Michael Standish lives in the Boston area and he has written numerous tool reviews and other short articles for several well known woodworking magazines. I am pleased to announce that Michael has agreed to do independent testing of our cyclones and to lend his expertise to our web pages. From time to time he will be adding material to this page, so check back often.

Ed Morgano

August 1, 2006

Who Is This Guy?

I’ve been working with wood for almost 35 years. I’ve been aggravated by shop dust almost ever since, and real apprehensive about it since being hospitalized for dust-related pneumonia fifteen years ago. Between then and about a year-and-a-half ago, I squandered more money than I’d like to admit to by following half-fast advice about how to deal with woodworking debris.

 

I was asked around that time for an article—I’ve also been writing tool reviews for various woodworking and carpentry magazines for about a dozen years—evaluating small single-stage dust collectors. A couple of months after that, I did the research and testing for a more general survey (units that ranged from 1 hp single-stage to 5 hp cyclone) for the same magazine.

 

If I hadn’t found Bill Pentz’ site, my research attempts would probably have begun and ended with the same old rehashes of sales pitches that had steered me wrong for so many years. Bill has picked the vault’s lock so that we can push the door open and get our money back. Not only are his pages thorough, but they’re also generously sprinkled with links and other cross-references.

 

These have their own links and cross-references, which makes them rich sources, but things can get out of hand: I woke up one day to find I’d accumulated several thousand pages in books, magazines, catalogues and printouts.

 

I eventually got sick of studying all that material. Lately I’ve been concentrating on testing machines, to check on the difference between what a salesman tells me and what actually happens when you plug the thing in and let it rip. Whenever money and time have allowed, over the last year-plus I’ve done about 70 tests on different (motor size, fan size, filter type, separation method, etc. etc.) dust collectors.

 

Right now, I’m testing the ClearVue 18-in. diameter cyclone. I’m interested to see what happens to the airflow when the inlet pipe’s size is changed. The effects I’ll be looking at are especially important when it comes to deciphering the relationship between testing methods and advertising claims. Look for the results in about a week.

 

Michael Standish

 

August 27, 2006

 

When I wrote my introduction to this section, I described some tests I’ve planned, and foolishly capped it with “Look for [the results] in about a week.” Well, my apologies.

 

I’ve had terrible back-order problems with (and, truth to tell, have not done very well keeping track of) the last bits and pieces (adapter/reducers, couplings, etc.) that are needed for setting up a testing rig.

 

I should be posting my results (in tables and graphs) in a couple of weeks. I’ll also be describing just how my tests were conducted, as well as some thoughts about testing procedures in general. Thanks for your patience.

 

Michael

 

A note: According to my attorney, the following material is “Copyright © 2006 by Michael Standish.” On the other hand, as the philosopher William Birks Gillespie put it, “If you can hear it you can have it.” Splitting the difference, I’d say that anyone who wants to reproduce the following material (in unmodified form, while crediting me) is welcome to do so.

 

Some Effects of Duct Area on Test Results:

 

Dust collectors are usually tested by incrementally blocking their inlets and taking two pressure measurements for each restriction. Velocity Pressure (VP) determines air stream velocity (in feet per minute) and airflow volume (in cubic feet per minute). Static Pressure (SP) is a bit more complicated, but for our purposes it represents resistance against that airflow. A typical set of matched pairs of these pressure readings looks something like this:

 

Charting the figures from the table creates a performance curve:

Connecting the dots lets us calculate the effects of ductwork on performance: Reading across the chart from a selected cfm point to the intersection of the curve, and then down to the matching point on the SP baseline, shows that for 800 cfm (close to 4000 fpm in its 6-in. duct), our dust collector should manage about 7.75-in. worth of resistance before the airflow falls below our requirement. By plugging in established values for intake inefficiencies, filter back-pressure, and ductwork losses (from friction and turbulence in pipes, hoses, and connectors) we can determine the maximum run of ductwork for a given airflow requirement.

 

So far, so good. This is an excellent and proven method that’s been used for decades, but there are some potential trouble spots for dust collector comparisons. First, it relies on uniform test procedures. Apples to apples, as they say. This is not an insurmountable problem—if testing methods are clearly described, it’s easy to tell if you’re actually looking at equivalent curves—but it’s worth remembering that even honest testing can result in variations on the order of 10%.

 

Second, it’s a bit trickier to get a complete picture of what a performance curve actually tells you. To see what I mean, we can look at this chart, which compares the performance of two cyclone dust collectors, one with a 7-in. inlet, one with a 6-in. inlet:

The first thing we can see is that the maximum airflow for the 7-in. is about 1510 cfm, and for the 6-in. it’s around 1450 cfm; pretty similar performance so far, but we ought to look deeper. To do this, we need to establish some ground rules:

 

First, we’d like airflow of at least 800 cfm, to handle big dust producers like planers or dado-cutting table saws; but we need an airspeed of 4000 fpm (transport velocity) to get rid of all that dust. A dust collector that gathers plenty of debris but leaves piles of it sitting in the ductwork has not finished its job.

 

Second, because airflow depends on both airspeed and ductwork size, at 4000 fpm there’s a corresponding volume for every duct diameter; for 7-in. it’s about 1100 cfm, and for 6-in. it’s around 800 cfm.

 

This means that if any ductwork at all is needed—and it will be, whether it’s just a few feet of flex hose or a full-blown system of pipes—we shouldn’t be misled by that sexy-sounding peak cfm number. Instead, we should also be looking at Static Pressure, which gives us an index of how much ductwork can be handled.

 

From the curves on our chart, we can see that the usable maximum airflow for the 7-in. dust collector (the matching value for our minimum airspeed) is 1100 cfm (at 7-in. SP); for the 6-in. it’s 800 cfm (at 11.5-in. SP):

 

But once a dust collector is connected to a woodworking machine and filters, real-life entry losses and filter resistance typically amount to about 3-in. SP. So our 7-in. cyclone will start with an SP allowance of about 4-in. SP, while for the 6-in. cyclone it’s around 8.5-in. SP.

At 4000 fpm, the resistance value for 7-in. duct is 0.038-in. per foot, and for 6-in. duct it’s 0.045-in. per foot. Dividing the corrected SP allowances by their respective per-foot numbers yields maximum ductwork runs of 105-ft. for the 7-in. and 189-ft. for the 6-in.

While these lengths are for straight, smooth-walled pipe only, plugging in known values for various connectors (the Equivalent Foot Method) can show us what will happen when either cyclone is combined with a simple ductwork system:

In the case of our 7-in. cyclone, adding five elbows (at 13-ft. worth of resistance each, the equivalent of 65-ft.) would still allow 40-ft. of pipe. But we’re not quite done yet. If our final connection will require a length of flex hose, the usual rule of thumb is to triple the per-foot figure on account of the turbulence caused by the corrugations; so if we add 8-ft. of hose to our ductwork system, the net length of straight pipe is further reduced by 24-ft., to 16-ft.

 

For the 6-in. cyclone, adding five elbows (at 12-ft. worth of resistance each, the equivalent of 60-ft.) would allow 129-ft. of pipe; adding 8-ft. of hose to the system (24-Equivalent Feet) reduces the net length of straight pipe 105-ft.

 

And there’s another thing when it comes to using these dust collectors in a real-life shop: It’s unlikely that a non-industrial woodworking machine will have a 6-in. (let alone 7-in.) port. We’ll most probably see a 4-in. connection, which means we need to look at the effects of decreasing duct diameters.

Some Effects of Decreased Duct Area:

 

The appeal of 4-in. ductwork isn’t hard to figure. It’s readily available, comparatively cheap, and it fits the dust ports that are usually found on non-industrial woodworking machines.

 

But if using 4-in. pipe or flex hose is an easy way out, it’s not a good solution by any means, and the chart below shows why. It illustrates what happens to a simple 1.5 hp two-bag dust collector when its 6-in. inlet stub is replaced by the single 4-in. branch of a typical 6-4-4 wye connector (the “bonus” adapter that’s commonly included with such machines):

The test curves show that reducing the duct size from 6-in. to 4-in. cuts the peak volume from 835 to 510 cfm, about 60% of the initial airflow. What the graph doesn’t show directly is that besides wiping out 325 cfm, choking down the duct size produces some pretty horrible resistance increases, as we’ll see.

 

First, though, we should look at the fundamental relationship between airflow, airspeed, and duct size. Like the tech books say,

 

Q = V x A,

 

Where Q  stands for volume (cfm) , V for velocity (fpm), and A for duct area (cross section, in sq. ft.). The formula produces results like these: 

This suggests a couple of possibilities for manipulating duct size so as to get the performance we need:

 

On the Planet Of The Free Lunch, we could simply open up the ductwork until we get whatever airflow we want. Unfortunately, our 1.5 hp motor can’t handle it here on Earth.

 

All other things being equal, increasing the duct diameter does result in greater airflow (cfm) at a given airspeed (fpm), as shown in the table above. But for this approach to succeed, the motor will have to do more work—that is, move more air. If it’s made to do more work than it can handle, the motor’s operating amperage will exceed its amperage rating, and it will burn up.

 

We could switch to a bigger blower (motor-and-fan unit) but even so, enlarging the ductwork will sooner or later result in the airspeed dropping below our 4000 fpm transport velocity (the airspeed we need to reliably move the dust, as explained in the “ground rules” remarks in Looking at Test Results):

 

 

 

 

 

Note that no matter how huge the blower, there’s a duct size that will do the trick; for example, a 2000 cfm blower might seem pretty spectacular, but its airspeed in 10-in. duct would be less than 3700 fpm.

 

So if enlarging the duct is pretty limited, how about taking the opposite tack and using a sufficiently powerful blower to simply jam the air through smaller ductwork?

 

At first glance, this seems sort of promising:

 

 

 

 

 

That is, cfm (volume) increases as fpm (velocity) increases. Unfortunately, as airflow and airspeed rise, so does resistance (the losses from friction and turbulence). Even worse, these losses are proportional to the square of any increases in velocity; by way of example, doubling the airspeed will quadruple the losses. More specifically,

 

 

 

 

 

 

 

Even more to the point:

 

In Looking at Test Results, we saw a cyclone using 6-in. duct that produced 785 cfm and 4000 fpm at an adjusted Static Pressure of 8.5-in. At the corresponding resistance of 0.045 SP-in. per foot, it will support the equivalent of 189-ft. of straight pipe.

 

But if that same cyclone’s ductwork is shrunk to 4-in., it will need to move the air at 8992 fpm to produce that 785 cfm; at this velocity, the resistance (at 4000 fpm, 0.07 SP-in. per foot) is multiplied fivefold (to 0.35 SP-in. per foot), and it will only support the equivalent of 24-ft. of straight pipe. 

 

And it looks as though we’ve turned a powerhouse dust collector into a very expensive Dust Buster.

 

This is admittedly an extreme case, but it clearly shows that reducing our duct diameter will have unattractive consequences: With a smaller blower (like our 1.5 hp unit above), airflow will be diminished (e.g. from 835 cfm to 510 cfm) but resistance—although increased—can be kept more or less within reason. A larger blower might produce the kind of airflow we’d rather see (in the neighborhood of 800 cfm), but the associated resistance increases make this something of a self-defeating exercise.

 

All of which suggests that the best plan for minimizing these complications is to maintain the largest duct a blower can support (typically, this means matching the factory-supplied inlet’s diameter), for as much of the ductwork run as is possible.

 

Ideally, that ductwork run would include same-size dust ports and/or hoods, but this is easier said than done in real life (on account of budget and space limitations). When using a reducer is unavoidable, it should be introduced right at the woodworking machine in question.