Thread: DC enclosure and ducting set up

Interesting.I just measured mine at 0.84kPa = 3.37 WC.As mentioned in a post a year or 2 ago I fitted a Wok in the separator to try and keep the “large lumps” down in the bottom rather than go into the PF. I decided to remove the Wok and remeasure the back pressure.
Now down to 0.72 kP or 2.89 WC.
Think I’ll leave to Wok out.

Update. Measured the back pressure with no filter. 0.14 kPa or 0.56 WC

Originally Posted by Lappa

Interesting.I just measured mine at 0.84kPa = 3.37 WC.As mentioned in a post a year or 2 ago I fitted a Wok in the separator to try and keep the “large lumps” down in the bottom rather than go into the PF. I decided to remove the Wok and remeasure the back pressure.
Now down to 0.72 kP or 2.89 WC.
Think I’ll leave to Wok out.

Update. Measured the back pressure with no filter. 0.14 kPa or 0.56 WC

One issue I forgot about with measuring the filter pressure is where the tube is located and the orientation of the tube.
If you are getting a negative pressure then the Bernoulli effect is affecting your measurements.
This is cause by the air flow past the tube opening which can create a large negative pressure effect. The effect is most pronounced when the tune opening is at right angles to the flow.
Even if you are getting a positive pressure there may be a negative component.

having the tube opening directly into the flow will generate a overpressure so having the tube opening facing downstream of the flow is probably the safest.
The best place to measure is also when the air speed is slowest son for a PF that might be the top of the filter?
So before we all get too excited, maybe take this into consideration?

Originally Posted by Lappa

Interesting.I just measured mine at 0.84kPa = 3.37 WC.As mentioned in a post a year or 2 ago I fitted a Wok in the separator to try and keep the “large lumps” down in the bottom rather than go into the PF. I decided to remove the Wok and remeasure the back pressure.
Now down to 0.72 kP or 2.89 WC.
Think I’ll leave to Wok out.

Update. Measured the back pressure with no filter. 0.14 kPa or 0.56 WC

One issue I forgot about with measuring the filter pressure is where the tube is located and the orientation of the tube.
If you are getting a negative pressure then the Bernoulli effect is affecting your measurements.
This is cause by the air flow past the tube opening which can create a large negative pressure effect. The effect is most pronounced when the tune opening is at right angles to the flow.
Even if you are getting a positive pressure there may be a negative component.

having the tube opening directly into the flow will generate a overpressure so having the tube opening facing downstream of the flow is probably the safest.
The best place to measure is also when the air speed is slowest son for a PF that might be the top of the filter?
So before we all get too excited, maybe take this into consideration?

I just vaccuumed the filter out as best I can. Now I think about it it's probably the first time I've done anything like a proper clean of the filter, made me realise how bad it was.

Anyway, took a new measurement and pressure has dropped down to 3.22" WC, so a fairly big difference. Hopefully that will stop the filter blowing off.

Is this an acceptable pressure reading?

No negative pressure here and no excitement either.

Originally Posted by 3 toed sloth

I just vaccuumed the filter out as best I can. Now I think about it it's probably the first time I've done anything like a proper clean of the filter, made me realise how bad it was.

Anyway, took a new measurement and pressure has dropped down to 3.22" WC, so a fairly big difference. Hopefully that will stop the filter blowing off.

Is this an acceptable pressure reading?

My DC is a used unit I bought from a Men’s Shed. The PF was blocked badly (I have flow figures in the generic 2hp thread) and I spent considerable time with compressed air and had to manually dig out larger lumps that were stuck in the pleats.
The flow now is much the same as I had back then with the PF as clean as I could get it. Unfortunately I didn’t do any pressure reading between the impeller and the filter back then, only flow figures.

I don’t know what to expect for a new filter but your new readings are much the same as I had with the Wok in so a big improvement. My unit wasn’t blowing the filter off so you should be safe in that regard.

Would the very best place to measure be at the top of the PF? i.e. through the metal top?

edit: Does it matter what size tube I use? I searched the forums but didnt find anything. I was thinking of making one with some 10mm clear PVC tube, a board and water with red food dye.....?

Originally Posted by woodPixel

Would the very best place to measure be at the top of the PF? i.e. through the metal top?

Yep.
On mine, it can’t be a permanent fitting, as it gets the the way of the paddle handles.

This is going to be a long post and as I have already written it twice and least it I will write a bit and then save, and repeat, it so you might check in a few times as it will change and or be extended.

Originally Posted by Lappa

No negative pressure here and no excitement either.

Good there's no negative pressure - unfortunate there's no excitement

Originally Posted by 3 toed sloth

I just vaccuumed the filter out as best I can. Now I think about it it's probably the first time I've done anything like a proper clean of the filter, made me realise how bad it was.
Anyway, took a new measurement and pressure has dropped down to 3.22" WC, so a fairly big difference. Hopefully that will stop the filter blowing off.
Is this an acceptable pressure reading?

Vacuuming is good but air blasts are more effective. See below
Is it an acceptable pressure - probably not but if that's as good as you can get it then you have little choice. Most beige end DCs only generate about 8"W of total pressure and to be expending 3/8 of that pressure on the filter is going to hammer your air flow. By way of comparison I have twin large needle felt filters and I get about 0.5" of WC when they are clean which is about what BP suggests should be the case. Because most of the dust I make especially lately is from the table saw nd I cut a lot of Al with the TS and BS the dust is quite coarse so I see little increase in pressure drop with use.

Originally Posted by woodPixel

Would the very best place to measure be at the top of the PF? i.e. through the metal top?
edit: Does it matter what size tube I use? I searched the forums but didnt find anything. I was thinking of making one with some 10mm clear PVC tube, a board and water with red food dye.....?

Tube size doesn't matter but if it is too fat it's hard to bend into a U so this makes the pressure difference readings more difficult to see.
The top of the filter will be the best place for sure , air speed is low.
If the paddle handle is in the way maybe look to extend the shaft so that a trickle retic elbow can be employed underneath the raised handle.

Originally Posted by BobL

By way of comparison I have twin large needle felt filters and I get about 0.5" of WC when they are clean which is about what BP suggests should be the case.

Interesting that my system, with no filter attached, is higher than that. I wonder if the difference is due to the size of the opening in the separator centre?

Actually it's probably better I start a new post about PFs, I was going to append it to the previous one but I think this might raise some interest of its own.

The following is extracted from a very interesting research article on Air Blast Cleaning of PFs.
https://www.researchgate.net/publica...e-jet_cleaning

The main difference is the PF is housed inside a filtration chamber and the dust is filtered by the outside of a PF about 300mm OD and 600 mm tall. This makes it somewhat smaller than a standard 2HP DC PF.

Here is their setup.
The 0.3 to 0.7 MPa air blasts are ~150ms in duration and are set to go off when the Filter pressure reaches 1.6"WC. The air blast is applied to the clean side of the filter
The effect of this short blast is to knock some dust off the outside of the PF and it falls down through a hopper into a sealed bin.

Screen Shot 2019-06-01 at 5.16.46 pm.png

Here are their basic results

Let's just focus on the orange graph for now. The new filters start out with a filter pressure of 0.05 kPa (0.2"WC) and over about 500 seconds load up to 0.4 kPa(1.6"WC) this is a pretty fast load up and would be something that say a drum sander would make.
Then a blast of air is applied to the clean side of the filter and the effect is to reduce the filter pressure to about half (0.8"WC) and the whole cycle repeats - note how it takes successively shorter and shorter time to reach 1.6"WC and the filter pressure after the cleaning increases slowly over time - the typical pressures are in the 1-2"WC.

PleatedFiltercleaning.jpg

The Green and Pink graphs refer to the addition of a cone (see article for details)inside the filter to spread the air blast around and one type of cone improves things and one make things worse.
NB these cones have nothing to do with the cones in conventional DC - the ones in the research article are there to direct the flow of the short air blast.

Air blast tech for cleaning filters has been around for some time but this is the first time I have seen this improvement using cones.
This setup would be suitable for anyone with a large cyclone like a ClearVue where a relatively small PF can be use.

Before anyone attacks their PFs with direct blasts from a compressor hose be aware you can easily puncture holes in these filters. It's much safer to apply the the compressed air blast sideways or parallel to the filter. It won't be as effective as the enclosed tank system shown above but you will reduce the risk of make holes.

So with the filter as good as I can get it I'm still losing a lot of airflow. Might be time to renew the PF. I can't remember how long ago I bought it but it was a long time ago.

Damn, Bob, you posted that just as I was posting. Seeing your last comment I would love to use compressed air to clean and test the results. I'll think about it for now.

Just ask if someone on the forum has a compressor you can use for a few minutes.

Just be careful with any compressor cleaning. On needlfelt filters the comp air can be applied directly from the outside inwards - I actually turn my bags inside out and then scrunch up the filter onto the end of the compressor nozzle and gove it a good blast for 30 seconds or so.

My understanding of the way the compressed air works in the system I described above is that it produces a shock wave to slightly expands the filter and loosen the compacted dust. This won't be as effective using the filter as is done on conventional DCs filtering from the inside out as it will be hard to compress a cylindrical filter than to expand it.