Fume hood


A fume 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 or back into the room, 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

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.
In 1904 the newly built Chemical Faculty at the Technical University in Gdańsk was equipped with fume hoods made of wood and glass in auditoria, several lecture rooms, student laboratories and rooms for scientists. Sliding up and down front panel with glass protected from fumes and explosions. Each fume hood was illuminated, equipped with gas installation for heating and running water with a drain. Harmful and corrosive gaseous byproducts of reactions were actively removed using the natural draft of a fireplace chimney. This early design is still functioning after over 110 years.
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, and many aspects of his concept are incorporated in modern fume hood designs.
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, though from the 1990s onward, epoxy powder-coated steel, teflon and polypropylene coatings were being recommended by literature for use in fume hood and exhaust construction.

Description

A fume hood is typically a large piece of equipment enclosing six sides of a work area, the bottom of which is most commonly located at a standing work height. 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.
Two main types of fume hood exist: Ducted and recirculating. The principle is the same for both types: air is drawn in from the front side of the cabinet, and either expelled outside the building or made safe through filtration and fed back into the room. This method of airflow control is intended to:
  • protect the user from hazardous substances
  • protect the product or experiment
  • protect the environment
  • provide ventilation of the space
Secondary functions of these devices may include explosion protection, spill containment, and other functions necessary to the work being done within the device; these functions may be achieved through enclosure design, duct design, and optimal placement of the fume hood in a room.
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. Specialty enclosures for teaching may allow for additional visibility by constructing the sides and back of the unit from tempered glass, intended so that several individuals can look into a fume hood at once, though they often have poorer ventilation capabilities. Some demonstration models built for educational purposes are movable, can be transported between locations or are built on a movable island, and may be ductless; they are often built with less demanding restrictions on chemical resistance, but offer other advantages, such as lower energy costs.
Fume hoods are generally available in 5 different widths; 1000 mm, 1200 mm, 1500 mm, 1800 mm and 2000 mm. 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. These designs can accommodate from one to three operators. All modern designs are required to be fitted with air flow meters to ensure that the hood is working properly while in use.
For exceptionally hazardous materials, an enclosed glovebox or class III biosafety cabinet may be used, which completely isolates the operator from all direct physical contact with the work material and tools.

Build materials

The frame and build materials used for a fume hood are selected based on anticipated chemical and environmental exposures over the life of the equipment. 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, as the material is easy to decontaminate. Stainless steel is less common than other materials due to its chemical vulnerability and cost.
  • Polypropylene: A build material with greater chemical resistance than some contemporary materials. Also used in cleanrooms. Lower cost than mild steel or stainless steel, but less heat resistant.
Manufacturers variously construct sash windows out of safety glass, tempered glass, high impact polyvinyl chloride, or plexiglass. The most common configuration of a sash window is a type that slides vertically and is counterbalanced for ease of movement when using heavy glass. Setups that handle hydrofluoric acid may use a window made of polycarbonate.

Liner materials

The interior of a fume hood is often subject to damaging chemicals and elevated temperatures, and as such it is often lined with materials resistant to the chemicals and environment it is expected to be subject to. In most cases, only the working surface at the bottom of the enclosed space is made from a liner material, which is most frequently built from epoxy resin or stainless steel, but a fume hood may be lined with any of the following materials:
Most fume hoods are fitted with a mains-powered control panel and/or air flow-monitoring device. Typically, they will allow for the manual or automatic adjustment of internal baffles, but are required by ANSI 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
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

Most fume hoods for industrial purposes are ducted. A large variety of ducted fume hoods exist. In most designs, conditioned air is drawn from the lab space into the fume hood and then dispersed via ducts into the outside atmosphere.
To reduce lab ventilation energy costs, variable air volume systems are employed, which reduce the volume of the air exhausted as the fume hood sash is closed. This product is often enhanced by an automatic sash closing device, which will close the fume hood sash when the user leaves the fume hood face. The result is that the hoods are operating at the minimum exhaust volume whenever no one is working in front of them.
Since the typical fume hood in US climates uses 3.5 times as much energy as a home, 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.