Dust collection, all vacumn never presure added

[Bill Dufour]

I notice that people complain about needing higher vacuum and more airflow to capture fine dust from certain tools such as miter saws and bandsaws. Why is it that no one ever recommends adding a fan to blow the dust into the vacuum port. Many chop saws use the motor cooling air to blow dust into a collection bag. Many big bandsaws have a belt drive blower or compressed air connection to clear chips from the cut line.
People laugh at me when I say gas ovens used to have a chimney and now they just dump the exhaust into the kitchen breathing air. California outlawed non vented gas fireplaces years ago.
I have never seen a home kitchen vent hood with makeup air ducting.
It seems to me only dealing with vacuum is missing half of the equation.
There is some philosophical quicksand with vacuums. The higher the vacuum the less you are paying more for. The ideal of a perfect vacuum would have you paying for absolutely nothing.
Bill D

[Andy D Jones]

I see your point about directing dust towards the dust collector input, but only DC installations that vent outside have any issue with return air resistance (pulling a slight room vacuum), and that is usually a heating/cooling loss issue.

I know some really tight houses in extreme climates have air-air heat exchangers to recover heating/cooling losses while exchanging stale interior air for fresh exterior air, but the air needs to be well filtered, or the exchanger surfaces will foul, vastly reducing the exchanger efficiency.

I quit worrying about what CA bans a long time ago. Not my circus, not my monkeys.

-- Andy - Arlington TX

[William Hodge]

There is some philosophical quicksand with vacuums. The higher the vacuum the less you are paying more for. The ideal of a perfect vacuum would have you paying for absolutely nothing.
Bill D

This post made me think.
I have been looking for a direct vent central vac for quite a while. I want it to vent an orbital sander. I don't want to filter the dust, just shoot it into a shavings trailer.

The idea of a perfect vacuum to nothing could be achieved by having a tube into deep space, near the sun. The emptiness of space would create a massive suction that would silently collect everything that wasn't tied down. Hang on to the cat. In the interests of not fouling the Hubbell Telescope, the dust would be aimed at the sun, and get burned up.

[Alan Lightstone]

I have never seen a home kitchen vent hood with makeup air ducting.

Bill D

I have one. They do exist, but are very rare. I think it's 800cfm and I truly never worry about smoke when cooking.

Code down here requires it when going that high in vent airflow. Inspector didn't catch not having it, but added it anyway.

[Bill Dufour]

800 CFM would empty the conditioned house air in a hurry without that makeup vent.
Bil lD

[Doug Garson]

This post made me think.
I have been looking for a direct vent central vac for quite a while. I want it to vent an orbital sander. I don't want to filter the dust, just shoot it into a shavings trailer.

Seems like a direct vent central vac would be easy, my 25 year old Beam central vac has a 2" outlet pipe which I plumbed up to run outside. If you took out the filter bag (maybe add a screen to protect the impeller) you would have what you want.

[John K Jordan]

...Why is it that no one ever recommends adding a fan to blow the dust into the vacuum port.

With a bright light and dark background I've watched dust travel almost 2' from a sanded spindle on the lathe to the dust collector pickup. The vacuum pickup nozzle set up a smooth air current pattern that efficiently moved and piked up extremely fine dust. I think a fan would tend to upset this flow and disperse fine dust into the shop.

[Jim Dwight]

John's thought is consistent with mine, I think. An advantage of the DC is it does not push the dust where the DC cannot get it. If you try to push the dust to the collector you have to be worried about it going where the DC is not. If you can direct the fan well, it would help the situation. If you do not, it could do more harm than good.

One of the DeWalt planners sends it's chips into a trash can without a DC. That is another example of a manufacturer that seems to have addressed this thought.

[Larry Frank]

Using a bell mouth hood greatly helps getting dust from a turning.

[Bill Dufour]

I have seen big wood chippers that have a blower fan tied into the chip chute at an angle to help blow the chips into the truck. I think some huge artillery has been made this way with extra powder charges added along the barrel.
Bill D.

https://en.wikipedia.org/wiki/V-3_cannon

[John K Jordan]

I have seen big wood chippers that have a blower fan tied into the chip chute at an angle to help blow the chips into the truck. ...

I suspect a big difference between directing large chips with significant mass and picking up fine lung-damaging dust floating in the air. In his writings Bill Pentz discussed handling fine dust in detail.

Using a bell mouth hood greatly helps getting dust from a turning.

Larry, I think I remember discussing this before. This is what I use at the lathe, not quite a bell mouth:

lathe_dust_pickup.jpg

The adjustable arm lets me position it a little. The pickup works better than I had imagined. I've even seen some dust and chips come off the tool at the front of the lathe and curve over the top and head for the nozzle. This is with a powerful 5hp ClearVue cyclone. The noise from the air movement is quite loud!

JKJ

[Larry Frank]

John....You should try a real bell mouth hood. What you have is not the same. You have sharp edges and contour which set up turbulent flow. BobL on the Aussie forum has written extensively on this and he is one of the best dust collection experts. I posted this picture before but this is what I use with a 4" pipe and 5 hp DC. I do not notice the noise from it. I have it on an adjustable stand and aim it at where I am turning or sanding.

IMG_20181115_1711419_rewind_kindlephoto-3858650.jpg

[John K Jordan]

John....You should try a real bell mouth hood. What you have is not the same. You have sharp edges and contour which set up turbulent flow. BobL on the Aussie forum has written extensively on this and he is one of the best dust collection experts. I posted this picture before but this is what I use with a 4" pipe and 5 hp DC. I do not notice the noise from it. I have it on an adjustable stand and aim it at where I am turning or sanding.

IMG_20181115_1711419_rewind_kindlephoto-3858650.jpg

Thanks, I know it's not the bell shape and remember this discussion and your picture from before. The bell intake might be more efficient but for now, what I use works for me and is impressive to watch the pickup. (put down a dark background and use a bright light to watch the movement of even extremely fine dust) Fact is, by using hand scrapers instead of the power sanding most people use I avoid putting a lot of dust in the air. And wear a respirator. An interesting experiment would be to buy a second identical nozzle and heat and reform the edges into more of a bell shape and compare.