DC Question

[Jason Christenson]

I've just reconfigured the DC setup in my shop and I am left with a question. When I started my 1.5 hp DC unit was right next to my lathe, now it is about 20 feet away at the other end of the shop. Obviously it's not quite as effective as it was before. I'm thinking about splitting the difference and moving it about half way back but before I do I'd like to know how much suction is enough? And is there a simple way to determine if the suction I'm getting now is enough?

Inquiring minds want to know!

Jason

[Lance Norris]

Well... the simple answer is try it and see. If it pulls the chips and dust to your satisfaction, your good.

[Jason Christenson]

Ok, her's a follow up question. Is it best to run my lines at floor level, ceiling level or midway?

Jason

[David Giles]

Ceiling level. Floor level keeps limits roll around cabinets and is a tripping hazard. If the DC isn't strong enough to pull the chips vertically, best to clog up near the machine. Mid level ... I assume that you mean wall mounted at chest height...seems a waste of wall space.

[Jim Becker]

Ceiling level. Floor level keeps limits roll around cabinets and is a tripping hazard. If the DC isn't strong enough to pull the chips vertically, best to clog up near the machine.

DC is all about air flow. The chips and dust have no real appreciable weight so we really don't need to worry about vertical drops. But we do need to worry about maintaining enough air flow volume and velocity (CFM) to be able to move the chips and dust through the duct work. Every twist and turn reduces the total duct efficiency (and length figures into that, too). If the blower isn't strong enough to maintain the required CFM through the duct work to evacuate the dust and chips, then we can't be happy.

A typical 1.5hp chip collector does have limits on how effective it can be with installed duct work...they are "best" with just a short hose and will often work acceptably with small duct work designs as long as the blower inlet is opened up to 5"-6" and that size duct is used. 4" duct is very restrictive in volume with a maximum CFM of 300-350 just because you can't fit any more air in it to increase that number at typical velocities.

[Bill Pentz]

I've just reconfigured the DC setup in my shop and I am left with a question. When I started my 1.5 hp DC unit was right next to my lathe, now it is about 20 feet away at the other end of the shop. Obviously it's not quite as effective as it was before. I'm thinking about splitting the difference and moving it about half way back but before I do I'd like to know how much suction is enough? And is there a simple way to determine if the suction I'm getting now is enough? …

Jason,

Lots of people really don’t understand airflow very well. The amount of air that your blower is going to move depends upon how much resistance it must overcome. Blower technology is fairly mature so if you buy the same size blower with same size and type of impeller you will get nearly identical performance from any of the major commercial brands. Our small shop blowers are mostly not that well made so other than just a few we end up with the commercial fan tables showing us the maximum performance we can get against any given resistance. This resistance is known as static pressure. A typical 1.5 hp blower on a dust collector will have minimum resistance of our filter, short length of duct, and the overhead of a single large hood. A static pressure calculator will help us figure out the total resistance. Just about the lowest resistance we can get is with brand new fine filter bags and a real short connection with 6” ducting for a total of about 2.5”. If you use 4” diameter duct, go 20 feet away, up and down a wall, and have a well “seasoned” but clean filter your resistance will climb to just over 6”. If you go to the Cincinnati Fan steel pressure blower table at http://www.cincinnatifan.com/catalogs/SPB-401-internet.pdf and look up the values for your 11” backward curved (BC) impeller you will find at about 2” your blower with 5” inlet is able to move a real 606 CFM pulling only 0.76 hp. Checking that same table looking across the same line to below the 6” resistance column you will see that your airflow will drop to only 303 CFM pulling about 0.56 hp. This is why Delta recently shifted to using 12” blowers on their similar sized dust collectors. This larger impeller will help overcome that resistance and let your motor move more air. For a 12” diameter regular impeller this same table shows 818 CFM at 2” pulling 1.56 hp and 606 CFM at 6” pulling 1.28 hp. This means you could burn up a 1.5 hp motor in a least case resistance situation but your filter will soon build up enough fine dust in the pores that does not come out with gentle cleaning to “season”. If you bought a Dwyer Instruments 0-10” Magnehelic gauge off eBay and added a good matching 166 model Pitot tube you could do your own pressure, air velocity, and air volume measurements for under $200. Most don’t want to spend that much.

In terms of what you need, good “chip collection” meaning collecting the same sawdust and chips we would otherwise sweep up with a broom can be done with a system that provides at least 350 CFM at your larger stationary tools unless you are using good sized commercial tools. To pickup the sawdust and chips we need to go to a material handling table such as the on the Cincinnati Fan site. They share a good material handling engineering data sheet at http://www.cincinnatifan.com/catalogs/EngData-203-internet.pdf. This sheet is an excellent basic tutorial on using air to move or convey material, but requires a little patience to understand. If you read their Table 1 on this sheet you will find that they recommend a pickup velocity of 4500 feet per minute (FPM) with a minimum suction of 3.0” to collect sawdust and heavier chips. Light wood shavings can be picked up with only 3800 FPM and 2.0” of suction. These same numbers become very important because chips and sawdust do have a considerable weight in a dust collection system. In fact, one of the biggest problems areas in dust collection are vertical drops in our ducting runs plugging because we fail to move ample air speed in FPM to keep the heavier chips entrained, meaning airborne. Without a 4500 FPM pickup velocity, we are not going to get the heavier chips. Without at least a 4500 FPM airspeed in our vertical ducts, these ducts will plug eventually as this heavier material gathers. The same issue occurs in our horizontal ducts, but the needed airspeed is about 1000 FPM less to keep things moving horizontally.

With all that said we can ignore the bigger chips and pick them up by hand. That means 3800 to 4000 FPM will provide ample airspeed to collect the chips and keep them moving through our vertical ducts. How much air we have to move depends upon how big of an area we need to cover with our fast moving airstream. For most small shop stationary tools we know we can make most happy with covering about a 4” diameter circular area moving 4000 FPM airspeed. In a hood that flares open and uses a center or lower port we get by great with 350 cubic feet per minute (CFM) airflow. A 4” diameter hose at typical dust collection pressures moves right at 350 CFM so this is a good size for doing “chip collection” in small shops. In fact is plenty and is easily supported in a standard 4” duct. Most with 4” ducts and dust collectors sized 1 hp and larger who only collect from one tool running at a time are very happy with how well these units keep our tool surfaces and floors clear.

Good “chip collection” is where most of the woodworking industry has been since the twenties, but it works poorly for good fine dust collection. Careful air engineering testing shows for good fine dust collection we only need about an air velocity of about 50 FPM to overcome normal room air currents. That means our tools need upgraded with hoods that block all of the faster airflows or there is zero chance of good fine dust collection. Unblocked airflow will launch fine dust all over our shops. Decades of air engineering show we make so much fine dust that once it escapes collection as it is being made even big exhaust fans and air cleaners cannot remove this dust fast enough to avoid failing even the “easy” OSHA an air quality tests, let alone the five times tougher ACGIH or fifty times tougher medical recommendations which is now our new EPA standards and European standard. Because most small shop tools are older designs that do a poor job of fine dust control we need a better hood plus building a “bubble” of air around the working areas of our tool. The calculations are mess but you need about 800 CFM at our larger stationary small shop tools to pull in enough air to meet OSHA air quality standards, about 900 CFM to meet ACGIH standards, and about 1000 CFM to meet medical recommendations which are current European and EPA recommendations.

bill