Controlling lab ventilation
Maximizing health and safety performance while minimizing energy consumption is extremely challenging for lab environments, so here’s some expert guidance for design and specification to catalyze solutions for your projects.
Maximizing health and safety performance while minimizing energy consumption is extremely challenging for lab environments, so here’s some expert guidance for design and specification to catalyze solutions for your projects.
CSE: Lab ventilation systems are known to be energy intensive. What systems-design approaches help save energy?
Jacob Tsimanis : There are several major contributors to the energy intensity of lab ventilation systems, including one-pass air conditioning/ventilation system (100% outside air); large internal equipment heat gains; and a minimum ventilation rate required to support a safe environment within the laboratory. However, a number of system design approaches can help to save energy for lab ventilation systems.
One approach is to use a heat recovery system which allows reduction of the energy penalty from the one-pass air system. Additionally, the use of heat recovery systems allows extraction energy from the exhaust air for the outside air precool and preheat.
Another method is to use a variable volume (VAV) system. As the required airflows within the laboratory will vary with the ventilation demand and with fluctuation of the external/internal loads, the variable volume system is able to address the required fluctuation within the space by increasing or decreasing the flow based on the space demand. The use of VAV systems helps by reducing laboratory airflow from 10 to 12 air changes per hour (ACH) to a minimum of 6 ACH.
Use of a chilled beam system as a supplement to the conventional lab air system also produces substantial energy reduction. The combination of these two systems allows merging the minimum of six air change requirements delivered by the conventional lab air system and supplemental cooling/heating requirements delivered with a chilled beam system. The pumping energy of the chilled beam system is by far more efficient than the energy required for the air delivery by a conventional air system.
Traci Hanegan : Try starting at the programming phase to minimize the quantity or length of fume hoods or biological safety cabinets. Helping owners understand the cost and energy impacts of their preferences upfront could make them reconsider their needs. For example, a 6-ft-long biological safety cabinet could exhaust 500 cfm or 1200 cfm, depending on whether it is a Type A2 or Type B2. The owners may find that they can modify their work process to enable them to use a Type A2 instead of a Type B2. Or they could perform their Type B2 work in a 4-ft-long cabinet and complete the rest of their work in a separate 4-ft-long Type A2 cabinet, thereby reducing the overall airflow. Better yet, the owners may be able to safely use a recirculating cabinet without any exhaust to the outside.
I also suggest minimizing sash openings or revising the sash configuration, which can help save energy.
Another approach is to use snorkels or canopy exhausts where safe to do so, rather than a fume hood, to reduce exhaust airflow.
Efficiently planned spaces will result in less square footage for a lab. This reduces the volume of air required to meet air change requirements. This strategy is more on the shoulders of the architects and lab consultants. An engineer may suggest that lab and non-lab areas be separated as much as possible so that less area is required to meet lab air change requirements.
Consider heat recovery options such as heat pipes and run-around loops.
Use oversized ductwork, filters, and coils so that the velocities and subsequent pressure drops are reduced, which reduces fan energy.
Provide primary air for makeup to rooms and use recirculating fan coils or radiant cooling panels for the remainder of the cooling load. This does not work if the makeup air requirements exceed the airflow requirements for cooling.
Early planning can minimize the distance and number of elbows in duct runs, which reduces static pressure and, consequently, and fan energy.
Pay attention to the room construction details; a leaky room requires a much higher offset between the supply and exhaust airflows in order to maintain a strong differential pressure. The extra airflow wastes energy.
Take advantage of cascading airflows from clean to dirty areas to reduce the total amount of makeup air provided.
Include provisions for nighttime or unoccupied airflow settings in the design.
Ryan Sprangers : Educating and working with the owner to determine low, yet safe ACH requirements is an important first step. Next, investigate what the driving factor for the airflow in lab spaces (ACH, cooling load, or fume hood makeup) is and determine if it can be reduced or used to reduce the load elsewhere. For example, labs that are governed by ACH or fume hood makeup can be located on the exposures of the building that would otherwise have larger cooling loads due to the exposure. Offices and spaces that are governed by conventional loads can be located on the interior or north exposure, which helps reduce initial system sizes and reduces energy consumption over the lifetime of the building.
Diane Feliciano-Welpe : Designers should evaluate both initial design and long-term operating costs when evaluating energy saving approaches. Labs21 is doing a good job of obtaining benchmark data and developing best practices guides to help designers rethink some of the “rules of thumb” historically used. The following system design approaches should be considered:
For laboratories with a high concentration of fume hoods, VAV fume hood control still remains the most energy-friendly option when maintaining a sash management program. Facilities with predictable occupancy can usually benefit just as effectively with 2-state fume hood control without the initial first cost of a total VAV solution. Low-flow fume hoods are also a viable option and can be further enhanced with 2-state operation.
Collaborate with members of Environmental Health & Safety and lab directors/principal investigators to determine if lower ACH can be used for ventilation. They are best suited to determine the safety of reduced airflows based on the scientific program, practices, and procedures used within the facility. Evaluate the option of using a continuous environmental air quality monitoring system as a means to reduce ACH. This may be a big upfront cost, but again should be evaluated as a lifecycle cost.
Use duct pressure reset to control the VAV supply fan serving the lab areas in lieu of the traditional fixed static pressure setpoint. Design engineers have to size the fan for the design pressure. The actual flow needed from the fan at any other operating point is less. By dynamically adjusting the pressure to meet changing airflows, the fan is slowed down to meet the need of the zone requiring the most pressure, and you have the corresponding savings in fan power. For some reason this control methodology has not been readily implemented for laboratory systems, but with the requirement of meeting ASHRAE Standard 90 (which requires duct pressure reset for DDC controlled terminals) as a prerequisite to a LEED-NC rating, we will probably see, and should see, more of this in specifications.
There following should be considered, too: Seal ductwork and specify low leakage terminal devices to minimize fan capacity due to leakage. Use good design practices when laying out ductwork to minimize pressure drop and its associated fan power. Minimize sharp bends and transitions and use low pressure drop air terminals, diffusers and grilles. Finally, size ductwork to minimize high velocities.
Joe Brooks : A lab ventilation system, like any fan system, should be designed to achieve its objectives using the least amount of energy. In order to minimize energy use, the system should be designed to exclude any system effects that could adversely affect the fan; the most efficient components should be selected; control systems should be designed to minimize wasted energy; and the design should attempt to operate the system near the fan’s peak efficiency point (an efficient fan will do no good if it operates in a region of low efficiency). When possible, component performance should be certified.
AMCA has many certified ratings programs for equipment that is commonly used in lab ventilation systems. Inlet flow, outlet flow, power, and efficiency can be certified for all types of fans. Performance of airflow measurement stations and acoustical duct silencers can also be certified using the AMCA Certified Ratings Programs (CRP).
CSE: What approaches for fume hood controls help minimize energy intensity without compromising lab safety?
Tsimanis : The use of presence sensors helps to determine the presence of a person in front of the hood by detecting motion, and also commands the lab airflow control system from an in-use operating face velocity (e.g., 100 fpm) to a standby face velocity (e.g., 60 fpm) and vice versa. When the sensor detects someone’s presence and/or motion within the detection zone, it is commanding the system to the in-use face velocity within 1.0 seconds.
Additionally, the use of the variable volume control for a fume hood operation is based on the position of the fume hood sash. When the fume hood sash is fully open, the fume hood exhaust will be at the maximum position; and with the fume hood sash closed, the fume hood exhaust will be at a minimum position. This strategy allows reduction in the airflow with reduction in total energy use. The most cost-effective application for this concept is for a small lab with a single fume hood.
Sprangers : Unless the fume hood and/or lab housing needs to operate 365/24/7, incorporate VAV controls on the hood, which will allow the amount of makeup to be reduced when the hood is not in use or the lab is unoccupied. Designers need to beware of fume hoods located in labs that will operate 365/24/7 and are governed by ACH, as incorporating VAV controls on these hoods will increase initial system cost and will not reduce energy consumption.
Another effective approach is to have the BAS monitor fume hood airflow and/or sash heights. This allows building owners to monitor whether their staff is lowering fume hood sashes. On projects where fume hood airflow is controlled using pressure controls, the CFM can be trended and correlated to an average sash height. On projects where potentiometers (sash height sensors) are used to control airflow, the sash height can be trended directly and averaged over time. This could be trended on a hood-by-hood or lab-by-lab basis so that problem areas can be easily identified and corrected.
Feliciano-Welpe : Bar none, the safest, most energy-efficient use of a fume hood is a VAV-controlled fume hood with a closed sash. Implementation of a sash management program is paramount to encouraging good lab practices and the safety of the users. A closed sash can protect a worker from volatile reactions within a fume hood. The fume hood is a safety device.
2-state control of constant volume or low-flow hoods can also be energy-efficient with the use of a sash position switch incorporated into a sash management program for safety.
The BAS can be used for a sash management program by trending the position of the sash. We have several customers that have used this information very effectively as part of an annual safety training class.
CSE: What codes and standards actions in 2008 should engineers be aware of?
Hanegan : Engineers working on hospital pharmacies or similar work should be aware that the United States Pharmacopeia has updated standard 797 on Pharmaceutical Compounding %%MDASSML%% Sterile Preparations.
ASHRAE is currently in the process of updating its Laboratory Design Guide, which was originally published in 2001.
Also, ASHRAE recently published Guideline 4, in 2008, called Preparation of Operating and Maintenance Documentation for Building Systems; and Standard 180-2008, Standard Practice for Inspection and Maintenance of Commercial Building HVAC Systems.
Brooks : AMCA International has recently completed developing a laboratory method of test standard and certified ratings program for induced flow fans. The standard, AMCA 260, Laboratory Methods of Testing Induced Flow Fans for Rating, was written in response to laboratory ventilation industry needs for third-party performance ratings. The standard includes a method to determine the outlet airflow (induced flow) of an induced flow fan. The certified ratings program includes inlet airflow, outlet airflow (induced flow), power, efficiency, and outlet size.
CSE: What are the most serious design challenges for lab ventilation systems? What keeps a lab designer up at night?
Tsimanis : The pressurization control within the lab space is a critical design requirement. The airflow should be designed to minimize chemical smell and to provide the desired space containment. The emergency ventilation can be designed with many different strategies. The challenge in the design is to create a safe escape opportunity for the occupants in event of an emergency.
Hanegan : Trying to design exhaust systems without the benefit of airflow modeling. I am always trying to sell owners on the valuable results that modeling provides over hand calculations, particularly on buildings with complex geometries, to reduce the likelihood of exhaust air being re-entrained into a building.
The next is educating owners and contractors about how to properly replace and leak test HEPA filters and carbon adsorbers. Carbon adsorbers are particularly challenging because they are not changed based on pressure drop, which can be continuously and cheaply monitored, but on consumption of carbon, which must be periodically tested.
I also worry about construction quality when I am not at the jobsite. This includes welding of stainless steel ductwork. My experience has been that even welding done by certified welders needs to be carefully observed and that the ductwork needs to be rigorously leak tested.
Another concern: Construction of lab rooms and sealing of leaks. A leaky room makes it difficult to maintain pressure differentials.
Each building is a prototype full of complex systems that have not been fully assembled and tested before. Owners expect us to be innovative and perfect at the same time. I try to be innovative but worry about the perfect part because I’m human. Quality control on construction documents is really important, along with understanding and communicating owner expectations.
Sprangers : It is often difficult to determine exactly what an end user is going to be doing in a fume hood or lab, so more often than not conservative assumptions are made that can increase initial system cost and/or energy costs over the lifetime of the labs. Determining ACH is a prime example. Four to 12 ACH is a very common range that laboratories fall into or will be designed with, but there are labs that operate with 8 and actually need 15, or operate with 12 and only need 6.
For areas governed by the International Mechanical Code, it can also be difficult to determine whether a system or portion of a system needs to be classified as a hazardous exhaust system. Having a common and documented understanding between the owner, users, design team, and local code authority is a critical step to ensure that the system will not be over- or under-designed and/or get rejected during a code/permit review.
Feliciano-Welpe : Ensuring that the air balance for pressurization has been properly calculated. Designers can only use their best guess for the makeup air (offset) required for maintaining the lab negative (or positive). If the offset has been undersized due to excessively leaky construction, exhaust capacity may fall short. The same holds true for the reverse—tight room, oversized. We frequently see situations where fans are running at a design static pressure higher than design in order to meet flow requirements.
Maintaining pressurization during steady-state is challenging but usually accomplished for most facilities when all is said and done. The harder part is addressing the unfortunate, but undeniable, realities of power failures, return from power failures, partial power outages, equipment failures, etc. All of these situations need to be addressed and incorporated into the design.
How to operate during fan degrade mode (that is, fan systems—supply or exhaust—not running at effective capacities).
Brooks : Lab ventilation systems, as well as all fan systems, require well-designed components used within well-designed systems. A high-efficiency fan that is not properly selected or placed in a poorly designed system may not perform as desired. The system has to be designed to eliminate system effects. The days of procuring oversized fans to account for system uncertainties are (or should be) over. The system designer should be familiar with system effects in order to avoid them. Designers may have nightmares about procuring equipment that does not deliver on performance promises. Ensuring the equipment is certified by independent third-party organizations, such as AMCA International, can help allay that feeling.
CSE: What tricks and tips would you give lab designers for specifying systems and equipment that will help improve performance in the field?
Tsimanis : There are many items that are critical in the laboratory design process. System flexibility, serviceability, and controllability are several elements that are essential for the system’s performance success. The lab control system needs a fast response and a reliable control interface capable of providing a safe environment within the laboratory spaces. It is suggested that in the design phase of the project, the airflows in each space are carefully defined with allowance of the air infiltration and exfiltration from each space.
Hanegan : The best advice I have for specifying systems and equipment is to enforce the specification requirements. This means obtaining owner buy-in that the specifications will be enforced with the contractor (since the buck stops with the owner), writing a spec that is complete and has properly edited boilerplate, and being thorough in site observations. The benefit is that the owner gets what he pays for and the design performs as intended. It also provides an honest, level playing field among contractors and equipment suppliers.
I require certified welders for stainless steel exhaust ductwork, check certificates during the submittal process, and have rigorous leak tests for the ductwork.
I also pay close attention to duct sealing requirements; there’s nothing wrong with asking for higher seal class ratings than the minimum required by SMACNA.
Sprangers : Start by coordinating the big- picture items with your architect. The locations of lab exhausts and intakes drive the entire project in well-designed facilities. Determine as early as possible whether lab flow rates will be dictated by air change requirements, cooling loads, or peak fume exhaust flow rates. If air change requirements dictate the flow rates, there may be no savings for VAV or low-flow fume hoods. Another major influence is which types of energy recovery are deemed acceptable for which exhaust systems.
Feliciano-Welpe : Involve key equipment manufacturers/contractors in the design process early on to help evaluate potential issues. Focus on desired results and performance rather than product specifications that can “pigeon-hole” manufacturers into providing non-standard features that gain no benefit in achieving the desired results. They have also lived with facilities and their users long after the building has been designed/constructed and can give you insight to what has worked well, and more importantly what has not.
Get the commissioning agent involved early in the design process so there is cohesion between what has been designed and what will it be functionally tested to—that is, will the design meet the test requirements? Make actual commissioning documents (specific to the project—not “guide” documents) part of the bid documents. This is the hardest part for a control vendor to estimate. The more information available, the clearer the scope of work becomes, and everyone gets a better product.
Make the construction documents clear and specific for the intended facility. We all learn from our previous experiences, but each facility is going to be different with different challenges.
Brooks : Look for the AMCA CRP seal on the air-moving product (fans, airflow measurement stations, acoustical duct silencer, dampers, louvers, etc.) to ensure you get the performance expected.
A general rule to avoid system effect factors is to have a minimum 2.5 duct diameters of straight duct on the outlet, 5 to 8 duct diameters of straight duct on the inlet, and avoid inlet swirl.
Director of Engineering
AMCA International Inc.
Arlington Heights, Ill.
Director, Life Science Solutions
Siemens Building Technologies Inc.
Buffalo Grove, Ill.
Traci Hanegan, PE, LEED AP
Principal Mechanical Engineer
Ryan Sprangers, PE, LEED AP
Jacob Tsimanis, PE, LEED AP
Client Leader and Mechanical Engineer/Associate Partner
Syska Hennessy Group
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