Let’s consider inertial forces, now, that act on a particle, which leads to impaction, like a bug hitting the windshield of a car.
The windshield causes air to move up and over the car.
The force of drag will cause a small bug to follow the airflow over the car, avoiding the windshield.
In contrast, a big bug, like a grass hopper, resists drag because of its inertia.
If the inertia of the big bug is sufficient, it deviates away from the airflow to hit the windshield and make a big mess.
This process is called inertial impaction.
The concepts of particle relaxation time and stopping distance are used to describe inertial forces.
The particle relaxation time is how long it takes a particle to 'relax' to its new conditions.
It is proportional to particle density and diameter squared, which is exactly the same as we saw before with terminal settling velocity.
The stopping distance is the distance that the particle will move along its original direction until it relaxes to the new conditions.
It is calculated as the initial velocity times the particle relaxation time.
In the figure from the video, the small bug has little inertia.
It relaxes fast to new conditions, having a short relaxation time, or tau, and thus moves only a short distance toward the windshield,
having a short stopping distance, and it passes over the car without hitting the windshield.
In contrast, the big bug has lots of inertia. It is slow to relax, has long relaxation time,
has a large stopping distance, so it moves away from the airflow streamlines and hits the windshield.
So now we can use these relationships to compute something a bit more related to occupational health,
such as how far will a silica particle be thrown from a grinder?
Going back to the same table that we used previously with particle diameter in micrometers and nanometers in the first two columns,
the middle column shows particle relaxation time, tau, in seconds.
The last two columns show the distance thrown, or stopping distance, in both meters and feet.
Small particles relax very fast. It’s only until we get to the largest particles where we get appreciable movement.
A 10 micrometer particle is thrown only 0.02 feet, which is about a quarter of an inch.
In contrast, a 100 micrometer particle will be thrown much farther, about 2 feet.
So the eye protection required for the worker shown in the picture is to prevent ocular damage from impact of very large particles.
Inertial impactors use this inertial property of aerosols to remove large particles from an airflow.
Air is forced through a nozzle, which accelerates the air into an air jet.
The air jet impinges on an impactor plate, causing the jet to abruptly turn 90-degrees.
Small particles with little inertia relax to the change in direction and pass around the plate, often to be collected on a filter.
Large particles, in contrast, have enough inertia to hit the plate and be removed from the air.
In this way, small particles can be analyzed separately from larger particles.
Impactors are usually characterized by collection efficiency by particle size, such as shown in the upper right.
Large particles hit the plate almost every time, having a plate collection efficiency of near 100%.
Small particles, in contrast, almost never hit the plate, as they flow with the air flow, having a low particle collection efficiency.
The particle diameter for which 50% of the particles collect, is often referred to as the
particle cutoff diameter and is used to describe impactor performance.
The light blue personal environmental monitor shown in the lower right is designed to collect particles smaller than 2.5 microns onto a
filter after an impactor section that removes particles larger than 2.5 microns.
Cyclones are another type of important device that use inertial impaction to separate large particles from an airstream.
The airflow enters a cyclone and spins several complete turns before exiting.
Large particles hit the walls of the cyclone and move to a chamber at the bottom of the device, called a grit pot.
Small particles remain airborne and pass out of the cyclone.
'Respirable' cyclones are commonly used as lapel-type samplers to measure particle exposures with the smaller particles that
can enter the deep lung, exiting the cyclone and collected onto a filter for subsequent analysis.
Air is pulled through the cyclone using a belt- mounted air sampling pump.
Industrial cyclones for air cleaning can be very large and serve to remove large particles,
nominally larger than 10 micrometers or so, before being exhausted in dusty industries, such as woodworking.