Dust Collection by the Numbers #2: Terms and Definitions

 Blog entry by EarlS posted 04-29-2018 12:49 PM 828 reads 1 time favorited 2 comments
 « Part 1: Where to Start? Part 2 of Dust Collection by the Numbers series no next part

It’s been a few weeks since I first started this discussion on dust collection systems and the math/science behind it. There is a vast amount of information available out there and all of the variations on how to size a DC set up.

Here are some basic terms to get things started:

AIR FLOW:
cfm (cubic feet per minute) is the volume of air being moved by the blower. 350 – 45 cfm is required for dust removal on a table saw

fpm (feet per minute) is the velocity the air is moving. 3500-400 ft/min is the flow rate required to keep wood chips and dust suspended in the air stream and not settle out.

Cross sectional area is the area of the pipe/hose, opening that the air is flowing through- remember back to high school geometry. Cross sectional area of a 4” pipe is 0.087 ft^2, area of a 6” pipe is 0.196 ft^2 , 2 times the area of a 4” line

Radius is the distance from the middle of the pipe to the inside edge of the pipe (usually in inches). It is also 1/2 of the inside diameter of the pipe.

The area of a circle is pi x radius^2 or in easier to remember terms area of a circle is 3.14 x (diameter in inches/12) = area of a circle in square ft or ft^2.

The volume of air (ft^3/min) divided by the area of the pipe (ft^2) gives the velocity that the air is moving (feet/min). You can figure out any one of the variables if you know the other two.

Air flow determines how much sawdust you can move. Wood chips require 65 cfm/lb of chips moved

STATIC PRESSURE:
Static Pressure is resistance against air. A DC system is made up of ducts, fittings, a dust filters, maybe a cyclone and some blast gates, and so on. Static pressure, or system air resistance is a compilation of the entire system. One flaw can significantly cripple an entire system.

“WC or inches of water column is the typical pressure unit used in the US for low pressure air systems.

1” WC is the amount of pressure exerted by a column of water 1” high.

Pressure is lost due to friction caused by the air flow through the through the piping and components in the system.

PERFORMANCE CURVE:

Blowers have a performance curve that shows the relationship between amount of air flow (cfm) and pressure the blower can create (“WC). As the air flow increases the pressure decreases.

SYSTEM LOSSES

Think of a DC system as a blower with a long pipe on the inlet, the lowest pressure (highest vacuum) is at the inlet of the blower. As you move away from the inlet of the blower the vacuum inside the pipe is lost to friction until it reaches atmospheric pressure, at which point there is no vacuum.

There are standardized calculations for determining the pressure loss per ft of pipe. Pressure loss in the fittings, cyclone and other components of the system can be expressed in terms of “equivalent length” of pipe. That means that you can total up the “equivalent length” of pipe and multiply it by the pressure loss/ft of pipe to obtain the pressure losses of the system and compare it to the static pressure of the DC at the air flow rate required to determine if the blower has enough suction and air flow to move the sawdust.

To put all of this into some context; a dust collection system removes the dust and chips from a piece of equipment using a blower that generates enough air flow and suction to overcome the pressure losses of the system while maintaining adequate velocity to keep the wood particles suspended in the air stream.

I will continue to make a disclaimer that I am not trying to cover every aspect or dust collection nor am I an expert in dust collection. The intention is to provide some basic information that can help folks figure out how well their DC system is performing and how to identify where improvements can be made.

The next installment will take all of this and use it to evaluate my DC set up. It has some problems, as I discovered.

-- Earl "I'm a pessamist - generally that increases the chance that things will turn out better than expected"