Overview
Fume hoods are commonplace in almost every laboratory and play an important role in protecting the hood user by containing potentially harmful chemicals and associated vapors or odors. In research, medical, and education labs, fume hoods are typically used to handle and store chemicals.
It is important to understand fume hoods are designed to protect the user, but not necessarily the products being used in the hood – thus the primary goal of a fume hood is containment. This differs from other lab equipment, such as bio-safety cabinets or isolation gloveboxes, which are designed to protect both the user and the product inside of the cabinet.
In the engineering world, fume hoods are typically referred to as a Primary Engineering Control, or PEC. They are the first line of protection to ensure laboratory air remains free of potential chemical toxins or contaminants. Because of the health risks associated with many chemicals, it is evident proper installation, maintenance, and control of fume hoods are imperative.
Equally important to PECs, are Secondary Engineering Controls, or SECs. SEC refers to the environment the PEC, or hood, is located in. Typically, other HVAC equipment serving the room, but not directly associated with the fume hood, is considered part of the Secondary Engineering Control. The SEC can have a major impact on fume hood performance and user safety, therefore proper design, set-up, and commissioning of the SEC equipment is critical.
The risks associated with the improper installation and set-up of a fume hood can be high and jeopardize the health and safety of its users. It is best practice to have fume hoods tested and certified as they are manufactured, installed, and thereafter on an annual basis. These performance tests are typically referred to as ‘as manufactured’, ‘as installed’, and ‘as used.” Due to the critical nature of fume hoods and the associated SECs, owners and lab safety officers should always consult a certified professional to perform NEBB or ASHRAE 110 Fume Hood Performance Testing.
Fume Hood Configurations & Control Schemes
There are a variety of ways to design and implement fume hood control systems. Control system selection heavily depends on the type of fume hood desired by the user, the processes being performed in the fume hood, and the overall requirements of the SECs. Safety is the most important consideration when selecting a fume hood control scheme.
Secondary to user safety, other aspects such as environmental safety, comfort control, energy efficiency, maintainability, and cost should be considered. The following sections outline various fume hood control schemes and provide a broad overview of the benefits and drawbacks of each set-up. The outline is by no means comprehensive, but is intended to introduce popular industry practices for fume hood control and act as a springboard for further discussion.
Single Constant Volume Bypass Fume Hood with Dedicated Exhaust Fan
Perhaps the least complicated and most cost-effective set-up involves a single constant volume airflow (CV) bypass fume hood ducted to a dedicated CV roof-mounted exhaust fan. This fume hood configuration is commonly used in labs with a small number of hoods where control of the lab environment is not critical, and the hood can be turned off when not in use. The exhaust fan should be selected by a licensed engineer, and set-up by a TAB contractor, to deliver the airflow specified by the fume hood users based on the chemical processes performed in the hood.
Typically, fans are belt-driven and run at a constant speed without the need for a variable frequency drive (VFD). Commonly, the fume hood face is equipped with a switch to turn the hood off when not in use to conserve energy. As with most CV bypass fume hoods, as the sash is lowered below the sash operating height (typically 18” above the worksurface) air is bypassed into the fume hood at the top of the hood to maintain constant air flow. ANSI/ASSP Z9.5-2022: Laboratory Ventilation standard requires fume hoods to be equipped with a flow indicator, or airflow sensor, that alerts the user when fume hood airflow is reduced below a low limit threshold. These sensors are calibrated and set-up by the TAB contractor or fume hood certifier.
The most obvious benefits of this configuration are the low first cost, lifecycle cost, and simplicity of set-up. Once the fume hood is tested and certified, the hood operates with limited need for a control system or ancillary equipment. As with any belt-driven exhaust fan, proper periodic maintenance of the belts, bearings, motor, etc. are imperative to ensure proper operation of the fume hood.
Multiple Bypass Fume Hoods with Common Exhaust Fan(s)
Slightly more complex than a single bypass fume hood set-up, this configuration typically involves a dedicated roof-mounted exhaust fan, or set of fans, serving multiple bypass fume hoods equipped with manual balancing dampers.
The manual balancing dampers are used to balance exhaust airflow to each hood as specified by the design documents or manufacturer specifications. This fume hood configuration is most commonly used in labs with a small number of hoods where hoods remain on (no switch to turn fan off) for chemical storage, overnight processes, or pressurization requirements of the SEC. Overall, set-up and maintenance of fume hoods in this configuration is like that of a single bypass fume hood aside from the additional manual balancing performed during initial TAB.
Some installations may be equipped with an adjustable bypass damper on the exhaust plenum used to induce additional airflow at the exhaust stack to maintain at least 3,000 FPM air velocity. It is important to note if airflow to one hood is changed, typically via a manual balancing damper, it will affect flow to all other hoods and potentially jeopardize containment abilities and user safety.
TAB contractors and fume hood certifiers should be aware that raising, lowering, or closing the fume hood sash can slightly affect airflow at that hood by 5-10% of the total air flow. Variations in fume hood airflow exceeding 10% when the sash position changes, may point to an issue with the hood’s bypass air intake. The following configuration introduces a solution that eliminates the need for manual balancing dampers and limits variations in airflow as connected fume hood sashes are opened or closed.
Multiple Bypass Fume Hoods Equipped with Airflow Control Valves
Constant volume airflow control valves (AFCVs) are used to maintain constant exhaust airflow to the fume hood no matter the sash position of the hood or changes occurring at other devices connected to the exhaust system.
For bypass fume hood applications, manual AFCVs are commonly selected, and come factory set at the airflow specified on the design documents or by the hood user. After installation, valve airflow should be confirmed and manually adjusted if required. It is important to note AFCVs must operate within a pressure drop range to maintain airflow, therefore the drop across the valves should be confirmed during set-up.
Most valves can operate within a pressure drop range between 0.3”w.c. to 3”w.c., although some fast-acting valves can operate at a pressure drop up to 6”w.c. AFCVs that are electronically adjustable and equipped with valve position feedback, airflow readout, and low pressure drop/airflow alarms are available. However, the installation and controls cost required for these valves typically outweighs the benefits in bypass fume hood applications.
The use of AFCVs is more expensive compared to previously discussed configurations, however these valves greatly improve controllability of the fume hoods. Because of this, AFCVs are common in labs with a large number of fume hoods where hood sash positions, or other devices connected to the exhaust system, may vary consistently throughout theday. AFCVs are typically installed in research or medical labs where fume hood processes involve hazardous chemicals or are mission critical.
In this configuration, hoods are served by a dedicated roof-mounted exhaust fan typically equipped with a motorized bypass damper. For critical applications, it is best practice to specify multiple fans connected to a common exhaust plenum with each fan sized to handle the full exhaust flow of the hoods in the case that one fan fails. Fan(s) typically run at a fixed fan speed as the bypass damper(s) modulate to maintain a static pressure set point within the exhaust duct. Maintaining this set-point, determined by the TAB contractor, ensures the pressure drops across each AFCV remain within the recommended range.
Multiple Variable Volume Fume Hoods Equipped with Airflow Control Valves
As the name implies, variable air volume (or VAV) fume hoods are equipped with controls that vary the amount of airflow to the hood, typically as the sash position is changed. VAV fume hoods are common in labs, or lab buildings, that host a relatively large number of hoods and are best served by a dedicated set of roof-mounted, VFD-equipped, laboratory fans.
The benefits of VAV fume hoods are realized in the energy saving potential. Ideally, when hoods are not in use the sash is lowered and exhaust airflow through the hood is reduced by the control system. This reduction in airflow allows the fan(s) serving the hoods to reduce operating speed, amp draw, and thus energy. The first cost and complexity of a VAV fume hood control configuration is much greater than that of the previously discussed configurations; however, the energy savings over time can be quite large relative to the first cost.
At the Primary Engineering Control (PEC) level, VAV fume hoods are typically served by AFCVs that modulate in response to changes in sash position or changes in hood face velocity. Sash position is sensed by sash position sensors mounted on, or integral, to the sash or sash track. Generally, a hotwire anemometer installed on the inner side wall of the hood is used to indirectly monitor face velocity.
The operation and accuracy of both the sash position sensor(s) and hotwire should be confirmed during the certification process. High end AFCVs are fast-acting, in the order of seconds, and are commonly equipped with feedback that outputs the estimated airflow (in CFM) exhausted through the valve.
Additionally, these valves are typically provided with a differential pressure switch that is factory set to break and relay an alarm to the building management system (BMS) when the valve lacks sufficient duct static pressure to control airflow accurately and effectively. Most AFCV manufacturers now provide models that control accurately with only 0.3”w.c. of pressure drop across the valve – a small amount of pressure loss relative to the drop of the entire exhaust system.
According to ANSI/ASSP Z9.5-2022: Laboratory Ventilation, Section 4.3.2., an in-depth risk assessment should be performed by the laboratory safety officer, in conjunction with the design engineer or fume hood certifier, to determine the appropriate minimum air flow for each VAV fume hood. The assessment team shall consider the overall containment and removal abilities of the hood at reduced flows, the chemicals used in the hood, and the potential for explosion in the hood, among other factors.
The Secondary Engineering Control (SEC) approach can vary drastically from lab to lab when specifying a configuration with multiple VAV fume hoods. Lab heat load, air exchange rate, pressure, and safety requirements should drive the SEC decision-making process to determine the appropriate control scheme and necessary level of control. Although there are several SEC options, two common approaches are room pressure control and room airflow offset control. Depending on the application, size of the lab, number of hoods, etc., these two control approaches can be quite intricate. In the interest of brevity, an outline of these SEC schemes is provided below.
Room pressure control, as the name implies, utilizes digital room pressure monitors (PM) to monitor lab pressure. PMs are integrated into the BMS, and the control system responds to maintain the required room pressure setpoint(s). Commonly, the room is either served by a supply air AFCV or general exhaust AFCV that modulates to maintain the pressure setpoint.
Room airflow offset control relies on the AFCVs and control system serving the lab to tabulate the total airflow into and out of a room. The control system then modulates either the supply or general exhaust valve(s) accordingly to maintain an airflow offset that satisfies the room pressure requirement.
Conclusion
The goal of a fume hood is simple, to protect the user from harm; but achieving the proper level of protection can be a challenge. There are several fume hood control configurations and seemingly even more Secondary Engineering Control possibilities for chemical lab spaces. A well-thought-out fume hood design will not only consider the type of fume hood desired by the user and the processes being performed in the fume hood, but also the overall requirements of the SEC.
It cannot be overstated that safety is the most important consideration when determining the appropriate fume hood and lab control scheme. Secondary to user safety, environmental safety, comfort control, energy efficiency, maintainability, and cost should be considered. The various control approaches outlined above provide basic information for owners, hood users, designers, contractors, fume hood certifiers, and facility personnel to consider when selecting, setting up, and maintaining a laboratory control system.
