Thread: Dust Extractor ducting, 5 or 6 in?

No, you should be able to use 6" for the main line. The main line being a longer run, 6" will offer less resistance than 5" ducting.

I have always read that ducting size is dictated by the largest size in the system. So if the dusty has 5" duct on the exhaust side then the main line can only be 5". Otherwise you loose air velocity. The general rule is the further away from the dusty, the smaller the duct size. I have an exel file which has good information however it is too big to attach to this post. PM me if you like.
Bevan

Originally Posted by Enfield Guy

I have always read that ducting size is dictated by the largest size in the system. So if the dusty has 5" duct on the exhaust side then the main line can only be 5". Otherwise you loose air velocity. The general rule is the further away from the dusty, the smaller the duct size. I have an exel file which has good information however it is too big to attach to this post. PM me if you like.
Bevan

Hi Bevan,
originally the dusty was supplied with two 4 inch ports at the inlet which I replaced with a single five inch one. The cross section of a 5" and two 4" are 19 and 25 sq inches respectively, which doesn't respect the rule of inlet versus outlet, unless, only one 4" port is used at a time.

The original two port arrangement was connected to a 6" opening and I assumed that the twin port arrangement could be replaced with either a 5 or 6 inch port.

Someone said that, one could get away with using a smaller duct on the exhaust side compared to what's on the inlet side, as it is dealing with blown rather than sucked air.

My dusty is rated at 1700CFM (2Hp, 3ph) and has a 300 mm impeller. Some have said it's impossible to produce this much CFM with such sized impeller.

Zelk

Ok, according to the retailer, a smaller outlet than inlet on the fan housing should not cause problems in this case. Speaking to a fan company about this, they were not sure, as there are too many variables. I asked the company also, if a 300 impeller with a 2Hp motor was capable of producing 1700CFM and they thought it may be possible, where as BIll Pentz thought it was unlikely unless a larger impeller and motor were used.

My combination machine has a 5"port at the saw and a 2.5" port on blade guard. After the point where the 4 meter 2.5" duct branches of the main line there minimal use of 5" ducting to the saw. Between the dusty and the wye there is 3.5 m of 5" flexible duct.

My dusty (claimed 1700 CFM) can take a 6" hose, would I notice much difference replacing the 5" with 6" flexible hose?

Zelk

You probably won't notice any difference

Originally Posted by echnidna

You probably won't notice any difference

If air is being drawn from two openings which have a total cross sectional area of 25 sq inches ( sawguard duct- 5 sq in, saw duct- 20 sq in) into a main duct, I thought a 6" main duct with cross section of 28 sq in would be superior to that of a 5" which has 20sq in?
Zelk

Originally Posted by zelk

Ok, according to the retailer, a smaller outlet than inlet on the fan housing should not cause problems in this case. Speaking to a fan company about this, they were not sure, as there are too many variables. I asked the company also, if a 300 impeller with a 2Hp motor was capable of producing 1700CFM and they thought it may be possible, where as BIll Pentz thought it was unlikely unless a larger impeller and motor were used.

My combination machine has a 5"port at the saw and a 2.5" port on blade guard. After the point where the 4 meter 2.5" duct branches of the main line there minimal use of 5" ducting to the saw. Between the dusty and the wye there is 3.5 m of 5" flexible duct.

My dusty (claimed 1700 CFM) can take a 6" hose, would I notice much difference replacing the 5" with 6" flexible hose?

Zelk

The 1700 cfm will be for a "no-filter" or free air flow. For all practical purposes one you stick the filters on the air flow will be around 1200 cfm.

And I agree with bob, you won't notice a difference between 5 and 6". It's a bit of swings and roundabouts, 5" has slightly lower air flow but higher air speed which will capture slightly bigger bits flying away from the source s they are produced. 6" will have the opposite but clear more fines in the long terms as the fines are like a gas and rely just on sheer air flow. In this case I would look at cost.

Originally Posted by BobL

And I agree with bob, you won't notice a difference between 5 and 6". It's a bit of swings and roundabouts, 5" has slightly lower air flow but higher air speed which will capture slightly bigger bits flying away from the source s they are produced. 6" will have the opposite but clear more fines in the long terms as the fines are like a gas and rely just on sheer air flow. In this case I would look at cost.

That has to be as good a summation of it as I've ever seen.

It's interesting to note how much resistance to airflow is produced by a 3.5 metre, 5" flexible hose.

I did an experiment last night where I disconnected the ducting at various points to see what effect it would have on the air pressure expelled from the dusty. As the dusty is enclosed, using a simple meter, I gauged the pressure of the air exiting the vent. The filter was on and there was a small amount of dust in the bag. The filter was cleaned regularly.

Photo 1 shows the ducting. P2 shows the vent and gauge.

With all the ducting connected the gauge showed a reading of 1, disconnecting the hose at the saw showed a reading of 2, disconnecting the hose at the dusty showed a reading of 3. One can assume that the difference between 2 and 3 is due to air resistance created by the flexible duct. It makes me wonder whether changing to larger 6" duct with 25% greater cross sectional area would improve things. I removed the ' 6" to 5" ' reducer at the dusty and there was only a very slight increase in airflow.

The last photo shows how much of the dust intake port had to be covered to get a reading of 1. I also checked what effect the carpet in the duct ( for muffling sound) had, the gauge showed no difference to airflow.

I have shortened the flexible duct to 2.5 m, so the required 6 " ducting would end up costing $88, not including a reducer, is it worth it, especially if I intend to increase the duct size to the overhead guard to 3.5"?

I found out that a larger intake compared to outlet on the fan housing is required to ensure that the blower motor does not try to move too much air which will cause it overheat and burn up.

Zelk

Good work Zelk

Just a technicality but you are not measuring pressure but air flow rates - if you could calibrate your scale you could even measure real flow rates (if you lived near me I would let you borrow an air flow meter so you could do this)

Everything will be a trade off between maintaining a higher air speed (but lower flow) at source to capture stuff straight away - versus lower speed but higher flow when venting the shed. This is why opening all the ports on a DC system and leaving it running after the dust generation has occurred is a good idea.

Originally Posted by BobL

Good work Zelk

Just a technicality but you are not measuring pressure but air flow rates - if you could calibrate your scale you could even measure real flow rates (if you lived near me I would let you borrow an air flow meter so you could do this)

Everything will be a trade off between maintaining a higher air speed (but lower flow) at source to capture stuff straight away - versus lower speed but higher flow when venting the shed. This is why opening all the ports on a DC system and leaving it running after the dust generation has occurred is a good idea.

Thanks BobL,

Sounds like I may have do some more experimenting. I must say cutting through boards, the dusty does a reasonable job. As far as improving the quality of dust extraction, the limiting factors are not just the dust extractor and plumbing, but also the inherent design of the saw. I think at the moment there are quite a few things on my plate, such as, workshop air extraction, venting the dusty to outside and of course, improving dust extraction.

Zelk

My experience exactly, Zelk.

Recently did an upgrade of my dust collection system, and a critical part was making or re-making ports, hoods and shrouds at the tools themselves. The manufacturers don't do a very good job of design for air flow optimisation, and they nearly all use 100 mm ports.

BobL and Bill Pentz warn us all that getting the hoods/shrouds right can be mission critical, and in my case, it was. However, it has been worth the effort. The machines that once were my worst dust makers are now performing very well. Both BobL and Bill Pentz have some good info on port/hood design which is worth investigating.