Exhaust Stack Discharge Velocity Reduction Study for Labs21 2009
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The document summarizes a study conducted at the University of California, Irvine to safely reduce energy usage in laboratory building exhaust systems through techniques like increased stack heights, variable speed fans, and lowered minimum exhaust flow rates. Wind tunnel testing of exhaust stack designs showed energy savings could be achieved without compromising safety. Retrofitting three campus buildings with these methods resulted in estimated annual energy savings of $61,000 or more and payback periods of 1.6 to 5.3 years.
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Exhaust Stack Discharge Velocity Reduction Study for Labs21 2009
- 1. Safely Achievable Reductions in Exhaust Fan Energy in Laboratory Buildings Chet Wisner, President, Ambient Air Technologies Jay Hayashi, P2S Engineering Marc Gomez, Director, UCI EH&S Fred Bockmiller, Principal Engineer, UCI Facilities Chris Abbamonto, Energy Manager, UCI Facilities
- 3. Campus Energy $avings Challenge Recipe for Success Safety Management Visionary & Supportive Upper Management Engineers Facility Managers Patience Team Synergy Supportive Users / Researchers
- 5. Can We Build “Smart Labs” that Greatly Reduce Energy Use?
- 7. Smart Laboratory Concept Balancing Laboratory Safety and Climate Safety Create lab buildings that out perform ASHRAE 90.1 / CA Title 24 by 40-50%. Combine energy initiatives such as centralized demand controlled ventilation (CDCV), low flow (high performance) fume hoods, reduced building exhaust stack airspeeds, and use of energy-efficient lighting. Building Exhaust System Labs w/CDCV real time lab air monitoring 4 ach occupied 2 ach unoccupied Energy efficient lighting Labs with low flow fume hoods (as appropriate )
- 8. Centralized Demand Controlled Ventilation (CDCV) Utilizing real time lab air monitoring, reduce air changes in labs from approximately 6 ACH to 4 ACH while the lab is occupied and 2 ACH when lab is unoccupied.
- 9. Low Flow (High Performance) Fume Hoods Utilize fume hoods that are designed to operate safely at lower face velocities, i.e., 70 FPM rather than 100 FPM. Exhaust plenum Deeper work surface Unique airfoil design Advanced baffle design
- 10. Laboratory Lighting Controls Reduce Power Density by 50% Lab Area LPD from 1.1 to 0.55 Lab Prep LPD from 0.9 to 0.36 Prep Room LPD from 2.0 to 1.0 Corridor LPD from 0.6 to 0.3 - Daylight sensors for fixtures near windows - Occupancy sensing by lab bay
- 11. Lab Building Exhaust Fan Energy Reduction Building Exhaust System Slightly higher stacks Variable speed fans (wind responsive if necessary) Air handler with fresh air intake
- 13. Can We Reduce Lab Building Exhaust Discharge Rates & Achieve Real Energy Savings Without Compromising Safety? This Initiative
- 15. Lab Exhaust Diagram Animated Wind Exhaust Fan Bypass Damper Plenum Fume Hood Supply Fan Duct Balcony Re-Entrainment of Contaminated Air
- 30. Typical Timeline of Exit Velocity Requirements Typical Design 1% Design Required by Dispersion Required by High Air Change Rate Required by Low Air Change Rate
- 39. Thank You!
- 43. 2 3 = 8 a Energy Proportional to Velocity Cubed
- 48. BYPASS
- 49. NOT MANIFOLDED
- 50. MANIFOLDED
Editor's Notes
- MG D EH&S UCI Thanks for letting us share our…
- UC Irvine is a growing…
- We are constantly challenging ourselves to save energy. Our recipe for taking on these ambitious projects is to have a multi-disciplinary team of professionals focused on a common goal.
- This is our agenda for today. Lets take a few minutes to overview our Smart Lab concept
- So,
- The great news is that energy efficient labs are evolving in the right direction.
- Our goal at UC Irvine is to outperform ASHRAE 90.1 or CA Title 24 by 40 to 50%. We have a poster session on our Smart Lab concept and hope you have stopped by or stop by today so we can brainstorm together on this concept.
- We are attempting to do this utilizing CDCV with real time lab air monitoring…
- We think low flow fume hoods are also a part of the solution
- We are reducing our lab lighting power density by 50%
- Which brings us to this study…
- We want to take a few minutes to share our Smart Lab concept
- Wendell This study asks the question… The overarching principal in this study is to maintain a safe environment for occupants of the laboratory building, for occupants of neighboring buildings, roof workers and others in the nearby campus environment. Safety is the primary purpose of a laboratory exhaust system. And that purpose must not be compromised by actions taken to improve energy efficiency. With that said, our goal is to find the sweet spot where we balance safety and energy savings
- Wendell The reason we performed this study is we thought…
- Wendell Discuss components of system… Note the plumes and effects on dispersion of short/tall stacks and low/high exit velocities. So, we would likely select a taller stack for energy reduction which allows us to reduce the fan speed and exit velocity.
- We want to take a few minutes to share our Smart Lab concept
- Selection process considered a large array of building characteristics… Exhaust stack parameters including stack height, exit velocity, flow per stack, and momentum flux Additional considerations included… Whether the building utilized a variable air volume or constant air volume HVAC design Whether the laboratory design was supported by wind tunnel testing of exhaust dispersion
- Taken together, these three laboratory buildings provide a representative group, allowing reasonable extrapolation of the results that is reported here to other laboratory buildings on the campus, other UC campuses and potentially other California campuses.
- These three buildings were selected based on the following criteria: Representative Campus Environments Locations of the selected buildings are representative of two different types of surrounding campus environments, mature region which includes numerous laboratory buildings and significant topographic relief, and a newer region of campus which does not have buildings on one side and contains relatively flat terrain. Representative Exhaust Stack Parameters The exhaust stack parameters of the selected buildings are representative of the general lab building population Significant Lab Activity Each selected building houses a significant level of laboratory activity. Induced-Air Fans I nduced-air lab fans is a type of fan that has been used in a number of recent laboratory exhaust system renovations on the UCI campus and is a popular design option for labs on other campuses as well. Curved Metal Roofs is an important design feature repeated with variations on other laboratory buildings on the UCI campus. Manifolded Exhaust Another design feature that is repeated on the UCI campus. With this configuration it also provides enhanced in-stack dilution of hazardous releases and provides enhanced momentum flux to support larger plume rise and better atmospheric dispersion of the exhaust plumes. Modern Design Approach All three buildings are representative of modern design approaches.
- Chet starts here.
- Chet or Jay Show the plumes and effects on dispersion of short/tall stacks and low/high exit velocities. So, we would likely select a taller stack for energy reduction which allows us to reduce the fan speed and exit velocity.
- Whenever we reduce the exit velocities, we lose some safety margin. This can be offset in some cases through other measures such as raising stack heights, clustering stacks, etc., however, the tendency overall is to remove some of the safety margin. As a result, we must Be more comprehensive in the wind tunnel testing – including such things as taking care to ensure enough receptors are installed on roofs. Perform field commissioning of the exhaust plume dispersion of the renovated system. This provides an overall test of the safety and energy efficiency of the system. Educate the lab users in proper use of fume hoods and other exposure control devices which are used in the labs. Particularly, they should know how much dilution they can count on from the lab hazardous exhaust system and how to relate that to the activities they are performing in fume hoods.
- Wind tunnel testing is based on fluid dynamic similarity modeling—like wind tunnel tests of aircraft wings. Once the test is properly set up, we can measure in the wind tunnel almost anything related to wind and multiply it by the appropriate scale factor to determine the full-scale real-world value. The dilution of exhaust plumes is one of the wind-related parameters which can be well modeled in the wind tunnel. The wind tunnel is the only means available today to accurately model wind flow and related parameters around buildings. If you are considering using alternative methods such as desktop calculations or Computational Fluid Dynamics for this purpose, please talk to me off-line. I can provide references to articles in the technical literature which clearly describe the shortcomings of non-wind-tunnel approaches. The first step in setting up a wind tunnel test is to build an accurate scale model of the building under study and the buildings, terrain and trees around it. You need to extend the model out about ¼ mile in all directions to take account of the effects of upwind buildings, etc on the wind and turbulence at the building under study. Here we see the model used in this study for the Biological Sciences 3 and Natural Sciences 1 lab buildings. They are the blue buildings in the center. The model is at a scale of 1:200 and includes terrain and trees as well as buildings for ¼ mile radius. The accuracy of the model is directly related to the accuracy of the results. So it is important to put sufficient effort into recreating a model which accurately represents the existing buildings as well as those which are expected to be built within the next few years. The model is mounted on a turntable will is rotated by the wind tunnel operator to model different wind directions. In this photo, you are looking upwind and can see the wood blocks which are used to create the proper profile of wind speed and turbulence in the air approaching the test model—more about that a little later.
- Brass tubes are installed in the model to represent the exhaust stacks. The flow through these stacks must be properly scaled to produce accurate dispersion results.
- Tubes are installed in the model to take air samples at locations where the concentration of the exhaust plume might be important. The jargon term for these points is “receptors.” The locations are indicated by the numbered arrows. Note that some of the receptors use blue plastic fixtures which are designed to take their sample at a height of about 5 feet full scale. This is representative of the breathing zone for the average person.
- The air approaching the model must reproduce the vertical profile of wind speed and turbulence found in the real atmosphere. These profiles are the result of the eddies caused at the ground by obstacles to the flow—buildings, trees, vehicles, etc. We simulate these in the wind tunnel using an array of blocks on the floor of the wind tunnel upwind of the model. This part of the wind tunnel is called the approach section and must be long enough to allow an equilibrium profile (boundary layer) to be established in the flow before it reaches the test model. Longer approach sections allow more well-developed profiles. The diagram shows the AAT wind tunnel layout with 120 feet of approach section upwind of the test section. Here is a picture of the AAT wind tunnel showing the approach and test sections. The vertical profiles of wind speed and turbulence are measured to ensure they reasonably match those in the real atmosphere—as shown in these graphs.
- The data for each receptor/stack combination was taken for each wind direction of consequence for all wind speeds. These were plotted as shown in the graph. The blue line is for a 7.2-foot stack and the magenta line is for a 12.2-foot stack. Note 1) the taller stack produces smaller concentrations and 2) the maximum concentration occurs at the “critical wind speed.” Below this speed the plume rises higher leading to lower concentrations. Above this speed, the dilution cause by mixing with the greater amount of air passing by the stack reduces the concentrations. Both of these effects are present for all wind speeds. There effects add to produce the resulting curve. In a typical design project, the wind tunnel is used to find the maximum concentration up to the critical wind speed or the 1-% wind speed, which ever is higher. The 1-% wind speed is the 99 th -percentile speed at the site. We took the more complete set of data for this project to facilitate evaluation of design alternatives.
- The first step (which has some major ramifications later on) is to establish the “design criteria.” This specifies the minimum acceptable dilution for the exhaust plumes. It is most convenient to specify this in terms of “normalized concentration” because the normalized concentration does not change if the exhaust flow from the stack changes. The normalized concentration is simply the concentration of the pollutant at a receptor (µg m -3 ) divided by the emission rate of the pollutant from the exhaust stack (g s -1 ). This is solely a function of the plume dispersion. The ASHRAE recommendation for the design criteria for lab exhaust is 400 (µg m -3 )/ (g s -1 ). It generally provides a good starting point when little is known about the future activity in the lab—the most common situation. ANSI Z9.5 provides an alternative recommendation of 750 (µg m -3 )/ (g s -1 ). This is based on the idea that the lab exhaust system should ensure at least as much protection as is afforded a lab worker using a fume hood. We prefer the more conservative ASHRAE standard partly since it seems inappropriate to expose the general population, who may not have made a career decision to work in a lab, to the same level of pollution as a lab worker who supposedly knows and has some control over the risks. The recommended standards should serve only as a starting point. As more is known about the activities (existing and future) in a lab, more appropriate design criteria can be developed. A few consultants provide the service of evaluating the lab activities to develop an appropriate exhaust dispersion design criteria. Tom Smith at Exposure Control Technologies provided that service for this study. ECT can also provide consulting to lower the energy use inside the lab while maintaining or improving the safety of workers.
- We want to take a few minutes to share our Smart Lab concept
- Most of the time, the exhaust fans are running much faster (higher exit velocity) than is actually required. Here is a reasonably typical hour-by-hour look at the flow (CFM) required to meet plume dispersion requirements for a lab exhaust (Required by Dispersion). The hour-by-hour variation is due to variations in the wind speed and direction at the building. The spike at 1600 hours (4 pm) could be the result of a cold front passing over the building and producing high winds for a short time. (Required by High Air Change Rate) This curve shows the exhaust flow required to support a high air change rate in the imaginary building. You can see that at times the flow required to maintain the air change rate determines the required stack flow—other times the plume dispersion requirements dominate. (Required by Low Air Change Rate) This curve demonstrates the potential effect of lowering the air change rate in the building. The relative magnitude of these three curves can, of course, vary dramatically from building to building, but this chart serves well for demonstration. (1% Design) The blue line shows the continuous air flow that would be specified if the 1% wind speed was accurately accounted for and happened to occur on the day we’ve plotted. More commonly, the design (Typical Design) incorporates additional conservatism as represented by the orange curve. The static system designs (constant exit velocities) discussed here are best represented by the blue line—the 1% design. Using dynamic control systems to adjust the exit velocities in response to the wind speed and direction, the flow required can be reduced to the highest of the three lines at the bottom of the chart—the highest of the red, black or gray lines.
- NOTE – General Laboratory Savings are limited by the minimum exhaust flow required to obtain acceptable plume dispersion. Reducing the minimum air exhaust rate below 6 ACH will not affect these results. However, reducing the minimum air change rate will result in a smaller quantity of air serviced by the supply fans. NOTE – BSL 3 Lab Savings are limited by the minimum exhaust flow required to obtain acceptable plume dispersion. Reducing the minimum air exhaust rate below 6 ACH will not affect these results. However, reducing the minimum air change rate will result in a smaller quantity of air that needs to be conditioned for use in the building. The resulting savings in energy require to condition that air could be substantial.
- In order to ensure the safety of persons on the Natural Sciences 2 rooftop terrace and those on the Natural Sciences 1 roof and also save energy and on-going utility costs. NOTE: savings are limited by the minimum exhaust flow required to obtain acceptable plume dispersion. Reducing the minimum air exhaust rate below 6 ACH will not affect these results. However, reducing the minimum air change rate will result in a smaller quantity of air serviced by the AHU supply fans.
- We want to take a few minutes to share our Smart Lab concept
- As was discussed earlier on in the presentation: Establish design criteria/performance guidelines for the design and use of laboratory exhaust systems Wind tunnel screening study to prioritize the other existing laboratory buildings that can benefit from this type of study New and renovated designs should be audited to ensure consistency with wind tunnel results Field commissioning studies should be performed on new and renovated exhaust systems Wind tunnel firms need to install a sufficient number of receptors on the roofs of test models to ensure detecting maximum concentrations and should submit model construction drawings.
- Jay-
- The goal of our smart labs project is to find the sweet spot where we maximize energy savings without compromising safety. At this point all turn it over to Chris Abbamanto.
- Based on the evaluation of the existing exhaust system, identify reasonable alternative renovation strategies. These will be approaches which can reduce the exit velocities of the exhaust while maintaining safe plume dispersion. Variable frequency drives provide an efficient means to reduce the flow through fan with large energy penalties. Stack extensions are a common way to improve plume dispersion. In this photo, a set of lab exhaust stacks was originally design too short (the white part of the stacks). In response to recurring odor problems, the stack height was increased (the black portion of the stacks), plume dispersion was improved, and the odor problem was resolved. Extensive manifolding and clustering are also demonstrated by this set of exhaust stacks. (Use cursor to point out manifolding). You can see at least two exhaust streams manifolded into this stack. And the close grouping of stacks (clustering) allows their plumes to merge in the air above the exit plane. There are some additional requirements for clustering to be effective which I’ll be happy to discuss with anyone interested off line. With either clustering or manifolding, we end up with larger plumes which are more diluted and rise to greater heights above the roof. When two or more fans service the same plenum, a relatively simple action which can save fan energy is to redefine which fans come on at what speeds to exhaust the plume(s) to the atmosphere. For example, if only one fan of a pair is running (a typical N+1 redundancy), splitting the flow between the two fans/stacks would reduce the exit velocity by a factor of 2. Since the energy required to move the air is proportional to the cube of the speed, this can theoretically reduce the fan energy by a factor of 2 cubed—or eight. Before implementing such an approach, it is necessary to review the fan efficiency curves and perform wind tunnel testing to ensure that plume dispersion at the lower exit velocity continues to provide acceptable plume dispersion. Installation of VFD controllers can solve the common issue of inefficient operation of the fans as speeds are reduced. After the alternative renovation designs have been identified, wind tunnel testing is conducted to determine whether acceptable plume dispersion is achieved. Wind tunnel testing can be used to determine minimum acceptable design parameter to ensure safety. For example, how low can the exit velocity be and still provide safe plume dispersion?
- Based on the evaluation of the existing exhaust system, identify reasonable alternative renovation strategies. These will be approaches which can reduce the exit velocities of the exhaust while maintaining safe plume dispersion. Variable frequency drives provide an efficient means to reduce the flow through fan with large energy penalties. Stack extensions are a common way to improve plume dispersion. In this photo, a set of lab exhaust stacks was originally design too short (the white part of the stacks). In response to recurring odor problems, the stack height was increased (the black portion of the stacks), plume dispersion was improved, and the odor problem was resolved. Extensive manifolding and clustering are also demonstrated by this set of exhaust stacks. (Use cursor to point out manifolding). You can see at least two exhaust streams manifolded into this stack. And the close grouping of stacks (clustering) allows their plumes to merge in the air above the exit plane. There are some additional requirements for clustering to be effective which I’ll be happy to discuss with anyone interested off line. With either clustering or manifolding, we end up with larger plumes which are more diluted and rise to greater heights above the roof. When two or more fans service the same plenum, a relatively simple action which can save fan energy is to redefine which fans come on at what speeds to exhaust the plume(s) to the atmosphere. For example, if only one fan of a pair is running (a typical N+1 redundancy), splitting the flow between the two fans/stacks would reduce the exit velocity by a factor of 2. Since the energy required to move the air is proportional to the cube of the speed, this can theoretically reduce the fan energy by a factor of 2 cubed—or eight. Before implementing such an approach, it is necessary to review the fan efficiency curves and perform wind tunnel testing to ensure that plume dispersion at the lower exit velocity continues to provide acceptable plume dispersion. Installation of VFD controllers can solve the common issue of inefficient operation of the fans as speeds are reduced. After the alternative renovation designs have been identified, wind tunnel testing is conducted to determine whether acceptable plume dispersion is achieved. Wind tunnel testing can be used to determine minimum acceptable design parameter to ensure safety. For example, how low can the exit velocity be and still provide safe plume dispersion?
- After the chosen exhaust system renovation is installed, a commissioning test of the entire system including plume dispersion should be performed to ensure that the design criteria is achieved—and safe exposure levels are obtained. The commissioning test involves releasing tracer gas through the actual exhaust stack and measuring the resulting concentrations at key receptor locations downwind. This 1) provides an overall check of the entire exhaust system include dispersion and 2) ensure a margin of safety is actually achieved.
- How would you approach a comprehensive program to obtain these energy saving for all off the lab buildings on your campus?
- Since the purpose of this study is to provide results generally representative of laboratory buildings on the UCI campus – and potentially representative of those on other UC campuses – three buildings were selected after careful consideration of a list of nine laboratory buildings on the UCI campus. The nine buildings considered for inclusion in this study were: Biological Sciences Nat Sci 1 Nat Sci 2 Croul Hall Hewitt Hall Sprague Hall Cal-IT2 Engineering Lab Facility Engineering Gateway
- First establish a campus-wide guideline for how design criteria are to be specified for the lab buildings. This is fundamental to a successful effort. You also need to specify which areas are subject to the design criteria and which are not—roofs? Mechanical rooms? Jay- It would also be useful to have a campus-wide guideline regarding the required air change rate in lab buildings to facilitate calculation of exhaust air flow rate.
- To make the best use of progressively scarcer resources it is useful to conduct a campus-wide screen study to identify the “low-hanging fruit”—lab buildings for which small up-front investments can produce large energy and cost savings. The screening study consists of an abbreviated wind tunnel study and a preliminary mechanical review of hazardous exhaust systems. The wind tunnel study would primarily use qualitative test (smoke visualization) to identify the predominate flow and dispersion features, including a limited number of quantitative measurements (tracer gas studies) to verify the magnitude of suspected issues. Preliminary estimates should be made of the required up-front investment and cost/energy savings.
- Based on the results of the screening study, prioritize the buildings for renovation design and implementation. Perform the design effort as previously discussed. Be sure to include field commissioning of the exhaust plume dispersion to ensure that everything has been implemented according to the design and that safety and energy savings will be achieved.
- Wind tunnel testing of exhaust dispersion is done in two primary modes. The first is smoke visualization as shown in this video clip. This allows us to see the plume and gives us a good idea of the flow phenomena controlling where the plume goes and how fast it is being diluted. This is very useful in understanding what is happening and devising alternative exhaust design approaches. The second mode of testing is the use of a precision tracer gas and analysis of its concentration in air samples at receptors to quantitatively specify the exhaust plume dilution. This is the fundamental set of measurements which tell us whether the maximum concentrations at receptors will exceed those allowed by health or odor standards.