By Travis Haynam and Ed Ravert
Effectively controlling harmful process emis- sions is an important issue facing fabricators as governmental and environmental regulations become increasingly stringent. Left uncontrolled, these emissions present health hazards
to factory workers and result in reduced productivity. In addition, emissions can accumulate on
floors, shelves, ledges, equipment, inventory, finished goods, and other places throughout a facility, negatively affecting product quality, machine
reliability, and overall worker safety.
By using a properly designed air pollution control
system including the hood, fabricators can avoid
these hazards. Four key aspects of a properly designed air pollution control system are capturing
the contaminant, transferring the contaminant,
selecting the proper collection equipment, and
selecting the proper air-moving device (fan). If any
one of these elements is not properly designed, the
system will not perform at an optimal level.
When evaluating air pollution control systems
for their manufacturing process, fabricators typically pay more attention to the equipment specifications or ducting, often overlooking the hood.
However, the capture hood (see Figure 1) is where
the air pollution control system first engages the
process generating the emissions. The hood’s role is
as important in the overall system as the roles the
other three elements play (see Figure 2).
The hood design has a direct impact on the
airflow and pressure requirements for the total
system. An optimally designed hood can result in
less obtrusive ductwork, a smaller dust collection
equipment footprint, and lower-horsepower fans.
This results in overall lower system installation costs
as well as lower system operating expenses because
of reductions in maintenance and energy usage.
A best practice to approach hood design for new
processes or to evaluate whether existing hoods are
serving their purpose effectively is to first understand the hood requirements.
Hood Design Basics
Hood design seeks to balance several characteris-
tics to achieve optimal system performance. These
• Maximizing emission control.
• Minimizing airflow requirements.
• Minimizing pressure losses (energy).
• Minimizing the impact on process efficiency
and worker productivity.
Because each of these characteristics affects the
other, a fabricating company should begin hood
design with a thorough review of the process generating the emissions, the contaminant itself, and
operator or process interaction to help determine
the hood performance requirements. The particle
size, process momentum or energy possessed by
the contaminant at time of capture, and any potential contaminant hazards also need to be identified and factored into the design.
For example, heat generated during welding
processes causes the surrounding air and process
fumes to rise rapidly. This motion or energy needs
to be considered and accounted for in the hood
design. In addition to process considerations, external sources of air motion that can upset hood
performance—such as machinery motion, process
material motion, movement of operating personnel, and natural room air currents—should be considered in the design as well.
However, pulling too much air can be detrimental to hood design and performance. Using welding
as the example again, too much air can disrupt the
shielding gas, potentially lowering the weld quality.
On CNC machining center applications, pulling too
much air results in more particulate capture than
is necessary, which decreases filter life or shortens
maintenance cycles. Additional flow also can lead
to increased energy usage through higher pressure
loss at the hood.
Once the design parameters are collected and the
application is understood, fabricators can use this
information to identify the proper capture velocity
It’s key in designing a complete air pollution control system
for optimal performance
The hood design plays an important role in how emissions
are removed from a work area, such as this welding table.
Air pollution control equipment, air-moving devices,
ducting design, and hood design are the four keys to
designing an effective emissions removal system. Often
hood design is the key factor that is overlooked.