Fume hood - WikiHQ

A fume hood (sometimes called a fume cupboard or fume closet, not to be confused with extractor hood) is a type of local exhaust ventilation device that is designed to prevent users from being exposed to hazardous fumes, vapors, and dusts. The device is an enclosure with a movable sash window on one side that traps and exhausts gases and particulates either out of the area (through a duct) or back into the room (through air filtration), and is most frequently used in laboratory settings.

The first fume hoods, constructed from wood and glass, were developed in the early 1900s as a measure to protect individuals from harmful gaseous reaction by-products. Later developments in the 1970s and 80s allowed for the construction of more efficient devices out of epoxy powder-coated steel and flame-retardant plastic laminates. Contemporary fume hoods are built to various standards to meet the needs of different laboratory practices. They may be built to different sizes, with some demonstration models small enough to be moved between locations on an island and bigger "walk-in" designs that can enclose large equipment. They may also be constructed to allow for the safe handling and ventilation of perchloric acid and radionuclides and may be equipped with scrubber systems. Fume hoods of all types require regular maintenance to ensure the safety of users.

Most fume hoods are ducted and vent air out of the room they are built in, which constantly removes conditioned air from a room and thus results in major energy costs for laboratories and academic institutions. Efforts to curtail the energy use associated with fume hoods have been researched since the early 2000s, resulting in technical advances, such as variable air volume, high-performance and occupancy sensor-enabled fume hoods, as well as the promulgation of "Shut the Sash" campaigns that promote closing the window on fume hoods that are not in use to reduce the volume of air drawn from a room.

History

thumb|Wooden fume hood at Gdansk University of Technology (2016 picture of 1904 installation still in use)

The need for ventilation has been apparent from early days of chemical research and education. Some early approaches to the problem were adaptations of the conventional chimney. A hearth constructed by Thomas Jefferson in 1822–1826 at the University of Virginia was equipped with a sand bath and special flues to vent toxic gases. The draft of a chimney was also used by Thomas Edison to provide ventilation in his work around the year 1900.

The first known modern "fume cupboard" design with rising sashes was introduced at the University of Leeds in 1923. 13 years later, Labconco, now a prominent fume hood manufacturer, developed the first fume hood for commercial sale, reminiscent of modern designs with a front-facing sash window. Soon after, in 1943 during World War II, John Weber, Jr. developed a fume hood concept with a dedicated exhaust fan, vertically rising sash window, and constant face velocity in response to concerns about exposure to toxic and radioactive substances. This design would become standard among atomic laboratories at the time,

The first mass-produced fume hoods were variously manufactured from stone and glass, most likely soapstone or transite, though stainless steel was being used by at least the 1960s. Labconco introduced the concept of a fume hood lined with fiberglass to improve durability and chemical resistance, Fume hoods are most often found in laboratories that require the use of materials that may produce harmful particulates, gaseous by-products, or aerosols of hazardous materials such as those found in biocontainment laboratories. This method of airflow control is intended to:

protect the user from hazardous substances

Fume hoods are generally set back against the walls and are often fitted with infills above, to cover up the exhaust ductwork. Because of their recessed shape they are generally poorly illuminated by general room lighting, so many have internal lights with vapor-proof covers. The front of the device includes a sash window, usually in glass or otherwise transparent glazing, which is able to slide vertically or horizontally. they are often built with less demanding restrictions on chemical resistance, but offer other advantages, such as lower energy costs. The depth varies between 700 mm and 900 mm, and the height between 1900 mm and 2700 mm. Regions that use primarily non-metric measurements often follow construction standards that round these dimensions to the closest value in inches or feet. Several common materials used for the exterior construction of a modern fume hood include:

Mild steel, powder coated: The traditional method of building fume cupboards is from a zinc coated mild steel. The cost is often low, but has corrosion issues over time and a high carbon footprint to manufacture. Powder coatings may be made from epoxy or other plastics, such as polyvinyl chloride.
Stainless steel: Typically used in radioactive applications, in cleanrooms, or in ATEX environments, and EN standards to provide visual and audible warnings in the following situations:

Air flow is too high or low
Too large an opening at the front of the unit (a "high sash" alarm is caused by the sliding glass at the front of the unit being raised higher than is considered safe, due to the resulting air velocity drop)

Some control panels additionally allow for switching mechanisms inside the hood from a central point, such as turning the exhaust fan or an internal light on or off.

Ducted fume hoods

alt=Ducted Fume Hood|thumb|A ducted fume hood

Most fume hoods for industrial purposes are ducted. A large variety of ducted fume hoods exist. In most designs, conditioned (i.e. heated or cooled) air is drawn from the lab space into the fume hood and then dispersed via ducts into the outside atmosphere. the reduction or minimization of exhaust volume is strategic in reducing facility energy costs as well as minimizing the impact on the facility infrastructure and the environment. Particular attention must be paid to the exhaust discharge location, to reduce risks to public safety, and to avoid drawing exhaust air back into the building air supply system; exhaust requirements of fume hood systems may be regulated to prevent public and worker exposures.

Auxiliary air

Fume hoods with an auxiliary air supply, which draw air from outside the building rather than drawing conditioned air from the room they are placed in, have been controversial and are often not recommended. In addition to providing a non-conditioned environment inside the hood as compared to outside the hood, which may cause discomfort or irritation to workers, the sash is adjusted to an appropriate working height to achieve adequate face velocity.

Non-bypass CAV

The most basic design of a CAV fume hood only has one opening through which air can passthe sash opening.

A major drawback of conventional CAV hoods is that when the sash is closed, velocities can increase to the point where they disturb instrumentation, cool hot plates, slow reactions, and/or create turbulence that can force contaminants into the room.

Low-flow/high-performance bypass CAV

High-performance or low-flow bypass CAV hoods are a modern type of bypass CAV hoods and typically display improved containment, safety, and energy conservation features. These hoods include features such as sash stops on the window, automatic baffle control via sash position and airflow sensors, fans to create a barrier of air between the user and the enclosure, and improved aerodynamics to maintain laminar flow.

Variable air volume (VAV)

alt=A white metal enclosure with a partially-opened glass sash at front|thumb|A variable airflow (constant-velocity) fume hood, with a visible flow sensor

VAV hoods, the newest generations of laboratory fume hoods, vary the volume of room air exhausted while maintaining the face velocity at a set level. Different VAV hoods change the exhaust volume using different methods, such as a damper or valve in the exhaust duct that opens and closes based on sash position, or a blower that changes speed to meet air-volume demands. Most VAV hoods integrate a modified bypass system to a conventional fume hood system to achieve a variable exhaust volume in proportion to the opening of the hood's face,

In a survey of 247 lab professionals conducted in 2010, Lab Manager Magazine found that approximately 12% of fume hoods are VAV fume hoods. Chemical-resistant filtered canopy hoods are manufactured by select vendors, but are not ideal for worker safety, as the fumes they draw in from equipment underneath pass through a worker's breathing zone. or generally in a lab bench area where processes that require additional ventilation are performed. but are comparatively low maintenance. and such filters have to be replaced regularly.

Air filtration of ductless fume hoods is typically broken into two segments: Additional specific filtration techniques can be added to combat chemicals that would otherwise be pumped back into the room.

The advantages of using a ductless fume hood include their ease of implementation compared to ducted hoods, and the fact that conditioned air is not removed from the building. These factor alone provide measurable savings in energy usage. Columbia University, Princeton University, the University of New Hampshire, and the University of Colorado, Boulder either discourage or prohibit the use of ductless fume hoods. Additionally, while typically not classified as such, the manner in which biosafety cabinets are operated when not connected to a duct system is functionally the same as a ductless fume hood,

In a survey of 247 lab professionals conducted in 2010, Lab Manager Magazine found that approximately 22% of fume hoods are ductless fume hoods. and are comparable to laminar flow cabinets. The laminar flow within these devices is easily disrupted, more so than traditional fume hoods, which can result in exposure to hazards within the hood.

Floor-mounted

Also termed "walk-in" fume hoods, floor-mounted fume hoods have a working area that extends from the floor to the bottom of a connected exhaust duct for the use of tall equipment. Despite the name of "walk-in", entering a floor-mounted fume hood in operation while it contains hazardous materials poses a significant risk to the user; they are only intended to be entered for the initial setup of equipment. Regulations may require that any exhausted material is filtered through a regularly replaced HEPA or activated carbon filter to avoid environmental release of radioisotopes.

Scrubber

Some fume hoods are equipped with scrubber systems designed to absorb particularly hazardous chemical fumes before they are exhausted, whether for environmental or user safety concerns. and also produces hazardous wastewater. and all corners may be altered to be coved or rounded to further reduce the potential for buildup of crystals. A drain is integrated into the design for removal of wastewater solution. This design was first developed by the United States Bureau of Mines in 1964, and is sometimes referred to as an "acid digestion hood".

Energy consumption

Because fume hoods constantly remove large volumes of conditioned (heated or cooled) air from lab spaces, they are responsible for the consumption of large amounts of energy. Fume hoods are a major factor in making laboratories four to five times more energy intensive than typical commercial buildings, and these energy requirements are exacerbated in hot and humid climates. Energy costs for a typical hood can range from $4,600/year in Los Angeles to $9,300/year in Singapore based on differences in cooling needs. Several other institutions report on programs to reduce energy consumption by fume hoods, including:

Hibernation

In 2020, Cornell University sought to reduce energy consumption during times of reduced occupancy (caused by a response to the COVID-19 pandemic) by shutting off airflow to many HVAC systems, including those connected to fume hoods. The process of shutting off, or "hibernating", these fume hoods turned out to be difficult to implement unilaterally across equipment of different models and ages, and only produced significant cost savings when applied over a period of more than 3 months. University of Nebraska–Lincoln, and Massachusetts Institute of Technology.

Use of sensors

Person detection technology, such as motion detectors and occupancy sensors, can sense the presence of a hood operator within a zone in front of a hood. Sensor signals allow ventilation controls to switch between normal and standby or "setback" modes that consume less energy. these technologies can adjust ventilation and lighting use to effectively minimize wasted energy in laboratories. However, there are safety concerns with reducing airflow in fume hoods through sensor signals if the sash is left open; some programs combine the principles of "Shut the Sash" campaigns with variable flow ventilation by using technology to actively remind users to close the sash of a fume hood that is not in use. Comprehensive controls on a laboratory may necessitate the use of a mechanical sash controller module that will automatically close the sash and shut off ventilation in concert with motion sensors.

Construction and installation

A worker building the frame of a fume hood|thumb|right

Fume hoods are typically constructed with a superstructure encasing the various supporting members and inner lining of the hood. This superstructure is often built out of sheet metal, which has apertures punched into it to allow for access to plumbing and electrical receptacles or devices.

Ducted fume hoods have additional specifications necessitated by their design compared to ductless models. Seams in metal exhaust ductwork must be welded, excluding the outer end where a fan or blower is positioned. Depending on design choices and HVAC capabilities, the blower may be installed within or above the hood, or it may be positioned at the exhaust point, usually the roof of the building.

Maintenance

alt=A line drawing depicting a worker in front a of a fume hood viewed from above, with arrows showing airflow direction|thumb|Improper monitoring of fume hood velocity and movements within the enclosure may create a [[wake (physics)|wake that can expose workers to hazardous materials from inside the fume hood.]]

Fume hoods require regular maintenance to ensure consistent functionality; this is in addition to the standard precautions and measures taken during regular operations and ideally involves daily, periodic, and annual inspections:

Daily fume hood inspections entail visual inspections of the fume hood for improper storage of material and other visible blockages. Airflow is often monitored for these daily inspections by taping a piece of tissue paper to the open face of the hood such that it will be drawn inward; if the tissue is not pulled inward, the hood exhaust is not functioning.
Periodic fume hood function inspections require the measurement of capture or face velocity with an anemometer. Fume hoods and other local exhaust devices may be smoke tested to determine if the contaminants they are designed to remove are being adequately captured and exhausted.), maintenance as recommended by the device manufacturer, or upgrades to bring the device into compliance with standards, local building codes, or to meet the specific needs of users. Organizations such as the National Institutes of Health may include wording that requires the retrofitting of already installed fume hoods in updates to their regulatory guidelines. Mechanical changes to ventilation systems or any one fume hood may also affect different devices connected to the system, which can warrant inspection and validation of connected devices after seemingly unrelated work.