Source for 3″ round in-duct hvac filter?
All-
Trying to locate a source that makes in-line odor (carbon) filters for 3-4″ flex duct. Nothing fancy, just something that would be semi-removable, or could pop into the end of the duct run- or even a very small filter box with connections for the round ducts on either side that would take a small rectangular filter.
Any ideas?
Thanks.
Replies
Usually, duct filters are ####bad idea. They restrict the airflow to the point of damaging furnaces.
No furnace involved, this is for straight ventilation from one particular source, in a lab situation.
t5hey can stagnate air and mke the exchanger run hot and suffer an early demise..
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
see previous reply
I agree with rich and IMERC. I spec and install a lot of real carbon filters - 1000-pounds units and bigger - for use in vapor streams. Something with a low enough pressure drop would hardly even be there, it would have to be so thin and loosely packed. That means very little contact time. That means very little adsorbtion (Yes, ADsorbtion, not ABsorbtion.)
So the downsides are: more back pressure, lower air flow, clogging on the filter, fire hazard depending on the temps (carbon can spontaneously combust like oily rags).
Upside: a tiny fraction of volatile organics removed from the airstream. Don't do it.
You could install a real adsorber, but it would have be in a larger vessel and still, the pressure srop would be very hard to keep low enough. I'm usually using units at a 5 to 20 " water column back pressure. You've only got 1/8" or so to play with in duct work.
Go back a step: why do you want to do this?
"or even a very small filter box with connections for the round ducts on either side that would take a small rectangular filter."
That IS a possibility. Like a furnace filter housing to hold a carbon filter version of a furnace filter. Pretty low back pressure because of the large surface and low air speed through the filter. But it gets back to that short contact time problem.
Also, heft any filter you consider. How much is actually carbon? Like a pound? Maybe 4 ounces? Vapor phase carbon has a adsorbtion efficiency of about 10% to 30% by weight. i.e. 16 ounces of carbon will hold 2 to 5 ounces of gasoline vapors, body odors, etc. Twice as high for adsorbing chlorine, but my point is they are often just window dressing as they don't have enough capacity to adsorb much.
Why I want to do this. . .
There is a lab room that receives HEPA/charcoal filtered makeup air, but the exhaust (3-4" ducts, each with own fan and outlet) is not. Since the exhaust does not go up a stack, I am trying to think of a way to easily put on a temporary filter whenever a particularly bad smelling compound - like a mercaptan - is being used, so that it does not bother those immediately outside. It can be sniffed way below its exposure limit. Not so much a safety issue as it is comfort outside.
Hi, back from Europe now.
For occasional use of low, low concentrations of a nuisance odor (mercaptans would defintely fall in that catagory), yes, a low-back pressure, high surface area carbon filter could help.
Note that while adsorbing 80-90% of the organic is an improvement, it may or may not be enough to avoid detection by building occupants. Mercaptans are pretty stinky and we tend to notice them due to their (articifical) association with natural gas.
"I smell gas!". No, you don't. You may smell the trace gasses added for you to smell, but you don't smell CH4.David Thomas Overlooking Cook Inlet in Kenai, Alaska