Aerosol Transport – Inertia

:: Section 9

Inertial Impaction

III. Concept of Cutoff Size

In an ideal situation, an impactor has a "sharp cutoff", i.e., all particles greater than a certain size are collected while all particles smaller than that size pass through. This size is called the cutoff size (d_p50). However, in reality, such a relationship is unlikely. This is because the impactor is not really a sieve, and the nozzle size is much larger than the particles, so there are always some opportunities to have oversize paritcles getting through. On the other hand, chances for undersized particles to be collected also exist. The following figure is the actual and ideal impactor cutoff curves.

So, for real-world applications, we use the Stokes number that gives 50% collection efficiency. For the impactor meeting recommended design criteria, the Stokes number for 50% collection efficiency (Stk₅₀) is 0.24 for circular jets and 0.59 for rectangular jets.