Hi,
I'm interested in knowing what sorts of material folks use to make after-market dust filters for their various devices which wouldn't normally have any. This seems to almost be a necessity when these kinds of devices are deployed in environments that are overly dusty and dirty (it should also be implied that these environments are all in-doors and would have less than ideal airflow and climate control).
A material that is too dense will hider airflow and cause an immediate increase in inlet temperature, which would exacerbate a potentially threatening temperature situation in environments where the ambient temperature is already in the mid to high twenties and above (that's 77 - 86F+ for my American friends ;)). A material that is not dense enough won't do a very good job at filtering.
Do folks just hack up HEPA filters or something?
One important question: how often is the equipment accessed for maintenance?
I've had reasonably good luck with air filter media coated with a
tackifier, similar to the Dustlok media here
http://www.filtersales.com/pagout.htm?id=Pad%20Media
It seems like what happens with it is heavier airborne fibers (lint, hair)
get caught up in the first few fibers of the media, not obstructing
airflow, and allow the finer dust to travel deeper into the media where it
sticks to the tacky layer at the back. It lasts a good long while. It's
single use though, so it has to be replenlished every now and then.
Foam rubber media tends to have trouble with surface/airflow area vs pore
size.
The best option, though, will be to enclose the equipment in a cabinet that
can be pressurized by one or more fan forced+filtered inlets. Middle
Atlantic makes rack cabinets and fan panels that can be used to pressurize
them that way. If you get a cabinet that takes a standard furnace filter,
I've had good luck with the off the shelf 3M Filtrete Ultra Allergen
filters, they have a TON of surface area with great fine dust capture and
very low airflow resistance, even when you're drawing the air through them
really way too fast. 
It sort of depends on what kind of stuff you're trying to filter out.
Panty hose actually makes a reasonably good filter for larger stuff, but Tom's question about how often are you going to service it comes into play, since you need to remove the debris that it catches periodically in order to avoid obstructing the air flow excessively.
OTOH, you also have to have some thought towards what are the benefits of not having the internals of the system coated with dust, vs. slightly reduced air flow.
Tom's suggestion of a pressurized cabinet is a good one of course, but that's not possible in all situations.
hth,
Doug
One important question: how often is the equipment accessed for maintenance?
Who knows 
Maybe it becomes someone's full time job to go do regular checks and maintenances of every POP? Maybe after an appropriate filter is found, a relatively low temperature threshold monitor is set up in an NMS. When this threshold is reached, it would probably be safe to assume a dirty filter (or some other condition that would require a visit) and someone could be dispatched to replace it.
I've had reasonably good luck with air filter media coated with a tackifier, similar to the Dustlok media here http://www.filtersales.com/pagout.htm?id=Pad%20Media
It seems like what happens with it is heavier airborne fibers (lint, hair) get caught up in the first few fibers of the media, not obstructing airflow, and allow the finer dust to travel deeper into the media where it sticks to the tacky layer at the back. It lasts a good long while. It's single use though, so it has to be replenlished every now and then.
Foam rubber media tends to have trouble with surface/airflow area vs pore size.
The best option, though, will be to enclose the equipment in a cabinet that can be pressurized by one or more fan forced+filtered inlets. Middle Atlantic makes rack cabinets and fan panels that can be used to pressurize them that way. If you get a cabinet that takes a standard furnace filter, I've had good luck with the off the shelf 3M Filtrete Ultra Allergen filters, they have a TON of surface area with great fine dust capture and very low airflow resistance, even when you're drawing the air through them really way too fast.
Unfortunately a cabinet isn't possible due to a variety of issues.
Hi,
I'm interested in knowing what sorts of material folks use to make after-market dust filters for their various devices which wouldn't normally have any. This seems to almost be a necessity when these kinds of devices are deployed in environments that are overly dusty and dirty (it should also be implied that these environments are all in-doors and would have less than ideal airflow and climate control).
A material that is too dense will hider airflow and cause an immediate increase in inlet temperature, which would exacerbate a potentially threatening temperature situation in environments where the ambient temperature is already in the mid to high twenties and above (that's 77 - 86F+ for my American friends ;)). A material that is not dense enough won't do a very good job at filtering.
Do folks just hack up HEPA filters or something?
It sort of depends on what kind of stuff you're trying to filter out.
Small-ish stuff. Your every day, run of the mill fine grain dust, tracked-in dirt & sand, some construction particulate (metal shavings, etc).
Panty hose actually makes a reasonably good filter for larger stuff, but Tom's question about how often are you going to service it comes into play, since you need to remove the debris that it catches periodically in order to avoid obstructing the air flow excessively.
Yup. Depending, either a vacuum or a straight-up replacement of the 'filter', I'd suspect. Or maybe just a good shake in some cases.
OTOH, you also have to have some thought towards what are the benefits of not having the internals of the system coated with dust, vs. slightly reduced air flow.
Indeed. The internals can definitely handle non-metalic dust, as well as a pretty wide temperature range (caused by either reduced airflow or an increase in ambient temperature, or both), so I'd imagine it would be a appropriate balance between the two.
I've had decent luck with window air conditioner filters available at
your local home despot. Trim to size with scissors. Periodically
replace.
HEPA they are not, but they'll keep out the worst of it without
restricting air flow (at least not until they're really dirty) plus
they're cheap and readily available.
If you have a more or less closed room (like a closet), sometimes it's
enough to just buy a freestanding hepa filter at walmart, clean the
room with a shop vac once and then leave the filter running in the
room.
Regards,
Bill Herrin