[OP]
Contributor
I am finalizing my dust collection setup and would like to install a pressure differential gauge to determine when the filters need to be cleaned. My question is what range should I be looking at? What is a typical pressure difference between clean and dirty filters?
- Mike
Contributor
A 0 - 5" magnahelic is perfect for this application. My system runs about 1" when the filter is clean and I service it when it gets over 2" w.c.
[OP]
Contributor
Perfect! That's what I needed to know!Originally Posted by John Lanciani
- Mike
Contributor
I bought a 0-5" on ebay and it works great.
Member
See my thread here: LINK
If at first you don't succeed, redefine success!
[OP]
Contributor
Thanks for the replies, fellas.
I am using a dual canister Powermatic 075 dust collector with two Wynn filter canisters. Should I just tap off the top of one of the filters? Would a gauge on each filter make sense or is that overkill?
- Mike
Member
I would have one on each. Especially if you get the gauges on eBay for cheap.
If at first you don't succeed, redefine success!
[OP]
Contributor
If I ran a line from the top of each filter to a tee, then ran one line to the gauge, would it give me a balanced (average) of the two?
- Mike
Contributor
No.
If you tee either the low, or high pressure side of two process variables to a single D/P indicator, you will get a reading indicative of the maximum D/P across a given filter. You won't know which one it is.
"The first thing you need to know, will likely be the last thing you learn." (Unknown)
[OP]
Contributor
Thanks...two gauges it is.
- Mike
Member
I think the answer is actually "yes".No.
If you tee either the low, or high pressure side of two process variables to a single D/P indicator, you will get a reading indicative of the maximum D/P across a given filter. You won't know which one it is.
Consider two chambers with slightly different pressures. If you connect a small diameter tube between the two chambers the pressure difference will cause a small flow from the high pressure side to the low pressure side. One end of the tube is at the higher pressure and the other at the lower pressure. There must then be a linear gradient of pressure along the tube since the flow has to be the same at any point in the tube. The pressure at the center is thus halfway between the two endpoint pressures and is therefore the average of the two chamber pressures.
For this to work properly the tee must be at the midpoint and there can be no leaks. Longer transverse tubes are probably better than shorter.
Beranek's Law:
It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.
Contributor
David
You obviously have some background in process measurement.. I've been an instrument and controls technician for 30 years In various nuclear power plants.
The magnehelic he is going to use is a static D/P cell. If you tee either process variable they will appear as a single process input.
They're a simple rugged little device. I don't think they're sophisticated enough to react in the manner you detailed. We have electronic flow devices that can do it, but not a simple mechanical cell.
I'll grab a few at work next week, and try them in out in a few configurations if I can find time.
We replace these low D/P cell on. Periodic basis. I never thought to grap a few out of the scrap bin for home use. I'm afraid it would take some fun out of wood working, if I incorporated the day job into it.
Member
Yes, the cell measures a single variable. That variable is the static pressure at the center of a small duct with flow. That length of duct does the averaging. There is only one input to the cell.
There is one thing I forgot: the entrance loss on the high pressure side. So while the pressure at the tee is the average of the pressures at the ends of the tubes, it's actually the average of the pressures just inside either end and so lower by an amount equal to the entrance loss. Oops.
Beranek's Law:
It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.
Member
One thing about Magnahelics is one of the tubes is connected to the part of the gauge under the viewing glass. It's possible for dust to migrate in the tube and dirty it up. Typically it's not a problem unless the lines are really short. You can put a tuft of cotton, cloth or cigarette filter (if you have those things handy) in the line to block the dust. The reading response will slow down but that shouldn't be a problem for a dust collector.
Of course if you're using the side ports remember to block the back ports. And visa-versa.
If you have a Magnahelic that has a dirty window, they can be cleaned if you're careful. The pointer is very delicate hollow tube, avoid touching it. The clear plastic cover has an o-ring seal that MUST be intact when you're done. Trying any more than that probably will come to a bad end.
This also means if the clear plastic cover has a crack in it it will leak and the gauge will not work properly.
Just for TMI: Capsulhelics are made for those applications where getting dust under the window is a problem. But they cost a lot more.
And for WTMI: In the same series, Photohelics are used when you need a switch output based on differential pressure setting. But I haven't seen one used in a new installation since the 1970's, remember them to be expensive too.
-Tom
Member
The system appears to let air go to either side at all times, so wouldn't the two filters get dirty at approximately the same rate? If one side is cleaner, it will let more air through, causing it to get dirty faster to equalize, won't it? I can't imagine there is a significant difference in pressure between the two.
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