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Reduce Energy Costs with Standby Lab Exhaust Fans January 2013 / White paper by Victor Neuman, P.E. Application Engineer
Make the most of your energy SM
Summary Introduction............................................................................................................3 Low hanging fruit: quick payback............................................................................4 Moving down the fan curve ....................................................................................4 Start with safety and proceed to energy savings.....................................................5 Example of OpEx vs. CapEx...................................................................................7 Conclusion.............................................................................................................8
White paper
Introduction Laboratory buildings for chemistry, medicine, biology, and related disciplines are intense users of energy for their operation. Some research universities use 60% of campus energy for their science buildings alone. Universities, institutions, and corporations are looking for quick paybacks while still reducing operating costs and improving sustainability.i The biggest single user of energy in the laboratory is the heating, ventilating, and air-conditioning (HVAC) systems. Ventilation is needed to protect building occupancies from chemical, biological, and radiological hazards. Air-conditioning is needed to provide the exact operating conditions necessary to support scientific research and to be in compliance with current Good Laboratory Practice and other codes and standards.ii Only a small number of techniques have been applied to saving energy and operating cost in the chemical exhaust portion of the air-conditioning system. A broad overview of some techniques that can be used to save lab exhaust energy is given in a white paper previously published by Schneider Electric, titled “Optimizing Lab Exhaust Systems to Reduce Energy Use and Costs.” This paper looks at one very specific technique which can be applied to lab exhaust energy reduction. The current practice is to have two chemical exhaust fans for each exhaust duct. One is the operating fan and the second is the standby fan. The most common system has the operating fan running at 100% of design volume while the standby fan is off and shut off by a backdraft damper. However, significant energy savings can be achieved by running both the operating fan and the standby fan together, each at 50% of design flow. In simpler terms, let’s refer to this technique as two 50% exhaust fans. Modern Chemical Fume Hood
i
“An Introduction to Low-Energy Design.” Laboratories for the 21st Century, U.S. Environmental
Protection Agency, DOE/GO-102000-1112. http://www1.eere.energy.gov/femp/pdfs/lowenergy_508.pdf ii
ANSI/AIHA, American National Standard for Laboratory Ventilation, Standard Z9.5-2011. Reduce Energy Costs with Standby Lab Exhaust Fans | 3
White paper
Low hanging fruit: quick payback The technique we are advocating in this paper is to run both the lead and lag chemical exhaust fans simultaneously at 50% of design flow. This technique is typically be considered “low hanging fruit” as it has a low capital cost to implement as well as a quick payback and recovery of capital – while still having significant energy savings. Typically, the timeframe to implement this is between 1-3 days, allowing for minimal interference with ongoing operations. Two 50% exhaust fans will work in either a “constant volume” or a “variable volume” air-conditioning system. For this paper, the discussion is restricted to the constant volume system. Owners can expect the ROI for typical two 50% exhaust fan projects to be in approximately one year or less. This quick payback, together with the short construction time, makes these exceedingly attractive projects.
Moving down the fan curve The technical reason why two 50% exhaust fan projects save energy is that we are moving down the fan curve. If fans used energy in a straight line or linear fashion, there would be no savings when going from a single 100% exhaust fan to two 50% exhaust fans. But the fan volume curve is based on a cube, or exponent of 3. If you reduce the flow in a fan by 10%, the power usage is reduced by 27%. A 10% reduction in flow leaves 90% of the flow remaining. The governing fan equation would look like this: 0.9 x 0.9 x 0.9 = 0.729 The technique profiled here is to run two fans at 50% flow. The ideal fan law relationship would be: 2 fans (0.5x0.5x0.5) = 0.25 This represents a potential energy savings of 75%. However, as with many real systems, the realized practical energy savings are less than the theoretical maximum. The realizable energy from moving down the fan curve follows an exponent of between 2 and 2.5. The low side of energy savings would follow a curve with an exponent of 2 as follows: 2 fans (0.5 x 0.5) = 0.5 This represents a savings of 50%. Reduce Energy Costs with Standby Lab Exhaust Fans | 4
White paper
Start with safety and proceed to energy savings As with any engineered system, human safety must be satisfied before considering energy savings. It is highly recommended that a risk assessment study be performed for any operating chemical, biological, or radiological exhaust fan. This includes the laboratory fume hood exhaust fans discussed here. Such a study evaluates the amount and toxicity of all chemicals and air-borne hazards likely to be found in the exhaust system. A credible spill scenario is constructed and analyzed in terms of dilution rates to receptor sites like doors and windows. The major risk with chemical exhaust fans is that, for certain wind speeds and directions, the exhaust plume exiting the fan will be pushed over so that it enters the outside air intake of air handlers, enters open doors and windows, or impacts maintenance workers on the roof, people on the ground, and people in adjacent buildings. Several years ago more than 4,000ASHRAE 110 fume hood safety tests were conducted by Thomas C. Smith of Exposure Control Technologies. The end result was that more than 5% of the fume hoods failed for the sole reason that a crosswind on the roof pushed contaminants back into the building.
CPP Wind Tunnel Risk Assessment
The most accurate type of risk assessment uses a wind tunnel and models of the building and surrounding buildings and terrain features. The ASHRAE Handbook chapter titled, “Airflow Around Buildings” has a complete description of how such a quantitative simulation of toxic concentrations is done. Consulting design firms are not usually able to complete this risk assessment of the exhaust fans in chemical/biological/radiological laboratory buildings. Two worldwide global firms that do conduct these studies are www.cppwind.com and www.rwdi.com. In our search for energy savings and operating cost reduction, we start with our wind tunnel risk assessment study. (If required due to budget constraints, the 2 firms above can do more limited studies by hand.) The best result is if the fan and stack systems are safe in all wind speeds and directions at full flow. If the chemical exhaust fans do not pass, there are several possible remediation measures iii: • Greater exit velocities • Taller stacks • The introduction of dilution air at roof level • Relocation of the exhaust fans to a new location • Treatment of the exhaust stream with filters, scrubbers, etc. As a rough guide, chemical exhaust fans with volumes of air of 200-2,000 cubic feet per minute (cfm) are often marginally safe at best. A one fan-one fume hood iii
ASHRAE, ASHRAE Handbook-HVAC Applications, Chapter 44, Building Intake and Exhaust Design, 2011 Reduce Energy Costs with Standby Lab Exhaust Fans | 5
White paper
system with a 1,000 cfm volume of air exiting at 3,000 feet per minute from a 10 foot tall stack will impact the roof 50 feet from the base of the fan in a 20 mile per hour wind. Exhaust fans of 60,000 cfm or greater usually have acceptable safety. But these guidelines or rules of thumb are not a replacement for a proper risk assessment study. To achieve our energy savings, the risk assessment study should include chemical exhaust flows of both one fan at full flow and two fans at half flow. The best result is if the two fans at half flow is also safe – then implementation of this control strategy can proceed. If the two fans at half flow are not safe for all wind speeds and directions, there are several choices. • Remediate with the measures shown above like taller stacks or more dilution air • Abandon the energy savings from running two fans • Implement a wind sensitive controller The June 2011 ASHRAE Journal article by Carter, Cochran and Reifschneideriv has more information on wind sensitive controllers. In brief, since the problem only occurs when the wind blows above a set velocity, the energy savings are only achieved at low wind velocities. At higher wind velocities, the controller will switch to one fan at full flow. You may ask, “won’t both fans fail simultaneously without any redundancy?” There will be some concern that since we are now running both fans, that both fans could fail simultaneously. This energy savings strategy seems to eliminate the standby fan or lead-lag fan operation. However, it has been found that it is not reliable to run one fan all the time and to never run the backup fan. As a result, the lead fan and lag fan (or standby fan) are exchanged regularly. In fact, in most cases both fans are given equal run times in alternation. Thus, the degree of backup and redundancy is equal between lead-lag and running both fans simultaneously.
iv
”Saving Energy in Lab Exhaust Systems”, by John J. Carter, Member ASHRAE; Brad
C. Cochran, Member ASHRAE; and Jeff D. Reifschneider, ASHRAE Journal, June 2011 Reduce Energy Costs with Standby Lab Exhaust Fans | 6
White paper
Example of OpEx vs. CapEx We will examine a large chemical laboratory building. It is a teaching facility and conducts research at a Midwestern USA location. As is required by safety, the chemical exhaust fans serving laboratory exhaust and chemical hoods normally never shut off. Electrical costs average $0.10 per kilowatt hour. To enable us to do a more detailed study of fan performance of all the many lab exhaust fans available, let us suppose that this building is served by 2 identical Greenheck Corporation 44AFSW 41 model fans. In the original design, one fan was on at 100% of design at 60,000 cubic feet per minute (cfm) and a pressure capability of 2.0 inches of water guage. The second fan was off on standby. As designed and originally installed, the main fan was using 70.81 brake horsepower. In our example, the energy conservation team of the university modifies the building automation system so that both the main and standby fans
Twin City Fan Dual Exhaust Fans
run simultaneously. Now, instead of one fan at 100% of design volume, we have 2 fans running at 50% of design volume. Using the manufacturer’s published fan curves, we see that the new control scheme results in each fan running 30,000 cfm at 2.0 inch water gauge. Each of the 2 fans is 16.2 bhp for a total of 32.4 bhp. This change saves 38.41 bhp; more than half of the original running energy for one exhaust fan is saved by running 2 fans at 50%. What might be an average cost of making this change? As discussed previously in this paper, it is important to perform a risk assessment of the exhaust fan running at 50% flow as it relates to chemical dilution and site geography. It would be better to run a wind tunnel simulation, but let us assume one step down from that for a manual calculation study costing $17,500. In our hypothetical university, as is common practice, the variable speed drives have already been installed, one per exhaust fan. If the university staff do the reprogramming of the building automation system, the cost will be minimal. Assume, to be on the safe side, that an outside contractor reprograms the building controls for the new operating mode for a cost of $10,000. The resulting cost benefit analysis is: • Base Case: 70.81 hp x .746 kw/hp x 8760 hrs x 0.10/kwh = $46,274 per year to run one fan at 100% volume • Energy use for 2 fans at 50% = 32.4 hp x .746 kw/hp x 8760 hrs x 0.10/kwh = $21,173 to run two fans at 50% volume Annual Savings = $25,100 Installation Costs = $27,500 Simple Payback = 1.1 years Note: The cost of the risk assessment study does not increase greatly with multiple fans. A larger project would have a quicker payback. Assumes no wind sensitive controller or taller stacks are needed. Reduce Energy Costs with Standby Lab Exhaust Fans | 7
Conclusion Since it can be done safely for two fans running at 50% instead of one fan at 100%, the answer is clear – pick the low hanging fruit. This paper concentrates on constant volume buildings but this technique can be applied to both constant volume and variable volume buildings. Savings are typically estimated at 50% as opposed to the 80-90% savings from variable volume wind sensitive controllers. The potential energy savings can be used to cost justify risk assessment studies for all chemical exhaust fans.
About Schneider Electric As a global specialist in energy management with operations in more than 100 countries, Schneider Electric offers integrated solutions across multiple market segments, including leadership positions in Utilities & Infrastructures, Industries & Machine Manufacturers, Non-residential Buildings, Data Centres & Networks and in Residential. Focused on making energy safe, reliable, efficient, productive and green, the Group’s 130,000 plus employees achieved sales of 22.4 billion euros in 2011, through an active commitment to help individuals and organizations make the most of their energy.
© 2013 Schneider Electric. All rights reserved.
www.schneider-electric.com
Schneider Electric One High Street, North Andover, MA 01845 USA Telephone: +1 978 975 9600 Fax: +1 978 975 9674 www.schneider-electric.com/buildings WP-LS-STANDBY-LAB-EXHAUST-US.BU.N.EN.1.2013.0.00.CC
January 2013
Make the most of your energy SM
Summary Introduction............................................................................................................3 Low hanging fruit: quick payback............................................................................4 Moving down the fan curve ....................................................................................4 Start with safety and proceed to energy savings.....................................................5 Example of OpEx vs. CapEx...................................................................................7 Conclusion.............................................................................................................8
White paper
Introduction Laboratory buildings for chemistry, medicine, biology, and related disciplines are intense users of energy for their operation. Some research universities use 60% of campus energy for their science buildings alone. Universities, institutions, and corporations are looking for quick paybacks while still reducing operating costs and improving sustainability.i The biggest single user of energy in the laboratory is the heating, ventilating, and air-conditioning (HVAC) systems. Ventilation is needed to protect building occupancies from chemical, biological, and radiological hazards. Air-conditioning is needed to provide the exact operating conditions necessary to support scientific research and to be in compliance with current Good Laboratory Practice and other codes and standards.ii Only a small number of techniques have been applied to saving energy and operating cost in the chemical exhaust portion of the air-conditioning system. A broad overview of some techniques that can be used to save lab exhaust energy is given in a white paper previously published by Schneider Electric, titled “Optimizing Lab Exhaust Systems to Reduce Energy Use and Costs.” This paper looks at one very specific technique which can be applied to lab exhaust energy reduction. The current practice is to have two chemical exhaust fans for each exhaust duct. One is the operating fan and the second is the standby fan. The most common system has the operating fan running at 100% of design volume while the standby fan is off and shut off by a backdraft damper. However, significant energy savings can be achieved by running both the operating fan and the standby fan together, each at 50% of design flow. In simpler terms, let’s refer to this technique as two 50% exhaust fans. Modern Chemical Fume Hood
i
“An Introduction to Low-Energy Design.” Laboratories for the 21st Century, U.S. Environmental
Protection Agency, DOE/GO-102000-1112. http://www1.eere.energy.gov/femp/pdfs/lowenergy_508.pdf ii
ANSI/AIHA, American National Standard for Laboratory Ventilation, Standard Z9.5-2011. Reduce Energy Costs with Standby Lab Exhaust Fans | 3
White paper
Low hanging fruit: quick payback The technique we are advocating in this paper is to run both the lead and lag chemical exhaust fans simultaneously at 50% of design flow. This technique is typically be considered “low hanging fruit” as it has a low capital cost to implement as well as a quick payback and recovery of capital – while still having significant energy savings. Typically, the timeframe to implement this is between 1-3 days, allowing for minimal interference with ongoing operations. Two 50% exhaust fans will work in either a “constant volume” or a “variable volume” air-conditioning system. For this paper, the discussion is restricted to the constant volume system. Owners can expect the ROI for typical two 50% exhaust fan projects to be in approximately one year or less. This quick payback, together with the short construction time, makes these exceedingly attractive projects.
Moving down the fan curve The technical reason why two 50% exhaust fan projects save energy is that we are moving down the fan curve. If fans used energy in a straight line or linear fashion, there would be no savings when going from a single 100% exhaust fan to two 50% exhaust fans. But the fan volume curve is based on a cube, or exponent of 3. If you reduce the flow in a fan by 10%, the power usage is reduced by 27%. A 10% reduction in flow leaves 90% of the flow remaining. The governing fan equation would look like this: 0.9 x 0.9 x 0.9 = 0.729 The technique profiled here is to run two fans at 50% flow. The ideal fan law relationship would be: 2 fans (0.5x0.5x0.5) = 0.25 This represents a potential energy savings of 75%. However, as with many real systems, the realized practical energy savings are less than the theoretical maximum. The realizable energy from moving down the fan curve follows an exponent of between 2 and 2.5. The low side of energy savings would follow a curve with an exponent of 2 as follows: 2 fans (0.5 x 0.5) = 0.5 This represents a savings of 50%. Reduce Energy Costs with Standby Lab Exhaust Fans | 4
White paper
Start with safety and proceed to energy savings As with any engineered system, human safety must be satisfied before considering energy savings. It is highly recommended that a risk assessment study be performed for any operating chemical, biological, or radiological exhaust fan. This includes the laboratory fume hood exhaust fans discussed here. Such a study evaluates the amount and toxicity of all chemicals and air-borne hazards likely to be found in the exhaust system. A credible spill scenario is constructed and analyzed in terms of dilution rates to receptor sites like doors and windows. The major risk with chemical exhaust fans is that, for certain wind speeds and directions, the exhaust plume exiting the fan will be pushed over so that it enters the outside air intake of air handlers, enters open doors and windows, or impacts maintenance workers on the roof, people on the ground, and people in adjacent buildings. Several years ago more than 4,000ASHRAE 110 fume hood safety tests were conducted by Thomas C. Smith of Exposure Control Technologies. The end result was that more than 5% of the fume hoods failed for the sole reason that a crosswind on the roof pushed contaminants back into the building.
CPP Wind Tunnel Risk Assessment
The most accurate type of risk assessment uses a wind tunnel and models of the building and surrounding buildings and terrain features. The ASHRAE Handbook chapter titled, “Airflow Around Buildings” has a complete description of how such a quantitative simulation of toxic concentrations is done. Consulting design firms are not usually able to complete this risk assessment of the exhaust fans in chemical/biological/radiological laboratory buildings. Two worldwide global firms that do conduct these studies are www.cppwind.com and www.rwdi.com. In our search for energy savings and operating cost reduction, we start with our wind tunnel risk assessment study. (If required due to budget constraints, the 2 firms above can do more limited studies by hand.) The best result is if the fan and stack systems are safe in all wind speeds and directions at full flow. If the chemical exhaust fans do not pass, there are several possible remediation measures iii: • Greater exit velocities • Taller stacks • The introduction of dilution air at roof level • Relocation of the exhaust fans to a new location • Treatment of the exhaust stream with filters, scrubbers, etc. As a rough guide, chemical exhaust fans with volumes of air of 200-2,000 cubic feet per minute (cfm) are often marginally safe at best. A one fan-one fume hood iii
ASHRAE, ASHRAE Handbook-HVAC Applications, Chapter 44, Building Intake and Exhaust Design, 2011 Reduce Energy Costs with Standby Lab Exhaust Fans | 5
White paper
system with a 1,000 cfm volume of air exiting at 3,000 feet per minute from a 10 foot tall stack will impact the roof 50 feet from the base of the fan in a 20 mile per hour wind. Exhaust fans of 60,000 cfm or greater usually have acceptable safety. But these guidelines or rules of thumb are not a replacement for a proper risk assessment study. To achieve our energy savings, the risk assessment study should include chemical exhaust flows of both one fan at full flow and two fans at half flow. The best result is if the two fans at half flow is also safe – then implementation of this control strategy can proceed. If the two fans at half flow are not safe for all wind speeds and directions, there are several choices. • Remediate with the measures shown above like taller stacks or more dilution air • Abandon the energy savings from running two fans • Implement a wind sensitive controller The June 2011 ASHRAE Journal article by Carter, Cochran and Reifschneideriv has more information on wind sensitive controllers. In brief, since the problem only occurs when the wind blows above a set velocity, the energy savings are only achieved at low wind velocities. At higher wind velocities, the controller will switch to one fan at full flow. You may ask, “won’t both fans fail simultaneously without any redundancy?” There will be some concern that since we are now running both fans, that both fans could fail simultaneously. This energy savings strategy seems to eliminate the standby fan or lead-lag fan operation. However, it has been found that it is not reliable to run one fan all the time and to never run the backup fan. As a result, the lead fan and lag fan (or standby fan) are exchanged regularly. In fact, in most cases both fans are given equal run times in alternation. Thus, the degree of backup and redundancy is equal between lead-lag and running both fans simultaneously.
iv
”Saving Energy in Lab Exhaust Systems”, by John J. Carter, Member ASHRAE; Brad
C. Cochran, Member ASHRAE; and Jeff D. Reifschneider, ASHRAE Journal, June 2011 Reduce Energy Costs with Standby Lab Exhaust Fans | 6
White paper
Example of OpEx vs. CapEx We will examine a large chemical laboratory building. It is a teaching facility and conducts research at a Midwestern USA location. As is required by safety, the chemical exhaust fans serving laboratory exhaust and chemical hoods normally never shut off. Electrical costs average $0.10 per kilowatt hour. To enable us to do a more detailed study of fan performance of all the many lab exhaust fans available, let us suppose that this building is served by 2 identical Greenheck Corporation 44AFSW 41 model fans. In the original design, one fan was on at 100% of design at 60,000 cubic feet per minute (cfm) and a pressure capability of 2.0 inches of water guage. The second fan was off on standby. As designed and originally installed, the main fan was using 70.81 brake horsepower. In our example, the energy conservation team of the university modifies the building automation system so that both the main and standby fans
Twin City Fan Dual Exhaust Fans
run simultaneously. Now, instead of one fan at 100% of design volume, we have 2 fans running at 50% of design volume. Using the manufacturer’s published fan curves, we see that the new control scheme results in each fan running 30,000 cfm at 2.0 inch water gauge. Each of the 2 fans is 16.2 bhp for a total of 32.4 bhp. This change saves 38.41 bhp; more than half of the original running energy for one exhaust fan is saved by running 2 fans at 50%. What might be an average cost of making this change? As discussed previously in this paper, it is important to perform a risk assessment of the exhaust fan running at 50% flow as it relates to chemical dilution and site geography. It would be better to run a wind tunnel simulation, but let us assume one step down from that for a manual calculation study costing $17,500. In our hypothetical university, as is common practice, the variable speed drives have already been installed, one per exhaust fan. If the university staff do the reprogramming of the building automation system, the cost will be minimal. Assume, to be on the safe side, that an outside contractor reprograms the building controls for the new operating mode for a cost of $10,000. The resulting cost benefit analysis is: • Base Case: 70.81 hp x .746 kw/hp x 8760 hrs x 0.10/kwh = $46,274 per year to run one fan at 100% volume • Energy use for 2 fans at 50% = 32.4 hp x .746 kw/hp x 8760 hrs x 0.10/kwh = $21,173 to run two fans at 50% volume Annual Savings = $25,100 Installation Costs = $27,500 Simple Payback = 1.1 years Note: The cost of the risk assessment study does not increase greatly with multiple fans. A larger project would have a quicker payback. Assumes no wind sensitive controller or taller stacks are needed. Reduce Energy Costs with Standby Lab Exhaust Fans | 7
Conclusion Since it can be done safely for two fans running at 50% instead of one fan at 100%, the answer is clear – pick the low hanging fruit. This paper concentrates on constant volume buildings but this technique can be applied to both constant volume and variable volume buildings. Savings are typically estimated at 50% as opposed to the 80-90% savings from variable volume wind sensitive controllers. The potential energy savings can be used to cost justify risk assessment studies for all chemical exhaust fans.
About Schneider Electric As a global specialist in energy management with operations in more than 100 countries, Schneider Electric offers integrated solutions across multiple market segments, including leadership positions in Utilities & Infrastructures, Industries & Machine Manufacturers, Non-residential Buildings, Data Centres & Networks and in Residential. Focused on making energy safe, reliable, efficient, productive and green, the Group’s 130,000 plus employees achieved sales of 22.4 billion euros in 2011, through an active commitment to help individuals and organizations make the most of their energy.
© 2013 Schneider Electric. All rights reserved.
www.schneider-electric.com
Schneider Electric One High Street, North Andover, MA 01845 USA Telephone: +1 978 975 9600 Fax: +1 978 975 9674 www.schneider-electric.com/buildings WP-LS-STANDBY-LAB-EXHAUST-US.BU.N.EN.1.2013.0.00.CC
January 2013
