Maker Pro
Maker Pro

How do electrostatic air cleaners work?

M

mike

Jan 1, 1970
0
There seems to be some ambiguity in terms, so, for the purpose
of this discussion...

An ELECTRONIC air filter uses kilovolts to create an electric
field that charges particles so they get attracted to a surface.

An ELECTROSTATIC air filter is a passive device that generates
the electric field by passing air across some dielectric medium.
The advertising is full of vague marketing speak with dimensionless
graphs and no real statements as to how they work for pollen particles.

Most pleated furnace filters have some electrostatic features.
For some vendors, it's their primary focus. The Filtrete line
mentions almost as a footnote.

I can't imagine much voltage being generated, so I set about to
measure it. I inserted electrodes under the outer polyethylene
(I think it's poly) mesh on either side and blasted air thru it.
I measured zero volts.
Ok, maybe not much current.
I put a .1uF high voltage cap across the electrodes and let it charge
for a while. I touched a scope probe to the cap with a trigger
level of 1V. I expected to see the peak voltage and the decay
of the .1uF and the 10M scope input.

I saw zero.

Is my measurement technique faulty?
Am I measuring the wrong thing?
Or are electrostatic filters made of snake oil?

Thanks, mike
 
T

The_Giant_Rat_of_Sumatra

Jan 1, 1970
0
Is my measurement technique faulty?
Am I measuring the wrong thing?
Or are electrostatic filters made of snake oil?

Your brain is faulty if you are trying to take a voltage reading from
an INSULATOR!

Meters read generated, constant source values, not electrostatics.

Elesctrostatic cleaners use about 6kV typically.

Your electronics training was snake oil.

Two plates. Charge them. They attract each other, and morph the charge
of particles within the field between them such that they get attracted
to one plate or the other. Pretty simple.
 
M

mike

Jan 1, 1970
0
Snake oil. You can easily make an electret, i.e. a dielectric with a
frozen-in electric polarization. The poled PVDF film used in billions
of microphones and front porch lights is the most common example, but
there are lots of others: triglycine sulphate, lithium tantalate, lead
zirconate-titanate (PZT), lead lanthanum zirconate-titanate (PLZT) and
deuterated triglycine fluoroberylate have all been used fairly widely
for piezo- and pyroelectric elements, and there are built-up examples
such as PN junctions and floating-gate EPROMs.

All of these work by having a static dielectric polarization inside
them with no applied voltage--so far, so good.

_However_, you can't maintain an E field in air that way. It's just
like rubbing a balloon on your hair: the triboelectric charge produces
an external E field large enough to stick the balloon to the
ceiling--for a little while. Then the charge gets neutralized by
attracting air ions, conduction via adsorbed water, and also slightly by
charged dust. The neutralization process occurs until there's zero net
charge on the balloon, at least if we neglect any triboelectric source
term due to air friction. (I don't know if this is significant in air
filters. It is for raindrops.)

Similarly, an electret attracts free charge to its surfaces until the
external E field is cancelled out. But there's more to it than that.

In the electrostatic limit,(*) the voltage between any two points is
just the line integral of _E_ dot _ds_ along any path joining them.
Thus if the external field is cancelled out, so is the internal field,
because otherwise you'd get a different answer for the voltage depending
on whether the path of integration went through the material or around
the outside, which in electrostatics is impossible.

If you think about the PN junction case, this becomes pretty
obvious--even in electrostatics, you can't make a kilovolt just by
soldering 800 diodes in series. External charge produces an overall
slope that cancels out the effects of all the series-connected bandgaps,
and makes all the Fermi levels line up at the terminals.

Cheers

Phil Hobbs

(*) If there's any dB/dt happening, curl E is nonzero, so the integral
of _E_ dot _ds_ along a closed path is also nonzero in general. That's
how transformers work, after all.
Thanks for the inputs.
The stuff on Deutsch-Anderson equation was interesting, but way overkill for
what I need and didn't address my concern.

I can restate my question as follows.
In a passive electrostatic furnace air filter, (one without any applied
power to generate an electric field) where (physical location in the
material
stack) is the field and how is it generated?

The filter stack is as follows:
Metal screen
plastic mesh
1/4" grey filter-looking stuff
1/4" blue filter-looking stuff
plastic mesh
Metal screen.

The two metal screens are electrically
connected by the aluminum frame.

There are filters with similar "electrostatic"
claims that have plastic frames and no metal screens.

I had it in my head that the electric field was generated by the
triboelectric effect of the air flowing thru the plastic mesh.
I expected to find kilovolts/cm as in an active electronic air filter.
I didn't find it.

Another theory was that the field was permanently "trapped"
in the material by solidifying it in a strong electric field
to align the polar molecules.

Because of the small physical size of the molecules, it's not hard to
imagine localized fields of kilovolts/cm, but the effect would be
very localized.


I did some experiments with an incense stick smoke particle source.
Placed the filter horizontal and put the incense stick under it.
Tried a simple fiberglass filter, the passive electrostatic filter,
and a Filtrete allergen filter with "electrostatic properties".
In all cases, most of the smoke passed thru the filters.

I was too lazy to take the electronic filter out of the furnace
at 4AM and don't want the house to smell like incense.

I did place an air ionizer under the incense smoke generator.
In that case, virtually all the smoke was trapped in the simplest
fiberglass filter.

I'm unable to experiment with the triboelectric effect. My visual
detection of the smoke plume doesn't work with high air flow.

I need a simple particle counter. I'll start another thread on that
subject.

Thanks,
mike
 
T

Tim Williams

Jan 1, 1970
0
Talked with a sales rep today, who said he once did a job for a power plant.
Their precipitators are large, of course: on the order of 50kV and 5A. They
turn them up until corona just starts giving way to sparks, then back them
off a few percent. He also said, before the days of low-sulfur coal and all
that, the precipitators would consume a small percentage of the total plant
power output. That's a *lot* of fly ash.

Tim
 
G

gregz

Jan 1, 1970
0
mike said:
Thanks for the inputs.
The stuff on Deutsch-Anderson equation was interesting, but way overkill for
what I need and didn't address my concern.

I can restate my question as follows.
In a passive electrostatic furnace air filter, (one without any applied
power to generate an electric field) where (physical location in the material
stack) is the field and how is it generated?

The filter stack is as follows:
Metal screen
plastic mesh
1/4" grey filter-looking stuff
1/4" blue filter-looking stuff
plastic mesh
Metal screen.

The two metal screens are electrically
connected by the aluminum frame.

There are filters with similar "electrostatic"
claims that have plastic frames and no metal screens.

I had it in my head that the electric field was generated by the
triboelectric effect of the air flowing thru the plastic mesh.
I expected to find kilovolts/cm as in an active electronic air filter.
I didn't find it.

Another theory was that the field was permanently "trapped"
in the material by solidifying it in a strong electric field
to align the polar molecules.

Because of the small physical size of the molecules, it's not hard to
imagine localized fields of kilovolts/cm, but the effect would be
very localized.


I did some experiments with an incense stick smoke particle source.
Placed the filter horizontal and put the incense stick under it.
Tried a simple fiberglass filter, the passive electrostatic filter,
and a Filtrete allergen filter with "electrostatic properties".
In all cases, most of the smoke passed thru the filters.

I was too lazy to take the electronic filter out of the furnace
at 4AM and don't want the house to smell like incense.

I did place an air ionizer under the incense smoke generator.
In that case, virtually all the smoke was trapped in the simplest
fiberglass filter.

I'm unable to experiment with the triboelectric effect. My visual
detection of the smoke plume doesn't work with high air flow.

I need a simple particle counter. I'll start another thread on that
subject.

Thanks,
mike


I think some, many, or most plastic tapes use a charge to hold the sticky
on. You can even make a electrostatic speaker using two plates, front and
back of the tape. It's not very efficient though.

I to have experimented with the various white filters. Believe me, it's
starts looking brown in a smokers house. It will also pick up radon
particles, and after a couple days It will show elevated radioactivity. I
don't know how they work, but the material can be washed with something
like greased lightning, and reused, and seems to work just as well.

I have used ion generators around the furnace thinking it would help pick
up particles with the filter, and anything else.

The filters with the finer mesh do restrict airflow more than those you can
start to see through.

Greg
 
Top