Math for carbon filter sizing?

[Alistair_N]

Hello all.

I have very recently found myself in a place where I can finally start working on my own grow. I've been reading up on here and elsewhere extensively for a while, getting my plans together, preparing for as perfect a DIY stealth micro I can pull off.

Stealth, of course, requires an appropriate filter, and here I am missing the one piece of the puzzle I still need: how to appropriately size said filter.

I've read all the lit on fan sizing, ventilation design, etc. I've studied the formulas and implemented them into my design. What I've not found is a formula for sizing a carbon filter vs. fan flow & pressure.

For example, something like:

% delta cfm = some function of [[fan mmh2o], [square area of filter], [depth of filter]]

Does that make sense? I'm really hoping to start my grow soon, but I'm not making a move until I feel confident my design will work, and I need this information. It needs to be so stealth my housemate will never know, so smell is the biggest worry.

Does anyone have any pointers, or a link to something I may have just missed? If any of you actually have such a formula, maybe include it in this thread and not just a link to it, so future searches can find it easy-like?

Thanks in advance.

 

[tokinafaty420]

The filter and the fan must be matched up. Drawing too much air or too little through the filter will not allow the filter to work correctly. That is why you want to purchase the correct size exhaust fan for your room first and match up the CFM rating on the filter to the fan.

 

[Alistair_N]

The filter and the fan must be matched up. Drawing too much air or too little through the filter will not allow the filter to work correctly. That is why you want to purchase the correct size exhaust fan for your room first and match up the CFM rating on the filter to the fan.

Yeah, that I know. Which is good, for a pre-fab filter. I need to build my own, for space reasons. I want to know how to get that CFM rating based on the things I listed.

And obviously every filter is different, the only way to know for sure is to build and test. But there's got to be an approximation rule to use as a guide.

 

[tokinafaty420]

Yeah, that I know. Which is good, for a pre-fab filter. I need to build my own, for space reasons. I want to know how to get that CFM rating based on the things I listed.

Copy the size from the professionals. They already did all the research work for you. A 265 CFM can filter is about the size of a 5 gallon water bottle.

 

[ijimunot]

Hard to say it depends on the material used to build the filter. Inner wall, outer wall, liners, dust filter the carbon itself and how well its packed in the filter. I would build a filter larger than what I need with an oversize fan and use a speed regulator on the fan and set it where it works best. A lot of ifs.

 

[Alistair_N]

Copy the size from the professionals. They already did all the research work for you. A 265 CFM can filter is about the size of a 5 gallon water bottle.

Alright.

So, let's say for example I have a fairly small PC-type fan of 50cfm. My space is pretty small, maybe 5cu.ft. Obviously, that's 10 changes per minute unobstructed. The lowest I'll want to go is 1 change/min, max maybe 3, so I want to end up with modified cfm between 5 and 15. I'll be able to determine lossage from my ventilation design easily enough, but I have no way to know how much a filter of a specific size will reduce CFM given a certain static pressure.

I'm wondering if anyone has the nitty-gritty on this, or has done/knows of any work to spec out a rough formula.

Knowing what a prefab filter's CFM rating is can be helpful if I just want to match up a standard blower, but if I want to design a system with unconventional parts with the least trial-and-error, it's exceptionally nice to have some math behind it.

And of course there is a lot of 'ifs,' I'm not looking for hard math.

Maybe this just isn't something that has been done yet? Per tokinafaty420's suggestion I am looking at prefab filters and the recommend CFMs and trying to see if I can work something out from that.

 

[Green9]

Alright.

So, let's say for example I have a fairly small PC-type fan of 50cfm. My space is pretty small, maybe 5cu.ft. Obviously, that's 10 changes per minute unobstructed. The lowest I'll want to go is 1 change/min, max maybe 3, so I want to end up with modified cfm between 5 and 15. I'll be able to determine lossage from my ventilation design easily enough, but I have no way to know how much a filter of a specific size will reduce CFM given a certain static pressure.

I'm wondering if anyone has the nitty-gritty on this, or has done/knows of any work to spec out a rough formula.

Knowing what a prefab filter's CFM rating is can be helpful if I just want to match up a standard blower, but if I want to design a system with unconventional parts with the least trial-and-error, it's exceptionally nice to have some math behind it.

And of course there is a lot of 'ifs,' I'm not looking for hard math.

Maybe this just isn't something that has been done yet? Per tokinafaty420's suggestion I am looking at prefab filters and the recommend CFMs and trying to see if I can work something out from that.

Good initiative, but you seem to be overthinking this. Just get large enough exhaust fan(s) and a speed regulator. 1 intake fan 80mm and 2 120mm exhaust fans to be sure that it can pull enough air through the filter, silently..

For micro grows there are several solutions for DIY scrubbers. To get a formula of any value you have to standardize on type first of all

 

[Jaybird2203]

This is abit old but I'm going to STRONGLY recommend you read Redgreenery's Ventilation 101 thread below

http://www.icmag.com/ic/showthread.php?t=112862

 

[gdbud]

Take a look at DIY - Build a Carbon Scrubber For Dummies!

 

[Pyramid]

The problem with PC fans is they dont work as well when there is a static pressure situation like with a carbon filter. They only pull when unobstructed. If you use a pc fan the most you can really put in front of it is a carbon sheet filter for and air purifier or something really small like 1/5 inch of carbon bed

 

[Alistair_N]

Thanks jaybird2203 and gdbud. I am very familiar with Redgreenery's post; I have it bookmarked for study. Lots of good info there. I'd seen the data in the DIY scrubber post before; the problem is that so many figures are built for the centrifugal blower type fans with nice high static pressure ratings. Smaller PC-style axials don't have much data available, in terms of filter sizing.

I guess, really, my main point is that I'm not looking for 'what size filter should I use?' and more 'is there any way to determine an approximate efficiency loss given an average filter of a certain size?'

From what I'm reading, and the posts above, probably not. At least, not that's currently available to low-end DIY applications. Trial and error it is, then!

 

[Ted_Hedgehog]

Hi Alistair,

I've recently had similar questions to yours regarding DIY carbon filters with PC fans, I found this thread by ScrubNinja to be very helpful, check it out if you haven't already -

http://www.icmag.com/ic/showthread.php?t=131842

I can't claim to be an expert on these things and no doubt there are far knowledgable posters on here that could pick my theories and maths apart, but this is just some stuff that may, or may not help you work out what is suitable for your needs.

'Canfilters' do some of the smallest prefab filters around, by smallest I mean handling the lowest cfm, they state that they can be used with PC fans, have a look on their website there's some handy info.


~warning dodgy maths approaching~

Can 1500 4" : Max exhaust 25 (ish) cfm - Size 10"L x 5.5" diameter with a 4" hole,

If we subtract 4" from 5.5" that leaves us with 1.5", divide that by 2 equals 0.75" which we can assume is the depth of carbon used, sound about right?

Now if we were to cut the filter length ways and lay it out flat we would have a flat carbon filter of about 10"L x 17"W x 0.75"D or 127.5cu.ins of carbon which theoretically could still handle 25 cfm, maybe a bit more.
(the 17"W is roughly the circumference of the filter)


Can 2600 4" : Max exhaust 42 cfm - Size 18"L x 5.5" diameter with a 4" hole, (this one also states a pressure drop @ max cfm of 42 Pascals or 4.28 mmh2o which is a handy bit of info)

Again using the same dodgy maths as above - 5.5"dia - 4"hole = 1.5" / 2 = 0.75" carbon depth.

Cut and laid flat would give a carbon filter of about 18"L x 17"W x 0.75"D or 229.5cu.ins of carbon that could theoretically handle 42 cfm.


~warning dodgy theory approaching~

Depending on your needs you could tweak the variables; L W D, to come up with something that works for you, after all you only need around 5 cu.ft/pm extracted and the Can 1500 can handle 5x that much.

So 10"L x 17"W x 0.75"D = 127.5cu.in @ 25 cfm

or 10"L x 12.75"W x 1"D = 127.5cu.in @ <25cfm (extra pressure caused by deeper carbon)

or 10"L x 25.5"W x 0.5"D = 127.5cu.in @ >25cfm (less pressure due to thinner carbon)

Remember the 127.5 is just a constant of that filter it won't necessarily effect what you decide to do, I'm just using it for illustration.


Other considerations - Carbon Granules or Pellets

Granules are preferred as they provide a greater surface area for odor absorption, however due to them usually being finer they will also compact more meaning that it will be harder to pull through.

Rhino (Phat) filters use granules and their smallest filter can handle around 200cfm and lasts for 2 yrs or more. (PC fan won't do)

Pellets are larger but have less surface area for odor absorption due to the squishing of the pelletizing process, they don't compact as much making them easier to pull through, though more may be needed to compensate.

CanFilters and most budget filters use pellets and last about a year, and one of the smallest as we've seen above is good for 25cfm. (PC fan will do)



Wow, didn't mean to waffle on so much, your probably more confused than I am now.

As I said I'm no expert and I can't give you any definite ways to do this or that, cos I don't think there are any, if you were using a whole room it would probably be easier, but with a small, stealth concern, we have to deal with a lot more voodoo magic to make it work and let's face it, it's more interesting.

Ted

P.S. The Can 1500 is only about £25 so you could just buy one, wouldn't be as satisfying... but a lot simpler.

 

[Alistair_N]

Wow, didn't mean to waffle on so much, your probably more confused than I am now.

Actually, you've helped me clarify a piece of the puzzle I was missing. Thanks! And thanks for the ScrubNinja link; I had (d'oh!) completly forgotten about it.

I was mistaken on the relationship between the variables. A filter isn't going to affect the fan directly, it is going to affect the pressure. Given a specific fan's static pressure rating, there is a CFM (also, rpm, voltage, etc) where the pressure is optimized when a filter is added to the system. I had it backwards in my head.

*excitedly going back to the drawing board*

 

[good drown]

can 50 and a 6" fan will be plenty for a 400w or more, and will contain the smelliest strains. if space is a factor, the cooler the hood the closer they can get without light burn

 

[BTLSfan]

Delta fans are strong enough to push through a 4 inch pro carbon filter. There are threads around here about it.