The concept of maintaining a minimum air
change rate in labs has been around a long time,
and speaks to the need to maintain adequate
dilution of any potential pollutant released in the
lab. The term air changes per hour (ACH) is often
misused. It’s sometimes listed as a constant number on room data sheets, which imply the ACH
for a given room is constant in all operational
states. With the advent of variable-volume airflow
control, it’s important to establish the minimum
ACH for each space. This is a value agreed upon
with users, operations and EH&S groups during
the programming phase, and is often based on
standard practice at that particular institution.
This minimum ACH value is meant to maintain
a safe dilution rate in the room, even when there’s
no thermal or direct exhaust load requiring
greater airflows. For variable-volume systems,
the maximum ACH is calculated room-by-room
based on internal heat gains, humidification
control requirements, direct exhaust loads and
building envelope effects.
There’s no direct code guidance on minimum
ACH other than for special chemical, cryogen
or other hazardous storage rooms as defined by
building and fire codes. For most current labs,
the minimum ACH typically falls within a range
of 4 to 10 ACH; and the actual selection of this
number is typically based on benchmarking the
practices and standards of other institutions.
POLLUTANT SENSING AND DCV
The concept of demand-controlled ventilation
(DCV) for labs has gained much traction recently. The idea behind DCV is to establish an unoccupied minimum ACH value that can be applied
when labs aren’t occupied to reduce the amount
of fresh air conditioning required to maintain
a base level of ventilation. By sensing a range of
common pollutants within a space, as long as the
threshold values for these pollutants aren’t triggered, the minimum ACH can be maintained at
a preset lower limit value, typically 2 to 4 ACH.
In some cases, such as animal labs where the
pollutant(s) of concern are well established,
this scheme works well and is often accept-
ed by EH&S and facilities groups. In other
cases, such as general chemistry labs, getting
agreement on which pollutants are likely to be
present can hinder general acceptance of this
technology. The pollutant sensing limits of the
technology can also be a prohibiting factor, as
not all chemicals can be readily detected.
LAB EXHAUST ENERGY RECOVERY
It has become common practice to apply
heat- and energy-recovery systems to lab outdoor air-conditioning systems, particularly in
the case of once-through air systems. Generally
speaking, there are two types of systems: sensible heat recovery, which relies on the temperature difference between the exhaust and fresh
airstreams; and total energy recovery, which
includes both sensible heat recovery, as well as
latent (moisture) heat exchange.
Sensible heat recovery, usually in the form
of glycol water loops, heat pipes or air-to-air
devices, work well where indoor/outdoor
temperature differences are large, such as cold
weather regions. They also don’t allow any
direct contact between the airstreams, which
eliminates any concerns related to contamination of incoming air by exhaust pollutants.
Total energy recovery is typically in the form
of an enthalpy wheel containing a medium
which absorbs and releases both sensible heat as
well as the latent heat associated with moisture
in the airstream. These devices have a minimal
potential for cross-contamination, as well as
sensitivity to certain chemicals. Generally, total
energy devices have higher heat-exchange effectiveness than sensible devices, and are superior
at heat exchange in warm, humid climates.
Early discussion with both EH&S and facilities management on the best approach for a
given project is key to finding the right balance between energy efficiency and safety.
The design of any lab involves a complex
interaction of numerous technical and management stakeholders. The process of setting
project requirements and engaging stakeholder
involvement is fundamental to the success of
these projects. When it comes to setting energy-efficiency goals for a lab project, it’s important to rigorously assess the safety risks associated
with the measures needed to achieve these goals.
This can only be done through a collaboration of
design and operational professionals starting in
the early planning stages of the project.
Josh Yacknowitz, PE, associate principal, leads
the Science and Industry Business for Arup in the
Americas. He has been involved in lab facilities
design, both as a mechanical engineer and interdisciplinary design project manager.
Pushing energy-efficient lab
continued from page 8
ed to first cost, operating cost, maintainability
and constructability. The important common
caution here is they all tend to add first cost and
installation complexity, and need a good deal
of care when it comes to selection, coordination
and design. These approaches require early
attention during the decision stage of planning
by both EH&S and facilities, as well as early
constructability input from a CM if available.
In any lab with a significant number of fume
hoods, the specification and selection of these
devices has a major influence on the MEP systems design. A number of safety and operational
issues must be understood first, including the face
velocity performance standard to be applied, the
face velocity control strategy and the relationship
between fume hood and lab general exhaust.
Typically, fume hoods are manifolded together within a lab control zone and are capable of
variable-volume control, allowing the central lab
exhaust systems to operate in a variable-volume
mode. This allows the outside make-up air volume to similarly vary, reducing fan and thermal
energy usage. Traditional hoods normally operate
in the 80 to 120 fpm face velocity range, which has
been an accepted standard for safe containment
of pollutants for many years. High-performance
hoods are certified by manufacturers to operate
as low as 60 fpm safely. While high-performance
hoods have become more widely accepted, it’s
still an issue that must be agreed upon with the
owner’s EH&S group, as not all institutions make
the mental progression to go with the lower face
velocity. In addition, local fire authorities may
have requirements for regular testing and calibration of hood airflows, and the calibration criteria
needs to be agreed upon early in design.
; ALTERNATIVES TO FLAME-RETARDANT BUILDING MATERIALS
Perkins+Will has released new research in response to the need for architects
and interior designers to develop a better understanding of flame retardants and
their impact on health. Flame retardants in the built environment are associated
with a range of health impacts including cancer, endocrine disruption and neuro-developmental problems. Many flame retardants are persistent, bioaccumulative
and/or toxic, and their use in buildings is largely avoidable. The white paper, titled
“Healthy Environments: Strategies for Avoiding Flame Retardants in the Built Environment” and developed by Perkins+Will’s Healthy Materials Group and Science
Fellow Michel Dedeo, identifies both new and existing opportunities to design
healthier buildings without compromising fire safety or code compliance.