Building an AC filter

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Building an AC filter

Postby Formica » Fri Jul 01, 2011 11:50 am

I am going to be using a small window AC unit in a trailer while on playa this year. I am concerned about the amount of dust that'll get into it, and how to reduce or prevent this.

My first idea is to build a trapezoidal intake which goes on the front of the unit, and has a HEPA furnace filter at the front. The objective is to make sure that the unit can draw enough air, and to keep the filter away from the intake, since said intake is also the primary heat exchanger for the unit. Still, I'm concerned about making the fan work too hard, and about heat build up potentially causing a fire hazard or damaging the compressor if the heat exchanger can't radiate as well as it should.

Anyone have advice on the topic? I've seen plenty of AC units on-playa that are running unfiltered, but I shudder to think about what one good dust storm could do to the interior of the unit if all that alkaline dust gets inside...
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Re: Building an AC filter

Postby jkisha » Fri Jul 01, 2011 12:54 pm

Don't do it.

I can tell you from personal experience it will not work. Your AC unit will overheat and blow the fuse.

Let me relate our A/C experiences. The first year we decided to build a Yurt, we also decided to air condition it. So we got a small AC unit and a small 1200 Watt generator. We tested it all here in Los Angeles and then we got to the playa, the AC kept blowing out the fuse on the generator. That year we discovered that the higher up in altitude you get, the less output your generator has. Our AC unit was nothing more than a fan that year.

So the next year we went out and bought a big 5,000 watt generator and were prepared to spend sleeping in and taking cool respites into the yurt. As we did the previous year, we built this great box with filters and encased the entire AC unit. After about 20 minutes, the AC stopped working. Fuck, what could possible be wrong now! Fortunately someone in our camp suspected the problem. So we immediately removed the box from around the unit, waited until it had cooled down enough for the switch to reset, and never had another problem.

Surprisingly, the unit wasn't effected at all by the dust. When we brought it home we just blew out the small amount of dust that was in the unit and we were ready to go for next year. It's been three years, this will be the fourth for the AC unit and it works as good as the day we bought it.
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Re: Building an AC filter

Postby CapSmashy » Tue Jul 05, 2011 2:25 pm

Unless you are standing there shoveling dust into your A/C unit, you should not have much, if any issue with the factory filter on the front. If its the foam kind, you might give it a rinse every few days, but otherwise, don't try to over engineer it.
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Re: Building an AC filter

Postby gyre » Tue Jul 05, 2011 8:43 pm

Formica wrote:I am going to be using a small window AC unit in a trailer while on playa this year. I am concerned about the amount of dust that'll get into it, and how to reduce or prevent this.

My first idea is to build a trapezoidal intake which goes on the front of the unit, and has a HEPA furnace filter at the front. The objective is to make sure that the unit can draw enough air, and to keep the filter away from the intake, since said intake is also the primary heat exchanger for the unit. Still, I'm concerned about making the fan work too hard, and about heat build up potentially causing a fire hazard or damaging the compressor if the heat exchanger can't radiate as well as it should.

Anyone have advice on the topic? I've seen plenty of AC units on-playa that are running unfiltered, but I shudder to think about what one good dust storm could do to the interior of the unit if all that alkaline dust gets inside...

Trapezoid?
Can you explain that?

I don't know if this can be done, but a hepa filter would not be the way.
All filters gain higher efficacy and lower flow as they collect dirt.
If you compare filters, the difference in flow between a 95% filter and a 99-99.9% (hepa) unit is dramatic.
That flow won't get better.
A true hepa filter capable of flowing enough air would be quite large.
These are typically units cased in steel with many folds, and about two feet deep.
They are actually quite reasonable in cost, as they are widely used, and can last forever with prefiltering.
They are incorporated into central air units usually.

Another factor is radiant heat reflection and insulation, which can be caused just from close proximity to the unit.

A simple mesh screen, as on a flywire door, restricts air flow about 50%, so square footage must be increased accordingly.

I would suggest that the most open material you can get away with, would be the filter material to use.
Perhaps the open weave fibreglas filters that do virtually nothing in normal use, would work, or open cell foam?
A filter like this would need attention as it would become restrictive quickly, just as any prefilter will, on a proper filter.

Perhaps a different approach, similar to the intakes used on vehicles designed for unpaved countries and on some usa cars, with an articulated intake to trap dust by gravity, or prevent it being drawn in?

Larger units, as they used to be designed, had a slower fan and wouldn't be so prone to collect dust, I would think.
Even large louvres might allow dust to drop out for the most part.
We have a shop here that custom makes them, and very cheaply too.
A cover that curves down would have a similar effect.

If you aren't working within the design capacity of the ac unit, you may need to add a fan or fans to it.
Ac units are always most efficient with the fan on high.
You would want to always use a filtered unit on high.

I'm not saying this is a good idea, or practical.
Just some suggestions.

I think I would use a mesh filter only, like the very fine mesh used in well pump filters, or several layers of a more open mesh.
Perhaps different materials could be used under that for catching dust, even open weave cotton?
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