3 Energy-saving strategies for laboratory exhaust
systems
Three ways to reduce energy consumption of laboratory
exhaust systems by at least 50%.
By Robert A. Valbracht,
Loren Cook Co., Springfield, Mo., Brad C. Cochran, CPP Wind Engineering
and Air Quality Consultants, Fort Collins, Colo. and Jeff D.
Reifschneider, CPP Wind Engineering and Air Quality Consultants, Fort
Collins, Colo. --
Consulting-Specifying Engineer, 10/1/2009 1:00:00
AM
There is a huge untapped energy saving potential in
our nation's research and teaching laboratories. According to the U.S.
Environmental Protection Agency (EPA), a typical laboratory consumes up
to 10 times the energy/sq ft of an office building, while specialized
laboratories may consume up to 100 times more energy.=B9 Due to the
requirements for high air change rates of 100% fresh air, the
ventilation system uses a high percentage of this energy (up to
80%).
A laboratory ventilation system can be broken down
into three parts: the fresh air supply, the conditioning components
(e.g., temperature, humidity, and filtration), and the exhaust system.
The fresh air supply and conditioning systems account for approximately
60% of the ventilation system energy consumption and have been the focus
of laboratory designers for the last several decades. Variable air
volume (VAV) air handling units have become the norm in laboratory
design to minimize airflow to match the building's ventilation demands,
which can vary throughout the day. Heat recovery systems are now also
the norm, particularly in northern climates, to reduce the energy
consumption of the conditioning systems.
Unfortunately, energy-saving opportunities for the exhaust
system are often overlooked, even though these systems account for the
remaining 40% of the ventilation system's energy consumption (and about
30% of the laboratory building's total energy
consumption).
This article examines three strategies that can safely
reduce the exhaust system's energy consumption by at least 50%. These
strategies can be used during either new construction or renovation of
existing laboratories.
APPLYING VAV TECHNOLOGY
Historically, laboratory buildings have used constant volume
(CV) exhaust ventilation systems, even when VAV systems are used on the
supply side. When the building ventilation requirements reduce the need
for supply air, bypass dampers are used to add additional airflow
through the exhaust system to keep the fans operating at full capacity.
When properly designed, a CV system will limit the concentration of the
exhaust plume that is re-entrained into a nearby air intake to safe
levels, but at the cost of high energy consumption.
Using
state-of-the art engineering techniques, controls, and exhaust fans,
exhaust ventilation systems can now optimize energy consumption by
applying VAV technology to the exhaust side. However, the VAV system
must be designed so that it does not compromise the air quality present
at nearby air intake locations or other sensitive locations. Building
exhaust may be re-entrained if existing CV systems are blindly converted
to VAV systems without a clear understanding of how the system will
perform at the lower volume flow rates.
In order
to safely employ a VAV system, one must understand the entire purpose of
the exhaust ventilation system. An exhaust system not only removes
contaminated laboratory air from the building, it also serves to
discharge the exhaust away from the building so that fumes do not
re-enter though air intakes or affect sensitive locations. This is
achieved through the proper combination of stack height and the accurate
calculation of exhaust discharge momentum. So how do you determine the
proper combination of stack height and exhaust discharge momentum? By
using an engineering technique called exhaust dispersion
modeling.
The preferred state-of-the-art method for conducting
an exhaust dispersion study is through the use of physical modeling in a
boundary-layer wind tunnel. Wind tunnel modeling is conducted by
releasing a precise amount of tracer gas from exhaust stacks on a scale
model of a laboratory building and measuring the exhausted tracer
concentrations at air intakes and other sensitive locations. Figure 1
shows an example of an exhaust dispersion study being conducted in a
wind tunnel.
Using information from the wind tunnel modeling, three
different strategies can be applied to maximize the energy-saving
potential of a VAV system.
STRATEGY ONE: STANDARD VAV EXHAUST
The first
design option is a standard VAV system where no bypass air is used and
the exhaust flow rate is based entirely on the building's airflow
demand. These systems must be designed so that safety is maintained at
the minimum volume flow rates. This typically involves either using
taller stacks or optimizing the placement of air intakes to minimize
re-entrainment of the exhaust. For a 50% turndown ratio, which can
typically be achieved during unoccupied hours, this might result in an
increase of 5 to 10 ft in the stack height. =46rom a controls
standpoint, this is likely the simplest system to use, particularly for
retrofit of existing laboratories, but it will produce only limited
energy savings.
STRATEGY TWO: VAV EXHAUST WITH WIND
SENSOR
The second design strategy involves connecting the
Building Automation System (BAS) to nearby wind speed/direction sensors.
The performance of an exhaust stack is impacted by the wind speed at the
top of the stack. For high-volume flow stacks there is a direct
relationship between downwind concentrations of the exhaust plume and
the local wind speed. As the wind speed increases, the plume rise
decreases, increasing downwind concentrations. For lower volume flow
stacks there is a critical wind speed that results in the maximum
downwind concentration (the wind speed that results in limited or no
plume rise). Similarly, when the wind is blowing from directions where
there are no sensitive receptor locations nearby, the volume flow rates
through the system can be reduced.
During a typical exhaust
dispersion study, the exhaust stacks are designed to achieve acceptable
plume concentrations at the critical wind speed and wind direction.
Thus, by definition, the systems are over-designed for all other wind
speed/wind direction combinations. When this design strategy is used,
the exhaust dispersion study is expanded to provide the minimum exhaust
flow rates as a function of the local wind conditions. The BAS
determines the current building loads and the minimum exhaust flow rate
based on the current wind conditions and then sets the exhaust volume
flow rate based on the larger of these two values (see Figure 2). To
ensure the reliability of the system, multiple wind speed/direction
sensors may be used and yearly calibrations should be
conducted.
STRATEGY THREE: VAV EXHAUST WITH IN-SITU
MONITOR
The third approach includes the use of a VAV system
with in-situ concentration measurements in the exhaust duct. When the
monitor does not detect any adverse chemicals in the exhaust stream (see
Figure 3), the exhaust system is allowed to operate at a reduced volume
flow rate. While there may be an increase in the plume concentrations at
the nearby air intakes, air quality will not be adversely impacted since
the exhaust plume is essentially =93clean.=94 When adverse chemical
concentrations are detected in the exhaust stream (see Figure 4), the
system increases the exhaust volume flow rate to achieve acceptable
levels at the air intakes.
When the plume is =93clean,=94 the volume flow rate
can typically be reduced by 50% to 75% and more closely correspond to
the building load.
Data collected at operating research laboratories with
in-situ monitors indicate that emission events that would trigger the
higher volume flow rate requirements typically occur no more than
approximately one hr/month. Thus, a typical system will be able to
operate without the need for bypass air more than 99.9% of the time,
resulting in significant energy savings.
The cost
for installing an in-situ monitoring system will be somewhat greater
than the wind speed/direction sensors, if the monitoring system is not
already used within the laboratory. If a monitoring system is already
installed, the additional cost to add sensors within the exhaust stream
is minimal.
ENERGY CONSUMPTION CASE STUDY
The
energy consumption for a typical laboratory was calculated for each of
the three VAV operating strategies described above along with a CV
system. The case study laboratory is configured with four exhaust stacks
operating at a maximum volume flow rate of 40,000 cfm each, and a
maximum building load of 120,000 cfm and a minimum turndown ratio of 50%
during off-hours. For the CV system this corresponds to an n+1 system
where only three of the four stacks are in operation. For the three VAV
scenarios all four stacks are used. (If one fan is down for maintenance,
the system can still operate at 100% load with just three of the four
stacks operating.)
Table 1 demonstrates the energy savings that can be
achieved for this case study laboratory. It is assumed that the standard
VAV system is designed to allow the volume flow rates to be reduced to
60% of full load (24,000 cfm/fan, 96,000 cfm for the system). For the
VAV systems with the wind sensors and with the in-situ monitors, the
minimum flow rates were set at 37.5% of full load (15,000 cfm/fan,
60,000 cfm for the system).
In general, the annual energy savings that one can
reasonably expect from employing a standard VAV system is approximately
$0.50/cfm of total exhaust flow. By adding either wind sensors or
in-situ monitors the savings can increase to around $0.75/cfm/yr. The
savings with the wind sensors will vary depending on the local wind
speed distribution, with greater savings for areas with lower-mean wind
speeds and less for those areas with higher-mean wind
speeds.
CONCLUSION
Laboratories possess a tremendous potential for energy
savings. As stated earlier, the exhaust system accounts for 30% of a
lab's total energy consumption. A properly applied VAV exhaust
ventilation system has the potential to reduce that total energy
consumption by 15%. When properly designed, a VAV system can provide
these savings without adversely impacting the air quality at downwind
air intake locations or sensitive locations. A wind tunnel-based exhaust
dispersion study will identify the specific energy-saving opportunities
available for a new or existing laboratory.
System
type
Annual energy
consumption
Annual cost (assumed
$0.12/kWh)
Table 1: Annual energy
consumption for a laboratory exhaust ventilation case
study.
Constant
volume
814 MW
hrs/yr
$122,200/yr
Standard VAV
321 MW
hrs/yr
$48,100/yr(20% system
turndown)
VAV w/ wind
sensors
200 MW
hrs/yr
$30,000/yr(up to a 50%
system turndown)
VAV w/
in-situ monitor
163 MW
hrs/yr
$24,400/yr(up to a 50%
system turndown)
Reference
1. U.S.
Environmental Protection Agency (EPA). =93An Introduction to Low-Energy
Design.=94 Laboratories for the 21st Century, U.S. EPA, Office of
Administration and Resources Management, DOE/GO-102000-1112, August
2000.
Author Information
ROBERT A.
VALBRACHT is vice president of engineering at Loren Cook Co., and chair
of the AMCA Standard 260 Committee. BRADC. COCHRAN is a senior associate
with CPP Wind Engineering and Air Quality Consultants and a member of
the AMCA Standard 260 Committee. JEFFD. REIFSCHNEIDER is a senior
engineer with CPP Wind Engineering and Air Quality
Consultants.
AMCA Standard
260-07
The Air Movement and Control Assn. International, Inc.
(AMCA) recently approved a new Test Standard and Certified Ratings
Program that can help assure the accurate calculation of exhaust
discharge momentum when using induced flow fans. AMCA Standard 260-07,
Laboratory Methods of Testing Induced Flow Fans for Rating, defines test
methods for determining the outlet airflow and velocity, both of which
are crucial to accurate momentum and plume rise
calculations.
