September 30, 1995
Polyvinyl chloride (PVC) coated galvanized
addressed. The designer must comply with the
ducts may be used for fume hood exhaust in lieu
VA HVAC Design Manual requirements for sound
of stainless steel, except for perchloric hoods,
calculations. Sound attenuation in the ductwork
which still require welded stainless steel ducts.
should be provided, as required, to achieve
As required by OSHA, the exhaust air from fume
desired sound levels.
Maintain air velocity
hoods should be terminated 3000 mm (10 ft.)
through the exhaust ducts within the ASHRAE
above the roof level with a minimum discharge
recommended range of 5 to 6 m/s (1000 to 1200
velocity of 15 m/s (3000 fpm). (More information
fpm). Lower range of velocity is preferred to
on fume hoods is contained later in narrative
achieve desired sound levels. Specifications
discussion.)
shall reference compliance with the Associated
Air Balance Council or National Environmental
System Type:
The system may be either
Balancing Bureau testing and balancing
constant volume or variable air volume (VAV)
procedure for fume hoods.
based on life cycle cost analysis. Use of room
by-pass is generally preferred by users in lieu of
The 5 dB room attenuation credit should not
the integral hood by-pass. The use of room by-
be taken in hard surfaced rooms, as is the case
pass versus integral hood by-pass should be
in many laboratories. Noise levels of NC-50 to
verified with the user, and requirements in PG-
NC-55 are not uncommon in laboratories.
08-1 also should be verified and adjusted
Biohazard safety cabinets with their internal
accordingly to ensure the project specifications
HEPA filters and fans may have sound ratings of
reflect the user's requirements.
63 to 67 dB (approximately NC-60) and are
inherently noisy. Sound ratings for fume hoods
The constant volume system is less
are not published but are believed to be in the
complicated and more easily balanced to
range of NC-40 to NC-50.
Where it can be
maintain the necessary room negative pressure.
shown that the design noise level (NC-45) cannot
VAV systems should be designed for a minimum
be achieved, the designer may specify a higher
12 air changes per hour with a reduction to 6 air
value subject to VA approval.
changes per hour for reduced loads.
The
reduction to 6 air changes per hour should also
Local and Remote Alarms:
Coordinate with
be verified to match the hood exhaust
research personnel and engineering staff to
requirements. Controls for sash position and air
identify critical points to be monitored with
flow control valves should be of industrial grade
readouts and alarms at the Engineering Control
quality. The designer should note the following
Center (ECC) and/or local panels. For example,
ASHRAE statement, "The decision to select a
the temperature in controlled temperature rooms
variable volume exhaust system should not be
and the status of air flow for fume hoods should
made without the understanding and approval of
be continuously monitored, and a local audible
the research staff and local safety officials. The
and visible alarm, and alarm at the ECC for each
level of sophistication and ability of the
fume hood and biological safety cabinet should
maintenance staff to maintain such a complex
be provided. See individual guide plates under
system is also an important consideration."
Sections 4, 5, and 6 for other requirements that
must be met.
Future Capacity:
Refer to equipment sizing
Cooling Loads: Common instrument rooms and
criteria in the HVAC Design Manual.
ultralow freezer areas contain numerous heat
Shafts: Exhaust air from fume hoods, ducts
producing items which must be reflected in the
carrying flammable vapors, and piping shall not
cooling loads.
be installed in the same shaft used for movement
Air Distribution: The Microtome/Cutting Room air
of environmental air as per NFPA 90-A.
distribution system must be designed so that
Noise: Excessive noise and inadequate testing
there are no drafts at the cutting tables.
and balancing of the air system, especially the
fume hoods, are common problems that must be
Page 2-6
VA Design Guide -- Research Laboratory
Previous Page
