A
fume hood or
fume cupboard is a large piece of scientific equipment common to chemistry laboratories designed to limit a person's exposure to hazardous and/or unpleasant fumes. Fume hoods were originally manufactured from timber, but now
epoxyEpoxy or polyepoxide is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives.- Chemistry :...
coated
mild steelCarbon steel, also called plain carbon steel, is steel where the main alloying constituent is carbon. The AISI defines carbon steel as: "Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten,...
is the main construction material. Two main types of unit exist,
ductA duct may refer to:* Duct , various ducts in anatomy and physiology* Duct , for transfer of air between spaces in a structure* Duct tape, a kind of adhesive tape* Ducted fan, motor for aircraft...
ed and recirculating. With the ducted type, old
asbestosAsbestos is a naturally occurring silicate mineral with long, thin fibrous crystals. The word asbestos is a borrowed Greek adjective meaning inextinguishable. The Greeks termed asbestos the miracle mineral because of its soft and pliant properties, as well as its ability to withstand...
vent pipe has been superseded on health grounds, typically with
PVCPolyvinyl chloride, commonly abbreviated PVC, is the third most widely used thermoplastic polymer after polyethylene and polypropylene. In terms of revenue generated, it is one of the most valuable products of the chemical industry. Around the world, over 50% of PVC manufactured is used in...
or
polypropylenePolypropylene or polypropene is a thermoplastic polymer, made by the chemical industry and used in a wide variety of applications, including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and...
.
A
fume hood or
fume cupboard is a large piece of scientific equipment common to chemistry laboratories designed to limit a person's exposure to hazardous and/or unpleasant fumes. Fume hoods were originally manufactured from timber, but now
epoxyEpoxy or polyepoxide is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives.- Chemistry :...
coated
mild steelCarbon steel, also called plain carbon steel, is steel where the main alloying constituent is carbon. The AISI defines carbon steel as: "Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten,...
is the main construction material. Two main types of unit exist,
ductA duct may refer to:* Duct , various ducts in anatomy and physiology* Duct , for transfer of air between spaces in a structure* Duct tape, a kind of adhesive tape* Ducted fan, motor for aircraft...
ed and recirculating. With the ducted type, old
asbestosAsbestos is a naturally occurring silicate mineral with long, thin fibrous crystals. The word asbestos is a borrowed Greek adjective meaning inextinguishable. The Greeks termed asbestos the miracle mineral because of its soft and pliant properties, as well as its ability to withstand...
vent pipe has been superseded on health grounds, typically with
PVCPolyvinyl chloride, commonly abbreviated PVC, is the third most widely used thermoplastic polymer after polyethylene and polypropylene. In terms of revenue generated, it is one of the most valuable products of the chemical industry. Around the world, over 50% of PVC manufactured is used in...
or
polypropylenePolypropylene or polypropene is a thermoplastic polymer, made by the chemical industry and used in a wide variety of applications, including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and...
. The principle is the same for all units; air is drawn in from the front of the cabinet by a fan, and either expelled outside the building or made safe through
filtrationFiltration is a mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...
and fed back into the room.
Construction and location
Fume hoods (fume cupboards) 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 2400 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.
AlarmAn alarm gives an audible or visual warning about a problem or condition.Alarms include:* burglar alarms, designed to warn of burglaries; this is often a silent alarm: the police or guards are warned without indication to the burglar, which increases the chances of catching him or her.* alarm...
control panels are common, see below.
Fume Hood Exhaust Options
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.
- Constant Air Volume (CAV)
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.
- Variable Air Volume (VAV)
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.
Sash Counterbalance Systems
- Cable & Pulley Systems - Typically an aircraft grade stainless steel cable runs over independently positioned pulleys to the counterweight. Cable counterbalance systems can bind when a user lifts the sash from one end of the hood, as the cables will travel across the pulleys at different rates. This system requires regular maintenance as the cables will fray or break over time.
- Chain & Sprocket Systems - Typically a hardened chain (similar to a bicycle chain) runs over sprockets to the counterweight. The sprockets are attached onto a single axle which allows them to turn at the same time. This system will travel smoothly no matter where the user chooses to lift the sash along the hood width. This is especially important on longer hoods where the user may be lifting the sash at one end. This system has an indefinite lifespan with little to no maintenance.
Fume Hood Liners
- FRP - Fiberglas Reinforced Polyester (most common)
- Epoxy Resin
- Square Corner Stainless Steel
- Coved Corner Stainless Steel
- Phenolic Resin
- Cement Board
Recirculating fume hoods
Mainly for educational use, or where the design of a building does not permit the fitting of external ductwork, 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.
Pre-filtration
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.
Main filtration
After pre-filtration, the fumes are sucked through a layer of activated charcoal which absorbs the majority of chemicals that pass through it.
AmmoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
and
carbon monoxideCarbon monoxide is a colorless, odorless and tasteless gas, yet very toxic to humans. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond...
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
- Ductwork not required.
- Temperature controlled air is not removed from the workplace.
- Contaminated air is not pumped into the atmosphere.
Cons
- Filters must be regularly maintained and replaced.
- Greater risk of chemical exposure than with ducted equivalents.
- The extract fan is near the operator, so noise may be an issue.
Ducted fume hoods
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
- Fumes are completely eradicated from the workplace.
- Low maintenance.
- Quiet operation, due to the extract fan being some distance from the operator.
Cons
- Additional ductwork.
- Temperature controlled air is removed from the workplace.
- Fumes are dispersed into the atmosphere, rather than being treated.
Low Flow / High Performance
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
HVACHVAC is an acronym that stands for the closely related functions of "Heating, Ventilating, and Air Conditioning"-the technology of indoor environmental comfort...
operating costs.
Radioisotope Hood
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.
Acid Digestion Hood
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).
Perchloric Acid Hood
These units feature a waterwash system in the ductwork. Because
perchloric acidPerchloric acid, HClO4, is an oxoacid of chlorine and is a colorless liquid soluble in water. It is a strong acid comparable in strength to sulfuric and nitric acids...
fumes settle, and form explosive crystals, it is vital that the ductwork is cleaned internally with a series of sprays.
Waterwash
These fume hoods have an internal wash system that cleans the interior of the unit, to prevent a build-up of dangerous chemicals.
Scrubber
This type of fume hood
absorbsAbsorption, in chemistry, is a physical or chemical phenomenon or a process in which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material. This is a different process from adsorption, since molecules undergoing absorption are taken up by the volume, not by the surface...
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.
Use
To determine whether a chemical is likely to require a fume hood for safe usage, its MSDS should be consulted. If there is any doubt, a hood should be used.
An operating and maintenance manual should be provided with a new fume hood, which will detail full usage instructions for a new user.
If you already know about the hood you are going to use, begin by making sure no one else is using it or has left things in it. If it's free collect what you need to be in the hood (
reagentA reagent or reactant is a substance or compound consumed during a chemical reaction. Solvents and catalysts, although they are involved in the reaction, are usually not referred to as reactants....
s and/or the experimental apparatus if the
productA product is a substance that forms as a result of a biological- or chemical reaction. While the end product of some chemical reactions may be the result of a relatively rapid reaction, nanoseconds to seconds, chemical equilibria in complex systems may require years or even centuries to be...
s will give off noxious fumes).
If the light in the hood is too dim to see (It probably will be; even if there's enough light in the room, you will be working in your own shadow) then turn on the internal light.
Stand (or sit if there is a stool) where you will be likely to be when you are working, then lower the glass front as far as you can without making it impossible to get your arms under it and work around.
Control panels
Most fume hoods are fitted with a
mainsMains is the general-purpose alternating current electric power supply. The term is not often used in the United States and Canada. In the US, mains power is referred to by a variety of formal and informal names, including household power, household electricity, domestic power, wall power, line...
-powered control panel. Typically, they perform one or more of the following functions:
- Warn of low air flow.
- Warn of too large an opening at the front of the unit. Known as a "high sash" alarm, this 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.
- Provide a method of switching the exhaust fan on or off.
- Provide a method of turning the internal light on or off.
Specific extra functions can be added, for example, a switch to turn a waterwash system on or off.
Maintenance
Fume hood maintenance:
- Daily fume hood inspection
- Visually inspect the fume hood area for storage of material and other visible blockages.
- If hood function indicating devices are not a part of your fume hood, place a by piece of soft tissue paper at the hood opening and observe it for appropriate directional flow into the hood.
- Periodic fume hood function inspection
- Capture or face velocity will be measured with a velometer or anemometer.Hoods for most common chemicals must have an average face velocity of per minute at sash opening of or higher.Face velocity readings should not vary by more than 20%. A minimum of six readings shall be used determine average face velocity.
- Other local exhaust devices shall be smoke tested to determine if the contaminants they are designed to remove are being adequately captured by the hood.
- Annual maintenance
- 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.
External links