So I finally did some testing.. In the water pump thread I mentioned a water scrubber for use to purify air from a fume hood.
I first decided to make a filled column and started out by making some filling by cutting small pieces of a 16mm PVC pipe.. After making about half
litre of filling, my laziness made me to search google for easier solution.. and there I found another design: a plate scrubber!
Figure 5 shows the scrubber. It contains 3 holed plates, with hole diameter ranging from 3,2 to 9,5 mm. They say that a decent scrubber needs 2 to 3
plates and maybe even 4 to 5 to meet future (environmental) needs..
So I started off directly building one of those since it seems to be easily improvised... heres my plan:
The small PVC pipe is there to control the level of water on the plate.
The material of bucket is polypropylene. The holes was done by a welder, 4mm in diameter. Theres about 300-500 holes... they were done surprisingly
fast
During testing just a little amount of water dripped through the holes (at 1/3 of the fans effect). So the water consumption is alot less than in
usual filled scrubber, couple of milliliters in some minute. This fact is also good since I can skip the water pump for the recirculation. Just gonna
use a big reservoar with water.. and collect the contaminated water into another reservoar.
Also, this thing doesn't need any good spray nozzle to distribute water!
I still however don't know how to do a drop remover (demister).. any idea? A glasswool sheet would probably create too much resistance..
Gonna post some pics when the stuff is done..Magpie - 20-8-2005 at 07:26
Nice going frofot. Your bubble plate design is basically that of a perforated plate distillation column.
For a demister pad I suggest the plastic scrubber pads used for kitchen pot scrubbing. I would start with a thickness of one pad (~2 cm) and see how
it works. I'm assuming your main concern is acid resistant material so recommend the pad be plastic.Fleaker - 20-8-2005 at 14:49
Very nice. My friend and I are incorporating an activated charcoal column for the absorption of organics. The more I look at the water scrubber, the
more useful it looks. Thanks for posting your progress.Magpie - 20-8-2005 at 21:14
Frogfot,
The scrubber is fine but I have some concerns about your system design.
How many plates will you have? If you have 3 plates and the water depth on each plate is 1 cm this provides a total pressure drop of 3 cm of water
(1.2" H2O). Then you will have to add demister, ducting, and fitting losses to this value. [FYI my blower will only pull 0.75" H20 at my
design gas flow rate of 450 cfm. This delta P will all be eaten up by my 8" ducting and fittings. ]
What kind of blower will you use and what is your design pressure drop and flow rate? [FYI my blower is an "open-one-side," belt driven
squirrel cage type running at 1000 rpm with a 9" diameter (22.5 cm) inlet.]frogfot - 20-8-2005 at 23:46
I have about 100 W (radial) kitchen fan. I'm not able to calc the theoretical pressure drop.. it would be too complicated when taking into account the perforated plate.. I'm basically doing this by trial and
error.. or how do they say..
If this fan wouldn't push through enough air, I'll change to another. I have in mind one fan. It's 210W, max pressure of 530Pa,
2150RPM, 570m3/h. Problem is, it costs 225$, and it would be not smart to by it not knowing if it could do the job..
I'm currently watching for such fans on my local recycling center. Though I havn't seen anything above 100W yet..
I've searched some catalogues but couldn't find similar stuff.. gotta visit some stores on monday. I'll "simulate" the
scrubber with steel-wool since it has a bit similar structure to those pads.
Lol, I had this idea to install a fixed propeller instead of the demister... this would twist the air and force all the drops to the walls of the pipe
by centrifugal force.. or maybe not.. anyway, this would make construction too complicated..12AX7 - 21-8-2005 at 00:02
Would be better to do a Dyson style centrifuge. I'm guessing it needs more velocity than you have pressure to make, though.
TimMagpie - 21-8-2005 at 12:12
The scrub pad you show looks like a more expensive and tighter pad than the one I had in mind. I found one under my kitchen sink. It is a
"Tuffy" brand and has a loose weave with holes that are about 3 mm in diameter. I think they are designed to use on Teflon cookware. Sorry
I don't have a digital camera to take a picture. I can't imagine it costing more than $0.50.
Your desired blower specs look pretty good. I will attempt to show a table comparing them with those of my blower:
As you can see you are getting quite a bit more pressure drop due to the higher rpm. This 530Pa is equivalent to 5.4cm H2O. So if you use 3cm drop
across 3 plates you will have 2.4cm H2O left for demister, ducting, and fitting losses. It is a balancing act. If you exceed these losses your flow
will drop until it comes back into balance to match your fan curve (graph of pressure drop vs gas flow). Try to get a copy of the fan curve before
you buy the blower.
[My blower installation is still in progress. I have the blower and discharge plenum installed. I just picked up the 8" PVC suction fittings.
This morning I installed 2 pipe hangers for support of the suction piping. Tomorrow I will pick up the PVC suction pipe at my local agricultural
irrigation dealer. ]Chris The Great - 21-8-2005 at 16:40
Very nice job! I am gonna build me one of these. I picked up a massive blower for $20, it used to be from a furnace. Can't remember the stats
off hand but this thing blows like crazy
If using a resovoir, would it be a good idea to add other chems to the water, for example sodium hydroxide if you're gonna be venting alot of
acid gas or bromine, or NaOCl for easily oxidized stuff? Just make sure you only use the special chemical solution only once, or you might be in for
some nasty surprise, such as when HCl goes into your NaOCl.
Thoughts?Magpie - 21-8-2005 at 20:12
Frogfot,
If you give me your duct length, diameter, and number/type of fittings (elbows, etc) I will calculate that part of the pressure drop for you. I set
this all up in an EXCEL spreadsheet when designing my blower system. It will be easy to substitute in the parameters for your ducting. I would need
the values for both the suction and discharge ducting.frogfot - 22-8-2005 at 03:52
Quote:
Would be better to do a Dyson style centrifuge.
Googled a bit, and if I got it right the air suppose to be injected under inclination into a cylinder.. which makes it spinning. Maybe it can work
with low pressure too, but construction will be a bit complex.
Just visited several stores in search of the perfect scrubbing pad... but all they had was usual sponges
So, I'll use the steel wool as a temporary solution. I'll forget about the rusting for now.. or maybe cover it in paint...
I consider bying a new fan only as the last way out. Btw, the page that sells these fans doesn't seem to have the pressure-flow curve..
Quote:
If using a resovoir, would it be a good idea to add other chems to the water, for example sodium hydroxide if you're gonna be venting alot of
acid gas or bromine, or NaOCl for easily oxidized stuff?
In the beginning I thoat using dilute NaOH or Na2CO3 if I used a recirculating pump (to protect it from corrosion). But now it doesn't seems to
be that important. It'll be cheaper without too.
My fume hood shouldn't give off any quantitative amounts of toxic gases.. It'll only take care of gases that come out for example when
I'll open a chlorine gas generator to vent it.. and stuff like that.
Most of the long experiments will be done in airtight vessels with an outlet through bubblers and a coal filter.
Quote:
If you give me your duct length, diameter, and number/type of fittings (elbows, etc) I will calculate that part of the pressure drop for you.
That would be cool if it's not to much work.
The setup will be like this:
fume hood
50 cm pipe, 11cm ID
11 cm connection, 9,8cm ID
scrubber
150 cm pipe, 11cm ID
11 cm connection, 9,8cm ID
fan connected to the outside
The hose measurements are at maximum.. maybe I'll set them up in other way to make them shorter..
EDIT:
Oh, forgot to mention that I've finished the second plate. One can see already that the capacity of my current fan is too low. It can hold max 1
cm water layer on each plate, and not all holes are bubbling.
EDIT2:
Humm, just a thoat, adding soap should increase the scrubbers efficiency by increasing the surface area of water and decreasing it's density (by
creating lots of bubbles...).. Well, maybe the bubbles will not survive some of the contaminants in the air..
[Edited on 22-8-2005 by frogfot]Magpie - 22-8-2005 at 12:42
frogfot,
I ran your numbers through my spreadsheet at three different flow rates, 200, 400, and 570 m3/h. I assumed you have 2 entrances, 2 exits, 1
expansion, and 1 contraction, and that your inner pipe wall is smooth. The results are:
Now to this you will have to add: 1) total depth of water in your scrubber, cm, 2) loss through perforations in the plate, and 3) demister loss.
When you get this total pressure drop you find it on your fan curve and that will tell you what your flow will be.
As you already know increasing diameter, decreasing length, eliminating fittings all will decrease your pressure drop for a given flow.
If you need other piping scenarios calculated just let me know. I won't start charging until I get tired of doing it. Twospoons - 22-8-2005 at 20:36
Quote:
Originally posted by frogfot
started out by making some filling by cutting small pieces of a 16mm PVC pipe..
Did you consider simply using the plastic spikey balls used in aquarium bio-filters? They're pretty cheap, and have a huge surface area.frogfot - 24-8-2005 at 11:10
Magpie, thanks for calcing that out. 200m3/h is probably more than enough for my hood. Btw, I use homemade vent pipes.
Quote:
Did you consider simply using the plastic spikey balls used in aquarium bio-filters?
Never seen such. It may get expensive since I'll need 10-20 litre of these
Anyway, I'll settle with plate scrubber design.
Tested the scrubber by placing an open flask with 25% ammoia at the air intake. Well, it didn't help much, the outgoing air was unbeareable
I tested it with one plate (2cm water) and with two plates (1cm water on each) - equal result.
Conclusion: more powerfull fan and higher water depth.
I really dunno what water depth the commersial scrubbers use per plate...
The adress I posted before says that each plate steals pressure of 2,54-3,81cm H2O (that's probably including the perforated plate).Twospoons - 24-8-2005 at 17:34
Ok, you're doing a plate design
For everyone else, this is what I was referring to: bio ballsQuince - 8-10-2005 at 04:52
A good place to get blowers is from old ham radios; the centrifugal blowers are used for forced air cooled ceramic tubes.Quince - 17-12-2005 at 20:18
BTW, you can put two blowers in series to increase the pressure drop.
Mineral wool can work fine as a demister if you just stretch sufficiently a thin layer, and use a coated window screen as strengthening support on
both sides.
it would be too complicated when taking into account the perforated plate.. I'm basically doing this by trial and
error.. or how do they say..
]
if it's not to much work.