Hi. My name is Chinomso Ibe. I am a graduate research assistant at the University of Minnesota.
This hands-on activity demonstration is on the visualization of local exhaust ventilation airflows.
I developed this activity along with Pete Raynor from the University of Minnesota and Tom Peters from the University of Iowa.
For this hands-on activity, you will need: A Wizard Stick Fog Generator including its Super Zero Fog Fluid, 6 double-A batteries,
a vacuum cleaner and vacuum cleaner extension hose – the one used here is a one and a half inch diameter extension hose,
PVC fittings of a 2 by 2 by 1.5 inch all-hub sanitary reducing tee, 1.5 by 1 inch reducing bushing,
and a 2 inch by 2 feet plain end pipe can be used to fit the dimensions of the extension hose of the vacuum cleaner.
A quarter sheet window cake box, two clamp stands, something to measure distance such as a ruler or measuring tape,
something to write with such as a pencil or pen, a pair of scissors, and a screwdriver are also needed.
The first part of this activity will focus on visualizing airflow patterns in a room.
Sometimes, airborne exposures are invisible to the eyes so airflow visualization is the art of making air flow patterns visible to us.
The Wizard Stick is a fog generator and we will use it to visualize airflow patterns in this activity.
To set up the Wizard Stick, you will need 6 batteries, a screw driver, and the Super Zero Fog Fluid.
First, unscrew the battery compartment using the screwdriver and load the 6 double-A batteries.
Close the battery compartment.
Then, unplug and fill the fluid tank of the Wizard Stick about three-quarter full with the Super Zero Fog Fluid. Plug the fluid tank closed.
To operate the Wizard Stick, press the first or top trigger for a few seconds to activate the
vapor generator. Then, squeeze the second trigger firmly to generate a steady stream of non-toxic fog.
In calm or steady air, wave your hands to visualize the pattern of the fog in the room.
To see the direction or path the fog takes, use the Wizard Stick near operating vents such as exhaust air vents, or supply air vents.
You can also place the Wizard Stick near cracks in doors to see how air is moving in or out of the room.
The next part of this activity is to understand how different hoods move or capture airborne contaminants using air flow visualization.
Local exhaust ventilation systems include a hood to draw in contaminated air, a duct to convey the air, and a fan to move the air.
There are three common types of hoods: a capturing hood, a receiving hood, and an enclosing hood.
This activity will demonstrate how these types of hoods work, or don’t work, using air flow visualization.
First, let’s create a set up that mimics a local exhaust ventilation system.
To mimic a hood and conveying duct, plug the 1.5 inch diameter reducing
bushing tightly into the 1.5 inch hub of the reducing tee.
Link the 2 inch diameter hub of the reducing tee into the 2 inch diameter plain end pipe.
Then, attach one end of the 1.5 inch diameter extension hose into the reducing bushing.
Connect the other end of the extension hose to the vacuum cleaner.
When switched on, the vacuum cleaner acts as the fan that moves the air.
The air velocity in the vacuum cleaner must be enough to move air into the hood and ducts.
We will begin by testing the effectiveness of the capturing hood.
Capturing hoods are located near an emission source without surrounding or enclosing it.
An example would be hoods used for welding operations or hoods by wood grinding benches.
For the capturing hood set up, mount the PVC pipe ensemble horizontally on the clamp stand.
Adjust the height of the ensemble so that the pipe opening is at the same height as the Wizard Stick nozzle.
Using the ruler to measure horizontal distance, move the Wizard Stick so that it is 2 feet away from the pipe opening.
Turn on the vacuum cleaner and press the Wizard Stick trigger firmly to generate a steady stream of fog.
Watch the movement of the fog and estimate the fraction that is captured: none, almost none, quarter, half, three quarter, almost all, or all?
With a 2 feet distance, none of the fog is captured.
Repeat the same for a horizontal distance of 1 foot.
Here again, none of the fog is captured. For a horizontal distance of 6 inches, a quarter of the fog is captured
For a horizontal distance of 3 inches, half of the fog is captured
For a horizontal distance of 2 inches, almost all of the fog is captured
And finally, for a horizontal distance of 1 inch, all of the fog is captured
We see here that the capture of the fog will improve as the source gets closer to the hood.
When the hood is too far, competing air currents in the room will reduce the ability of the hood to
capture the fog from the source, in this case, the Wizard Stick.
Therefore, a capturing hood is effective as long as the distance from the source to the hood is very close.
Next, test the effectiveness of the receiving hood.
Receiving hoods use the buoyancy or upward movement of hot air to capture contaminants.
This means that the contaminant must have some initial velocity.
An example of a receiving hood is a canopy hood located over a melting surface.
To set up the receiving hood, mount the PVC ensemble vertically above the Wizard Stick.
Adjust the location of the ensemble so that the PVC pipe opening is directly above the Wizard Stick nozzle.
Using the ruler to measure vertical distance, move the receiving hood ensemble so that the
pipe opening is 1 foot away from the Wizard Stick nozzle.
Turn on the vacuum cleaner and press the Wizard Stick trigger firmly to generate a steady stream of fog.
Watch the movement of the fog and estimate the fraction that is captured.
With a distance of 1 foot, about three-quarters of the fog is captured.
Repeat the same for a vertical distance of 6 inches. Almost all of the fog is captured.
For a vertical distance of 3 inches, all of the fog is captured.
For a vertical distance of 2 inches, all of the fog is also captured.
We can also see here that fog capture will improve as the source gets closer to the hood.
However, the receiving hood is likely to be able to capture all of the smoke at a longer distance than the capturing hood.
This is because the receiving hood takes advantage of the energy or natural momentum of the contaminant.
Since the momentum of the fog is moving upward, the receiving hood located above the
Wizard Stick will capture the fog somewhat more effectively than that of the capturing hood located to the side.
Finally, test the effectiveness of the enclosing hood.
An enclosing hood is a ventilated box that has either all sides fully enveloping a contaminant source, such as a glove box,
or it can partially envelope a contaminant source with one or more sides open such as a laboratory fume hood.
For the enclosing hood set-up, first fold the window cake box into shape.
Then, use the pair of scissors to cut a large hole approximately the diameter of the 2 inch PVC pipe on the long side of the cake box.
Also, cut a small hole approximately the diameter of the Wizard Stick nozzle on the short side.
Partially remove the cardboard on the other long side of the cake box. If necessary, use a clear tape to secure the shape of the box.
Next, mount the PVC ensemble horizontally.
Then, secure the cake box on a clamp stand so that the edge of the PVC pipe fits into the large hole on the long side of the cake box
and the Wizard Stick nozzle fits into the small hole at the bottom of the cake box.
Turn on the vacuum cleaner and press the Wizard Stick trigger firmly to generate a steady stream of fog.
Watch the movement of the fog and estimate the fraction that is captured. In this case, all of the fog is captured.
What if the cake box was removed and the ensemble is now acting like a capturing hood?
We can see that less fog is captured without the cake box than with the cake box.
The main difference is that, with the cake box, the air is contained and the smoke cannot escape.
This allows the velocity in the hood to move the air without much difficulty.
Without the cake box, however, the smoke easily escapes, especially as the distance
between the Wizard Stick and the hood opening is far apart.
Due to this distance, the velocity supplied by the hood is not sufficient to capture the fog into the duct.
I hope this video demonstration on airflow visualization has helped you understand how to
the capturing, receiving, and enclosing hood work. Thank you for watching!