Minimum CFM on carbon filter

[apartmentblower]

hi all

i know that if you use too high of a CFM fan on a carbon filter, its odor killing effectiveness will be lessened due to the fact that the carbon would not have ample time to clean the air as it passes through.

But what happens when you use too low of a CFM? The correct carbon filter for my application is made for a larger sized fan, both flange and CFM wise than my fan. Due to unalterable (really!!!) circumstances, a fan of less cfm and with a smaller flange w/ adaptor will be used.

 

[FreezerBoy]

If the filter is "larger" than the fan, airflow will decrease and temps will rise. At some point fan damage may occur if the difference is pronounced enough.

 

[apartmentblower]

If the filter is "larger" than the fan, airflow will decrease and temps will rise. At some point fan damage may occur if the difference is pronounced enough.

thanks,
The filter will be inline and the fan is pushing (weird, i know) so im not concerned about heat issues.
So I assume for my application it is fine? Just carbon will lose effectiveness sooner w/ a longer contact time, right?



second question: anyone know what the maximum CFM that can be forced by a fan through a 4" duct is?

third question: as duct size lengthens, CFM of air flowing through drops, correct? So does the fan somehow start working slower? It surely can't, fans always run at the same speed. So what causes the CFM to drop? air has to go somewhere right, it cant just sit in the duct and wait for its air-buddies to catch up or else the duct would get too full, so the air always has to go somewhere

maybe im too stoned


i promise if you are bewildered at why im asking these strange and almost useless questions, i will post pics when im done and all will be clear

 

[Mr. Bongjangles]

I agree with Freezerboy.

I also suspect another potential problem with going below the manufacturer's minimum cfm rating, which is uneven flow through the filter itself.

I imagine with a small fan and large filter, the airflow would be concentrated in the end of the filter closest to the fan/duct, as there will not be enough power to create even suction throughout the carbon bed.

If only 1/3rd or 1/2 of the carbon was thus being used to scrub the air, then the lifespan of the filter would be reduced by a similar amount, and the drop-off in effectiveness could potentially happen so fast as to cause a security problem.

Maybe not.. Perhaps the reduction in airflow (vs what the filter is designed for) in general would make the lifespan/effectiveness similar in the end, even if only part of the filter was being touched. I can't say for sure, but did want to bring up the issue as it came to mind.

I guess if you have to do it that way, be sure to keep an extra close eye on the exhaust and check for odors getting through on a regular basis.

 

[JackTheGrower]

hi all

i know that if you use too high of a CFM fan on a carbon filter, its odor killing effectiveness will be lessened due to the fact that the carbon would not have ample time to clean the air as it passes through.

But what happens when you use too low of a CFM? The correct carbon filter for my application is made for a larger sized fan, both flange and CFM wise than my fan. Due to unalterable (really!!!) circumstances, a fan of less cfm and with a smaller flange w/ adaptor will be used.

Damage? How?

Pardon the

How could drawing less CFM through a given carbon area be harmful?

If a higher CFM can pass then it's automatic that a lower CFM can pass through a given carbon material.

Carbon is carbon.. Is there some sort of change in absorption for slower speeds.. If any I would assume better..

Also I base all my statements on 4mm activated carbon pellet not Granular activated carbon.

 

[JackTheGrower]

I agree with Freezerboy.

I also suspect another potential problem with going below the manufacturer's minimum cfm rating, which is uneven flow through the filter itself.

I imagine with a small fan and large filter, the airflow would be concentrated in the end of the filter closest to the fan/duct, as there will not be enough power to create even suction throughout the carbon bed.

If only 1/3rd or 1/2 of the carbon was thus being used to scrub the air, then the lifespan of the filter would be reduced by a similar amount, and the drop-off in effectiveness could potentially happen so fast as to cause a security problem.

Maybe not.. Perhaps the reduction in airflow (vs what the filter is designed for) in general would make the lifespan/effectiveness similar in the end, even if only part of the filter was being touched. I can't say for sure, but did want to bring up the issue as it came to mind.

I guess if you have to do it that way, be sure to keep an extra close eye on the exhaust and check for odors getting through on a regular basis.

It just sounds silly to say a lower CFM is dangerous.

Silly to say that only part of a filter is used with a lower pressure.. I think it's safe to say that it's pressure because it's how many cubic feet are being moved per minute right? That cubic foot hasn't got smaller because the fan is a lower cfm.

If nothing else dust on the filter will make sure some paths are more effort then others as it collects thus air will follow the path of least resistance and pull from the whole filter even if just over time with lower CFM..

I do agree that there seems to be an ideal of carbon to CFM.. I would recommend trying to be close but not over that.


But damage? I doubt it..

Anyone with experience is this?

Jack

 

[dongle69]

Having a filter rated at a higher cfm than your fan is the proper way to do it.
Your fan doesn't have to work as hard and your filter will last longer.
Although, it depends on how much of a cfm difference you are talking about.
The MaxFan guys specifically told me to run a bigger filter (2100 cfm) than my fan (1700 cfm).
apartmentblower, what are the listed cfm's of your fan and filter?

 

[JackTheGrower]

thanks,
The filter will be inline and the fan is pushing (weird, i know) so im not concerned about heat issues.
So I assume for my application it is fine? Just carbon will lose effectiveness sooner w/ a longer contact time, right?



second question: anyone know what the maximum CFM that can be forced by a fan through a 4" duct is?

third question: as duct size lengthens, CFM of air flowing through drops, correct? So does the fan somehow start working slower? It surely can't, fans always run at the same speed. So what causes the CFM to drop? air has to go somewhere right, it cant just sit in the duct and wait for its air-buddies to catch up or else the duct would get too full, so the air always has to go somewhere

maybe im too stoned


i promise if you are bewildered at why im asking these strange and almost useless questions, i will post pics when im done and all will be clear

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

Redgreenry has a thread.

Good luck..


Post an update on how it went.


Jack

 

[FreezerBoy]

Maybe im too stoned

Don't make me come back there and wash your mouth out with soap young man.

If the filter is "too large" it means the carbon will last longer. If too "small" it will fail sooner.

Ducts, scrubbers, mufflers add back pressure which affects cfm flow dependent on fan type. Axials move the most and centrifugals the least, until you add back pressure. Then Axials become useless and Centrifugals kick ass.

If you really want to study this try Ventilation 101

Damage? How?

Pardon the

How could drawing less CFM through a given carbon area be harmful?

I was referring to the outside possibility of burning out the fan. It's the old Blade Runner line: "The lamp that burns twice as bright burns but half as long." Drive your Beetle at an average speed of 55, it will last 20-30 years. Drive at an average speed of 150 and you could kill it in 20-30 days.

How much "larger" would the filter have to be to do harm? I don't know but, they give us those specs for a reason. While I'd suggest a filter that's slightly "larger" the key word is slightly.

 

[Mr. Bongjangles]

It just sounds silly to say a lower CFM is dangerous.

Silly to say that only part of a filter is used with a lower pressure.. I think it's safe to say that it's pressure because it's how many cubic feet are being moved per minute right? That cubic foot hasn't got smaller because the fan is a lower cfm.

If nothing else dust on the filter will make sure some paths are more effort then others as it collects thus air will follow the path of least resistance and pull from the whole filter even if just over time with lower CFM..

I do agree that there seems to be an ideal of carbon to CFM.. I would recommend trying to be close but not over that.


But damage? I doubt it..

Anyone with experience is this?

Jack

The depth of the carbon bed creates resistance. In a larger filter, it will be a deeper bed of carbon which has more resistance to overcome.

In the extreme circumstance, forcing a small fan to fight the resistance on a large filter could potentially cause damage. Is the fan gonna explode? Prolly not. Will its lifespan be reduced? Prolly so. I'd consider that "damage."

If you took a 12'' fan and put duct reducers on it and brought its intake hole down to 4'', what would happen? The fan's efficiency would be reduced, and it would end up working harder to move a similar amount of air. The motor would likely get a good bit hotter than it would otherwise.

Not an exact parallel for what is happening with the small fan/large filter, but I think it illustrates how an increase in resistance is not a good thing for the fan.

As for my idea about the uneven flow, well, it is just that, an idea.

Think it silly if you want, but nothing in your response illustrates how an underpowered fan could create even suction in a huge filter.

Won't the path of least resistance be the part of the filter closest to the opening? If there is only a small amount of airflow, I don't see how that would even itself out and create consistent negative pressure in the filter.

 

[JackTheGrower]

Maybe I don't get it but if a rated CFM fan needs a minimum X surface area and I provide it some multiple of that surface area I'd say it has less resistance to deal with because it can draw from more surface if some of the surface has more resistance then others due to uneven carbon or dust blocking pathways.
I agree that an extreme depth of carbon could be a problem such as with using small particles or dust. I don't fear 4MM pellet.

I should test my theory on the 8 inches of 4 MM pellet in a 6 inch duct and a 85 CFM Radio Shack 4 inch muffin fan. Alright I will.. Geeeze just when I wanted to be lazy.

That a small fan can be over worked for a given job is a given. With the cost of carbon I would want to get the most out of my investment.


BBIAB...


Jack

 

[FreezerBoy]

With the cost of carbon I would want to get the most out of my investment.

Check Ebay for a guy named thirdmillenium $1.99+shipping. He's selling granules which are preferred to pellets due to increased surface area.

 

[JackTheGrower]

10 inches of 4mm pellets in a 6 inch duct and a 85 CFM Radio Shack fan

10 inches of 4mm pellets in a 6 inch duct and a 85 CFM Radio Shack fan

Okay I went and did it but I ended up with 10 inches of pellets in a 6 inch duct. I was just lazy and left it in there rather then get my hand dirty.

Results
Did it work? Yes.
How well? Output seemed to be 1/8th to 1/4 of the fan without carbon. I could smell a trace of the Nag Champa but not the full effect of the stick.


Would I use this in real time? Yes if it's in a closed space and the plants aren't too stinky. That fan is really too small for my needs. If I double it with two fans I think it could improve the pull through it. I also would only use 8 inches..



So how deep can we get those granules?

I don't doubt the surface area of the granules it's just that I'm set up for pellets and I'm comfortable with them.


Jack