Dust collector fans
Dust collector fans are material handling fans and, ideally, they should be designed to withstand the passage of not only saw dust but also chunks of wood and, perhaps, the odd screw or nail. This is the heart of your dust collector. Take a good look at it before buying your collector.
Dust collector fans can be made of various materials including steel, cast aluminum and plastic. If you look at how a unit is constructed you may see that there are plastic or steel grids over the inlet and possibly the outlet openings. These grids are there to stop you from putting your fingers into the impeller when the fan is running. However, they also cut down on the fan’s efficiency. In some instances, such as a plastic fan impeller, the grid is there to keep large pieces of wood from hitting the blades and damaging the fan impeller. If the fan has these grids on the suction or discharge side you won’t have too much luck sucking shavings away from your thickness planer. In the case of one fan we tested the grids were plastic and resulted in the loss of about 40 cfm (cubic feet per minute) in flow for each grid.
Fan Impeller Design
In the photos you will see the two basic designs and their differences. Notice that one is a cast aluminum impeller. The lead photo shows a “backward-inclined” fan impeller. This design is very efficient and quiet. However, the blades are numerous (usually 9 to 16) and because they are close together, large pieces of wood will damage the fan impeller and housing (you may be able to see some evidence of this type of damage to this impeller). This design of impeller is capable of moving large volumes of air. Originally, some of the manufacturers used this type without the grids in front of them. After many were damaged, they changed designs.
Now the most popular design of impeller is the “radial” fan impeller. The one pictured is cast aluminum.
This design features blades that radiate straight out from the hub of the impeller. There are generally 6 to 10 blades spaced quite far apart.
Manufacturers switched to this type of design because large pieces of wood pass through easier, causing less damage. Although this results in less air volume, they are generally stronger. They are available in steel, aluminum or plastic. Avoid the plastic impeller if there is risk of any large pieces passing through it. The aluminum impeller won’t create sparks if struck by metal debris, but it does make the motor work harder. It lacks the flywheel effect created by the heavier steel impellers.
The fan impeller used primarily in the air cleaner units is a “forward-curved” design. There are usually 24 to 64 blades. They operate at lower speeds than the material handling fans as they are just meant to move air. Because of their lower speed there is far less noise.
Fan Housing Construction
You also need to consider the construction of the fan housing. Can the air flow smoothly through the fan housing and out the discharge without hitting any major obstructions? Is the fan casing sealed so that fine dust can’t escape? Most of the dust collector fan casings are of scroll type and have squared corners. The discharge end of the scroll is usually square graduating to a round opening. Look and see how it is built. The more gradual the transition the better. As with any pipe fitting for dust collection: the less gradual the transition, the more the resistance to flow and the poorer the performance.
Photo #4 (Above) shows not only the impeller (modified backward-inclined) but also the discharge transition piece.
When purchasing a dust collector have a close look at the fan and ask what material it’s made of. Also ask what the “cfm” (cubic feet per minute) rating is for the unit. Find out if the rating is for the fan only or for the entire set-up as displayed. Some manufacturers quote “free air” ratings, which is the rating of the fan without filters or ducting of any kind hooked up. Free air ratings are of no use to you, especially when you’re trying to determine the size of air cleaner you need for your shop.