Basic Documentation

Siemens Industry, Inc. Page 7 of 14
Document No. 149-488
The online model may be the only approach
appli
cable to Venturi air valves. The cone is free to
move back and forth in the throat of the Venturi, so
the controller really doesn’t know where it is, and
can’t determine if the valve is fully open or not.
Given that duct pressure reset is still not widely
applied, it has been suggested that a good
implementation is eligible for Innovation in Design
points.
Exhaust Fan Power
In many laboratory HVAC systems, the energy
consumed by the exhaust fans is unaffected by
reductions in airflow through the building. That’s
because the exhaust system is designed not only to
remove air from the building, but to eject it in a way
that disperses the plume so it doesn’t blow into
another building. To accomplish this dispersion,
designers usually rely on a constant air velocity at
the top of the exhaust stack. When energy
conservation measures reduce the airflow out of a
laboratory, a bypass damper opens in the exhaust
system, pulling outdoor air through the fan and
sending it out the top of the stack. This uses much
less energy than conditioning the air and blowing it
through the building, but it presents an opportunity
for further conservation.
One solution is to apply multiple exhaust fans, each
with their own discharge stack. When a fan runs, it
blows enough air to disperse the plume. When
demand is low, some of the fans shut off completely;
the ones still running run at full flow. A staged fan
system still has a bypass damper, but it’s smaller,
smoothing out the flow between the fan stages.
Reliable control of a staged exhaust system can be
complicated, but the energy savings often justifies it.
The fact sometime overlooked in discussions of
exhaust dispersion is that performance depends on
the weather. High winds challenge the exhaust
plume, and can cause poor dispersion. Standard
practice is to design the stack for the worst expected
wind conditions and run it that way all the time. The
best practice guide from Labs21
13
raises the
possibility of adjusting operation dynamically
according to the weather. Such an approach
depends on careful analysis by exhaust dispersion
experts. Done well, it would certainly qualify for
Innovative Design points.
13. U.S. Department of Energy, Laboratories for the 21st
Century: Best Practice Guide, "Modeling Exhaust Dispersion
for Specifying Acceptable Exhaust/Intake Designs",
DOE/GO-102005-2104 (May 2005).
Commissioning
Commissioning is the process that turns the promise
of a Green design into the reality of a Green
building. Design is crucial, but until it’s executed it
accomplishes nothing.
Commissioning is so central to success that
fundamental commissioning is prerequisite (EA P1)
to a LEED-NC rating. ASHRAE’s proposed standard
(S189P) also calls for commissioning.
14
Table 3 lists specific commissioning tasks
particularly relevant to laboratory facilities.
The LEED prerequisite emphasizes installation and
acceptance tests to verify that the systems operate
as intended. They stress that the test requirements
be communicated in the contract documents. This is
the only practical way to incorporate them into the
construction process.
Experienced laboratory ventilation contractors are
familiar with the tests likely to be applied, and are
equipped to estimate and execute them, report the
results and follow up with any necessary corrective
action. The mechanical contractor, the BAS
contractor and the TAB agent need to coordinate
their activities with the commissioning agent for
effective commissioning. At a minimum (LEED
prerequisite) this is communicated in the contract
documents. In the best situation these contractors
contribute to commissioning plans during design.
They can offer practical input on test protocols and
work flow arrangements.
If the building operation staff also participates in the
acceptance test program, it gives them a tangible
feel for how the systems are supposed to perform.
They draw on this experience when they need to
analyze, troubleshoot, and maintain the building in
years to come.
Enhanced commissioning (LEED EA Credit 3)
extends the scope in both directions, back to the
design phase and forward to occupancy and
verification. This is consistent with the
commissioning concepts defined in the ASHRAE
Guidelines.
15
To succeed with this approach you
need partners who grasp systems, not just parts
suppliers.
14. ASHRAE, BSR/ASHRAE//USGBC/IESNA Standard 189P.
15. ASHRAE, Guideline 1-1996, "The HVAC Commissioning
Process".