Dust collection duct sizing

[Eric Marsh]

I have a Rikon dust collector. I am considering modifying to size of the inlet to the fan from 5” to 6” . I would also run 6” ducts through out the shop. The problem I have is that the hose from the fan to the separator is only 5” and I cannot change this. So my question, is it worth while increasing the size of the inlet ducts when I can do nothing to the outlet side of the fan?

[Rod Sheridan]

Eric, that question can only be answered if you can determine how much airflow you expect through the 6 inch duct.

With the airflow, you'll be able to calculate the airspeed and determine if the 6 inch duct is too large.....Regards, rod.

[Peter Christensen]

If you are willing to do surgery on your dusty you can make the outlet bigger with sheet metal, MDF/plywood or fibreglass. There is a sticky in the Aussie Woodwork Forums on how to get the most out of a 2hp DC. Basically and ideally open the outlet and attach it directly to the bag ring and eliminate the hose completely. Also when you open the inlet shape the lip facing the impeller to a bell mouth. That's in that long thread too.

[Bill Dufour]

If you open the inlet and/or the outlet you risk overloading the motor and burning it out. you need to do some math and find out more about the fan you have.
Bill D

[Jim Weisbo]

Not sure if it is too late to get in on this, I too have been converting to 6 inch starting with my d/c inlet, mitre saw and table saw (6 x4x5 "Y" feed ...4 inch to overarm and 5 inch to base)? Want to try and get everything out of my old jet 1.5 hP d/c and thinking since I have replaced bag with cannister, why not change outlet to 6 inch and straight hose (currently 5 inch with sweeping 90 degree turn)?

Also, looking at a duct suppliers website, they explain frictional losses in a system are additive ... doesn't that mean changes to reduce static pressure on outlet side have effect of increase air velocity on inlet side and of course at the end of hose that connects to the machine.

[james manutes]

If I understand your post "5 inch between DC and separator " - you wonder about the 5" Dust Deputy in line in an otherwise 6" ducted system . If so I'd say the smaller portion of that configuration will determine your results . What 5" ducts will support likely would be lacking in velocity needed in the 6" size . Which Rikon DC you have is very important . As for the "outlet side of the fan" , on most DC's that opening is 5" x 6" ( 30 sq. in.) . Five inch ducts are 19.8 sq. in. , six inch are 28.3 sq. in. - that should mean you won't have significant back pressure on the"outlet side " .

[David L Morse]

As you and Jim suspect, a pressure loss on the outlet side has the same effect as a pressure drop on the inlet. The fan just sees a path from inlet to outlet. It doesn't care about where you happen to be observing from.

To compare the effect of the restrictions on the outlet compared to a six inch pipe on the inlet the method of equivalent pipe length can be helpful. So here are my measurements of equivalent pipe length for the various pieces on the outlet.

Rectangular to 5" round transition: 51'
5 " hose: 53'
Separator: 128'

Total: 232'

So, just changing from 5" to 6" on the inlet will have very little effect. To make a useful difference the outlet path needs some modification.

However, as Bill pointed out, you need to pay attention to the motor. Horsepower demand from the fan rises directly with air flow rate (CFM). Temperature rise increases with the square of power and thus with the square of CFM. You'll need to check the current draw and make sure it doesn't go above the motor's full load current if you make major modifications.

Pictures are always fun. These fan curves are for the Harbor Freight unit with the Rikon 12" fan instead of the original 10" and should be the same as yours if it's the "2 HP" model.

[Michael W. Clark]

Hi David, Can you elaborate on your temp vs HP comment. Are you referring to temp rise across the fan?

Also, what are the different curves in your graph? Are they different speeds?

[David L Morse]

Hi David, Can you elaborate on your temp vs HP comment. Are you referring to temp rise across the fan?

Also, what are the different curves in your graph? Are they different speeds?

The temperature I'm talking about there is the temperature of the motor windings. That's of concern because the aging rate of the motor insulation is an exponential function of temperature. There's a rule of thumb that says for every 10C increase in temperature insulation life is reduced by half. Temperature rise is the dominant factor in power rating.

The graph is static pressure as a function of volume flow rate measured in a duct on the fan inlet, close to the inlet. This is consistent with the method commonly used for characterizing hobby level dust collectors, as seen in various magazine reviews and vendor documentation. The curves are for various outlet configurations. For example, the curve labeled "1" represents the outlet configured as shown in the photo labeled "1".