yes, but I would think that a lower velocity per each square centimeter should therefore catch more particulates.

let's change the test to have 2 identical filters, one having to filter 10cf/m and the other 20cf/m (just rough numbers). logic would dictate that the one filtering less air, and air at a lower velocity, should do a better job of filtering than the one having to filter 20cf/m.

increasing the filter size should accomplish the same thing, because making the filter larger means that for any square centimeter of filter, there is less air passing through it and at a lower velocity.

 

I don't think velocity is the relevant principle at all in an open loop system such as used in a car. In a closed-loop, such as filtering air in a home, it is, because effectiveness is determioned by the velocity as it translates into AEPH (air exchanges per hour).

In a car, the filter has one shot, and velocity is a factor but a minor one - it will either be caught or not, mostly determined by the porosity of the paper, or the ability of "depth" filters, like guaze and foam, to grab it. This is a function of the size of the particule and the filter medium - it is not like the air filter is alive, and has a reaction time.

 

really?

let me build another example:

we have dust-filled air being forced through filter mediums (any medium).

some of the air is forced through at a leisurely 1cf/m. for another filter, the air is forced through at a 10cf/m. another, 100cf/m.

which is going to catch more particles? note: the filter has to decellerate the particles to catch them, so which particles have higher inertia? (and yes, inertia for these particles is very very tiny, but then again, they are just being caught by paper or foam)

when the particles pass through the filter, the higher velocity they have, the more "straight-through" they will pass. but at a lower velocity, they can meander through the filter, and are more likely to get caught up in the medium.

 

Sorry, I cannot see it - dirt is not an airplane being guided through the labyrinthine ways of the filter. A foam filter works by having no straight through inlets - since the dirt of any significant size cannot change direction fast enough, it gets trapped. In the gauze filter, the slots and the "fingers" either trap it or do not - in fact, now that I think about it some, the fingers might do better at low velocities, since they would not be as likely to be deflected away from the airflow.

I know for a fact that HEPA filter's efficiency is inversely proportional to velocity.

In a paper element, there is no negotiation through the media - the holes either catch it or do not - I would think at higher velocity dirt already trapped or dirt newly acquired might actually break-thru the media at higher velocity, like a car blowing through a chain link gate.

Here are some links you may find interesting:
http://www.visteon.com/about/features/2001...01/061901.shtml
http://www.bobistheoilguy.com/airfilter/airtest1.htm

 

good readings.

 

[QUOTE]Originally posted by Road Rage
Sorry, I cannot see it - dirt is not an airplane being guided through the labyrinthine ways of the filter. A foam filter works by having no straight through inlets - since the dirt of any significant size cannot change direction fast enough, it gets trapped. In the gauze filter, the slots and the "fingers" either trap it or do not - in fact, now that I think about it some,

 

jeez, why didn't I think of this example earlier:

using identical filter mediums (the same OEM filter paper used in our cars), we have the Honda engineers build the following:

Filter A: 1' x 1' sq.
Filter B: 10' x 10' sq. <-- a gigantic filter frame, but this is just for illustration

these filters have no pleats, they're just huge flat pieces of paper stretched across a square frame.

the engineers then build the necessary plastic pieces to fit to these filters that reduces down from the diameter of the filter to the intakes of our motors' intake.

now, we have 2 hypothetical motors sitting next to each other-- one with Filter A, and the other Filter B-- and we bring these motors up to some fixed rpm that then pulls in, let's say 60 cf/m of air. (1 cubic foot of air per second)

for Filter A (1' sq), a cubic foot of air passes through it every second.

but look any 1' sq. section of Filter B: only 1/100 of a cubic foot of air passes through that section. therefore, this air is travelling more slowly through the actual filter medium, despite the fact that the motor itself is still pulling 60cf/m of air.

since the air is travelling more slowly, I suggest the filter will be able to trap particles better.

---

another example: put on a gauze face mask and go out in a dust storm. tape the edges of the mask to your face. ask your instincts: how are you going to breathe? are you going to suck air through that mask rapidly, or are you going to breathe slowly?

I think most people would breathe slowly because they know that if they suck hard on that mask, they'll pull more dirt through it than if they were breathing slowly.

so once again, I think we are in agreement. I'll quote what you wrote:

Road Rage: "I know for a fact that HEPA filter's efficiency is inversely proportional to velocity. "

translation: higher velocity = lower efficiency.

lower the velocity of the air passing through the filter medium, and the filter works better. per my extreme example above, having a larger filter reduces the velocity of the air passing through it, so it should therefore filter better.

also, since the air can pass through the larger filter easier (lower velocity), it should not affect performance, and may even increase performance by reducing the amount of air resistance.