Topic: Laboratory Exhaust
How to Design Laboratory
Ventilation
by Dan Jore | March 9, 2021
The COVID-19 pandemic exposed the need for more
information in fighting contagious diseases. Research on its
causes, controls and cures spiked and continue still as the
world attempts to protect against such viruses. Of course,
laboratories work on a variety of projects in addition to
contagious diseases and all need varying levels of protection
to maintain a safe work environment. The need for safe
laboratory ventilation always exists, regardless of the type of
research conducted. However, questions remain when it
comes to specifying these ventilation systems, specifically
laboratory exhaust systems. What is the proper level of
protection to specify?
The answer is, it depends. A one-size-fits-all approach to
laboratory ventilation is not the answer as the level and type
of research done in laboratories varies. A high school lab
does not have the same requirements as a laboratory doing
research on infectious diseases. Fortunately, there is help.
The American Society of Heating, Refrigerating and AirConditioning Engineers (ASHRAE) have released a
publication that will help. Classification of Laboratory Design
Levels is a guide that addresses:
• Design and operation of laboratories
• Interactions with laboratory airflow control systems (LACSs)
that provide conditioning
• Control of environmental air quality in a laboratory
The classification guide breaks laboratories down into five
levels of increasing hazard (Table 1) and describes the type
of equipment and controls needed for each design level.
The laboratory ventilation design levels (LVDL) are:
Table 1
An illustration in the shows a typical lab (Figure 1) with
primary air control components and features. The illustration
provides a description of these items, why each is important,
and how these may be utilized at each design level.
Figure 1 – represents a simplified diagram of a laboratory and LACS showing critical
components. (Image courtesy of The American Society of Heating, Refrigerating and
Air-Conditioning Engineers. (ASHRAE))
ASHRAE’s guide covers the entire spectrum of ventilation for
laboratories. For simplification, let’s focus on items to
consider when designing laboratory exhaust system and the
LVDL. These include fan redundancy, protection of fan drive
components from the airstream, a bypass air plenum and
damper, drive type, fan discharge, and system height.
Number of Exhaust Fans – Fan Redundancy
Quantity of fans in an exhaust system depends on the
amount of air being exhausted, LVDL recommendation on
equipment redundancy and the results of a risk assessment
performed during the design process.
Single fan use in LVDL 0, 1 or 2 laboratories is acceptable
and if the laboratory risk assessment is a low. The use of a
single fan is often with dedicated exhaust to a single hood or
space.
ASHRAE LVDL 3 or 4 recommends equipment redundancy.
Redundancy provides a back-up fan in the event of a failure
to maintain containment in the laboratory. This
recommendation is an important consideration as the hazard
level increases along with the hazard potential of the work
being performed in the facility.
Multiple fans offer system redundancy and higher system
airflow capacities when multiple hoods are manifolded
together. In addition, multiple fans provide the control option
to stage fan operation, reducing operational costs when risk
levels are monitored. Ventilation rates for occupied or
unoccupied laboratories per ASHREA 62.1 increase with
LVDL or by the effectiveness. While safety is the primary
concern, different air change rates (ACH) rates present an
opportunity to save on operational costs through reduced
treatment to supply air and also exhaust fan energy saving
through staging fan operation.
Reference the Greenheck Fresh Air blog post on laboratory
exhaust redundancy for descriptions of the different types.
Airstream Protection
Personnel protection is at the center of the LDVL discussion.
The airborne discharge of chemicals or hazardous materials
from the laboratory poses a risk for individuals on the rooftop.
An aspect sometimes overlooked is the handling of exposed
airstream components when performing maintenance or
repairs. A fan system with a sealed airstream provides a
protective cover and gasketing for drive components, such as
the motor, shaft, bearings, and sheaves to protect against
contaminants, but still within the airstream. This design is
acceptable when the degree of hazard severity is between
negligible to moderate depending on the chemical. Service
may require coming in contact with parts sealing the drive
components. The alternative to a sealed airstream is an
isolated one. Drive components are completely outside of the
contaminated airstream. Two housing designs that feature an
isolated airstream are the scrolled centrifugal and bifurcated
housing designs, allowing for safer inspection or service by
maintenance personnel.
Appropriate personal protective equipment should be worn
when servicing a laboratory exhaust system.
Exhaust Fan Discharge
Exhaust discharge becomes more critical as the design level
increases. Three different discharge types are available that
address design level requirements, add to the height of the
system and produce a high-velocity discharge that helps in
the dispersion of the contaminated plume.
A high plume nozzle is an engineered cone producing higher
discharge velocities and increasing the exhaust effluent
plume height Effective plume height, which includes the fan
system height and exhaust effluent plume height, is important
in preventing contaminated exhaust effluent from being reentrained into the laboratory or adjacent buildings through
make-up air systems or windows.
A high plume dilution wind band combined with a highvelocity nozzle entrains outside air, diluting the exhaust
effluent and directs exhaust airstream up away from the
building. Dilution provides for lower contaminant
concentration levels downwind of the exhaust system.
A variable geometry nozzle maintains a high-discharge
velocity in a variable volume laboratory application without
passing air through the bypass air damper. Energy costs are
reduced by slowing the fan speed and not bypassing outside
air while still having a high discharge plume height.
Bypass Air Plenum and Damper
Dilution is a control strategy for all five LVDL categories. It
limits exposure risk inside the laboratory and also reduces the
contamination risk from the discharge exhaust effluent.
Dilution comes with the price of high ventilation rates, and
combined with laboratories often requiring 100% outside air, it
is a large operating expense. This expense is where an
exhaust system with a plenum and bypass air damper can
help.
As a system control strategy to reduce the operating costs
associated with dilution rates and high-level LVDL categories,
many laboratories utilize a variable volume exhaust system.
Variable air volume (VAV) laboratory systems save operation
costs by reducing the amount of conditioned air exhausted
during periods when high ventilation rates are not required.
The building’s laboratory exhaust requirements such as
effective plume height and dilution levels are maintained
during these periods utilizing unconditioned air, from outside
the building, to make up the difference with a bypass damper
and plenum. An exhaust system’s bypass damper(s) percent
open position is modulated to balance laboratory airflow rates
and also satisfy a constant airflow through the exhaust fan.
The VAV control strategy, including a plenum and bypass air
damper, reduces operating expenses and maintains
laboratory safety both inside and out.
System Height
System height, including stack height, is mentioned as a
component of modern laboratory control systems in the
ASHRAE guide. It is a safety consideration in laboratory
exhaust systems for all LVDL classifications to protect
maintenance personnel. Both NFPA 45 and the ANSI/AIHA
Z9.5 - Laboratory Design Guide, recommend a minimum of a
ten-foot-high system discharge height protecting individuals
on the roof deck from inhaling or contacting contaminated
exhaust effluent or particulates.
Drive Type
Fans used in laboratory exhaust utilize one of two drive
system types; belt or direct drive. All the LVDL categories
discuss redundancy and control strategies to lower operating
costs in laboratories. Lower operating costs also relates to
the choice of drive type utilized in the exhaust system.
Belt drive systems have a motor, fan drive shaft, highperformance bearings, and a set of belts and sheaves.
Recommendations call for belt drive exhaust system to utilize
a minimum of two belts with a 2.0 drive service factor. This
two-belt minimum provides for system redundancy and
reliability. Should a belt break, the remaining belts can handle
100% of the load without losing containment in the fume
hood.
Direct drive systems have fewer components for potential
failure and require less maintenance. Fans with a direct drive
system are more efficient, eliminating losses through the fan
belts, pulleys and fan shaft bearings. If paired with a variable
frequency drive (VFD) provides for potential operational cost
savings when adjusting the fan speed to match varying flows
from the laboratory.
Additional Considerations
Other features also play a role in determining the best
laboratory exhaust system with corresponding laboratory
LDVL and overall laboratory risk assessment. These include
footprint, overall height, sound, and weight.
Learn more about laboratory exhaust systems
here or redundant lab exhaust systems. You can also learn
more about LDVL by downloading the Classification of
Laboratory Design Levels. It is free to download on the
ASHRAE website or from TechStreet.
LABORATORY EXHAUST