3 Energy-saving strategies for laboratory exhaust systems

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.

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. From 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 “clean.” 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 “clean,” 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.

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.

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