How much you need depends on variables such as inertia, changes in an enclosure's direction, the total volume of the space, and friction caused by obstructions. It's harder to move dust-laden air when there are steel beams in the way, for example. Two machines may have the same cfm rating, but one machine may have two to three times more horsepower and quite a few more inches of static pressure. It can be a critical difference. You have to match what you need to what you buy.
Contractors play it safe when they use machines with the most inches of static pressure.
However, well-designed enclosures minimize both the static pressure and cfm your machine has to deliver. The ideal enclosure, according to Cignatta, resembles a narrow tunnel. Such spaces are easier to ventilate than large, tentlike enclosures. By reducing your cfm and static pressure requirements, you can get by with a smaller, cheaper, more portable dust collector. Manufacturers have made strides in making dust collectors smaller, quieter, and more efficient, said contractor Eric Campbell, who owns eight machines and has two more on order.