dust extractor air volume

[wallace]

Hi all, I want to improve my dust extraction. At present I have a 1hp chip extractor going into a 45 gallon barrel and venting outside. Useing 4" flexi hose. This setup dosent keep up with my sawbench. Hence I need to improve. If I were to add another extractor on the same line would this double the suck. Or does it not work like that? Also what does the static pressure mean and how does it affect the flow. Also if I were to use 6" ducting would an extractor with a 4" inlet alter things? Any thoughts appreciated.
Mark

 

[CHJ]

Increasing the size of the ducting may not be benificial as the flow rate for a given volume of air in the duct will be lower and can result in debris just dropping out of suspension in the ducting.

Adding another exctractor in parallel on the same ducting is most likely to result in one fighting the other or starving the other for air flow and in worst case can result in the weaker one going into compressor stall and not doing any work.

As you say your extraction is external to the shop can you fit a much coarser filter bag, mine's fitted with a home made one out of coarse sheeting and it about doubled the airflow.
Or are you saying you have no filtration and just vent the dust straight out of the shop after drop-boxing the bulk of the shavings/dust.

 

[wallace]

Thanks for that, I was thinking 6" ducting because thats the size on my saw, and I thought it might work better. I dont have any filters, just the barrel acting as a drop box and then exiting. Do you think these are suitable for dust extraction?

http://www.ebay.co.uk/itm/Industrial-Ex ... 4603354f9c

Mark

 

[Bluekingfisher]

I'm not expert on the matter but the fan you have shown is around 1/3HP will that be enough to draw chips and dust from your saw?

Would it not be worth considering a dedicated 1.5HP barrel chip extractor for the saw like this?

http://www.worldofpower.co.uk/sip-dust- ... googlebase

It may save you a lot of agg??

 

[siggy_7]

In response to your question about static pressure, this is what pressure drop the extractor can generate - the higher the static pressure, the better the performance will be as more resistance is added to the network. If you want to understand the physics of it, spend a few minutes reading about the Bernoulli equation on wikipedia. A high static pressure does not necessarily mean a high flow rate in a free system however.

The key thing to understand about dust extraction is that you have two parameters to consider - pressure drop and flow rate. Increasing the size of the ducting reduces the pressure drop for a given flow rate - so if your extractor is putting all its energy into overcoming losses in small ducting, increasing the size will be beneficial. With 4" ducting for the airflow your extractor will generate, moving to 6" will cause more problems than it solves because as CHJ says the flow speed (i.e. air velocity - as distinct from flow rate which is the volume of air moved) will reduce, and when the flow speed drops too low particles will drop out of suspension and start blocking your ducting.

To keep up with a table saw, especially one with a 6" duct, I would recommend that you look for a high volume, low pressure extractor (often called "chip extractors"). This is because with large ducting and big dust collection chutes like you get in table saws, what you want is a lot of flow rate through a flow network that has low pressure losses. For use with power tools, where the ducting and collection apertures are smaller, the pressure losses increase - so for that type of scenario you need static pressure rather than flow rate, which is why high pressure low volume vacuum extractors (which are basically big hoovers) work better. The disadvantage of a chip extractor is that fine filters cause a big pressure drop unless they have a lot of surface area, which is why you see them with such large filter bags which even then aren't very fine compared with what a vacuum type extractor can happily cope with. As you are apparently venting outside your shop though, this isn't an issue.

In summary - to improve your situation, fit a bigger chip extractor with a high flow rate and coarse/no filters venting outside your shop.

 

[wallace]

Thanks for all that chaps, I do have a very good twin motor nilfisk which copes really well with my planer and bandsaw. My big concern is the wadkin table saw. I tried an experiment by plumbing it into my 1hp chip extractor with a short run and venting straightoutside and it was useless. I've been scouring motor repairers looking for a substantial impellar I could use and make something alla bill pentz. I have a flange mounted 2.5hp motor that would be perfect if I could find a suitable fan. I have noticed that the commercial extractors of say 1hp are the same as a 2hp the only differance being motor rating. Do they run faster or are the smaller ones restricted to stop the motors burning up?
Mark

 

[desmoengine]

remember also that if you are venting the extractor exhaust out of the workshop ,you will need to provide a means of air replacement,else you just lower the air pressure in the shop and reduce the pick up of particles. open a window or door or fit vent grill that will pass the same volume of air as can be extracted.
dave w

 

[desmoengine]

dont want to hi jack the thread but has any body bought built uses one of the cyclone central intercepter units.
if so whats your reckoning on performance.
dave w