solder fume extractor HEPA filter was backwards. ok to turn around?

[julianhigginson]

so, I bought a 2nd hand metcal BVX-201 with very low use a few months ago, and am now getting it setup in my lab.

But when I just opened it up to look at it, seems that the HEPA filter has been installed backwards (text is right way around, airflow direction arrow definitely the wrong way around) and used like that for, I assume, as long as this filter has been in there.

Now I'm wondering, do I just flip it back the right way and use it? or should I man up and spend the extra $ to replace it upfront?

I'm not at all an expert on what a HEPA filter even is, let alone how it works, and what would happen if it was reversed and then turned the right way around, so I'm all ears if someone who does know about these things wants to tell me about anything horrible that could happen if I do flip it and continue to use it.

PS, it has been used with the pre-filter (as designed) which is looking a bit grey and was covered in dusty crap before i vacuumed that off, so there's not likely to be anything large particle wise on the "out" side of the filter, just whatever small stuff made it through the pre-filter originally.

[Jeroen3]

I don't think the direction of a HEPA filter matters much. I mean, a piece of high density cloth is still a piece of high density cloth when you reverse it.
Aside from maybe some construction details that prevent the filter to fold close on pressure.

[julianhigginson]

Yeah I saw that write up, and couldn't work out why a HEPA filter would even have a direction based on that information, so figured that I was still missing something...

Maybe it is as simple as mechanical support. Or maybe even for these filters, it's just arbitrarily assigned, since other filters are directional, so people expect these to be?

Sounding like I should just leave it backwards now..

[Jeroen3]

The filter will have many V-ish shapes to increase the surface area. When reversed, this V shape could collapse under pressure if only supported on one side.

I guess it would have arrows on each side if it was important.

[rs20]

So the concern here is this: will the particulates captured on the "intake" side of the filter, now flipped to the "outlet" side, be ejected from the filter and spread throughout the room? And if releasing a bunch of particulates like that wasn't a problem, why have the filter at all?

On that basis, I would prefer to just get a new filter, but if I didn't want to pay for that, I'd avoid flipping the filter after installation.

[Richard Crowley]

The directional arrow designates the mechanically stronger direction for higher-flow applications.  Likely irrelevant for low-flow applications like solder fume extractors.

Reversing a used filter (of any kind) would appear to completely defeat the reason for implementing the filter at all.  Rather like using both sides of the toilet paper.  Pretty gross.   

[richnormand]

Could it be also that the filter has a pre-filter layer, activated charcoal layer, then a the proper hepa layer, then a strength mesh to keep it intact once it starts to clog up. Each might have its own mechanical webbing for strength too in the back flow?
Difficult to tell.
If you replace it it would be interesting to post a cross section on the filter before disposal to see if it is symmetric.

[julianhigginson]

I'll take some photos when I get a chance. Pretty sure it's just a HEPA filter..  there's a first filter called the "pre filter" which is just like thick felt. It sits on top of the HEPA filter, add they recommend changing that regularly, and changing HEPA once a year... (Based on every day 8 hour usage I guess)

They do have a totally different filter assembly that has charcoal in it I believe, but it's more for working with solvents, not solder fumes.

[julianhigginson]

here's some pictures.

first up, the wrong way.



then right way


then with prefilter bedded down on the HEPA filter



so... it Looks like there is some kind of "tissue paper" layer on the nominal output filter side. (which has been my filter's input side)

the amount of time I've spent thinking about this, it would have been cheaper to just replace the filters and know I'm good..
:-)

[richnormand]

Thanks for the photos julianhigginson. 

I commented about filter direction since I have seen many large (read: very very expensive) HEPA filter assembly systems with multi-filters that had to be in the right direction for filtering efficiency and filter lifetime reasons, including electrostatics.
The good ones also had a real time computer laser particle count monitor between stages and a differential air pressure monitor and flow meter to protect the unit. If the pre-filter was not working well the HEPA would clog up within a month in some cases.