VSD Retrofit For Lab Fume Hood Exhaust System
When you’re a student working with toxic chemicals, jarring alarms are not exactly a desired part of your process.
When you’re a student working with toxic chemicals, jarring alarms are not exactly a desired part of your process. Likewise, when you’re a facilities director, an annual expense of $30,000 to run a third unnecessary fan for a lab environment is something you would gladly shed. Controls and drives recommissioning at this Canadian university not only eliminated those scenarios, it paved the way for broader cost reduction plus better experiences for facility staff and occupants alike.
Control recommissioning in existing buildings presents an excellent opportunity to not only save energy, but to fix operational deficiencies and work with the building’s occupants to reduce complaints. Our recent work with the University of Victoria (UVic) in British Columbia is an example of how installing a VSD and instituting advanced controls measures can achieve significant and persistent results.
In this case, an operational deficiency associated with the fume hood exhaust system was causing a significant health and safety concern with lab users. To manage the consistent complaints, the exhaust system was manually overridden. While the override solved the health and safety concern, it resulted in an additional 300,000 kWh of electricity being used annually. SES Consulting worked closely with UVic’s facilities management team to develop a solution to fix the operational deficiency to reduce energy consumption, eliminate the health and safety concern, reduce maintenance costs associated with excessive fan run hours, and enable the university to continually improve the performance of their building.
This energy conservation measure (ECM) was one of several measures identified during a recommissioning study on one of UVic’s science buildings. This study was carried out as part of a provincial electrical utility (BC Hydro) sponsored continuous optimization program (COP). Continuous optimization or ongoing recommissioning focuses on ECMs with simple paybacks of less than two years. The process includes the following key phases:
- Baseline energy analysis
- ECM investigation
- ECM implementation
- Coaching and training
- Ongoing measurement and verification
The science building was constructed in 2008 and is home to several science departments. The building has five floors and includes several large lecture theatres, offices, labs, meeting rooms, and food services. There are two major wings; the lab wing and the office wing, each with its own independent HVAC system. The lab wing has over 55 variable flow fume hoods that share a common constant volume exhaust system consisting of four 60 hp axial exhaust fans. The system provides adequate flow with either two or three fans depending on the fume hood demand. The fourth fan provides redundancy.
Make up outdoor air for the constant volume fans is supplied via bypass dampers located just upstream of the exhaust fans. As fume hood demand decreases, the bypass dampers open to supply make up outdoor air and maintain constant flow and the static duct pressure setpoint. As fume hood demand increases, make up air reduces. Once the fume hood demand decreases below a given threshold, the system was designed to switch from three-fan mode to two-fan mode.
Figure 1 presents the BAS graphic for the fume hood exhaust system prior to implementation of the ECM. The programming for automatically enabling or disabling the third fan has been manually disabled. In this example, one of the OA bypass dampers is open to 91%, making this an excellent opportunity to shut off the third fan.
During the investigation phase, the first and obvious question was, “Why was the auto duty fan change manually overridden?” Based on discussions with the building operator, the system was overridden due to operational constraints during fan startup and shutdown. During fan startup or shutdown, brief but dramatic pressure drops occurred in the exhaust riser, resulting in loud banging noises and vibrations throughout the exhaust ducts. There were also reduced exit velocities in the fume hoods. While the fume hood exit velocity remained above zero and still provided suction, the velocity in several fume hoods was sufficiently reduced to trigger on-board alarms. Imagine being a student, intently focused on a lab experiment with toxic chemicals, when you hear a sudden BANG, and then the fume hood alarms go off. Unfortunately, this situation was quite common, created a lot of fear among facility users, and was obviously unacceptable. Facilities management had little choice but to continuously operate the system in three-fan mode, even during periods of low fume hood demand.
The continuous operation of the third fan consumed approximately 400,000 kWh annually. Even with BC’s relatively low electricity rate of approximately $0.06 per kWh for commercial clients, the third fan was costing the university nearly $24,000 every year in electricity consumption and an additional $5,000 in demand fees. Based on our initial review of fume hood and bypass damper trend logs that were archived in the BAS, we estimated that the exhaust system would likely operate in two-fan mode for most of the year, representing a significant energy savings opportunity. The question remained: how do we fix the operational constraint and save energy while providing safe operation of the exhaust system? Perhaps an even bigger challenge: how do we convince the client, and in particular the occupants that this will work, when it’s their health and safety on the line?
The financial case for the ECM was strong, with an estimated simple payback of less than two years, even while ignoring demand savings and maintenance savings associated with the excessive run hours on the third fan. Facilities management was definitely interested and the project team was assembled, including the following key members:
- Engineers from SES Consulting
- UVic facilities staff, including:
- Energy manager
- Maintenance shop manager
- BAS operator
- Health and safety representatives
Problem solving recommissioning measures is an iterative process that requires agile project management: frequent testing, revising, and refining to determine the operational limitations of the system. Under the guidance and supervision of facilities management, SES Consulting worked closely with the BAS operator to perform experiments on the system (with adequate warning to occupants). Our goal was to better understand the underlying dynamics and brainstorm options for enabling and disabling the third fan based on demand, while eliminating any significant impact to the occupants. Equipment trend logs and real-time energy monitoring were used to identify the key issues, document the findings, and present potential solutions to the team.
Figure 2 is a screenshot from the energy monitoring information system (EMIS), and highlights some early testing results. The goal of this experiment was to begin quantifying the actual ventilation demand for the third fan. Throughout the testing and implementation phase, occupants were notified that our team was modifying the system to improve energy efficiency. In general, the lab managers were supportive and appreciated our efforts. The solid line is the real-time power demand, whereas the dashed line is the building’s “energy baseline,” representing what the buildings typical demand is based on. Factors include (but are not limited to) outdoor air temperature, wind speeds, and wind direction.
On December 2, prior to VSD installation, the system was reset to operate in two-fan mode. When required, the third fan would enable and remain on indefinitely. The system remained in two-fan mode for over a month and switched to three-fan mode on January 14. Based on these initial results, we updated the energy saving and simple payback estimates. This was critical to getting further buy-in from facilities to invest more time and money into the project.
In retrospect, the solution was fairly straightforward. A VSD was installed on the third fan to control the ramp times during startup and shutdown. Controlling the ramp time allowed us to manage the pressure drop and eliminate the loud noises, vibrations, and reduced exit velocities in the fume hoods. It should be noted that the VSD does not run at less than 100% flow outside of startup and shutdown since this would result in a reduced exhaust plume height. Since the plume height is momentarily reduced when the fan speed is less than 100%, switching from three-fan mode to two-fan mode is disabled during periods of high winds. To prevent short cycling, a limit was set on mode switches per day.
The results of this project are quite significant. In addition to significant energy savings, facilities management has improved relationships with the building occupants, dramatically reduced the run time on the third fan, and increased their in-house skills and knowledge on how to efficiently operate their buildings. Since the ECM was implemented, there have been no complaints from lab users about the fan’s startup and shutdown due to negligible impacts.
Figure 3 presents the typical daily profile following implementation. It demonstrates how often the system operates in two-fan mode (blue) and three-fan mode (grey). The data confirms our initial assumption that fume hood demand correlates closely with typical business hours. The third fan is generally not needed between 6 p.m. and 6 a.m. on weekdays. There is even less demand on weekends, holidays, exam periods, and summer break.
The percentage of time that the exhaust system operated in two-fan and three-fan mode each month is presented in Figure 4. As expected, the system only required two-fan mode for approximately 70% of the time. Annual electricity savings of approximately 300,000 kWh was achieved between March 1, 2013, and February 28, 2014 — an annual savings of $18,000. The capital cost to implement the project was approximately $19,000, resulting in a simple payback of 1.1 yrs based on energy savings alone. Additional savings related to demand and maintenance are not included in this calculation but are anticipated to be quite significant as well.
The two-fan mode run times began to increase in November 2013 and remain at approximately 90%, suggesting that fume hood demand decreased even further. This dramatic additional savings of 20% is quite interesting and initially caused some concern. How was it possible that such dramatic changes occurred? Did something change? Was the system working incorrectly? Following a brief email correspondence with UVic’s team, they identified several potential reasons for the increased performance, including:
- Several broken fume hood flow valves were fixed.
- The BAS operator added some additional improvements to the sequence of operations.
- Control upgrades were implemented on several other fume hoods located in the building, allowing for reduced fume hood flow rates afterhours.
- A ‘Green Labs’ occupant engagement program began and is still in progress.
The new VSD and control sequences enabled the UVic team to continuously commission the system and reap the rewards. Assuming trends continue these additional recommissioning measures will result in an additional annual savings of 50,000 kWh.
Often, facility operators are skittish about implementing any measures that may affect building occupants’ health, safety, or comfort, even if they represent some of the best energy savings opportunities. Approaching these challenges as a problem solving team ensures all concerns are addressed. Careful testing and refining of the measures is key to establishing and maintaining the trust of those who will be most affected by the changes. Having operations intimately involved in the refining, testing, and ongoing monitoring will ensure that measures can be maintained, and if issues do arise, they can be addressed competently without simply reversing the changes. In addition, effective communication and engagement with the occupants was a critical component to the success of this project.
This project is just one example of how an effective recommissioning team can save energy while providing a solution to operational deficiencies and improving occupant comfort. In this case, the installation of a VSD on one of the fans in the laboratory fume hood exhaust system resolved a longstanding operational deficiency and safety concern that had resulted in continuously operating a 60-hp fan when it was typically unnecessary.
The presence of the VSD allows the BAS to automatically shut down the fan based on actual fume hood demand with no negative impact on the fume hood system. Initial annual energy savings of 300,000 kWh and $18,000 in electricity savings have been achieved with a payback of only 1.1 yrs. Since this ECM was implemented, the team at UVic has refined the fume hood system even further, resulting in an additional annual savings of 50,000 kWh. In addition to the direct energy savings, this recommissioning project was successful in improving user satisfaction with the facility and engaging facilities staff in an ongoing process of continuous improvement. These results set the stage for ongoing savings and improved performance.