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Reply by crank49

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Posted on To use a cyclone or not

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crank49

4032 posts in 2996 days


#1 posted 09-27-2015 08:05 PM

I’m going to try to explain static pressure without getting technical.

All pumps, including fans, do two things that each affect each other. They move a volume of something from one place to another and they do this by creating a pressure difference between the two places.

If you create low pressure in a certain place, then the surrounding air will rush in and try to balance the pressure. Suck the air from around a spinning saw blade and surrounding ambient air will move into that space.
The smaller the opening you try to pull the air through the faster it tries to move; at least up to the point there is not enough pressure to make it go any faster.

The higher the pressure difference the fan creates, the faster the air moves in to replace what’s now flowing throught the duct toward the fan. This is important because the speed of the flow has to be at least as great as the speed of the dust and chips being thrown out of the saw kerf for the dust to get captured.

Whether the fan is positioned before the filter, or after the filter (or separator) make no difference to the relationship between flow and pressure. It is, very wise to put the filter or separator befor the fan to save wear and tear on the fan itself.

You can build a very simple gage to show the pressure available on any fan. It’s called a u-tube manometer. You need nothing more than a few feet of clear tubing and a drill and some water. Look it up on Google or similar search engine and you will find tons of examples. Suggest looking for DIY u-tube manometer.

Caution, a shop vac based dust collector might produce pressure of over 90 inches of static and will suck the water out of your tube unless it is over 90 inches tall. Better to buy dial type pressure gages for these.


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