Energy-Saving Exhaust Systems

Exhaust Ventilation Systems Can Now Be Designed to Optimize Energy Consumption by Employing VAV Technology

In this day and age, there is a tremendous emphasis on energy conservation. Individuals talk about various ways to save energy (turning off lights, riding a bicycle, etc.), yet there is a huge amount of unrealized energy savings available in our nation’s research and teaching laboratories. A typical laboratory consumes up to 10 times the energy per square foot of an office building, while specialized laboratories may consume up to 100 times more energy.1 Due to the requirements for high air-change rates of 100 percent fresh air, a high percentage of this energy usage (up to 80 percent) is associated with the ventilation system. The ventilation of a laboratory can be broken down into three systems: the fresh air supply system, conditioning (temperature, humidity, filtration, etc.), and the exhaust system.

The fresh air and conditioning systems account for approximately 60 percent of the ventilation system energy consumption and have been the focus of laboratory designers for the past several decades. Variable air volume (VAV) air-handler units have become the norm in laboratory design to minimize airflow to match the building’s ventilation demands, which can vary throughout the day depending on the laboratory occupancy and the fume hood activity (when VAV fume hoods are installed). Heat recovery systems have also become the norm, particularly in northern climates, to reduce the energy consumption of the conditioning systems.

The exhaust system, which accounts for the other 40 percent of the ventilation system’s energy consumption, has often been overlooked when considering energy-saving strategies, even though it may account for about 30 percent of the laboratory building’s total energy consumption. The conventional wisdom has been that the exhaust system must operate at full load conditions 24 hours per day, 365 days per year.

This article will address three strategies that can be employed either during the design of a new laboratory or during the renovation of an existing laboratory to safely reduce the energy consumption of the exhaust system by at least 50 percent, which equates to a 15 percent reduction in the laboratory’s total energy use. To put this into perspective, using statistics provided by Laboratories for the 21st Century1, if half of all American laboratories reduced their energy consumption by 15 percent, this would result in an annual energy reduction of 42 trillion British thermal units. This is equivalent to the energy consumed by 420,000 households, $625 million, 9.5 million fewer tons of carbon dioxide emitted, removing 650,000 cars from U.S. highways, or saving 28 million trees from harvest.

Historically, laboratory buildings have utilized constant volume (CV) exhaust ventilation systems, even when VAV systems are employed 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 load conditions. For a conventional fan system this correlates to an exit velocity of at least 3000 fpm. If entrained flow exhaust stacks are installed, the exit velocities may exceed 6000 fpm. 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 now have the opportunity to be designed to optimize energy consumption by employing VAV technology on the exhaust side. A VAV system allows the airflow in the exhaust ventilation system to match (or nearly match) the supply airflow requirements of the building. However, care needs to be taken that the VAV system is designed so that it does not compromise the air quality present at nearby air intake locations or sensitive locations. This may occur 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, but it also serves to discharge the exhaust away from the building such that fumes do not reenter the building through air intakes or impact sensitive locations. This is achieved by the proper combination of stack height and exhaust discharge momentum. If a short stack height is used, a high exhaust discharge momentum is necessary to transport the exhaust safely away from the building (typical of an entrain flow exhaust system). Alternately, if the exhaust stack is taller, a smaller amount of exhaust discharge momentum is necessary to transport the exhaust safely away from the building. Since the exhaust discharge momentum is directly related to the energy consumption, a taller stack will always require less energy to safely discharge the contaminated laboratory air. So how do you determine the proper combination of stack height and exhaust discharge momentum? This is defined 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 an atmospheric 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 sensitive locations. An example of an exhaust dispersion study being conducted in a wind tunnel is shown in Figure 1. Additional information on conducting wind tunnel studies to evaluate the performance of exhaust systems can be found in the July 2005 ASHRAE Journal2 and in the Laboratories for the 21st Century’s best practice guideline3.

Figure 1. Example of an exhaust dispersion study being conducted in a wind tunnel.

Standard VAV exhaust system

Using the information from the wind tunnel modeling, three different strategies can be utilized to maximize the energysaving potential of a VAV system. The first is a standard VAV system where 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 taller stacks or optimizing the placement of air intakes to minimize re-entrainment of the exhaust. For a 50 percent turndown ratio, which can typically be achieved during unoccupied hours, this might result in an increase of 5 feet to 10 feet in the stack height. From a controls standpoint, this is likely the simplest system to employ, particularly for retrofit of existing laboratories.

VAV exhaust system 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 overdesigned 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. A flowchart for this system is shown in Figure 2. To ensure the reliability of the system, multiple wind speed/direction sensors may be used and yearly calibrations should be conducted.

Figure 2. Flowchart of the control strategy for a variable air volume exhaust system interconnected with local wind speed and wind direction measurements

VAV exhaust system 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, 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.” To ensure safe operating conditions, the maximum allowable concentration levels at the nearby air intakes are limited to a value of 1500 mg/m3 per g/s, as illustrated in Figure 3. This corresponds to the concentration limit prescribed by ANSI4 for the maximum concentration present at a manikin standing in front of the fume hood. Therefore, under the worst-case conditions, the concentration of a “clean” plume at a nearby air intake is no greater than the maximum allowed concentration of contaminated air present in front of the fume hood. When adverse chemical concentrations are detected in the exhaust stream, the system increases the exhaust volume flow rate to achieve the design criteria of 400 mg/m3 per g/s, as illustrated in Figure 4. The 400 mg/m3 per g/s design criteria is the standard that is typically applied to laboratory exhaust operating under worst-case conditions.

By relaxing the design criteria to 1500 mg/m3 per g/s when the plume is “clean,” the volume flow rate through a typical exhaust system can be reduced by 50 percent to 75 percent of the flow rate required to achieve the 400 mg/m3 per g/s. This means that if adverse chemical concentrations are not detected in the exhaust stream, the laboratory exhaust system can operate at flow rates corresponding to the building load.

Figure 3. Illustration of an exhaust system operating with an in situ monitor when the monitor does not detect adverse chemical concentrations in the exhaust stream

Figure 4. Illustration of an exhaust system operating with an in situ monitor when the monitor detects adverse chemical concentrations in the exhaust stream.

Data collected at operating research laboratories with an in situ monitor indicate that emission events that would trigger the higher-volume flow rate requirements typically occur no more than approximately one hour per month. Thus, a typical system will be able to operate without the need for bypass air more than 99.9 percent 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 savings

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 percent 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 percent load with just three of the four stacks operating). Table 1 demonstrates the energy savings that can be achieved for this case study. It is assumed that the standard VAV system is designed to allow the volume flow rates to be reduced to 60 percent of full load (24,000 cfm per 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 percent of full load (15,000 cfm per fan, 60,000 cfm for the system).

Table 1. Laboratory Exhaust Ventilation Case Study—Annual Energy Consumption

In general, the annual energy savings that one can reasonably expect when employing a standard VAV system is on the order of $0.50/cfm of total exhaust flow. By adding in either wind sensors or in situ monitors the savings can increase to around $0.75/cfm per year. The savings with the wind sensors will vary depending upon the local wind speed distribution, with greater savings being available 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. An energy savings of 15 percent or more is available through the use of VAV exhaust ventilation systems that are designed to minimize exhaust airflows to meet building demands. When properly designed, a VAV system can provide these savings without adversely impacting the air quality at downwind air intake locations or sensitive locations. The specific energy-saving opportunities that are available for a new or existing laboratory can be determined by conducting a wind tunnel–based exhaust dispersion study.

References

1EPA, “An Introduction to Low-Energy Design,” Laboratories for the 21st Century, U.S. Environmental Protection Agency, Office of Administration and Resources Management, DOE/ GO-102000-1112, August 2000.

2Carter, J., R. Petersen, and B.C. Cochran, “Specifying Exhaust Systems,” ASHRAE Journal, American Society of Heating, Refrigeration & Air Conditioning Engineers, Inc., Atlanta, GA, July 2005.

3EPA, “Best Practices: Modeling Exhaust Dispersion for Specifying Acceptable Exhaust/Intake Designs,” Laboratories for the 21st Century, Environmental Protection Agency, Office of Administration and Resources Management, DOE/GO- 102005-2104, May 2005.

4ANSI/AIHA, American National Standard for Laboratory Ventilation, Standard Z9.5-2003, 2003.