Replies

1. 
   RandMCapozzi | Nov 05, 2004 07:16pm | #1

   Brian,

   Do you have a "squirrel cage" type fan often used in HVAC systems or do you have an airfoil impeller? Squirrel cage blowers (they look a lot like hamster wheels) will move a lot of air (CFM) but are very poor for dust collection systems because they can not generate the necessary pressure to over come the resistance of ducting and hoods.

   Airfoil impellers look similar to squirrel cage fans but their blades are airfoil shaped (like an airplane wing). They are very efficient in DC systems.

   If you have an airfoil impeller then that will work fine (as long as the size is appropriate for the amount of air and SP you need). If you have the squirrel cage, don't waste your money trying to get it to work. Use that fan instead as an air cleaner and get yourself a centrifugal pressure blower fan.

   As to your other question, with a cyclone positioned in front of the fan, you shouldn't need to worry about impeller impacts or clogging or quickly filling filter media. The cyclone will result in enough air velocity drop to let the "big stuff" fall out of the air stream and into a catch can/barrel. Only the very fine dust would make it out of the cyclone and past the fan. The filter on the back side of the fan then only needs to filter out the fine dust and doesn't get prematurely filled with wood chips/shavings. This is the primary advantage to cyclone systems. Of course cyclone systems result in higher static pressure in the system. This means you need a bigger impeller and motor all other things being equal.

   I recommend reading through Bill Pentz's (http://billpentz.com//woodworking/cyclone/index.cfm) site if you haven't already. It has quite a bit of useful information regarding dust collection.

   Good luck,

   --Rob

1. 
   brianmexico | Nov 05, 2004 09:55pm | #2

   Rob,

   Thanks for the very informative explantion of the different types of fans. I think I understood from your explanation that both types are the same, the only thing that changes in the cross section of the blades, one being straight and the other being a semicircle? The type that I have is the semicircle kind. I am going to have a look through the link that you passed along. By the way, is there any simple way that I can measure the pressure created by the fan that I have? I'm not sure how this is measured or how I can determine the effectiveness of my fan. I know that just the fan and motor weigh about thirty pounds and if I put it on the floor and turn it on it's strong enough to actually blow itself around. I think I'll set up a mock up with PVC and see how it goes.

   Thanks for the advise,

   Brian

1. 
   TKanzler | Nov 05, 2004 11:31pm | #3

   For static pressure at zero flow, just block off the outlet with a piece of plywood, insert a length of clear tubing into the hole and bend it down and then up (like a sink trap), fill with some water, leaving enough tube above the water level so that it doesn't come out (you'll only need a couple of inches for that style blower), mark the level with the fan off, then remark with it running.  The difference is the static pressure at zero flow, which is the highest it will generate.  You may get a variable reading with the tube being that close to the fan, and it'll be more accurate at the end of a 10' length of duct (eliminates effects from gusting and such).

   It won't be very high, though.  Those fans are designed to move air at lowish static pressure, and the flow rate drops like a stone with increases in static pressure.  You really need a high-pressure radial blower operating at high speed, which is why DC's use that style, of course.Be seeing you...

2. 
   JohnWW | Nov 07, 2004 09:18pm | #5

   Brian,

   You can skip going through the trouble of making tests, the performance you will get out of the impeller, provided it is matched to the motor's size, is entirely dependent on the motor's horsepower.

   A one horse motor will move enough air to serve one machine putting out a moderate amount of chips, through a short piece of 4 inch hose.  If you want to put in longer runs of permanent and larger diameter duct work, connected to several machines, with a few bends in the pipes, and a cyclone, then you are going to need several horsepower.  Your two horse motor is on the lower end of power you will need.  No amount of fiddling and wishful thinking will overcome the basic physics of air volume and velocity being  greatly dependent on the horse power driving the fan.

   The fan you have, with the closely spaced blades, isn't the type you need for the job, it is designed for high volume, low resistance air moving, and it will quickly clog even if you use a cyclone.  There are a few sites on designing ductwork and choosing the proper size fan and motor, they'll give you the information you need to do the job right.

   John W.

   Edited 11/7/2004 1:25 pm ET by JohnW

   Edited 11/7/2004 1:31 pm ET by JohnW

1. 
   brianmexico | Nov 08, 2004 12:36am | #6

   John,

   Thanks for the info. After going through the above mentioned website and spending a few hours surfing through various links I had also come to the conclusion that this fan I have, though very big and powerful, won't cut it for dust collection. I'm building a new shop and because of this want to start off on the right foot with a good dust collection system. The fan that I have I will either use for a hanging air-filter system or I will try do build a dedicated sanding table that vents directly to the outside of the building. I was a smoker for thirteen years and quit a few years back, that combined with the woodworking has left me very conscious of the risks of lung disease, so even if it costs me a few more bucks I want to do it right. I think I may just go ahead and buy the oneida system and upgrade to some better filters or bags.

   Keep makin shavins,

   Brian

2. 
   stancalow | Nov 06, 2004 02:50am | #4

   brianmexico

   at the site that was mentioned (Bill Pentz)  there is an extensive plan and instructions for making your own cyclone.