necessary to effectively capture the contaminant
with a hood. Figure 3 outlines the American Conference of Governmental Industrial Hygienists (AC-
GIH) recommended capture velocities for typical
applications. Identifying the required capture velocity is the first step in defining a successful hood
design, but it must be combined with the proper
type of hood to achieve the desired performance.
Selecting the Proper Hood Type
Capturing the contaminant at or near the source,
referred to as local ventilation, is always recommended where practical because it requires the
least amount of energy and does not allow the
contaminant to migrate throughout the rest of the
factory or into the workers’ breathing zone. Different applications require different hood styles
or approaches to make local ventilation practical.
Therefore, matching the proper hood type with the
application process is also essential for achieving
the desired performance.
Although hoods are available in many sizes and
shapes, the three basic categories are exterior capture hoods, enclosures, and receiving hoods.
Exterior Capture Hoods. Exterior hoods capture
air contaminants that are being generated from a
point outside of the hood. Examples of these types
of hoods are extraction arms, slotted hoods, and
simple open-ended ducts. These hoods are effective if the contaminant is released with low momentum and within the hood’s effective reach.
Typical applications for exterior capture hoods are
processes such as thermally generated fumes and
bag dumping or filling stations.
An advantage of exterior hoods is that they generally use less airflow when compared to other,
larger hoods and, therefore, use less energy. They
also are relatively simple to design and inexpensive
to fabricate and install. The low airflow can result in
reduced duct and equipment sizes. A disadvantage
of exterior capture hoods is that significantly more
airflow is required the farther away the hood is
from the emission source. This means that, in most
cases, exterior hoods need to be relatively close to
the emission source to be effective and efficient.
Best practices for using external hoods include
using a perimeter flange as well as a gradual taper
from the hood to the duct. Incorporating these
features will focus the hood capture velocity to-
ward the extraction source and reduce the pressure
loss at the hood. Figure 4 shows the developed air-
flow patterns of several hood geometries and the
impact of these features.
Enclosure Hoods. Enclosure hoods surround or
contain the emission source with one side of the
enclosure left completely or partially open. These
hoods are most often applied when local ventilation is impractical because of interference, size of
parts, or where emission generation rates are very
high. Enclosure hoods are commonly used in applications such as abrasive blasting, spray booths,
CNC machining (see Figure 5), conveyor loading,
enclosed bucket elevators, crushers for mining, and
vibrating screens.
The primary advantage of an enclosure hood is
that it locally contains the emissions, preventing
migration and protecting the workers. With its
small opening to draw an inrush of air through, a
relatively large space can be controlled efficiently.
Disadvantages of an enclosure hood are that it is
not always practical or cost-effective to enclose
the entire process. If workers need direct access to
parts and need to be inside the enclosure while the
process is producing emissions, additional personal
protective equipment is required.
An enclosure hood design is based on the ingress
velocity of all open areas within the hood. Sufficient capture velocity must be achieved across
these openings to ensure that the contaminant
will not escape the enclosure. However, finding
the proper balance is essential, because pulling too
much air is not only detrimental from a pressure
drop standpoint, but also can increase the loading
on the collection equipment, which in turn shortens the filter life.
Receiving Hoods. Receiving hoods are designed
to take advantage of the momentum or the force
of the contaminant to capture it within the hood.
Common examples are the hood on the discharge
side of a grinding wheel that leverages the inertial
forces to carry the air contaminants into the hood
(see Figure 6). Another example is an overhead
canopy on thermally generated fumes that uses the
thermal rise to capture the contaminant.
Advantages of receiving hoods are that they typically capture the fume at or near the source and
leverage the process parameters to collect the material. As a result, low flow rates can regularly be
achieved with this approach. A disadvantage is that
these hoods rely on a consistent process and ambient conditions to operate correctly.
Completing the Hood Design
Using the process information collected, including the recommended capture velocity and the
hood type that best matches the process and performance requirements, fabricators can specify
the hood geometry they need. Resources such as
ACGIH’s Industrial Ventilation manual provide
recommendations and the equations necessary to
complete the design for many types of hoods and
specific processes. Although these recommendations may not precisely match the application, a
general approach or strategy can be taken and applied using the specific performance requirements.
After the hood design is completed, the process
of designing the ductwork and selecting the proper
dust collection equipment and fan can begin.
Travis Haynam is director of business development and Ed Ravert is senior application engineer at United Air Specialists Inc., 4440 Creek
Road, Cincinnati, OH 45242, 800-252-4647, info@
uasinc.com, www.uasinc.com.
Different applications require
different hood styles or approaches
to make local ventilation practical.
Figure 6
A receiving hood is properly placed on this grinding table
so that it can easily capture the contaminants that fly
its way.
Figure 4
The airflow patterns of selected exterior hood types are
shown.
Figure 5
An enclosure hood, such as the one on this CNC machining center, is used in applications where a large amount
of emissions are generated.
Energy of Dispersion Examples Capture Velocity (FPM)
Low Evaporation from tanks; degreasing 75-100
Average Intermittent container filling; low-speed conveyor transfers; welding;
plating; pickling
100-200
High Barrel filling; conveyor loading; crushers 200-500
Very High Grinding; abrasive blasting; tumbling 500-2,000
Figure 3
The American Conference of Governmental Industrial Hygienists issues recommended capture velocities for several
fabricating activities. Source: Industrial Ventilation, a Manual of Recommended Practice for Design, 27th Edition,
ACGIH.
