Thread: How much extractor do we need?

Originally Posted by elanjacobs

As for cyclone efficiency, at what particle size? Will it separate 80% of sander dust as well as the bigger stuff, or is dust that fine just going straight through?

Unfortunately we don't know what the particle size profile is of the dust coming off your sander.
A standard particle counter cannot be shoved into the dust stream coming away from the sander as this will ruin it , as happened to BP's expensive counter.
A full particle size analysis of the sander dust would be quite expensive.

We can make some guesses based on what other people measure when sanding takes place (see graph below).
Despite what we might think sanding produces very little total MASS of material below 5 microns
In the example below mass is described by the green and purple points and shows that 99% of the mass of sanding dust is above 10 microns
If the cyclone is 90% efficient above about 10 microns (this is highly possible see below) it will then easily collect more than 80% of the mass that dust.

In terms of the numbers of particles it's a different story, now we follow the red and blue points. Even though the represent only 1% of the mass most of the dust particles are actually smaller than 10 microns with a peak at ~5 microns. More importantly these are the ones that will hang around much longer in the air and cause teh health problems. Stuff above about 10 microns falls to the floor very quickly and is not really a problem. But lest leave that aside for the moment.


Sanderdust-profiles.jpg

Now we need to look at cyclone efficiency for various particle sizes.

The following diagram is pretty typical of large industrial cyclones ie between 55 and 90% of the dust mass is collected at 5 microns and 80 to 97% at 10 microns.
Note the sizes of the cyclones and large pressure drops involved.
The more air and dust a cyclone has to handle the less efficient it becomes.
How does your dust load compare to the 85 lb/hr for these cyclones?

Cycl1.jpg


Remember 99% of the mass of sanding dust is above 10 microns with more than 50% of the mass being above 30 microns where the cyclone efficiency is between 92 and 97%

Lets assume
95% collection for the 50% of the mass above 30 microns,
90% collection for the 49% of the mass between 10 and 30 microns
50% collection for the remaining 1% of mass less than 10 microns
This averages out to about around 92% efficiency - which is pretty typical for cyclones

If your shop makes 20kg of dust per hour, 92% extraction still leaves 1.6 kg/hr coming out the end of the cyclone which is too much to vent especially back into a shed so filtering will still be needed.

A really good cyclone like a Clearvue captures 99% of the dust above 5 microns which (if it could handle 20kg/hr) would still leave 200g of fine dust coming out the end of the cyclone which is also too much to vent direct to air especially inside a shed. Outside a shed if there was a good breeze blowing it might be OK but otherwise thats 8kg per working week which would certainly be noticeable after a few still days.

I hope that explains the sorts of issues involved. What it looks like is that if you decide to use a cyclone you will still need some sort of a filter system, scaled according to your actual sawdust production

Please bear in mind that I had a very small envelope to do these calcs.

You really notice how bad the air is after you've been breathing clean air for a couple of hours. I just hope it's not too late for me, my throat is already sensitised

That's not good - you will need to watch that given it's your livelihood.

Hmmm, so a cyclone might not end up being viable... I'm assuming you can't have post-cyclone filtration dropping back in to the same bin. If only floor space wasn't an issue....

The CNC comes out of lease in June, so that's when things will actually be able to happen. We'll get a few people out closer to that time and see what their sales pitches are, until then I'll live in my mask

Originally Posted by BobL

Unfortunately we don't know what the particle size profile is of the dust coming off your sander.
A full particle size analysis of the sander dust would be quite expensive. But perhaps warranted in this case

We can make some guesses based on what other people measure when sanding takes place (see graph below).
Despite what we might think sanding produces very little total MASS of material below 5 microns
In the example below mass is described by the green and purple points and shows that 99% of the mass of sanding dust is above 10 microns
If the cyclone is 90% efficient above about 10 microns (this is highly possible see below) it will then easily collect more than 80% of the mass that dust.

Remember 99% of the mass of sanding dust is above 10 microns with more than 50% of the mass being above 30 microns where the cyclone efficiency is between 92 and 97%

Lets assume
95% collection for the 50% of the mass above 30 microns,
90% collection for the 49% of the mass between 10 and 30 microns
50% collection for the remaining 1% of mass less than 10 microns
This averages out to about around 92% efficiency - which is pretty typical for cyclones

If your shop makes 20kg of dust per hour, 92% extraction still leaves 1.6 kg/hr coming out the end of the cyclone which is too much to vent especially back into a shed so filtering will still be needed.

hi Bob, great information as usual, but can you expand a bit in respect to Elan's problem which is excessive clogging of his bag house filters. Which if I recall correctly are located outside his work space.

In your previous life and post retirement studies, do you have a feel for the particle size range which would result in the too rapid clogging of Elan's bag filters?
Whilst I follow the health benefits of collecting the sub-5 micron dust at source, from what Elan has written, his system can do this when the filters are clean, or will once he opens up a port or two, but the system then degrades rapidly once the filters start to clog.

As I understand what you have written, Elan could potentially dedicate a cyclone to the twin drum sander and exhaust it into his existing bag house filters which are located outside the work space. But this would be a false step if filter clogging is primarily associated with sub-5 micron particles. However, if filter clogging is mostly associated with particles between 10 and 30 microns, then even a relatively in efficient cyclone could dramatically reduce the frequency of [the expensive] bag cleaning.

BTW, our filters are 1 micron. Or so they say

Originally Posted by ian

hi Bob, great information as usual, but can you expand a bit in respect to Elan's problem which is excessive clogging of his bag house filters. Which if I recall correctly are located outside his work space.

I don't think the filters are outside his workspace. From what I can see Elan's filter problem is just that the amount of fine sawdust (versus chips and curlies) produced is just too the filters cleaned more often.

In your previous life and post retirement studies, do you have a feel for the particle size range which would result in the too rapid clogging of Elan's bag filters?

This is a really good point. One might think that if cyclone that is 80% efficient at removing total dust from the air is made 90% efficient then this should double the time between filter cleaning. However those efficiencies apply to all dust but not all dust contributes to filter clogging to the same extent.

All dust particle sizes affect filter performance to some degree. To start with, some dust on the filters improves the filtration efficiency with very little loss of flow. Needlefelt bags and pleated filters when completely clean are typically 95% efficienct at 5 microns range but after being conditioned their efficiency can increase to 99.5% with still only a small loss of flow. After this flow performance gradually degrades as that dust layer gets thicker.

In terms of affecting air flow the bleedingly obvious applies, the finer the incoming dust, the thinner a layer of dust over the filters required. In the case of curlies, the layer probably has to be meters deep, Dust in the 30 - 10 micron range has to be cm thick, while in the case of <10 micron dust it just needs to be mm deep. In practice even basic DCs provide some degree of particle separation with few chips and curlies ending up clogging filters because they fall out of suspension in the collection and filtering area. Usually the dust that ends up on filters is <30 microns. The distribution of these particles is dependent on what WW processes are undertaken. For something like sanding most of numbers of particles on the filters will be <10 microns, though the mass of these particles will usually be much smaller than the >10 micron filter clogging dust.

Whilst I follow the health benefits of collecting the sub-5 micron dust at source, from what Elan has written, his system can do this when the filters are clean, or will once he opens up a port or two, but the system then degrades rapidly once the filters start to clog.

I doubt that even when the filters are clean there is enough air flow to collect sufficient <10 micron dust to meet acceptable long term health risks. Elan is going on by what he sees, but only the use of a wearable particle counter could determine what is happening,
While improved collection is important, to start with Elan sounds like he would like to be able to operate for longer with the flows a clean filter system provides. because that is his immediate aim I will stay with this like of discussion.

As I understand what you have written, Elan could potentially dedicate a cyclone to the twin drum sander and exhaust it into his existing bag house filters which are located outside the work space. But this would be a false step if filter clogging is primarily associated with sub-5 micron particles. However, if filter clogging is mostly associated with particles between 10 and 30 microns, then even a relatively in efficient cyclone could dramatically reduce the frequency of [the expensive] bag cleaning.

As I said above filter clogging is typically associated with dust <30 microns.

Based on what I said above, a cyclone would still help because it will remove substantial amounts of dust in the <30 micron range and even a useful amount of dust below 5 microns

My back of the envelope calc above indicates that a decent basic cyclones should remove 92% of sanding dust <30 microns.
This should extend the periods between filter cleaning by about a factor of 12.
In practice it will be better than this because not all Elan's dust is from sanding

A really efficient cyclone like the BP cyclone is supposedly 99% efficient for >5 microns.
Even for fine dust generation like sanding, relatively little (<1% of ) of the dust is <5 microns.
Combing these two factors would suggest that periods between filter cleaning would be extended by by between 50 and 100 times.
I don't know if anyone makes an industrial scale BP design or a high efficiency cyclone.

The filters and the entire unit is outside and venting to the atmosphere

Originally Posted by BobL

I don't think the filters are outside his workspace.

from post #11

The DC is already on common land without permission, we can't take up any more space outside and there isn't space to take inside.

Originally Posted by BobL

Based on what I said above, a cyclone would still help because it will remove substantial amounts of dust in the <30 micron range and even a useful amount of dust below 5 microns

My back of the envelope calc above indicates that a decent basic cyclones should remove 92% of sanding dust <30 microns.
This should extend the periods between filter cleaning by about a factor of 12.
In practice it will be better than this because not all Elan's dust is from sanding

A really efficient cyclone like the BP cyclone is supposedly 99% efficient for >5 microns.
Even for fine dust generation like sanding, relatively little (<1% of ) of the dust is <5 microns.
Combing these two factors would suggest that periods between filter cleaning would be extended by by between 50 and 100 times.
I don't know if anyone makes an industrial scale BP design or a high efficiency cyclone.

hi Bob,
what I'm interpreting from this is that Elan could potentially (note I'm only saying potentially) utilise around half the footprint of his bag house filter for a cyclone -- that discharges into the existing waste bin -- with the cyclone's exhaust directed to the remaining section of the bag house. And if that were feasible / possible he would greatly reduce the frequency of filter cleaning while staying within his existing DC footprint. While at tegh same time improving dust collection from inside the factory where he works.

Originally Posted by ian

hi Bob,
what I'm interpreting from this is that Elan could potentially (note I'm only saying potentially) utilise around half the footprint of his bag house filter for a cyclone -- that discharges into the existing waste bin -- with the cyclone's exhaust directed to the remaining section of the bag house. And if that were feasible / possible he would greatly reduce the frequency of filter cleaning while staying within his existing DC footprint. While at tegh same time improving dust collection from inside the factory where he works.

Humm their raises a bunch of complications

The collection bin of a cyclone is usually before the impeller hence is under vacuum relative to atmosphere. The collection bin of a DC is after the impeller hence is under positive pressure relative to atmosphere - the two systems cannot share the same bin.

If the Cyclone bin is placed after the impeller there will still be pressure differentials developed that could reduce the effectiveness of the dust separation by the cyclone when both are running. When only one is running it would blow air/dust back out through the other into the workshop. This could be over come by some sort of one way air tight valve/gate system - starting to be more trouble than its worth.

Our DC is negative pressure, the fan is after the filters.

Originally Posted by elanjacobs

Our DC is negative pressure, the fan is after the filters.

How integrated are the filters with the sawdust bin?
Presumably the filters are set up so that when they are rattled the dust falls into the collection bin?

Couple of photos with labels of the DC might help?

IMAG2858.jpg
Everything above where the pipe enters is filters. The idea is that the heavier stuff drops straight in to the bin and the filters get the rest

Thanks Elan.

The following is a possibility
A better layout could be obtained by interchanging the cyclone and filter bank (and turning the filter bank around) to better line up the large ducting.
A specialist in this area could maybe come up with even a better layout
So you keep your current system and interpose a large cyclone.
The current bin becomes the cyclone bin and you could use a much smaller bin for the filter bank, the whole filter bank could be lowered so it is closer to the ground to fit under the gutter box.


Cycl2.jpg