Wrong by Design

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Wrong By Design

Ten Critical Points in Laboratory Exhaust Ventilation Design and Installation

BY MAHDI FAHIM

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Laboratory fume hoods are the most important exposure control devices (ECDs) in research laboratories where hazardous materials, including toxic, corrosive, carcinogenic, and allergenic substances, are used. In the hierarchy of controls, engineering controls, such as fume hoods, are considered the most effective preventive method, compared to administrative controls and personal protective equipment (PPE), for eliminating or significantly reducing exposures to hazardous materials. Fume hoods and ECDs are expected to effectively capture and contain hazardous chemicals and exhaust them into the outdoor environment. When using hazardous materials, researchers heavily rely on fume hoods and other approved local exhaust devices to protect their health and safety. Occupational and environmental health and safety professionals may conduct only limited quantitative exposure monitoring based on the assumption that the laboratory exhaust systems are working effectively. It is necessary to ensure that these devices are meeting safety expectations.

Laboratory exhaust systems and their components are complex. If OEHS professionals do not understand and consider in advance certain selection, design, installation, test, calibration, and preventive maintenance criteria, they may compromise system performance, negatively impact hood containment, and jeopardize users’ safety. This article lists the top 10 areas OEHS professionals should consider when designing or installing new laboratory exhaust systems or modifying existing systems.

1. CHOOSE THE RIGHT LABORATORY EXHAUST SYSTEM There are several factors to consider when deciding between variable air volume (VAV), constant air volume (CAV), high-performance, or other exhaust systems. Short-term energy savings or needs should not be the sole focus or the main driver. To balance safety and efficiency, think about the following factors:

Flexibility. The system’s design should be able to accommodate future exhaust needs, new exhaust devices, and additional capacity in both supply and exhaust without making the system oversized and adding unnecessary costs to the project. The excess exhaust capacity that each system needs should be assessed case by case and may differ from one project or building to another. If the system is designed to use only recirculating (that is, filtered or ductless) fume hoods, savings in energy costs may be significant, but the design may constrain new research opportunities as demand for experiments and materials increases or changes.

Compatibility. The exhaust system, as well as the hood or ductwork materials, should be compatible with both current applications and future needs. Specific processes, such as open acid digestions, high volume or heated perchloric acid use, iodination, or hydrofluoric acid evaporation in large and open scales, may require different hoods, ductwork materials, ductwork designs, and minimum exhaust flow. Hoods for some applications, such as perchloric acid use, will require CAV systems that are not connected with the rest of the building exhaust and run on individual fans. If other requirements, such as a water washdown system and special fan and ductwork material, are not considered early in the design phase, adding them later will be cost prohibitive or may not be feasible. OEHS professionals need to gather information from researchers to ensure they communicate expectations and receive a hood or system suitable to their needs.

Costs. These include increased costs for system testing, maintenance, calibration, and parts replacement. Complex exhaust systems include hardware and software that must be upgraded or replaced on a routine basis. Including these needs in budget and cost estimates early in planning may significantly impact design decisions.

Labor. Additional labor may be necessary for testing and maintenance. New VAV or detection-based control systems are complex and may require third-party involvement or well-trained and experienced in-house team engagement. Designing or modifying an exhaust system without an experienced mechanical, electrical, and electronic maintenance team is a recipe for disaster.

2. INVOLVE ALL STAKEHOLDERS IN THE INITIAL STAGES OF PLANNING AND DESIGN OEHS practitioners should connect users’ needs and safety or compliance requirements with designers’ and engineers’ ideas and perspectives. Most users are not aware of differences in fume hood designs and applications, linear length, and depth. By talking to users and researchers and understanding their needs, OEHS professionals can help them select the right types and sizes of exhaust devices. Not only can this increase research efficiency and enhance user safety, but it can also open up opportunities to reduce initial and long-term energy costs. Often, a smaller benchtop hood that is less costly and requires less exhaust flow compared to a full-scale fume hood may be all a researcher needs to set up an experiment or enclose an instrument.

Maintenance team members and their supervisors also need to be involved in the initial stages of planning and design. This will give them an opportunity to better understand the system’s components, preventive maintenance needs, and long-term calibration and adjustment requirements. OEHS professionals must also plan initial training for exhaust system users and maintenance teams. The scope of the project should include official training for the maintenance team before testing, balancing, and commissioning is completed and the system is fully functional.

It is OEHS professionals’ responsibility to ensure that all future users of the fume hoods and other local exhaust devices are fully trained on the devices’ safety components, proper use, functions, and limitations. The training program must reach new users before they use the fume hoods. Establishing refresher training sessions and methods of communicating or reporting system malfunctions, shutdowns, modifications, and failures, as well as work order procedures, is crucial for sustaining a laboratory safety program.

3. DOCUMENT VENTILATION SYSTEM REQUIREMENTS AND SPECIFICATIONS OEHS professionals should develop an in-house laboratory design and construction document that includes all design requirements, including these major elements:

Supply air diffuser types and location requirements for reducing cross drafts. Supply and exhaust diffuser layout is also important for achieving efficient mixing and avoiding the short-circuiting of supply or exhaust air.

Fume hood requirements. These may include requirements for sash stops, flow monitors, maximum sash heights, design sash heights, sash mechanisms (that is, vertical, horizontal, or combination mechanisms), hood work surface materials, spill containment, face velocity, and VAV response times for the ventilation hood and the surrounding room.

Cabinet requirements. Specify acceptable types of base storage cabinets and the exhaust connection requirements for corrosive cabinets. For example, should the cabinets connect directly to the exhaust duct, or to the leading exhaust pipe inside the fume hood? Flammable base cabinets are not usually connected to exhaust systems.

Acceptable room exhaust control mechanisms. For example, night setback controls or sensors that lower exhaust until motion is detected or the light switch is turned on can reduce general exhaust to pre-approved rates when the lab is unoccupied. The air changes per hour (ACH) for unoccupied spaces needs to be defined on a case-by-case basis depending on risk.
The acceptable cross-draft tolerance for each hood. This value should be calculated based on a percentage of the hood design face velocity, such as 30 percent of the hood’s average face velocity, not a preset value, such as 30 feet per minute.

Pictured: Measuring downdraft velocity across the HEPA filter in a clean room laminar flow fume hood. This complex face velocity evaluation is usually misunderstood and balanced improperly.

Conditions within which the system’s diversity factor is acceptable. The diversity factor refers to the assumption that not all hoods will be used at any given time. Designing the system with the expectation that it will never need to accommodate the simultaneous operation of every hood may result in energy cost savings, but it may also pose significant safety risks to users. If the hood exhaust system is designed with a diversity factor, additional provisions are needed for routine test protocols, documentation, and training. Separate training will need to be developed for OEHS professionals, users, and maintenance team members. Exhaust stack types, heights, and locations to prevent re-entrainment. An exhaust dispersion study is highly recommended to ensure that the exhaust plume is not drawn back into the building air intake. The main causes of exhaust re-entrainment include exhaust stacks being located too close to the building air intake, stacks being too short, and stacks having low velocity. Adding new structures adjacent to an existing building may have a significant impact on the exhaust plume pattern. As-manufactured (AM) test certificate requirements. Any selected hood must have an AM ANSI/ASHRAE 110 test certificate describing what testing criteria the hood passed, as specified in ANSI/ASHRAE 110, Methods of Testing Performance of Laboratory Fume Hoods. Requirements for all other local exhaust devices. These include gas cabinets, in-house built enclosures, slot hoods, and snorkels. To define the minimum flow required for these devices, conduct a risk- or hazard-based analysis, or refer to resources from organizations such as ACGIH, the American National Standards Institute (ANSI), and the Scientific Equipment and Furniture Association (SEFA). Emergency power requirements for fume hood exhaust fans. Care must be taken to avoid excessive negative pressurization of rooms when a VAV supply system is shut down during a fire or other emergency. The number of exhaust fans that remain connected to emergency power must be sufficient to provide minimum exhaust flow to all fume hoods when their sashes are fully closed. A process for approving modifications to existing laboratory exhaust devices or systems. Define the scope of the projects that may trigger the need for a new testing, adjusting, and balancing (TAB) or repetition of the ASHRAE 110 test. Documentation should also include the following elements: • critical elements of applicable codes and standards • ventilation rates or ACH criteria for each type of lab or space • VAV fume hood minimum exhaust flow settings (that is, when the sash is closed) • acceptable sash or flow control mechanisms, including automatic sash closure, zone presence sensors, light sensors, or emergency purge buttons • a list of pre-approved fume hoods, including special-design hoods • requirements for hoods that are compliant with the Americans with Disabilities Act (ADA) and locations of each ADA-compliant hood on every floor and in every building • a list of pre-approved fume hood monitors and flow alarms • code requirements for exhaust duct materials, exhaust manifolding in occupied spaces, and joints or connection types, such as welded or flange connections • exhaust fan types and specifications • TAB and commissioning requirements for the heating, ventilation, and air conditioning (HVAC) system • references to other applicable design and construction documents, including laboratory design and plumbing for emergency eyewashes and safety showers 4. ESTABLISH A DETAILED, AS-INSTALLED ANSI/ASHRAE 110 TESTING PROTOCOL Third-party testing contractors must satisfy requirements stipulated in the lab’s ANSI/ASHRAE hood testing protocol and provide reports on measurements, pass or fail criteria, and recommendations. This protocol should include: The number of hoods that must undergo ANSI/ASHRAE 110 testing. Does the testing need to be performed on all installed hoods, as is typical, or only on a representative portion? Test challenge requirements. Specify whether contractors should document the sash positions of other hoods in the room while the test is being performed on any given hood. Manifolded exhaust systems are challenged to ensure that exhaust flow meets design criteria and is sufficient to support all fume hoods when their sashes are open simultaneously at a designated height, such as 18 inches. This test must be planned differently if the system is designed with a diversity factor. Required specifications. These include face velocity targets and minimum, maximum, and average tolerance, low or high flow alarm setpoints, acceptable cross-draft ranges, cross-draft measurement references, and room pressurization requirements. Specify areas, such as clean rooms, where the room pressurization setting is different than in labs and needs to be positive relative to connected or adjacent spaces. Pass or fail criteria for the ANSI/ASHRAE 110 tracer gas test. OEHS professionals may select more stringent pass or fail criteria for the ASHRAE 110 tracer gas concentration compared to levels recommended by other standards, such as ANSI Z9.5, Laboratory Ventilation. They may also define and document maximum or peak tracer gas leak concentration criteria. A pre-testing checklist. Make sure that installation is complete, the system is fully functional, and the room or hoods are balanced and meet design specifications. This checklist should be signed by the project manager, mechanical engineer, and OEHS professional and presented to the third-party testing agency in advance. Qualifications for third-party hood testing companies. Not all third-party testing companies have the tools and expertise to meet a lab’s test protocol requirements. Look for experienced and certified technicians who perform fume hood or ASHRAE 110 testing. 5. ESTABLISH HAZARD AND RISK-BASED ACH CRITERIA FOR EACH SPACE When establishing these criteria, consider their feasibility and issues related to management of change. In some cases, hood density and room environment optimization may require greater ACH needs, leaving no margin to save energy costs by reducing ACH. Performing a risk assessment will also help to specify design criteria such as exhaust stack velocity, type of fume hood or ECD, hood face velocity, exhaust duct velocity, and VAV minimum flow rate. 6. DEFINE A VAV HOOD MINIMUM FLOW RATE USING ANSI GUIDELINES Selection of a lower minimum flow is based on a risk assessment performed by an OEHS professional, as specified by ANSI Z9.5. For special hazard hoods or for applications that release high volumes of hazardous chemicals, including corrosives, inside the hood, the upper range of the minimum flow rate needs to be considered. Good change management practices are critical when there is potential for major changes in processes and types, volumes, concentrations, and physical states of materials, as well as the frequency of hazardous material use. The VAV control system must be able to maintain the maximum five-second response time based on the lower minimum flow set point. Hood minimum flow must be documented and verified during routine tests. A building automation system (BAS), if used properly, can verify hood minimum flow. 7. DOCUMENT THE MINIMUM REQUIRED EXHAUST FLOW FOR ALL ECDS Design flow rates for other ECDs, unlike those for fume hoods, are not usually well documented, which makes it harder to perform routine tests and performance verification. For each ECD, define and provide the as-installed measurement for the minimum exhaust flow. It is good practice to label each device with a nameplate or durable sticker displaying the target design flow. 8. REVIEW ROOM LAYOUT DRAWINGS Evaluating the layout of the room may help OEHS professionals identify and correct potential excess cross draft sources. Practices to address cross drafts include: • installing hoods away from doors, high traffic areas, and other areas where high-velocity air currents may occur • positioning supply air diffusers at least 6 feet from the hoods • using non-directional or low-throw-velocity supply diffusers 9. REQUIRE RISER DIAGRAMS FOR EACH BUILDING OR SYSTEM Riser diagrams should indicate the capacity of each exhaust fan and the locations, types, design flow rates, and number of exhaust devices connected to each fan. The riser diagram, along with the supply air capacity diagram, provides crucial information for future additions or modifications to the laboratory ventilation system. 10. REVIEW CODE REQUIREMENTS This review should include ductwork material, hazardous exhaust recirculation limitations, and approved heat recovery systems. Consider local code requirements. Heat recovery systems such as enthalpy wheels should not be used on hazardous exhaust. The recirculation of hazardous exhaust may be allowed within certain spaces only with the review and approval of an OEHS professional. Ensure that hazardous exhaust cannot be recirculated among different spaces. ESTABLISHING A LAB VENTILATION MANAGEMENT PROGRAM To maintain success and effectiveness, the elements discussed in this article need to be implemented as an institution’s lab ventilation management program (LVMP). ANSI Z9.5 provides guidelines for establishing an LVMP where all stakeholders work together to create and maintain a safe, compliant, sustained, dependable, efficient, and effective lab environment. MAHDI FAHIM, MS, is the environment, health, and safety manager at Biogen. Send feedback to The Synergist.

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