|
Location of the Fume Hood is of prime importance, since a variety of conditions
will effect the performance of the Hood. When selecting a location for the
Fume Hood, operator convenience, work flow and exhaust duct locations should
all be considered. In addition, an attempt should be made to keep the hood
away from the line of traffic. It is both inconvenient and dangerous to install
a Fume Hood so that the operator is forced to work in the line of traffic movement.
The presence of cross-drafts will adversely effect the performance or the Fume
Hood. For this reason it is a good idea not to locate the Fume Hood near open
doors and windows. High velocity air diffusers located directly in front of
the Fume Hood could actually draw fumes out of the Hood and into the room.
Although there is no single preferred method for delivery of make up air, it
is a good idea to locate the Fume Hood as far from the air diffuser as possible
so that the air first sweeps through the laboratory working area and then into
the Fume Hood.
Ducts are excellent conductors of sound.
Anchoring the ductwork securely to the building structure
and sizing the ductwork so as not to exceed velocities
in excess of 2000 FPM will help to reduce duct noise.
The exhaust discharge should be located
at least 10' above the roof structure and have a
zero pressure weather cap. Covered weather caps that
increase static pressure and interfere with the flow
of discharged air are not recommended.
As a rule, each Fume Hood should be
exhausted by its own exhaust blower, as shown in
Figure 3. When multiple Hoods are manifolded to a
single exhaust blower, as shown in Figure 4, imbalance
in the exhaust airflow can occur as Hoods are turned
on and off. To overcome this situation, all Hoods
could be turned on and off together. However, this
may result in an unsatisfactory arrangement from
the standpoint of energy efficiency.
Multiple Hoods which are controlled
by a variable air volume (VAV) control system may
be manifolded to a single exhaust blower. The VAV
controls maintain the proper air balance and provide
excellent energy efficiency, by reducing the amount
of air being exhausted when the Hood is not in use.
Care should be taken in the design
of the VAV system. The air being supplied to the
laboratory must also be variable to meet the changing
exhaust volume of the Fume Hoods. If not, over pressurization
of the laboratory could occur causing contamination
of adjacent areas.
Because the exhaust volume of the VAV
system can vary greatly, it is a good idea to select
an exhaust blower with some type of air bypass, as
shown in Figure 5. This will keep the fan motor from
overloading and maintain an acceptable stack velocity.
Fume Hood blowers and ductwork systems
should be included in the maintenance schedule. Dust
and other contaminants collected on the blades of
the fan can greatly reduce its capacity. Blowers
should be placed so that they are accessible for
periodic cleaning. Periodic examination of the ductwork
should also be made to detect any loose joints or
porosity due to corrosion.
Airfoil Bypass Hoods
The Airfoil Bypass Hoods have a double bypass to
provide constant exhaust volume and limit changes
in face velocity.
The upper bypass functions automatically
with the raising and lowering of the sash. The lower
bypass provides a continuous air sweep of the work
surface.
When hood top enclosures are used,
a bypass grille must be specified
Airfoil Variable Air Volume Hoods
The Airfoil Variable Air Volume Hoods (VAV) are designed to be used with exhaust
control systems provided by other manufacturers. In lieu of an upper bypass
the VAV Hood has a lintel which restricts the air intake of the Hood to the
sash opening and lower bypass.
The commercially available exhaust
control system will detect the movement of the sash
and adjust the volume of air required to maintain
a constant face velocity.
Factory mounting of the exhaust controls
is available when specified.
Airfoil Add Air Hoods
The Airfoil Add Air Hoods provide up to 70% of the Hood exhaust requirements.
With a bypass similar to the Airfoil Bypass Hood, outside air is introduced
through the add air plenum, thus minimizing the amount of conditioned room
air required.
When outside temperatures are
extreme, it may be necessary to temper the incoming
air.
|
