How Low Can You Go? Low Pressure Drop Laboratory Design Page: 3 of 11
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Ventilation Energy Use in Detail
The electrical power requirements of the ventilation system represents the combined supply and exhaust
fan power. Heating is provided by natural gas and is not represented in this graph. This fan power can be
estimated by the following equation:
Airflow in CFM x System air pressure drop, in inches w.g .
= Fan Power, in brake hp
6345, constants factor x Fan system efficiency, aflan x motor x ladrive
Reducing the energy consumed by a laboratory's ventilation system requires changing one or more of the
three variables in the equation above: Fan System Efficiency, Airflow, or System Pressure Drop. Table 1
offers a relative guide to the areas of opportunity to reduce the power requirement of a laboratory
ventilation system.
Table 1. Ventilation Energy Saving Potential
Typical Potential for
Ventilation Energy Savings
Parameter Versus Traditional Design Comment
Fan System Efficiency 5%-15% Minor potential, traditional
design often does okay.
Airflow 0%-60% VAV supply and exhaust systems
offer big fan and conditioning
energy savings over constant
flow; dependent on facility usage
(diversity).
System Pressure Drop 30%-65% Traditional design results in poor
laboratory systems; large
reductions are possible in
numerous areas.
Standard design practice usually results in a fan system efficiency of around 62%. Careful selection of a
direct drive fan and use of high efficiency motors can push that efficiency up to around 72%, at best
resulting in a power reduction of about 15%. While an important aspect of design, minimal opportunity is
available and, if energy efficiency is emphasized as important design criteria, conventional design
methods can optimize the fan efficiency.
The airflow through the system is typically set by the requirements of the facility, with three key design
decisions separating typical laboratory facilities from energy efficient ones. The largest design decision is
to use a variable flow exhaust system, rather than a constant flow system. Varying the supply and exhaust
flows based on actual usage immediately captures the significant savings possible from reducing the flow.
With a variable volume system, diversity in the exhaust load typically results in the majority of system
operation at a 25% or greater reduction in airflow. For the same ductwork size and similar fans, reducing
the airflow reduces the fan power by approximately the cube (a simplified fan law relationship). Using
the cube relationship, a 25% reduction of airflow results in about a 58% reduction in the fan power
required. In practice, the fan energy savings are slightly lower, but the fan law suggested savings are a
reasonable first estimate of the magnitude of the ventilation energy reduction potential. Additional
energy savings are realized by a 25% reduction in the amount of air that is conditioned.
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Weale, John; Rumsey, Peter; Sartor, Dale & Lock, Lee Eng. How Low Can You Go? Low Pressure Drop Laboratory Design, report, December 1, 2001; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc738853/m1/3/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.