd.

[Ken Garlock]

Hi Bill. I noticed that PSI is about to launch a couple new cyclones. Their spec sheet doesn't make sense to me. There are too many numbers that are the same for both the 2 hp. and the 3 hp. models. I think someone is P**ing on my shoes and telling me it is raining. :mad: Have you had a chance to look them over?:confused:

[Dave Hammelef]

Originally posted by Bill Pentz
.....You can easily calculate FPM by dividing the CFM by the size of the ducting in square feet. With 6" ducting that computes to 4047 FPM which is near perfect. Likewise, the flow into a 4" pipe fed by that 6" duct also calculates to well over the desired 4000 FPM engineering target.

The problem is what happens when that 400 CFM stream hits our 6" diameter main?

I contend that makes for a huge problem unless you open ducting down stream.

Thoughts?

bill
Bill Help, What CFM did you use to get the 4047 FPM. (I could calculate it but its easier to ask).

Ok know for part 2. So you say That 6" to 4" is fine makes sense to me the FPM will increase if the CFM stays the same and the Diameter decreases. Then you ask about the other way around. 4" to 6" and mention a 400 CFM stream (What 400CFM stream?)

Then you say that will be a huge problem unless you open ducting downstream. What kind of huge problem, and open ducting in which way closer to the cyclone/fan or further away down the 6" in another run, or down the 4" ?

I am really confused

Dave:confused:

[Bill Pentz]

Here's a link to the small dust collection overview article that I am working on that caused the upset. Maybe it will answer some of the questions.

And the bottom line here is Steve was right on! You get plugging in your main ducts if you don't have the equivalent of the same area open as your main duct. You can do that either with opening more ducts further down the stream or going with single sized main and down drops.


Dust Collection Overview (http://cnets.net/%7Eeclectic/woodworking/cyclone/dcoverview.html)


bill

[Dave Hammelef]

Bill, after reading your page I am still not sure I follow your examples. You seem to be always looking at the airflow and assuming the CFM or FPM (one or the other not both) stays constant regardless of pipe diameter.

My understanding is the motor/impellar can pull set CFM of air at a given pressure. So you must look at the pressure drop of your run to determine how many CFM you are moving, then you can calculate FPM.

This is not simple stuff, and unless you look at all the factors your making assumptions and genralizations that may not be true. The reason I can not breath through that hose is my impeller (Diaphram) is not designed to suck air at high pressure deltas. That small dia. hose has large friction losses which require high pressures to move the air through.

and your water tank example the tank is pushing at a set pressure if I pressurized the tank, and made that smaller hose strong enough to withstand the pressure I could send enough water out the larger hose attached to the garden hose to knock you over. (having not done the calculations this may not be physically possible becouse of the extreme pressures needed) but lets just assume that everything works at the high pressure needed.

By opening another inlet you are simply reducing the pressure drop, which will then increase your impellers CFM capability becouse as pressure drops increase so the CFM decreases. IF your second outlet was a mile downstream you would not see any impact becouse the pressure drop would virtually not change

[Bill Pentz]

Dave,

I agree with you. With enough pressure, you can cut steel with a tiny stream of water.

With our hobbyist sized dust collectors using a constant speed motor and fixed system, that pressure goes up to a max of about 12" with even a good 3 hp impeller. The result is the pipe resistance which made up from its length, diameter, and surface friction will define the airflow. And just as you said, if you open up another gate too far down the pipe, you get no more flow.

Once that airflow is defined by say a length of 4" pipe in our fixed systems, you are not going to get any more flow at all into your main, no matter how big it gets unless you open up another path. Unless you step up to a bigger blower with larger impeller and more hp you are not going to overcome that resistance. Moving 800 CFM through a 4" pipe is not rocket science at all, but takes a huge impeller and big motor to get the pressure up high enough. Those that use 2 hp motors and normal impellers are not going to get the job done with that sized pipe if their goal is to collect the fine dust on their larger machines using 4" duct. That's my point.

I hope I understood what you were saying.

bill

[Bill Pentz]

Originally posted by Ken Garlock
Hi Bill. I noticed that PSI is about to launch a couple new cyclones. Their spec sheet doesn't make sense to me. There are too many numbers that are the same for both the 2 hp. and the 3 hp. models. I think someone is P**ing on my shoes and telling me it is raining. :mad: Have you had a chance to look them over?:confused:

Ken,

Their specifications don't appear to be backed by a real product, but as I said with Grizzly, I'll reserve judgement until I see a real unit.

Just over a year ago I called them on my own nickel concerned about their cyclones plugging and their using the same sized impeller in three different horsepower DCs and cyclones. Their "engineer" told me I did not know what I was talking about because airflow depended on motor size not impeller. At that point I hung up not only my phone but my interest.

Based on some of the forum complaints I've read about their dust bags, blowers, and cyclones, I really would like to see an independent testing lab take a serious look at their horsepower, CFM, and filtering claims.

Do some archival post searching and you are going to find most of the happy campers are those who are first time users.

bill

[Doug Edwards]

Bill,

In order to maintain the velocity in the lines to the dust collector, you must have sufficient volume at that point. A 6" diameter line has an area of 0.196 ft2 and a 4" line has only 0.087 ft2. In order to maintain the velocity in the 6" line, you need 2.25 times the cfm. One thing that is commonly used to maintain flow in commercial systems is an air bleed at the end of the main. This is used to add the difference between the air needed at the source and the air needed in the main.

Using a bleed air source does a couple of things to help out. First, it dilutes the dust in the air to prevent high concentrations that can snag and create plugs. Second, it will maintain a constant velocity between the source and the collector. Since different equipment needs different levels of exhaust, the position of this damper may need to be changed for some machines.

An alternative is the use of a counterweight damper. This simple damper opens in relation to the static pressure in the line. If a source requires a lot of air to operate, the pressure at the end of the line is low and there is only a small amount of bleed air needed. If the source requires less air (i.e. a 4" bandsaw tap), the static pressure rises and more bleed air is sucked in as the damper opens. This keeps the airflow in the main relatively constant even though the air from the hoods changes. If setup properly, this damper will rarely need adjustment. Someday if I have time I will show you how to build one. You can look at them on some manufacturer's websites. They are really quite simple to put together. Theres a considerably heavier duty one at www.ruskin.com Enter PDR 92 in the search function.

A similar effect can be had by opening another hood upstream from the operating hood. Since a 6" main has about the same area as two 4" hood drops, opening a second hood so that the air is in the main before the tee for the operating equipment enters will keep the velocity constant.

Now that I've said that, I must tell you that the velocity to prevent plugging in a 6" duct is less than for a 4" duct. Even though the velocity drops from the 4" to the 6" by more than half, if the main is straight and there is no significant vertical travel, the dust may convey for years at 2500 fpm without a plug.