Enclosing hoods are designed to keep the contaminant inside the hood with escape prevented by limiting openings.
Examples of full enclosures include an abrasive blasting cabinet.
The worker puts a part to be cleaned into the cabinet through the access door.
The door is closed and the abrasive blasting process is done by the worker through gloves in the front of the cabinet.
The airflow for a full enclosure must be sufficient to keep air velocity moving inward
at all gaps and to evacuate the enclosure before it is opened.
Enclosing hoods also include partial enclosure, like the paint spray booth shown at right.
The velocity at the face of the paint spray booth is high enough to ensure that paint and solvent gases do not enter the workplace.
Here, we show full enclosures for machining of metal parts.The doors of the enclosure open, parts are manually or automatically inserted,
the doors close and machining is performed by a computer control. The doors open, the part is exchanged and the next part is machined.
This arrangement provides high effectiveness of control with low airflow. However, getting parts in and out often presents difficulties.
Here we show a full enclosure for a machining process.
The process is fully enclosed with a door that allows access to the machine for service or part change out.
Air from outside the enclosure is pulled in through any gaps in the enclosure.
The door is essentially a large gap, and when open, air velocity into the door is slow.
When the door is closed, the gaps are small and the inward velocity can easily keep contaminants inside the enclosure.
In this case, we show a part being drilled with oil used as a coolant. The oil and metal chips stay within the enclosure with the door closed.
If the door is opened too soon after the process finishes, then contaminants can escape out of the large open door.
This problem can be avoided by giving sufficient time for contaminants to purge from inside the enclosure before opening the door.
Chemical fume hoods are a very important type of enclosing hood commonly found in industry, research, and academia.
There are different types of chemical fume hoods. Traditional fume hoods that use constant volume airflow are the most common.
We will talk about them at length in the next slide.
Other types of fume hoods provide constant velocity at the face by variably controlling airflow.
Later in this presentation, we will discuss smaller specialty hoods that have become available for powder handling.
Chemical fume hoods are an important class of enclosing hood.
Shown here in side view, air is pulled from an exhaust duct with baffles and slots used to direct airflow.
Often, a storage cabinet below the work surface is held at negative pressure for safe storage of chemicals.
A sash is used to open and close the hood face, and its position changes the airflow patterns.
When the sash is fully up, most of the air enters through the hood face.
When in the middle position, some air passes through a bypass grill, the hood face, and an airfoil.
Finally when in the sash is in the lowest position, air enters the hood from the bypass grill and airfoil only.
These different arrangements facilitate good capture for a range of different situations.
The position of items emitting contaminants (or sources) affects the efficiency of the chemical fume hood.
Sources placed near the front of the hood can escape capture.
Instead, sources should be placed at least 6 inches and preferably 10 inches from
the opening, so that contaminants can follow a safe path toward the exhaust duct.
Although harder to reach, contaminants from sources placed near the back of the hood will typically stay away from the front opening.
As shown in the top view at left and the side view at right, chemical fume hoods are
designed so that the air enters perpendicular to the hood face.
However, a worker can dramatically influence this airflow pattern.
A low pressure region develops as air is pulled around the worker, generating a recirculation pattern in front of the worker.
For this reason, it's important to keep the sash as low as possible when working with hazardous chemicals.
With the sash too high, contaminants can easily escape and move into the person’s breathing area.
Lowering the sash reduces this problem by placing a barrier between the worker and the contaminant.
The lower sash also limits the amount of recirculation zone in front of the worker.
Avoid placing unnecessary items in the fume hood for storage.
These items can obstruct the proper air flow from the front of the hood to the rear slots.
Use a small table or shelf to elevate necessary items and allow air flow to flow under the table.
Never place anything on top of the airfoil or directly in front of the rear slots.
In addition, when using a chemical fume hood, avoid cross drafts,
such as those from foot traffic, fans, windows, doors, or ceiling air diffusers.
Drafts can dramatically change airflow patterns and cause contaminants to escape capture, entering the room.
Also avoid rapid movements, such as rapidly pulling your arms out of the hood.
Such actions can also change airflow patterns, allowing contaminants to again enter the room.
A biosafety cabinet is another common enclosing hood specifically designed to handle hazardous biological materials.
In this animation, we show a Class II biosafety cabinet.
A fan pulls air from the workspace floor, pushing it through high efficiency particulate air filters (HEPA filters).
Some of the cleaned air exhausts to the room and some flows downward.
Contaminated air from biohazardous materials is simultaneously pulled and pushed downward and then cleaned.
This simultaneous push and pull of air eliminates most of the adverse recirculation
patterns typical of traditional chemical fume hoods.
There are numerous other examples of enclosing hoods. Here a hood specifically designed for barrel filling is shown.
When material, such as a powder, is dropped into a barrel, it displaces air in the barrel.
If the material is dusty, the displaced air will contain many particles.
The barrel filling hood captures this particle- laden air before it enters the workplace.
Another example is a ventilated tunnel.
Here, we show a ventilated tunnel that fully encloses a dusty process, such as grinding, as parts moves along a conveyor belt.
The tunnel helps reduce the volume of air required to ventilate this type of a process.