Basic Documentation

Page 4 of 14 Siemens Industry, Inc.
Document No. 149-488
indicating the effectiveness of their sash
manag
ement program.
6
Further, the fume hood controllers themselves can
be programmed to remind users to close the sash.
For example, an audible alarm chirps periodically if
the sash is half open for more than 15 minutes. The
BAS becomes part of a plan that helps people
improve and maintain their Green practices.
These are reasons to make sure the fume hood
controls are well integrated into the BAS of the
facility. Powerful and flexible data processing
capabilities give the right staff members the tools to
manage hood use. A BAS service provider can
maintain and adapt the information infrastructure
when the building changes or lab users alter their
activities.
Low Flow Hoods
For many years, most chemical fume hoods were
designed to operate with a face velocity of 100 fpm.
This value was almost an unwritten standard in
laboratory ventilation. Several years ago, fume hood
manufacturers started designing hoods to contain at
lower velocities, down to about 60 fpm. These hoods
have been called:
 Low flow fume hoods
 Low velocity hoods
 High performance hoods
Since the airflow rate through the hood is equal to
the velocity times the open face area, lower face
velocity means the hood uses less air. Some lab
designers have taken this airflow reduction as a
substitute for VAV hood control. Others have gone
farther, combining low velocity with a VAV or CV2
control system for greater flow reductions. To some
extent, low velocity and VAV controls each diminish
the effect of the other, but at least one analysis
found sufficient value to use them together.
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Regardless of the flow control strategy (VAV, CV2,
or constant) we need to control the flow through the
hood. This means we need a flow regulating device
and a sensor. We also need a display to tell the
hood user if the hood is working correctly.
6. U.S. Department of Energy, "Metrics and Bench Marks for
Energy Efficiency in Laboratories".
7. Victor A. Neuman, "VAV vs. low-flow: What saves more?"
R&D Magazine (February 16, 2007),
http://www.labdesignnews.com/LaboratoryDesign/LD0610FE
AT_1.asp (Accessed April 29, 2008).
Setting Ventilation Rate
Sometimes the airflow rate through a lab room is set
by the need to dilute and remove contaminated air.
This is often referred to as the minimum ventilation
rate. As a mechanical design parameter, it can have
a significant effect on equipment sizing and on
energy consumption day after day. Consequently, it
attracts the attention of HVAC designers and energy
managers. As a safety parameter, it is one of many
factors affecting the air quality experienced by
workers in the room. Consequently, it also attracts
the attention of Health and Safety officials and
representatives of the lab users.
A Green design team needs to press the questions
"How can we best protect workers?" and then “How
much air do we need to do it?” Posing the questions
in this way makes it possible to keep members from
various disciplines working together for answers.
The process can break down if the question
becomes “How can we justify lower air change
rates?” or a battle between safety and energy
issues.
This step is so important that the Labs21 group
made it a prerequisite (EA P4). Too often, the design
process fails to challenge presumed requirements.
Ventilation rates have been selected according to
the customary air change rate quoted by the safety
department, or institutional standards, or established
design practice at the firm. Compared to these
cookie cutter practices, the Green approach is likely
to produce a safer lab AND lower energy
consumption.
That’s because just asking “How do we know how
much air we need?” causes lab managers and
safety officers to think about what the hazards really
are, and range of tools to mitigate them. They take a
fresh look at what chemicals they’re using, and how
they handle, store, and distribute them. This gives
the ventilation designers the opportunity for better
thoughts about how and where they put air in and
out of the room. Placed in this context, the question
“How much air do we need?” elicits a better answer.
The Green design process may lead to the
conclusion that there are certain times or events that
call for more ventilation than others. Variable
ventilation rates, based on inputs from lab users or
measured air contamination can be a reasonable
response.
Today’s designs may incorporate air quality monitors
into the BAS. Normally, the room runs at a low
ventilation rate because the lab air is usually quite