Evaluating Alternative Fume Hood System Technologies Using Advanced Building Simulation

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1 Evaluating Alternative Fume Hood System Technologies Using Advanced Building Simulation William J. Kosik, P.E., C.E.M. 1 Karl Aveard, AIA Associate 2 Heather R. Beaudoin, P.E., C.E.M. 3 Erik L. Olsen 3 1 Director of Engineering, OWP/P, 111 W. Washington Street, Suite 2100, Chicago, Illinois, USA, Tel , Fax , wkosik@owpp.com 2 Director of Business Development, Earl Walls Associates, 5348 Carroll Canyon Rd., San Diego, CA, Tel , Fax , kaveard@ewalab.com 3 HVAC/Energy Engineer, OWP/P 1. INTRODUCTION Recent design improvements in laboratory fume present new opportunities to reduce the ongoing energy costs of a research facility, as well as reduce the first cost of heating, ventilation, and air-conditioning (HVAC) equipment. Because laboratory energy costs may be 4-5 times those of a typical office building (Mills et al. 1996), opportunities for energy savings merit exploration. The energy required to heat and cool make-up air for laboratory fume is a large portion of laboratory HVAC energy costs, so fume hood technologies that allow for reduced air exhaust volumes, while maintaining laboratory safety, provide a primary opportunity for energy savings. Other approaches not presented here may also be used to reduce ventilation energy requirements, such as reducing operating time and applying heat recovery. Because the energy required to condition outdoor air varies with climate, the energy savings will differ based on geographic location. This paper provides a high-level shoebox analysis of the energy savings that might be achieved by applying different fume hood technologies in several U.S. locations. This information may be used as a departure point to justify the application of these technologies to specific projects and show the need for further, more detailed analysis. The advanced building simulation tools that may be used for a more detailed analysis are discussed, and the work presented is a demonstration of the type of analysis that may be performed with these tools. 2. LABORATORY VENTILATION BACKGROUND 2.1 Modular Laboratory Planning The primary focus of laboratory design is safety for the laboratory occupants, followed by simplicity. Both of these needs are addressed by modular laboratory planning, which assembles a series of laboratories from individual fixed-dimension modules. Figure 1 shows a typical laboratory consisting of two modules; the X dimension is 11 feet and Y is 28 feet. This modular approach provides a consistent, safe width for laboratory circulation, while simplifying layout of the complex building services required in a laboratory.

2 2X Y Figure 1 Typical two-module laboratory Because most new laboratories are planned with this modular approach, the remainder of this work refers to a typical two-module laboratory, similar to that in Figure 1. Although specific laboratory requirements and layouts vary, this modular analysis is used to maintain simplicity and generality. 2.2 Factors Affecting Ventilation Rates Proper ventilation is an important aspect of laboratory safety. Most laboratory HVAC systems are designed with no air recirculation, to prevent the spread of pollutants released in individual laboratories throughout the building. Because of this, all air supplied to a laboratory is considered outdoor, or ventilation, air. The amount of ventilation air required may vary considerably between designs, but is typically determined by code requirements, hood exhaust requirements, or the cooling load. Each of these possible drivers is discussed below. Code Building codes typically specify ventilation rates in terms of minimum air changes per hour (ach). Requirements vary according to laboratory type and use, but range from 6-12 ach (ANSI/AIHA 2003). These requirements are generally higher than ventilation rates for other types of spaces; for example, the ventilation portion of air supplied to a typical office may be 1-2 ach. Hood Exhaust Fume must maintain a minimum exhaust rate in order to ensure proper pollutant containment. Ventilation air must be supplied to a laboratory in order to make up this air exhaust from the room. Depending on the type and number of used, this requirement may or may not drive the total laboratory ventilation requirement. Cooling Load Ventilation air also provides cooling to a laboratory to counter heat generated internally by people, lights, and equipment. In spaces with high cooling loads, this requirement may determine the total ventilation rate. In most cases, however, the code or hood exhaust requirements determine the ventilation rate. Energy savings can therefore be achieved if the ventilation rate can be reduced to the cooling load requirement. In addition, if the cooling load does not drive the ventilation rate, the air entering the space may need to be tempered to avoid overcooling.

3 2.3 Fume Hood Technologies Several different types of fume are available and used in laboratories today. The general goal of newer technologies is to reduce the hood exhaust rate while maintaining pollutant containment. The four basic hood types analyzed in this work are discussed below. Constant Air Volume Constant Air Volume (CAV) are conventional that have been the mainstay in laboratories. The exhaust air from these remains constant, and does not depend on sash height. This means that at lower sash heights, the face velocity across the hood increases. There are CAV Bypass available that keep a constant velocity across the hood face and bypass air through an opening above the sash face. Variable Air Volume Variable Air Volume (VAV) can vary the amount of exhaust air based on the sash position. This allows the face velocity across the sash to remain constant. This also allows for a reduction in exhaust air during many times of operation, leading to energy savings. This type of hood requires additional controls for the fan systems in order to vary the ventilation rate and maintain appropriate pressure relationships. Low-flow These designs provide a reduction in required exhaust volume resulting from the traditional requirement of 100 feet per minute (fpm) at the sash full open vertical position. As an example, one manufacturer designed a unique sash which can restrict the opening size. The reduced dimensions and limited opening size result in lowering air flows on their 72" fume hood from 1290 cubic feet per minute (cfm) to 350 cfm. Low Velocity These designs provide a reduction in required exhaust volume and provide equivalent containment with the sash full open vertically at 60 fpm or less when compared to fume running at 100 fpm with the sash full open position. Examples of these include manufacturer's products that incorporate design improvements like special airfoils, modulating rear baffles, supplemental assist fans, protective air curtains and more. 3. ADVANCED BUILDING SIMULATION Advance building simulation tools may play an important role in the design process when lowflow or low velocity fume are used, both for determining energy savings and demonstrating laboratory safety. Crawley (2003) defined building simulation as software which emulates the dynamic interaction of heat, light, mass (air and moisture), and sound within the building to predict its energy and environmental performance as it is exposed to climate, occupants, conditioning systems, and noise sources. A defining feature of advanced building simulation tools is that the engineering models used are of a complex nature such that they can only be solved with a computer, whereas other traditional engineering calculations can theoretically be solved by hand. Although development continues on simplified tools for use by the broader design community (Lehar et al. 2003), the current state of the art generally requires an expert user to ensure accurate results are obtained with building simulation tools. In addition, most experts will only accept results from rigorously developed programs that have been validated with standard sets of test data.

4 Many types of building simulation tools exist, and a complete discussion is beyond the scope of this paper. Two tools that are particularly suitable for laboratory design and will be discussed here are energy simulation and computational fluid dynamics. 3.1 Building Energy Simulation Energy simulation performs a dynamic simulation (time is the independent variable) of energy flows throughout a building, including the building envelope, room air, and HVAC systems, and the interaction between these. This type of modeling can be applied to predict the total energy consumption of a building, as well as the energy savings provided by energy conservation measures such as low-flow fume. One source of the complexity of energy simulation is the numerous inputs required, including weather data, building geometry and construction information, occupancy (internal load) information, and HVAC system information. However, an energy simulation also provides many outputs which can be used to guide the design process, including space and surface temperatures humidity levels, HVAC parameters (flow rates, temperatures, etc.), and component, system, and whole-building energy consumption. 3.2 Computational Fluid Dynamics Computational fluid dynamics (CFD) simulates the entire field of airflow throughout a space, including air speed, direction, and temperature. CFD is a widely accepted design tool that is used for analyzing fluid flow in engineering applications across many industries, not just buildings. In laboratories, CFD can be used to determine that the air distribution design provides the desired ventilation and airflow patterns, which may be especially important when ventilation rates are reduced below traditional levels. Information required to perform a CFD simulation includes room geometry, sources of heat and pollutants, and details of air inlets and outlets. CFD outputs include the distibution of air temperature and velocity, pollutant concentrations, and performance metrics such as thermal comfort and mean age of air. 4. MODELING ENERGY SAVINGS FROM REDUCED-FLOW HOODS 4.1 Methodology The use of reduced-flow hood technologies can lead to energy savings from various elements of an HVAC system. There can be savings in the reduction of exhaust and supply fan energy and savings from the energy used to heat and cool the air used as hood make-up. Fan system savings will depend on the size of the systems serving the building, the individual fan types and sizes, and the controls. To keep the analysis simple, only the energy costs for conditioning the make-up air are considered here. Because laboratories typically use 100% outside air, the cost of conditioning make-up air for fume can be large. Ten locations throughout the U.S. were chosen for analysis. The locations were chosen as representative of different climatic regions throughout the country with a significant amount of lab space. They include Atlanta, Baltimore, Boston, Boulder, Chicago, Houston, Phoenix, Pittsburgh,

5 San Diego, and Seattle. The energy costs at these locations also vary widely. Figure 2 below shows the average costs for electricity and natural gas in these locations, as reported by Energy Information Administratoin (2001a, 2001b). The combinations of the climates and energy costs determine the savings that can be achieved by using alternate fume hood technologies. In some locations the savings are much more dramatic than others. To demonstrate the savings from conditioning the air for fume hood make-up, a simplified model was produced. The model is a typical double module chemistry lab. The lab is 22 x 28 with the ceiling at 9 ½. There is 1.5 W/ft 2 of lighting. There are two 5 fume in the module. To simplify the analysis, the space is taken as an internal space with no exterior exposures. The only load in the space is internal load generated by people, equipment, and lights. The operating hours for the lab are 6:00 AM to 6:00 PM Monday through Friday (a total of 3120 hours), but the building is exhausted 24 hours a day. The airflow to the space and the outside air temperature and humidity are the parameters required to analyze the costs of conditioning the make-up air. Because this analysis focused on a single parameter and because of the large number of locations evaluated, a simplified bin method was used to calculate the energy costs. The bin data was used to calculate the cooling energy, heating energy, and reheat energy required to condition the airflow to the space. This method can quickly estimate savings, but for a real building the power of an energy simulation program should be utilized. An energy simulation program can be used to calculate the complex interaction between the building, building systems, and the outside environment. $0.14 $1.00 Electricity $0.12 Natural Gas $0.10 $0.90 $0.80 $0.70 $/kwh $0.08 $0.06 $0.60 $0.50 $0.40 $0.04 $0.02 $0.00 Atlanta Baltimore Boston Boulder Chicago Houston Phoenix Pittsburgh San Diego Seattle $/Therm $0.30 $0.20 $0.10 $0.00 Figure 2 Average energy costs for U.S. cities, Cases Studied As a basis for comparison, the energy costs for conditioning make-up air for four different lab airflow scenarios were calculated. Two of these airflows were variable and based on internal loads in the space that peaked at 5 W/ft 2 and 10 W/ft 2. The 5 W/ft 2 case represents a typical lab equipment load, while 10 W/ft 2 respresents a laboratory with more intense energy usage. To

6 calculate the energy costs an average airflow was calculated based on the load and occupancy profile of a typical lab. The other two scenarios were constant flows based on code-required airflows of 8 air changes per hour and 12 air changes per hour. The costs of conditioning airflows for 3 different types of were also calculated. For this analysis it was assumed that the code required airflow is 8 ach (780 cfm) and the peak equipment load is 5 W/ft 2. The first hood type is a Constant Air Volume (CAV) hood. There are two in the lab each exhausting 775 cfm for a total of 1550 cfm. These exhaust 1550 cfm, 24 hours a day. The next hood type analyzed is the Variable Air Volume (VAV) hood. The VAV can also exhaust 775 cfm each for a peak exhaust flow of 1550 cfm. The VAV hood, however, can vary its exhaust rate. Given the code-required airflow of 780 cfm for 8 ach, the VAV airflow can vary between 780 cfm to 1550 cfm. Hood exhaust of 1550 cfm would only be required for a few hours a day. When the building is unoccupied (a total of 5640 hours) the hood exhaust is reduced to 780 cfm. An hour weighted average of 960 cfm was used to determine energy costs in this analysis. The last type of hood analyzed is a low-flow hood. The low-flow hood maintains a constant exhaust of 400 cfm per hood for a total of 800 cfm. Figure 3 below compares the maximum, average, and minimum airflows for the different laboratory airflow scenarios. Ventilation Rate (cfm) Maximum Average Minimum Load Load 8 ach 12 ach 2 CAV 5 W/ft 2 10 W/ft 2 2 VAV 2 low-flow Figure 3 Laboratory airflows for seven laboratory airflow scenarios

7 4.3 Results The reduction in airflow possible with different fume varies depending on the ventilation rate that must be maintained in the space, either per code or based on the load. Figure 4 below compares the annual energy costs of the seven different laboratory airflow scenarios for the city of Pittsburgh. The energy costs for 2 CAV is over 25% higher than the energy costs for a lab with 12 ach. If the ventilation rate is reduced to 8 ach, or low-flow are used, the energy cost is more than halved. If the ventilation rate can be reduced to the smallest load requirement, energy cost savings of nearly 80% can be achieved. The energy costs for conditioning make-up air also vary depending on location. Figure 5 below compares the annual costs, on a square foot basis, of conditioning make-up air in each of the analyzed locations using the three different fume hood technologies. It is easy to see the savings possible with VAV or low-flow fume. In Houston a laboratory could save $2.00/ft 2 per year by using VAV instead of CAV, and $2.77/ft 2 with low-flow. In locations with less extreme cooling or heating requirements the savings are less. In Seattle the savings are $0.67/ ft 2 with VAV and $1.11/ft 2 with low-flow, which can still be significant in a large laboratory building. Generally, alternative fume are more expensive than traditional CAV, however over the life-cycle of the hood, the VAV, low-flow, and low velocity will cost less to own and operate. It is also possible to realize first cost savings with alternative fume. The reduction in airflow for the will generally result in smaller air supply and exhaust systems and smaller $4.50 Normalized Annual Energy Cost ($/ft 2 ) $4.00 $3.50 $3.00 $2.50 $2.00 $1.50 $1.00 $0.50 $0.00 Load Load 8 ach 12 ach 2 CAV 5 W/ft 2 10 W/ft 2 2 VAV 2 low-flow Figure 4 Normalized annual energy cost of conditioning make-up air for different laboratory airflow scenarios in Pittsburgh

8 Normalized Annual Energy Cost ($/ft 2 ) $6.00 $5.00 $4.00 $3.00 $2.00 $1.00 $ CAV 2 VAV 2 low-flow Atlanta Baltimore Boston Boulder Chicago Houston Phoenix Pittsburgh San Diego Seattle Figure 5 Normalized annual energy cost of conditioning make-up air for different fume hood technologies in U.S. cities plant equipment such as chillers and boilers. In addition, the simplicity of a constant volume, lowflow system generally results in a lower control system cost when compared to a VAV system. A whole-building energy simulation can be used to provide a more detailed evaluation of the energy and capital cost savings potential of a specific alternative fume hood system. 4.4 CFD Applications The results above demonstrate the value of reducing laboratory ventilation rates to provide energy savings. However, laboratory safety takes precedence over energy savings. Therefore, it may be necessary to perform a CFD analysis to demonstrate that sufficient ventilation is achieved at the reduced ventilation rate, particularly if the ventilation rate is reduced to that required by the load. Although a comparative study of the ventilation performance of different air distribution designs is beyond the scope of the present work, an analysis of a single design has been performed to demonstrate typical results and the types of conclusions that may be made. Figure 6 shows the configuration modeled to represent a typical two-module laboratory with two fume. The 22 x 28 laboratory includes two fume exhausting 775 cfm each. Three diffusers supply the room with a total of 1,575 cfm at 65 F. Several generic heat sources are distributed through the lab, with a total equipment load of 5 W/ft 2.

9 supply diffusers fume hood lights door undercut heat sources general exhaust fume hood (back side) Figure 6 Laboratory configuration for CFD model Figure 7 shows the path of massless particles emitting from the face of a large piece of equipment. There is a clear recirculation zone where the particles are repeatedly entrained in the diffuser jet and do not take a direct path to the fume hood or general exhaust. If this were a piece of process equipment emitting harmful fumes or particles, there could be a potential hazard to the laboratory occupants, and further analysis would be warranted, perhaps followed by a change in the air distribution design or relocation of the equipment. This example demonstrates the type of valuable design guidance CFD analysis can provide. recirculation zone Figure 7 particle source Three minute massless particle trace from face of large equipment

10 5. CONCLUSIONS New fume hood technologies with reduced exhaust rates provide significant opportunities for energy savings without compromising laboratory safety and warrant further investigation. Future studies may evaluate a broader set of laboratory types to provide general guidelines outlining the energy savings potential of various technologies. In addition, advanced building simulation tools such as energy simulation and CFD may be used to provide design input and evaluate energy savings and ventilation effectiveness for specific projects. 6. REFERENCES ANSI/AIHA Z American National Standard: Laboratory Ventilation. Crawley, D.B Building Simulation Process: Overview and Resources ASHRAE Winter Meeting, Chicago. Energy Information Administration. 2001a. Form EIA-861, Annual Electric Power Industry Report. Energy Information Administration. 2001b. Form EIA-176, Annual Report of Natural and Supplemental Gas Supply and Disposition. Lehar, M.A. and L.R. Glicksman A simulation tool for predicting the energy implications of advanced facades. MIT Building Technology Division, Cambridge, Massachusetts. Mills, E., et al Energy efficiency in California laboratory-type facilities. Lawrence Berkeley National Laboratory, LBNL-39061, Berkeley, California.