Thread: How much extractor do we need?

Originally Posted by tonzeyd

I know its not an ideal solution and would feel sorry for whoever gets given the task, but wouldn't a work around be to have some pleb go out back with a stick and give the bags a good beating? Given that the DC works fine when the bags are clean?

I don't think they are bags abut pleated filters which don't exactly like being beaten.

Someone needs to develop a cheap flow warning that attached to each independent leg of commercial duct systems and goes off when the flow starts to drop. It could indicate that other parts of the system need to be shut down for instance or if multiple alarms are happening that filters need cleaning etc. If you wanted to have the sander and CNC on an independent system a CV Max only takes up one square metre of floor space and the simplicity of it means that the only thing that can go wrong is the motor committing suicide. With the system I am looking at at the moment I think we will get the cyclone off the floor and drop the debris into a container of some sort on the floor. The only reason I am looking at the suspended idea is that the existing ducting is very high and it is easier to get the cyclone to match it than the other way round.

Originally Posted by BobL

30I keep thinking that a large cyclone might be a better option? I'm not familiar with very large cyclones and it is my understanding that they use a very large (20") impeller and suitably sized motor and while they have plenty of suck that is not the only KPI required. The other aspect is fine dust removal by the cyclone itself (ie pre any final filter) which reduces the amount of very fine dust filtration required. This comes down to the ability of the cyclone not only to separate chips from air, but fine dust from air. Only cyclones designed in a special way like the Clearvue can do this. Clearvues are so efficient that they emit only a very small fraction of the very finest invisible dust so that they can be used without filters if they vent outside. The problem with a Clearvue is it will not be able to provide the flow required to service several of your larger machines at the same time.

It may be worth looking at a large industrial cyclone that still uses a final filter.
If the cyclone itself pulls out enough fine dust out of the air streams this may reduce or simplify the need for any final filter cleaning.

I'll definitely look into it, just need to find one with the motor mounted on top, not on the ground next to the unit.

Originally Posted by tonzeyd

I know its not an ideal solution and would feel sorry for whoever gets given the task, but wouldn't a work around be to have some pleb go out back with a stick and give the bags a good beating? Given that the DC works fine when the bags are clean?

It's been done a few times; full hazmat suit, gloves, taped wrists and ankles and a respirator. We rigged up a brush and air blower on a stick to get as far into the pleats as we could. The aim is to not have to do that.

In the air filtering business, loaded filters are usually monitored and alarms triggered using differential pressure switches like these
Series MP | Mini-Photohelic® Differential Pressure Switch/Gage combines the differential pressure gage with two SPDT switching set points. Designed to measure and control positive, negative, or differential pressures. | Dwyer Instruments
or these
Gage / Switches - Digital | Dwyer Instruments
These are useful because they provide visual/numerical indication of the actual pressures and set points
But despite what they say they are not low cost enough for most DIY operations.
There may be some cheaper chinese version available but I have not looked for these for some time

The $3 vacuum/pressure switches I'm using at home and at the mens shed could be used in the same way. The ones at the mens shed are also set up so that if too many gates are open the next machine will not even start. The mens shed is using a Clearvue so there is always flow but one of these budget level pressure switches could be setup to monitor the pressure between the impeller and filters, and atmosphere so that filter clog.

One problem is locating the pressure point between the impeller and filters because the air is generally moving quite fast at this point and so some sort of combination static dynamic/bernoulli type pressure is measured and then some sort of calibration is then needed.

I have a pressure monitoring point in my twin filter bag housing that I can switch to a digital pressure meter. This monitors the pressure between the impeller and filters, and atmosphere. The sorts of pressures I see are 0.4" of WC for clean bags which is surprisingly consistent with BPs pressure loss of 1" for a single 5 micron bag. Currently the pressure is 0.6" WC. Vigorous shaking of the the bags for 30 s reduces the pressure back to 0.4".

If teh bags are clean and I crush one of the bags down completely so only one is fully inflated and only a small amount of air gets out of the crushed bag the pressure goes up to 0.7"WC. Remember these will be some sort of dynamic/bernoulli type pressures but the fact that it increases when one bag is crushed down demonstrates that this pressure point can be used to monitor filter bags getting clogged. A pressure switch rigged to trigger a simple alarm at say 0.8" of WC could be easily implemented. What haven't I done this? Probably because I just don't do a lot of WW.
I occasional start longer tasks by checking how full teh bags are and shaking the bags to get maximum flow.

We found the older the bags became the quicker they clogged. I placed a sign on the front of the dusty saying, "Beat Old Bags Daily."
A dear old couple, regular customers, thought this quite inappropriate.

Elan
Having read through this thread, I think you need to go back to your boss and say ...

"I've done a bit of research and think that as a first step we need to increase the size of our dust extractor lines and install some auto opening blast gates."

I'm not sure that anyone above has calculated the size of pipe required to shift 12,000 cu.m per hour (I make that equivalent to around 7100 cu.ft. per min).
Bill Pentz's calculator suggests you need a 500 mm dia trunk main for that volume -- but I think Bill's calculator assumes a MUCH smaller extractor than what you have, so there could be a significant error in that estimate.
I'm guessing that your current extractor can cope with the existing dust load, allbeit with a bigger bin, if the dust mains are increased in size. I'd guess taht yor current main is 10 or 12 inches (250 or 300 mm).

Our current main is 16" for the flow level below 12k cu.m/hr (i think 8k)

You're missing the main point though Ian, we can't keep the filters clean in the current setup.

Originally Posted by elanjacobs

Our current main is 16" for the flow level below 12k cu.m/hr (i think 8k)

You're missing the main point though Ian, we can't keep the filters clean in the current setup.

thinking out aloud ...

that implies that your existing system is designed as a chip collector with a bag filter -- i.e. your problem might be solved by increasing the area of your filters and establishing a regular filter exchange regime. As mentioned above by QC Inspector, if you had multiple filters in parallel, you could contract someone to regularly clean the filters off site, switching the exhaust flow to a filter bank #2 while bank #1 was cleaned.

But ASSUMING that your current system can pull enough air for your revised work methods -- more sanding and CNC routing than solid wood planing -- perhaps you could install a big cyclone between the impeller and the main dust trunk? I'm thinking that auto-close / auto-open gates on each machine might compensate for the reduced air flow consequent to adding a cyclone. If the existing system was designed for the situation where most gates are permanently open, auto-open and close gates might easily compensate for the flow loss consequent upon fitting a cyclone.
It might be possible to locate the cyclone inside the factory (high up on a wall) and have a chute (that passes through the wall) to direct the debris to the current bin.

Originally Posted by elanjacobs

Our current main is 16" for the flow level below 12k cu.m/hr (i think 8k)
You're missing the main point though Ian, we can't keep the filters clean in the current setup.

It's really only half the point, even if the filters can be kept clean, 4" inch ducting and 4" machine ports cannot clear the fine dust at source from most wood working machines. It may look like it's working but particle counters will say otherwise.

The 7000 CFM will be the nominal flow to maintain 4000 fpm in a 16" trunk. 4000 fpm is the nominal linear speed to keep sawdust in suspension and is how DC ducting systems are usually specified. To achieve this on your system only requires a very low pressure, about 1" of WC (see red circle below) so it means your trunk line is a non-issue and it will not be the flow limiting step.

NB not all the blue numbers (FPM) on this diagram make sense as it has been cobbled together from bits and pieces of the same diagram.
Flowratesxy.jpg
Your trunk line sounds like it's more than big enough, I thought you said it was too small and had to be upgraded?
Is it the junctions on the main trunk that need upgrading?

It splits pretty early to smaller lines, but it's 16" in to the DC

Originally Posted by elanjacobs

It splits pretty early to smaller lines, but it's 16" in to the DC

Ah Ha!

While the existing dust plumbing might need an upgrade in size, an accountant looking at the set-up might conclude, that when it comes to fine dust the existing system is TOO efficient -- i.e. too much fine dust is accumulating in the filter bags.

But I think the core problem is SEPARATION of dust and debris before the flow gets to the filter bags.
Using Bill Pentz's calculator -- a 16 in main implies 5600 cfm, at 4000 fpm. This is equivalent to 9,500 cu.m per hour. So is the existing system 8k or 9.6 k per hour?
In my mind the question to be answered is, can a big cyclone separator be fitted in front of the impeller. If yes, that is where Elan's boss should invest his $$. If no, then you might look at an upgrade to the filtering set-up -- with the potential negatives already mentioned in this thread. Or, worse case, Elan's boss needs to install a new system.

I'm thinking that improvements to the trunk and sub-mains (and machine ports) might address residual fine dust COLLECTION. A quick test of the existing fine dust collection is -- how much dust settles out of the air over a weekend? If everything is covered with a fine layer of dust, then collection is an issue.

BTW, I assume Elan's workplace uses many hand sanders. What sort of dust collection do these sanders have?

Our current DC has 8 massive pleated cartridge filters (136 sq.m of filter area) with a vibration clean cycle that can't shake enough dust out of them.

We don't have space for both a floor mounted cyclone and baghouse DC. We also can't increase the footprint of the DC, which is about 2.2 x 1.6m. So, the options as I see it for the DC are: either a reverse pulse system that can keep itself clean enough to maintain flow, or a big-ass cyclone that's good enough to vent to the atmosphere without additional filtering. Given that the cyclone has no moving parts or consumables, I'm guessing it would be cheaper than a reverse pulse baghouse, which might mean we can afford to redo all or most of the ducting.

There are 3 ROS, each with their own Festool vac.

Also 4 Jet air filters hanging from the ceiling that have made a HUGE difference, might look at getting a few more.

Originally Posted by elanjacobs

Our current DC has 8 massive pleated cartridge filters (136 sq.m of filter area) with a vibration clean cycle that can't shake enough dust out of them.

We don't have space for both a floor mounted cyclone and baghouse DC. We also can't increase the footprint of the DC, which is about 2.2 x 1.6m. So, the options as I see it for the DC are: either a reverse pulse system that can keep itself clean enough to maintain flow, or a big-ass cyclone that's good enough to vent to the atmosphere without additional filtering. Given that the cyclone has no moving parts or consumables, I'm guessing it would be cheaper than a reverse pulse baghouse, which might mean we can afford to redo all or most of the ducting.

Have i got this correct, your existing DC is a 15 kW blower that vents into 8 pleated cartridge filters.
the big stuff drips into a bin, while teh finer stuff stays in the filters till shaken loose by the vibration clean cycle.

For me, the question is -- can you mount a cyclone canister on the inlet side of your existing blower? Something like a humongous version of one of these.


yes, it will seriously impact the efficiency of your DC, but from what you have previously described, you might have sufficient head room within that system to accommodate the losses.

The cyclone canister could be wall mounted near roof level.