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Thread: SCMS Dust Extraction
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21st September 2014, 10:21 PM #46.
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I have a light inside my enclosure but one thing I forgot to add was a window to see when the bags a re full.
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21st September 2014 10:21 PM # ADSGoogle Adsense Advertisement
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21st September 2014, 10:29 PM #47
Funny you mention that bob .. I was having a beer with a friend yesterday, and suggested that I could replace one of the structafloor panels with perspex, and got laughed at ! Not too late to install though ..
Glenn Visca
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24th September 2014, 07:19 PM #48
Picked up my louvred vent yesterday afternoon. Also took delivery of some lengths of 150mm PVC.
Here is the vent on the bench
... And here its installed. Lots of butyl mastic and I think it looks quite neat.
It's just a smidge over 2x the area of a 150mm duct ... I hope that's enough. The louvres are quite course, as is the grill mesh for rodent protection.
If not, I have positioned it such that I can add another (with some mucking around).
Am midway through cutting the hole into the internal enclosure walls to match.
Thanks for looking.Glenn Visca
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24th September 2014, 10:30 PM #49Junior Senior Member
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Nice. Looks very neat
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24th September 2014, 11:44 PM #50
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24th September 2014, 11:50 PM #51.
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The 2X duct cross sectional area recommendation for the vent assumes unrestricted flow.
If I knew you were going to use those louvre thingies I would have suggested 4X the area.
Those louvres are surprisingly resistive because they are equivalent to a 90º elbow but being such a tight bend they create a lot of turbulence.
Otherwise they do indeed look tidy.
Any chance of putting another one in the wall?
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24th September 2014, 11:57 PM #52Junior Senior Member
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Bummer...a killjoy!
(Just kidding, we all appreciate your help Bob)
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25th September 2014, 12:20 AM #53.
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Before you fork out for the extra louvre why not measure the pressure differential between the enclosure and the outside. Make up a 30 cm high U tube manometer from some clear PVC and add enough water with some food dye to a height of 100 mm up the side of the U. Leave one side of the U outside and put the other inside the enclosure - makes sure everything is well sealed up. If the pressure differential is less than 1/2" then you should be OK. Remember every inch of pressure lost means less overall air gets moved.
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25th September 2014, 08:58 AM #54
Hmmm ... While I understand the concepts here, and the requirement to NOT generate back pressure on the DC, I get all foggy when it comes to test and measurement, so I am about to ask a series of silly silly silly questions. Please, no laughter.
Any limits on the size of the tube used for the manometer ?
When you say put one leg of the manometer outside, I assume you mean poke through the Louvre.
This is a non machine test right ? I don't need to have the DC connected up and operating. We are just looking at the differential pressure between room and outside.
Is the half inch limit the aggregate of both sides of the manometer ?
More questions may followGlenn Visca
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25th September 2014, 11:17 AM #55.
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My take on all this is there are no silly questions.
Any limits on the size of the tube used for the manometer ?
I'd go for something like 6 to 12 mm diameter tube.
I use a short length of clear PVC and bend that into a U shape and then attach irrigation drip line to extend the sensing points to where I need it.
When you say put one leg of the manometer outside, I assume you mean poke through the Louvre.
This is a non machine test right ? I don't need to have the DC connected up and operating. We are just looking at the differential pressure between room and outside.
Is the half inch limit the aggregate of both sides of the manometer ?
More questions may follow
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25th September 2014, 11:24 AM #56
Thanks Bob.
Its going to take me a little while to get to into a position to perform that test as I haven't yet completed any of the ducting - let alone a complete seal of the enclosure.
As always - your advice is very much appreciated.
I have some 1/2 inch plastic tube somewhere that I use as a level when building pergolas, decks etc. ... but having just moved into this house last November, I am stumped if I can find it.Glenn Visca
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25th September 2014, 11:41 AM #57Glenn Visca
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25th September 2014, 12:31 PM #58.
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27th September 2014, 08:08 PM #59
Hi there,
Managed to do some testing today. Results are as follows:
THE DUT.
DC is rigged via Y to 2 x 150 mm ducts popping out the the top of the enclosure. I havent run the duct anywhere else yet in case I needed to open up more vent to outside.
During the test, both 150mm ducts were open.
The door to the enclosure was sealed by timber door stops pressed hard up against the door.
The base of the door was sealed with a rolled blanket from the inside of the enclosure. I will be buying a draft stopper tomorrow.
THE TEST RIG
Picture of U Tube Manometer below. Marks at 1 inch intervals. Tap opened during test ( of course ).
RESULTS
Test 1
Door closed. Test through small hole in wall of enclosure. Vent to outside open.
1/4" movement each side - so 1/2"
Test 2
Door closed. Test through external vent. Vent to outside open.
1/4" movement each side - so 1/2"
Test 3
Door closed. Test through small hole in wall of enclosure. Vent to outside CLOSED (block off)
3/8" movement each side - so 3/4"
Test 4
Door closed. Vent to outside open. Test tube poked through open end of 1 of the 150mm ducts to test suction. Other duct blocked (but only by laying piece of 6mm ply over the opening .. so probably not a great seal.).
1 1/32 inch movement each side - so 2 1/16"
Test 5
Door OPEN. Vent to outside open. Test tube poked through open end of 1 of the 150mm ducts to test suction. Other duct blocked (but only by laying piece of 6mm ply over the opening .. so probably not a great seal.).
1 inch movement each side - so 2"
OBSERVATIONS
As I don't have a remote switch yet, I have to open the door to the enclosure to turn on the DC. There is certainly a fair amount of pressure required to close the door once the DC is up to speed.
To say the enclosure is completely airtight would be a lie. Having fitted the door stop hard up against the door, I was a little surprised to feel air escaping around the door. I guess the pressure flexes the door just enough to let air escape. I can try some draft stop rubber or brushing, but seriously, the stops are pushed hard up. I wonder whether some sort of flexible flap (like plastic damp course) that will push against the door when pressurised might be better.
With hindsight, would have been better to have door open inwards to the enclosure, so when pressurised, it would push against the door stops rather than away. BUT, this would have cost me more space on an enclosure that is already about 1100 x 1700.
To be completely transparent ... I have also NOT duct taped the PVC joints either. I plan to.
I also noticed air escaping under the bottom plate of the stud wall. I will need to seal them with mastic I guess.
SUMMARY
There is definitely a small loss in suction by back pressure, which seems to equate to about 3%.
PROGNOSIS
Help me BOB :confused: . Any comments so far ?
Do you think test results are compromised by the leaks ?Glenn Visca
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27th September 2014, 08:49 PM #60.
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When you say "vent to outside" - do you mean the louvres are blocked or something else ?
The louvres need to be open at all times.
The fact that you sense "a fair amount of pressure" in closing the enclosure door, even with all your leaks, tells me the louvre opening is on the small side.
Can you take the louvres out and see it that makes a difference?
I know you have pop riveted the louvres in place - maybe put them back after testing using tek screws, Apart from weatherproofing it doesn't matter if they leaks - in fact it will help the situation.
Do you think test results are compromised by the leaks ?
The fact that you get at least 1/2" with all the leaks is consistent with what I said above and tells me the louvre area is not enough.
I'm not saying my enclosure is completely air tight ( I still sense a very slight pressure when I close the door) but my pressure loss between the inside of the enclosure and the inside of the shed is 1/5" or 0.2".
My enclosure opening to the outside is a foam lined "double S bend" (to control sound) with a rectangular cross section of 200 c 500 mm which is nearly 6x the cross section of a 150 mm duct
It doesn't matter that my enclosure leaks because it is outside but because your enclosure is inside it is important that it does not leak (or leaks as little as possible) so it is even more important that it is not at a very high pressure, otherwise more fine faust ladened air will get back into your shed.
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