The American University in Cairo

Laboratory SafetyChemical Fume Hoods

Introduction

• A hood is the most commonly used engineering control in the labs.

• Laboratory fume hoods are designed to protect the users from hazardous airborne contaminants that may be released in an experiment.

• They are designed to draw air into the hood and away from the user and the laboratory, to contain contaminant air, and to remove it from the building.

Description of Laboratory Hoods

• Hoods come in various sizes. But standard width are 4 and 6 feet.

• Three basic components: chamber, face, and exhaust.

Description of the Hood

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• The Chamber: It has a base or surface, the lining is chemical resistant.

• Water/gas/electricity/steam.• May have installed rinsing devices.

The Face

• Opening to the chamber, where air is drawn into the hood from lab.

• Has a slided sash.• The sash helps keep the contaminant air

inside the hood.• To reduce the risk, close the sash as much

as possible (18 inches).

The Exhaust System

• It draws air away from you.• The exhaust duct is located in the top rear

of the chamber.• A blower or high-powered fan draws the

air.• The most efficient position for the blower is

on the roof of the building (10 feet high).

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• Turbulence in the chamber is caused by irregular air patterns, and it increases the chances for back flow.

• An internal baffle split the air stream and reduces turbulence by keeping the air flow fairly uniform throughout the chamber.

• Newer hoods have air foils, or slits, which help keep the air velocity constant.

Laboratory Chemical Hoods

• Materials resist corrosion.• Located in the distal corners, away from

high traffic to avoid turbulence.• Hood exhaust 10 feet above any adjacent

roofline or intake within 50 feet.• The average face velocity 80 – 100 fpm at

18 inches sash height.

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• Positive steel mechanical latch for the sash to prevent opening above 18 inches without operator intervention.

• Velocities greater the 150 fpm are not acceptable.

• Gas, water, vacuum, etc. controls shall be located on the exterior of the hood, and shall be labeled and color coded.

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• An electronic airflow indicator – high alarm 150 fpm and low alarm at 80 fpm.

• On hoods with filters – indicator to indicate pressure drop across the filter.

• Fume hoods ductwork shall be operated with negative static pressure in the ductwork through all spaces in the building.

Types of Chemical Fume Hoods

• 1- Standard fume hoods.• 2- Bypass fume hoods.• 3- Auxiliary fume hoods.• 4- Perchloric fume hoods.• 5- Radioisotopes fume hoods.

Standard Fume Hoods

• Sash, rear baffles, blower and the ductwork which connects the hood to the blower.

• Velocity will vary according to the sash height.

• The air velocity may be high.

Bypass Fume Hoods

• Same as standard + a bypass.

• The operation of the bypass depends on the position of the sash.

• This design keeps the velocity at the base of the hood fairly constant.

Auxiliary Fume Hoods

• Same as bypass with the addition of an outside air supply. (additional blower and ductwork).

• Small amount of A/C room air is exhausted.

• Difficult to design.

Perchloric Acid Fume Hoods

• Constructed of stainless steel – equipped with water sprays to wash down any perchloric acid deposits which are potentially explosives.

• Should be used for perchloric acid only.

Radioisotopes Fume Hoods

• Interior is constructed from stainless steel to prevent absorption and accumulation of radioactive materials.

• Designed to be easily cleaned. (removable baffle for cleaning).

• Equipped with integral filter components. HEPA or carbon filters.

Proper Use

• Equipment and other materials should be placed at least six inches (15 cm) behind the sash.

• Do not use fume hood as a storage cabinet.

• Do not store chemicals against the baffles.

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• Minimize the pedestrian traffic immediately in front of the hood. Walking past the hood causes turbulence which can draw contaminant out of the hood into the room.

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• Adjust the sash height to the smallest opening (18 inches) (45 cm).

• Have an indication position for the sash.

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• PPE – glasses – lab coat – face shield• Electrical receptacles ex-proof for

flammable liquids.• Large items should be elevated at least 2

inches (5 cm) from the hood base to ensure airflow to the baffle openings.

• Regular maintenance and measuring air velocity.