Cheap, Effective Shop Air Filtration

[Julie Moriarty]

I wanted to see just how effective a simple box fan with good filters could pick up fine dust in the shop so I went to Amazon and ordered (2) Lasko 20" 3-speed fans and (2) 20x20x5 furnace filters with a MERV 13 rating (hospital rated). Then I went to HD and picked up a 3-pack of 20x20x1 cheapo filters. The cheap filters are added to help make the more expensive ones last longer. Total cost: around $115. I already had the wood.

I made some wood boxes to take the filters and attached them to the fans. The last two days I've had both running. During that time I ripped 240' of 4/4 mahogany, planed (22) 10'x2" mahogany, both sides, to 3/4" and cut only the stiles on the miter saw with one of the fans directly behind me, blowing air towards the DC catch area.

The planer is pretty good at picking up dust. The table saw, not so much, and slightly better is the miter saw, at least for fine dust.

Here's a comparison between a new filter (left) and one only two days old:

I was surprised...

For anyone interested, these are some pics of one of the fan boxes:

Without the filters (left) and with just the MERV 13 filter (right). The MERV 13 filter above has already seen 2 days use.

With the two day old cheapo filter covering the MERV 13 filter.

The nice thing about these is that I can place them wherever they will pick up dust the quickest. If I'm sanding on my workbench and the board isn't too long, I can place the fan right there to pick up the airborne dust. I love it when a plan comes through!

[Andrew Joiner]

I read a detailed article that interviewed an HVAC expert on air quality and furnace filters. The expensive filters block more particles and the cheap ones let more air and particles thru. The HVAC expert had years of experience testing filters and furnaces so the interviewer asked him" What filter do you use in your own homes HVAC system?"

The expert said he buys the cheapest filters and vacuums the dust off every month.

I don't have a dust filters in my shop, I just wear a mask. I do vacuum my home furnace filters monthly and toss them yearly. I think it would work for shop air filtration too.

[Jerome Stanek]

I use a good filter behind a cheap one on my unit The cheap filter gets the big stuff and then the expensive one gets the finer dust. I don't have to change the expensive one near as often as I used to.

[ian maybury]

Thanks Julie, it's great to see another trying stuff out. That way we all learn.

Sounds mind you like you're in trouble. Firmly on the bicycle of dust collection - having made the mistake of just half opening the 'see no evil' eye a whisker. It's a bit scary how obvious the dust on the two day filter is - it kind of underlines the need to catch it at source if at all possible. Wonder how those filters would compare to a cartridge filter even if the retention is about the same - the available sq ft of filter area is quite a big factor in how long they last before cleaning. Cleanability without damage is another factor.

I'm always impressed at how available and how cheap a lot of that sort of hardware tends to be in the US. Even the box stores seem to stock far larger product ranges than over here.

If the sir quality really is improving then it's fairly likely that you will notice it. I do anyway - I make the mistake of running my saw the odd time with the dust collection off and can immediately feel a prickliness in my nose and then my eyes start to water and get sore - especially some time afterwards. You don't always notice when you're intent on your work. I still have to buy a Dylos meter, so I've no idea what my air quality actually is. Chances are the saw produces a lot more fine dust than the planer, but i'm not sure if anybody has ever validated that with a meter or whatever.

ian

[Julie Moriarty]

Ian, the prickliness in the nose, as you describe it, turned into a full blown eruption for me. It just happened this year. I usually wore a dust mask, always when sanding, and never had any obvious bad reactions. Then this winter things went nuclear. And I became a dust eradicating fanatic.

Andrew, I worked construction for almost 40 years and have had many a conversation with the HVAC guys about filters, some talked like scientists. Seems there are as many opinions as there are choices in filters but they all agree that the cheapest filters don't catch the smallest particles. It's a personal choice as to weather that is important to you or not. The reason high MERV filters are so thick is because they need the extra total area so as not to choke the furnace. All the HVAC guys I've talked to over the years say a clogged cheap filter is more efficient than a new one because it lets so little through it. But its small overall area causes it to choke the furnace.

The latest hi-tech filter systems were getting the HVAC guys enthusiastic but they are only as good as the air that passes through them. If filter systems were 100% efficient you'd never have to dust your house. I'm sure a lot of dust that I find in the house begins in the workshop. Anything I can do to capture that before it leaves the shop is a plus.

When I saw how much dust the cheap filter had already collected, I knew that dust wasn't ending up on my coffee table.

[Ole Anderson]

I tried that and it worked somewhat as long as the filters were clean, but that fan type can't generate any significant pressure to force any quantity of air through even a partially used up filter.

[Phil Thien]

I tried that and it worked somewhat as long as the filters were clean, but that fan type can't generate any significant pressure to force any quantity of air through even a partially used up filter.

They actually don't do as badly as I thought they would.

I suggest that you tie a piece of ribbon (1/4" wide by 12" long, or so) to the output side of the fan, and observe how it blows around while the fan is running with no filters. Now observe the ribbon blowing with clean filters in place. Now as the filters load, you will see a change in how the ribbon blows, and you will know when it is time to clean/replace the filters.

If you want to move up one step, you can find approx. 450-CFM blowers that are quite small (about 9" cubed) that you could adapt into your design. They're typically used in the grow industries (I think for a lot of cash crop stuff, believe it or not). But they will work better with a loaded filter.

[ron hossack]

Your filter setup is nice and fancy. My Scottish roots show in mine. I just duct tape the cheap furnace filter to the fan.

My setup is because I do quite a few Fly Rod handles/grips from Red Fir bark. In less than 5 minutes the snow white filter becomes clogged.

[Jerome Stanek]

I have an old furnace fan that I use. I got it free from a heating cooling place.

[Julie Moriarty]

I tried that and it worked somewhat as long as the filters were clean, but that fan type can't generate any significant pressure to force any quantity of air through even a partially used up filter.

My shop is in the basement and it's cold down there. When the fans are running they create an air flow in the shop that, until my blood starts to flow, is a bit too cool for me. Any more air flow could chill me to the bone!

The 1" filters are about $2 each so I'll probably replace them fairly often. And I really like being able to locate the fans to where I'm working. Yes, it's low-tech, but as long as it's catching dust, I know that isn't getting in my lungs or being spewed throughout the house.

[ian maybury]

I guess that the nature of all filters is that they leak dust below certain size - that's what the MERV or HEPA rating number means. It's just that the amount is small - US HEPA filters (spec is 99.97% retention above 0.3 microns) leak maybe 0.03% of particles, and then only if they are smaller than 3 microns in size.

What's interesting is the question of what the realities are. 0.03% leakage still translates to some quantity of fine dust suspended in the shop air, and added to this is whatever dust we kick up off workshop surfaces and the like. The various H&S codes in industry meanwhile permit a TLV somewhere between 1 and 5 mg/m3 (miligrams per cubic metre) for the working day.

What's completely unclear (and one of the reasons why some of us have been pushing for the publication of much better data on dust systems on e.g. the Wood mag dust system thread) is how (a) a given level of dust system filter performance (HEPA or whatever) translates in practice into a given concentration of dust suspended in your shop air (it should be amenable to fairly simple maths as it's got to be down to volumes, air change rates and mixing), and (b) (re. your reaction Julie) how the body reacts or is likely to react to this.

My own experience suggests that shop air feels really nice and fresh if there's a decent airflow, and the filter quality is approaching HEPA. Whether it's 'safe' is another matter.

I've no idea for example if the 1 - 5 mg/m3 of wood dust air quality standard delivers zero immune/allergic/disease based responses (but doubt it very much given the way our immune systems work) - but suspect that it probably is deemed to reduce the risk to some acceptable level. Which in one way is fine, but a right PIA if you (speaking generally) turn out to be one of the tiny minority affected. Especially if not just one of the minority, but one that has just spent a lot of money on dust collection only to find that you have become sensitised, that even an air fed mask doesn't do the business, and that the only option is to avoid contact completely.

It's not a simple matter - in that the weight of dust per cubic metre of air alone doesn't determine that much. Dust particle size distribution within a given weight of dust (how much there is of each size of particle) has to come into the deal too. To take an extreme example - chances are that somebody throwing an off cut across the shop into a bin momentarily raises the wood dust content of the air in your shop above the TLV limit, but it's unlikely to have much impact on your health unless it catches you behind the left ear. On the other hand if that same chunk has been converted by your saw into fine dust and sprayed out into the air the effects may well be quite significant.

I'd love to see an industrial hygienist or whatever the competent person is discuss this issue (allowable levels of dust in the air) from a practically informed viewpoint - if that is such a thing exists. History is full of science calling limits like these only to be proven very wrong. Just how in practice do commonplace situations relate to the air quality standards, and how do we react to this?

One of the key differences (it seems) with a HEPA rated filter is that the standard requires that the air quality is achieved right from the moment of first use. i.e. there is no tolerance of any requirement to build up a 'cake' as in the case of bag filters and the like to enable making the numbers.

That's not to say that caking does not occur on pretty much all filters - it seems (but i don't really know) that the phenomenon must to some degree arise in all cases. My suspicion is that there has been work done in the case of HEPA filters and similar to slow down the rate of loss of airflow/build up of flow resistance with caking/the build up of dust...

Anyway. Just raising more hares...

ian

[Brian Brightwell]

That looks to be simple effective and cheap. I was wondering if the fans are reversible if you could take them outside and blow most of the dust off that way?

[Dan Hintz]

My setup is because I do quite a few Fly Rod handles/grips from Red Fir bark.

I don't care about the fan, I just want some of those pen blanks

[Alan Lightstone]

I have a similar fan with a cheapo filter on it. When I've turned it on with the Dylos running, it does lower the numbers - but not nearly as much or as quickly as my Jet 1000 air cleaner.

I consider it a cheap supplement, but nothing more. ll turn it on sometimes right next to a piece I am sanding or one I want to dry quicker.

[Julie Moriarty]

Update:
I've already replaced one 1" filter. In two days of work, the second one now needs replacing. And the other fan needs a new filter too. All that dust that didn't get into my lungs or get spread around the house. This may be the best $115 I've ever spent!