Fume hood

Fume hood

A common modern fume hood.
Other names	Hood Fume cupboard
Uses	Fume removal Blast shield
Related items	Laminar flow cabinet

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 epoxy coated mild steel is the main construction material. Two main types of unit exist, ducted and recirculating. With the ducted type, old asbestos vent pipe has been superseded on health grounds, typically with PVC or polypropylene. 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 filtration 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. Alarm control panels are common, see below.

Fume Hood Exhaust Options

• Auxiliary Air

This method is outdated technology.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.

• Belt Drive Systems -

• 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. 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

• 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^{[citation needed]}, 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.

Specialty Hood Types

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 HVAC 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 acid 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 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.

Use

This article contains instructions, advice, or how-to content. The purpose of Wikipedia is to present facts, not to train. Please help improve this article either by rewriting the how-to content or by moving it to Wikiversity or Wikibooks. (September 2009)

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 (reagents and/or the experimental apparatus if the products 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 mains-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 1-inch (25 mm) by 6-inch (150 mm) 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 100 feet (30 m) per minute at sash opening of 18 inches (460 mm) 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.

See also

• Laminar flow cabinet

• Vented Balance Safety Enclosure

External links

• University of Louisville's Chemical Hood User's Guide
• Information from the University of Bath in the UK

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