Chemical Engineering September 2010 - 53

also mix contaminated and clean air
similar to a mixed-flow impeller, with
useful dilution effects when released
into the atmosphere. But the centrifugal
fan approach requires a rooftop
stack with sufficient height, sometimes
as high as 40 ft or more. For the
same dilution and operating efficiency,
a mixed-flow impeller system will typically
rise only about 15 ft above the
roofline. Because of the lower height,
the mixed-flow impeller exhaust system
would not require the extra stabilizing
hardware needed with centrifugal
fan exhaust systems.
Tall exhaust stacks are not the only
structures associated with centrifugal
fan systems. Often, centrifugal fan
systems also need a rooftop penthouse,
mainly to protect service personnel.
Because centrifugal fan systems rely
on belt-drive motors - in contrast to
the more reliable direct-drive motors
of mixed-flow impellers - periodic
maintenance visits are required to adjust
belt tension or to replace pulleys
or failed belts. The penthouse enclosure
maintains safe, workable conditions
for maintenance personnel.
In terms of reliability, laboratory
workstation fume-hood exhaust systems
based on mixed-flow impellers
are relatively simple and maintenancefree
compared to the belt-driven fans
and blowers of typical centrifugal and
axial fan systems. The belts alone in
a belt-drive system, which can stretch,
loosen and break over time, require
regular maintenance and repair, resulting
in shorter expected operating
lifetimes. For example, the minimum
expected lifetimes (L10) of bearings
in belt-driven systems are typically
40,000 h, while the expected operating
lifetime of direct-drive motor bearings
can be at least 200,000 h for some
mixed-flow impeller systems.
A quieter solution
Noise management is also an important
consideration for most laboratories and
areas - in some cases requiring firms
to perform noise studies at their property
lines to determine the noise levels
of a facility's various systems, including
the laboratory exhaust system. The
lower speeds and inherently quieter
design configuration of the direct-drive
motors in mixed-flow-impeller exhaust
systems, compared to axial and centrifugal
fan systems, usually translate
into lower noise and vibration levels. In
addition, for applications where noise
must be tightly controlled, mixed-flow
impeller systems can be designed with
integrated noise attenuators that do
not increase the overall height of the
roof exhaust fan.
Axial and centrifugal fan exhaust
systems can also be equipped with
noise-reduction attachments, such as
a discharge silencer, added onto the
exhaust stack. But this adds to the already
tall stack height, and requires
additional mounting hardware to minimize
vibration induced in the building.
Even at their lower motor speeds, there
is little sacrifice in performance with
a mixed-flow impeller. It can provide
greater airflow than a centrifugal fan,
and greater air pressure than an axial
fan, at lower rotational speeds.
Compared to belt-driven centrifugal
and axial fans, direct-drive mixed-flow
impeller fans are energy-efficient and
cost-effective choices for fume hoods,
with about 25% greater energy efficiency
than belt-driven centrifugal fan
systems. Improperly aligned shafts,
pulleys, pillow blocks and motors in a
belt-driven system can rob it of power,
and can introduce reliability issues.
Also, the outlet nozzle of a mixed-flow
impeller system can be designed for
high exhaust-gas exit velocities even
at lower fan speeds and corresponding
reduced power requirements.
For further energy efficiency, it may
be necessary to specify a variable-speed
drive system. The use of variable-speed
drives can contribute to lower energy
costs, running at slower fan speeds at
night when laboratories are unoccupied.
In addition, if one fan fails in a
multiple-fan exhaust system, the remaining
fans increase speed to compensate.
This is critical in many biosafety
level (BSL) laboratories, where
redundancy is mandatory.
In addition, energy savings are also
possible by recovering some of the heat
normally lost through exhausted air.
Mixed-flow impellers can be equipped
with unique heat-recovery modules
that extract warmed air from the exhaust
flow and transfer the heat energy
to the intake of a building's ventilation
system, significantly reducing heating
fuel costs for the building (for every 1ºF
in added heat, there is a corresponding
3% reduction in heating fuel costs).
Ductless laboratory fume hoods
The three major laboratory fume-hood
exhaust systems discussed so far assume
the use of ductwork networks to
vent contaminated air from the laboratory.
In recent years, ductless laboratory
fume hoods have emerged as
lower-profile alternatives. In ductless
designs, the hood incorporates a carbonbed
or other type of filter to remove and
control contaminants. The filter media
must be matched to the types of chemicals
used in a laboratory. General-purpose
filters will adsorb a percentage of
most chemicals and gases, while particulate
matter filters may be needed
for some applications. In some cases, a
chemical impregnate may be needed in
the filter to alter the target chemical to
a less hazardous material.
A ductless, laboratory fume hood has
no external connections other than its
power cord, making it extremely portable
and useful for facilities that require
more than occasional redesign.
The power cord connects to electronic
systems that monitor airflow and filter
conditions for operator safety, as well
as provide visual and audible alarms
when needed. A ductless approach is
not a substitute for all ducted systems,
and should never be used in any chemical
laboratories involving unknown
results (for which the filter may not
work). Admittedly, ductless laboratory
fume hoods sacrifice efficiency for portability,
especially compared to a centrifugal
or mixed-flow impeller system,
and they require careful monitoring of
the carbon filter's condition so that replacement
can occur once the filter has
become saturated.
■
Edited by Scott Jenkins
Author
Paul A. Tetley is executive
vice president of MetPro
Corp. (160 Cassell Rd.,
Harleysville, PA 19438;
Phone: 215-723-4700; Email:
ptetley@strobicair.com).
Prior to his current position,
Tetley was vice president and
general manager at Strobic
Air Corp., a Met-Pro subsidiary.
Since joining Strobic Air
in 1989 as engineering production
manager, he has designed or invented
many innovative Tri-Stack fan systems, an
acoustical silencer nozzle, and a unique multifan
plenum system.
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