Professional Member
The flimsy little carbon pre-filters used in portable HEPA filters, as well as those designed for residential use (as a replacement for the 1" throw away type) have puzzled me for some time. For one thing, there isn't enough carbon mass to adsorb significant quantities of VOCs. Second, dust loading prevents the activated carbon from interacting with VOCs... And then I stumble upon this notice about a MERV 8 requirement in front of activated carbon filters:
" PREFILTRATION REQUIREMENTS FOR
ACTIVATED CARBON FILTERS
It is extremely important that gas phase filters be adequately protected from
particulate contamination. Particulate loading clogs the pore structure of the carbon
granules, seriously reducing their ability to capture molecules of gas and odor.
To maximize the effectivenesss of gas phase filters they should be prefiltered
with particulate filters having an average ASHRAE Dust Spot Efficiency of not
less than 30% (MERV 8). ..."
http://www.airguard.com/downloads/pr...ionrequire.pdf
So, I'm thinking that the carbon pre-filters are mostly for show. The residential carbon filters are more expensive, don't do either VOC or particulate filtration well, and don't last. Using a MERV 8 filter in front of them will tax most residential HVAC systems, unless they are greatly oversized. So, they're a losing proposition and bad value. Worse, they make HOs feel like they have protection from VOCs whereas they don't (well, not much anyway).
P.S.: let's keep talk of PCOs for another thread, before someone mentions them as a solution to this problem... I know they're shiny, have high margins and high maintenance costs and so are good for business.
Professional Member
pm
This reference is to granular carbon filters - not the carbon mats in air purifiers and pleated filters.
Carbon mats have their place. They do a reasonably good job of pulling out odors. The big problem is that there is not very much carbon so they load relatively quickly and need to be replaced fairly frequently.
Professional Member
Why would the carbon in mats be less affected by dust than the carbon in granules?Originally Posted by breathe easy
Professional Member
The dust will affect the performance of carbon pleated filters. However, it is not such an important point because the carbon pleats are intended for relatively short term use and have lower requirements for odor removal.
Professional Member
I note then that they are useful only for odor removal and not for VOC control. In other words, what you say is that dust isn't much of an issue because so little can be expected of them anyway. They are expensive too, typically 40% more expensive than a MERV 11 filter (priced at filtersnow.com). That sounds like a bad value to me...
Thanks
Professional Member
pm
Odors generally are VOC's.
Carbon pleated filters have their place. They work effectively if the users feel they reduce the odors (or VOC's) beneath their thresholds.
We have a number of applications where they work very effectively and are well worth the extra price. A few quick examples - most of our school district customers use them when they have had a fire nearby or a problem in a chemistry lab. We have some country clubs and restaurants that use them for reducing smoke odors. We have some customers that use them for air intakes that access parking garages or loading docks.
Another thing to consider is the catalytic effect of carbon on ozone. Most fresh air intakes in high ozone urban environments should use carbon pleated filters.
Professional Member
The difference between odors and VOCs is that odors are what we sense (and yes, some aren't VOCs but that's besides the point). Controlling odors from VOCs is easier than controlling VOCs, because there are VOCs we don't smell, and some are bad for your health before you can smell them:
"VOC exposures are often associated with an odor while other times there is no odor. Both can be harmful. "
http://www.health.state.mn.us/divs/eh/indoorair/voc/
"Users feeling" and customers using: I'm sorry but that's a classical fallacy, it's not because a lot of people believe in something that it's true, correct, or in this case, a lot of people buying don't make it a good value. It can be just ignorance or being gullible. Let's try to stick to the science please. I won't deny that carbon mats can help reduce odors somewhat, but that's very different from controlling VOCs.
About ozone, it's true that activated carbon can decompose it catalytically, but doesn't dust inhibit that effect as well? Is ozone able to bypass what impedes VOCs? If not, doesn't that mean that the MERV 8 requirement in front of activated carbon also applies if you want ozone decomposition, to preserve the active surface?
BTW, they are 60% more expensive, not 40%... I had divided by the carbon filter price instead of by the MERV 11 filter's price.
Professional Member
hmm... Sorry, I got excited and intense in my last post... Certainly if your customers think the carbon mats are worth the money, for odor reduction or whatever, it's better than nothing... I meant that it's difficult to get the mats to perform optimally, and given their price, that would have been desirable
Professional Member
Carbon filters are very effective in removing odors, ozone, and VOC's. They are designed to be installed behind electronic air cleaners in the slot provided so they don't load with particulate.
Last edited by jrbenny; 12-23-2007 at 04:07 PM. Reason: removed link to your sales site.
Regular Guest
Not sure if this link has been posted previously but it's one of the best third party studies of portable air cleaners that I have stumbled across.
http://www.ncembt.org/downloads/NCEMBT-061101.pdf
It would seem to confirm that thin carbon prefilters don't work well for VOC control.
Nathan
PS Based on the specs, P2 = IQAir HealthPro Plus, P4 = Blueair 501 and the P3 is likely the Frederich 90B. Can't figure out what P1 and P6 are though.
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