A common modern fume hood. |
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| Other names | Hood Fume cupboard |
|---|---|
| Uses | Fume removal Blast shield |
| Related items | Laminar flow cabinet |
A fume hood or fume cupboard is a type of local[1] ventilation device that is designed to limit exposure to hazardous or noxious fumes, vapors or dusts. A fume hood is typically a large piece of equipment enclosing five sides of a work area, the bottom of which is most commonly located at a standing work height.
Two main types exist, ducted and recirculating. The principle is the same for both types: air is drawn in from the front (open) side of the cabinet, and either expelled outside the building or made safe through filtration and fed back into the room.
Other related types of local ventilation devices include: clean benches, biosafety cabinets, glove boxes and snorkel exhausts. All these devices address the need to control airborne hazards or irritants that are typically generated or released within the local ventilation device. All local ventilation devices are designed to address one or more of three primary goals:
Secondary functions of these devices may include explosion protection, spill containment, and other functions necessary to the work being done within the device.
A general but non-specific term for some of these local ventilation devices is Laminar flow cabinet. This category may include clean benches, biosafety cabinets and other devices characterized simply by the laminar nature of their airflow. The term laminar flow cabinet, however, is insufficient to identify their actual design and use - some will protect the product but not the user, and others will protect both. Terminology for local ventilation devices has been, and remain, unclear and non-specific, and the reader is advised to take special care in their selection and specification based upon which of the three primary goals (listed above) are to be met.
Fume hoods typically protect only the user, and are most commonly used in laboratories where hazardous or noxious chemicals are released during testing, research, development or teaching. They are also used in industrial applications or other activities where hazardous or noxious vapors, gases or dusts are generated or released.
Because one side (the front) of a fume hood is open to the room occupied by the user, and the air within the fume hood is potentially contaminated, the proper flow of air from the room into the hood is critical to its function. Much of fume hood design and operation is focused on maximizing the proper containment of the air and fumes within the fume hood.
As most fume hoods are designed to connect to exhaust systems that expel the air directly to the exterior of a building, large quantities of energy are required to run fans that exhaust the air, and to heat, cool, filter, control and move the air that will replace the air exhausted. Significant recent efforts in fume hood and ventilation system design have focused on reducing the energy used to operate fume hoods and their supporting ventilation systems.
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Fume hoods were originally manufactured from timber, but now epoxy coated mild steel is the main construction material. Fume hoods (fume cupboards) are generally available in 5 different widths; 1000 mm, 1200 mm, 1500 mm, 1800 mm and 2000 mm.[2] The depth varies between 700 mm and 900 mm, and the height between 1900 mm and 2700 mm. These can accommodate from one to three operators. They are generally set back against the walls and are often fitted with infills above, to cover up the exhaust ductwork. Because of their shape they are generally dim inside, so many have internal lights with vapor-proof covers. The front is a movable sash, usually in glass, able to move up and down on a counterbalance mechanism. On educational versions, the sides of the unit are often also glass, so that several pupils can gather around a fume hood at once. Low air flow alarm control panels are common, see below.
This method is outdated technology. The premise was to bring non-conditioned outside air directly in front of the hood so that this was the air exhausted to the outside. This method does not work well when the climate changes as it pours frigid or hot and humid air over the user making it very uncomfortable to work or affecting the procedure inside the hood. This system also uses additional ductwork which can be costly.
This hood allows air to be pulled through a "bypass" opening from above as the sash closes. The bypass is located so that as you close the sash and reduce the sash opening, the bypass opening gets larger. The air going through the hood maintains a constant volume no matter where the sash is positioned and without changing fan speeds.
This hood works with sash positioning controls to let the HVAC system know how much the sash is being opened. The controls then let the system know to reduce or increase the fan speed and thus the volume of air that needs to be exhausted.
Mainly for educational or testing use, these units generally have a fan mounted on the top (soffit) of the hood, or beneath the worktop. Air is sucked through the front opening of the hood and through a filter, before passing through the fan and being fed back into the workplace. With a recirculating fume hood it is essential that the filter medium be able to remove the particular hazardous or noxious material being used. As different filters are required for different materials, recirculating fume hoods should only be used when the hazard is well known and does not change. Recirculating fume hoods are often not appropriate for research applications where the activity, and the materials used or generated, may change or be unknown.
The first stage of filtration consists of a physical barrier, typically of open cell foam, which prevents large particles from passing through. A filter of this type is generally inexpensive, and would last for approximately six months, dependent on usage.
After pre-filtration, the fumes are sucked through a layer of activated charcoal which absorbs the majority of chemicals that pass through it. Ammonia and carbon monoxide will, however, pass through most carbon filters. Additional specific filtration techniques can be added to combat chemicals that would otherwise be pumped back into the room. A main filter will generally last for approximately two years, dependent on usage.
| Pros | Cons |
|---|---|
| Ductwork not required. | Filters must be regularly maintained and replaced. |
| Temperature controlled air is not removed from the workplace. | Greater risk of chemical exposure than with ducted equivalents. |
| Contaminated air is not pumped into the atmosphere. | The extract fan is near the operator, so noise may be an issue. |
Most fume hoods for industrial purposes are ducted. A large variety of ducted fume hoods exist. Air is removed from the workspace and dispersed into the atmosphere.
The fume hood is only one piece of the lab ventilation system. As the recirculation of lab air to the rest of the facility is not permitted, air handling units serving the non-laboratory areas are kept segregated from the laboratory units. As a means of improving indoor air quality, some laboratories also utilize single-pass air handling systems, where air that is heated or cooled is used only once prior to discharge. Many laboratories continue to utilize return air systems to the laboratory areas to minimize energy and running costs, while still providing adequate ventilation rates for acceptable working conditions. The fume hoods serve to evacuate hazardous levels of contaminant.
To reduce lab ventilation costs, variable air volume (VAV) 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 actually working in front of them.
Since a six foot constant volume hood uses as much energy as three average homes in America, the reduction or minimization of exhaust volume is particularly beneficial 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 discharge location, so as not to risk public safety, or to pull the exhaust air back into the building supply air system.
| Pros | Cons |
|---|---|
| Fumes are completely eradicated from the workplace. | Additional ductwork. |
| Low maintenance. | Temperature controlled air is removed from the workplace. |
| Quiet operation, due to the extract fan being some distance from the operator. | Fumes are dispersed into the atmosphere, rather than being treated. |
Conventional fume hoods can consume three times more energy than an average American home. In recent years, laboratory fume hood manufacturers have developed and introduced energy-efficient low-flow/ high-performance fume hoods, designed to maintain or improve operator protection while reducing expensive HVAC operating costs. While there is no standardized definition of the terms "low-flow" or "high-performance," fume hoods that operate with less exhaust flow than would be required to produce 100 feet per minute with a full open vertical sash are typically considered to be "low-flow."[3]
This fume hood is made with a coved stainless steel liner and coved integral stainless steel countertop that is reinforced to handle the weight of lead bricks or blocks.
These units are typically constructed of polypropylene in order to resist the corrosive effects of acids at high concentrations. If hydrofluoric acid is being used in the hood, the hood's glass sash should be constructed of polycarbonate which resists etching. Hood ductwork should be lined with polypropylene or coated with PTFE (Teflon).
These units feature a waterwash system in the ductwork. Because perchloric acid fumes settle, and form explosive crystals, it is vital that the ductwork is cleaned internally with a series of sprays.
These fume hoods have an internal wash system that cleans the interior of the unit, to prevent a build-up of dangerous chemicals.
This type of fume hood absorbs the fumes through a chamber filled with plastic shapes, which are doused with water. The chemicals are washed into a sump, which is often filled with a neutralizing liquid. The fumes are then dispersed, or disposed of, in the conventional manner.
Most fume hoods are fitted with a mains-powered control panel. Typically, they perform one or more of the following functions:
Specific extra functions can be added, for example, a switch to turn a waterwash system on or off.
Fume hood maintenance can involve daily, periodic, and annual inspections.
Exhaust fan maintenance, (i.e.,lubrication, belt tension, fan blade deterioration and rpm), shall be in accordance with the manufacturer’s recommendation or as adjusted for appropriate hood function.
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