By: Vince McLeod, CIH and Glenn Ketcham, CIH
Issue: Dec. 2006/Jan. 2007
Untitled Document
About a year ago we wrote about the fundamentals of chemical fume hoods in response
to a reader question regarding storage inside the exhaust cabinet. In that article
we discussed the basic design principles and operation of chemical fume hoods.
Since exhaust hoods are one of the major expense items for most laboratories
they have a huge impact on continuing operational costs. Therefore, we decided
to provide you with information on some of the newer designs offering performance
and energy conservation.
Laboratory exhaust systems fall into three main classes: chemical fume hoods
for working with corrosive acids and bases, volatile solvents, and other
hazardous chemicals; biological safety hoods which can be designed to protect
the work
(clean air bench) or the worker (true biosafety cabinet); and standard exhaust
hoods typically used in mechanical or machine shops or production areas.
For this article we are going to limit our discussion to the chemical fume
hood,
what is found in almost all forensic investigative laboratories.
Laboratory
fume hoods are designed to protect the worker by containing and exhausting
harmful
or toxic fumes, gases, or vapors from chemicals used in the hood. An exhaust
blower is typically mounted on the roof or in an exterior penthouse so that
room air from the laboratory is pulled into and through the hood, creating
directional airflow into the hood. Remote mounting of the blower ensures that
the entire system is under suction so if a leak develops everything is drawn
in and nothing gets pushed out of the exhaust duct. The “pull” at
the hood opening is termed the “face velocity” and is usually measured
in feet per minute. Proper face velocity of the hood is critical in maintaining
protection for the worker. Too little flow allows room air currents or disturbances
to overpower the hood and draw chemicals or vapors into the room. Too much
flow can result in turbulence and eddies that also lead to contaminates escaping
the hood. Baffles and other aerodynamically designed components determine how
air moves into and through the hood. Contaminates inside the hood are diluted
with room air and exhausted outside by the hood’s duct system where they
are dispersed. The volume of air exhausted by the hood depends on a number
of factors, most important of which are hood size and design. With average
chemical fume hoods exhausting around 750 to 1000 cubic feet per minute of
conditioned air, you can see how hoods put a large load on the laboratory’s
heating, ventilating, and air-conditioning system and thus impact the operational
costs. Let’s look at some of the different chemical fume hood designs
available and their pros and cons.
CONSTANT AIR VOLUME
There are two basic types of laboratory fume hoods: conventional and by-pass.
Conventional hoods consist of a basic enclosure with a movable sash (or window).
Since the face velocity or “pull” is a function of the total
volume divided by the area of the sash opening, closing the sash on a conventional
CAV hood will increase the face velocity. The conventional hood’s performance
depends primarily on sash position. However, as the sash is closed down velocities
can increase to the point where they disturb instrumentation and delicate
apparatus, cool hot plates, and slow reactions or create turbulence that
can force contaminates into the room.
By-pass hoods contain openings above the sash in addition to an air foil sill
that will redirect the air flow as the sash is closed. The by-pass openings
reduce the changes in face velocity to a narrow range by keeping the area for
air flow equal (within the limits of the by-pass) as the sash is moved up or
down. Therefore, face velocities do not reach the detrimental levels of conventional
hoods. For this reason, by-pass hoods hold a major share of the market today.
Recent models of by-pass hoods, called high-performance or “low flow” hoods,
have improved containment and safety features as well as energy saving designs.
These design features vary by manufacturer, obviously, but generally have one
or more of the following: sash stops or horizontal-sliding sashes to limit
the openings; sash position and airflow sensors that can control mechanical
baffles; small fans to create an air curtain barrier in the operator’s
breathing zone; refined aerodynamic designs and variable dual-baffle systems
to maintain laminar (undisturbed, non-turbulent) flow through the hood. Although
the initial cost of a high performance hood is slightly more than a conventional
by-pass hood, the improved containment and flow characteristics allow these
hoods to operate at face velocities as low as 60 fpm which can translate into
$2000 per year or more in energy savings depending on hood size and sash settings.1
In
laboratory settings where the tasks might be very specific and unchanging for
the most part, a variation of the “low flow” hood referred to as
the reduced air volume (RAV) hood is an option to consider. This design incorporates
a by-pass block to partially close off the by-pass reducing the air volume
and thus conserving energy. Usually, the block is combined with a sash stop
to limit the height of the sash opening ensuring a safe face velocity during
normal operation while lowering the hood’s air volume. By reducing the
air volume the RAV hood can operate with a smaller blower which is another
cost saving advantage.
One downside to the RAV hood is that with its restricted sash movement and
reduced air volume, the realm of tasks and flexibility of use is also constrained.
Another major caution to note is the potential to override or disengage the
sash stop. If this occurs the face velocity could drop to an unsafe level.
To counter this condition, train operators to never override the sash stop
while in use, only for loading or cleaning the hood. In addition, an air flow
monitor is always recommended.
VARIABLE AIR VOLUME
The newest generations of laboratory fume hoods vary the volume of room air
exhausted while maintaining the face velocity at a pre-determined level.
Variable air volume (VAV) hoods change the exhaust volume using different
methods such as a damper or valve in the exhaust duct that opens and closes
based on sash position or a blower that changes speed to meet air volume
demands. Most VAV hoods integrate a modified by-pass block system that will
ensure adequate air flow at all sash positions. They are connected electronically
to the laboratory building’s HVAC so hood exhaust and room supplies
are balanced. In addition, VAV hoods feature monitors and/or alarms that
warn the operator of unsafe hood airflow conditions.
Although VAV hoods are much more complex than traditional constant volume
hoods with corresponding higher initial costs, they can provide considerable
energy
savings by reducing the total volume of conditioned air exhausted from the
laboratory. Since most hoods are operated the entire time a laboratory is
open, this can quickly add up to significant cost savings.
References
1. How to Select the Right Laboratory Hood System, Labconco Corporation, Kansas
City, MO. 2003.
Vince McLeod is a Certified Industrial Hygienist by the American Board
of Industrial Hygiene and the senior IH with the University of Florida’s
Environmental Health and Safety Division. He has 15 years of experience in
all facets of occupational health and safety and specializes in hazard evaluation
and exposure assessments.
Glenn Ketcham is a Certified Industrial Hygienist
with 20 years experience in the health and safety field. He is currently
the Risk Manager for the University of Florida. He has worked as a USDOL/OSHA
compliance
officer and has program management experience in general OSHA compliance,
laboratory and chemical safety, workplace ergonomics, loss prevention, disaster
preparedness,
and classical industrial hygiene.
