Draw thru vs blow thru filters

[Tom Bender]

To control fine (hazardous) dust it is better to have a system that draws thru the filter than one that blows thru. The difference is leaks, which any system can have. Fine dust goes right thru a cyclone and can only be stopped by a HEPA filter. It can be invisible. If you have a blow thru system be diligent about leaks.

[Matthew Curtis]

Is thert any evidence to support this claim?

I would think that leaks would be the biggest factor effecting performance regardless of direction?

[Andrew More]

Fine dust goes right thru a cyclone and can only be stopped by a HEPA filter. It can be invisible. If you have a blow thru system be diligent about leaks.

This seems to be a matter of speculation. How well sealed is the cyclone? Considering we can air seal most things, and air molecules are smaller than dust, I think it's possible.

Also, most cyclone systems I've seen are blow through, do you have an example which is not?

[Malcolm McLeod]

To control fine (hazardous) dust it is better to have a system that draws thru the filter than one that blows thru. The difference is leaks, which any system can have. Fine dust goes right thru a cyclone and can only be stopped by a HEPA filter. It can be invisible. If you have a blow thru system be diligent about leaks.

Looking at it strictly from the perspective of the filter, draw thru or blow thru really doesn’t matter. The filter experiences the same differential pressure in both cases. If it will miss dust in one, it will miss it in the other.

To be 100% draw thru, the blower has to be downstream of the filter(s). Any leak in the system would thus be ‘into’ the system. Arguably better, but still subject to the filter’s performance.

[ChrisA Edwards]

Is thert any evidence to support this claim?

I would think that leaks would be the biggest factor effecting performance regardless of direction?

Not sure if this answers your question. The bag below my filter got a split in it, about 2" long, most likely from me catching it with the end of a clamp and not noticing it.

After several sawing and planing operations, I noticed I could first smell the sawdust, usually the first noticeable sign there is fine sawdust in the air, then I noticed that the whole corner, where the Dust Collector sits was fogging up with a haze of fine sawdust.

Bag replaced and the problem was gone.

[Malcolm McLeod]

Also, most cyclone systems I've seen are blow through, do you have an example which is not?

Many Torit industrial systems are 100% ‘draw thru’.

[Doug Garson]

Pretty much every two stage system I have ever seen has the cyclone under negative pressure and the filter under positive pressure. You could probably come up with a design that keeps the filter under negative pressure but never seen one. If someone has please post a picture. Google two stage dust collection system and look at the images. https://www.google.com/imgres?imgurl=https%3A%2F%2Fcdn10.bigcommerce.com% 2Fs-ygsmo4m53s%2Fproducts%2F15933%2Fimages%2F8282%2F10 _810__28381__58995.1484176869.1280.1280.jpg%3Fc%3D 2&imgrefurl=https%3A%2F%2Fcanucktools.ca%2Fgeneral-3-hp-2-stage-dust-collector-10-810m1%2F&tbnid=B06DUFZxV6eEyM&vet=12ahUKEwjyvpiK07vnAhUGATQIHYIPB30QMygSegUIARDb Ag..i&docid=G35EqqfUCyDyIM&w=600&h=600&q=two%20stage%20dust%20collector&ved=2ahUKEwjyvpiK07vnAhUGATQIHYIPB30QMygSegUIARDbA g

[Andrew More]

Many Torit industrial systems are 100% ‘draw thru’.

Good to know. I drive by one every day on the way to taking my son to day care. I've often wondered how it works. It's a bit hard to tell, since all the good bits are in a large blue box.

[Matthew Curtis]

Not sure if this answers your question. The bag below my filter got a split in it, about 2" long, most likely from me catching it with the end of a clamp and not noticing it.

After several sawing and planing operations, I noticed I could first smell the sawdust, usually the first noticeable sign there is fine sawdust in the air, then I noticed that the whole corner, where the Dust Collector sits was fogging up with a haze of fine sawdust.

Bag replaced and the problem was gone.

I am not sure how this relates to draw thru vs blow through filters.

A 2" split in any bag would create a dust leak regardless of what style it is. I am surprised you only experienced a "fine dust" leak.

[Doug Garson]

Actually a 2" rip in a bag under negative pressure would not create a dust leak, still not sure there is a practical way to design a "draw thru" system for a woodworking shop. May be practical for a big industrial application. Again if anyone has a picture of a draw thru filtration system in a woodworking shop I'd love to see it.

[Malcolm McLeod]

...a draw thru filtration system in a woodworking shop I'd love to see it.

I’ve never seen one for hobby or even light industrial use, but again, the filters would need to be between the 1st stage (cyclone?) and the blower. That means the filters need to be in a housing that’s ducted accordingly. I think that drives the cost out of reach for most users who’d be reading this. Not to mention size.

Most if not all pleated cartridge filters that I was around in years gone by were designed for outside>in flow with ‘structural’ steel mesh on the inside to prevent collapse. This also allows for a directed air blast from timed high flow solenoids & on-board air reservoirs to momentarily reverse flow and ‘clean’ the filters. (I found they were ineffective - until you shut the blower off.)

The current crop of cyclone filters obviously flow inside>out. Mesh location has changed and I think they’re harder to clean than above.

It should not be terribly difficult to buy a hobby cyclone system, or the components, then build a ply filter housing. You’d just need to make sure the housing is rigid enough for the negative pressure, but still opens for cleaning and filter swap. ...if you have room?

[Matthew Curtis]

Actually a 2" rip in a bag under negative pressure would not create a dust leak, still not sure there is a practical way to design a "draw thru" system for a woodworking shop. May be practical for a big industrial application. Again if anyone has a picture of a draw thru filtration system in a woodworking shop I'd love to see it.

I assumed the bag was under positive pressure.

[ChrisA Edwards]

I assumed the bag was under positive pressure.

Correct, the bag sits after the blower and inflates with pressure when the system is turned on.

The bag, being after the cyclone, did not really affect the debris that was filtered out by the cyclone, only the super fine dust, that still made it through the cyclone first stage, was in the filter bag and this ws the stuff that was escaping through the bag cut. Not a lot, but over time, 20 to 30 minutes, it became noticeable.

[Doug Garson]

I assumed the bag was under positive pressure.
But you said "regardless of what style it is" and we were discussing positive vs negative pressure systems.

[Matthew Curtis]

But you said "regardless of what style it is" and we were discussing positive vs negative pressure systems.

Correct. I am not sure but I do not believe that there is a system where a bag would be under negative pressure unless it was in a vacuum chamber.

[Doug Garson]

Agreed, and sorry if I was nit picking. Getting back to the original post, yes in a draw thru (induced draft) system with the filter under negative pressure leaks are less of an issue than in a typical positive pressure system. In theory you could design and build such a system, in practice it would be more complicated, take up more space probably not include a bag and be more expensive.

[Jerry Bruette]

Wouldn't a rip or hole in a bag be a leak no matter if it's under positive or negative pressure? I would think that where you exhaust the air to would determine if it will be a problem or not. If you exhaust outside, no problem, if you exhaust inside it would fill your building with whatever comes through the hole in the bag.

[Doug Garson]

Yes a leak is a leak whether under positive or negative pressure. A problem either way just a different problem. If under positive pressure it would leak dust into the building. If under negative pressure it would reduce the volume of air available to remove dust from the equipment so maybe less of a problem but still a problem. Wouldn't matter if you exhaust outside or not.

[Andy D Jones]

Things to consider if constructing a negative pressure "draw-only" two stage collector:

The direction of the airflow across the cartridge filter should not be changed, since the filter is supported on one side (usually the outside) by metal mesh. Therefore, the airflow should continue to be from the inside to the outside of the filter.

So a "plenum" has to be built around the filter so that the blower suction is outside the filter, and the exhaust from the cyclone is to the inside of the filter.

I don't know what happens to the blower as the air density/viscosity (reduced by more-negative pressure) impinging on the impeller is reduced. Maybe not enough to be significant if the impeller was optimized to draw through a cyclone to begin with.

Andy - Arlington TX