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Fume Removal—Engineering Controls for Reducing Employee Chemical Exposure

Introduction

A fume hood or fume removal system is a device used to capture hazardous chemical vapors, gases, dusts, mists and metal fumes in a work process. The purpose of these systems is to eliminate or lower human exposure to these hazards. A fundamental safety and industrial hygiene principle is to control exposure to hazards through the use of engineering and administrative controls prior to the use of personal protective equipment. OSHA also states this in the Respiratory Protection Standard, 29 CFR 1910.134.

Fume hoods are one example of an engineering method used to control exposure to hazardous substances. A fume removal system will consist of at least two of the following components: Collection hood, ducting, air cleaning device and blower. Details of each of these components will be covered.

Regulations

There are government regulations and industry conformance standards which either directly or indirectly require the use of fume removal equipment. This section will identify and highlight many of those regulations.

The Air Contaminant Standard, Title 29 of the Code of Federal Regulations (CFR) Part 1910.1000, established Permissible Exposure Limits (PELs) for many common industrial and laboratory chemicals. A PEL is the highest average chemical concentration that workers can be exposed to based on an eight-hour work day and a forty-hour week. There are also substance-specific standards for Asbestos: 29 CFR 1910.1001, Cadmium: 29 CFR 1910.1027, Formaldehyde: 29 CFR 1910.1048, Benzene: 29 CFR 1910.1028, Lead: 29 CFR 1910.1025, and others, which identify the PEL and other regulations for each specific contaminant.

The Occupational Exposure to Hazardous Chemicals in the Laboratory Standard, Part 29 CFR 1910.1450, mandates that employers control employee chemical exposure below established PELs through the implementation of a chemical hygiene plan. Chemical hygiene plans are developed by employers for their specific facilities and applications. Statements regarding the use of fume removal equipment and maintenance of the equipment would be included in this plan.

The American National Standards Institute (ANSI) has several industry conformance standards which impact fume removal equipment. The ANSI Z9.5-2003 Laboratory Ventilation Standard deals specifically with the design and operation of laboratory ventilation systems. ANSI Z9.2-2006, Fundamentals Governing the Design and Operation of Local Exhaust Systems, deals specifically with industrial ventilation and fume removal applications. These two ANSI standards are referenced in 29 CFR 1910.94 Ventilation Standard.

In addition to these regulations, local building codes, fire codes and environmental regulations all need to be investigated prior to system design and installation.

Components of a Fume Removal System

A fume removal system will consist of two or more of the following components: A blower, ducting, air cleaning device and collection hood.

The blower is a major component of every system and must be selected carefully. Blowers need to be sized properly in order to remove the contaminant from the work area. Blowers are sized and rated by the amount of air, measured in cubic feet per minute, which they can move at a given amount of resistance. The resistance is termed static pressure and is measured in inches of water. It is the amount of resistance the fan must overcome in order to move air through the ventilation system. The amount of static pressure in a system is influenced by the type of collection device, length of ducting, number and amount of turns in the ducting and type of air purification device (if used). Consult a ventilation specialist to determine the correct blower size for each particular application.

Blower flywheels are available in a variety of materials (steel, aluminum, plastic) for different applications. Non-sparking flywheels should be chosen when working with flammable materials. Explosion-proof blowers are available for locations which could be potentially explosive. The motors on these blowers are designed and classified not to cause an explosion in certain types of environments.

Ducting is the material through which the air and contaminants are moved. Common materials are galvanized steel, stainless steel, PVC and other rigid or flexible plastic materials. Angles for fitting corners and reducers for connecting different size ducts are all available accessories.

Air purification devices will either be mechanical filters or chemical adsorption media. Mechanical filters are used to remove particulate contaminants. The type of filter selected will be determined by the application and particle size of the contaminant. HEPA filters are the most efficient type available and are capable of filtering particles of 0.3 microns in size. Mechanical filters will gradually become plugged with the particulate material and air flow through the system will decrease. Pressure gauges and air flow indicators can be used to determine when filters need to be changed.

The use of air purification devices prior to exhausting air outside your facility may be required by your local Environmental Protection Agency (EPA). Determine this prior to designing a system. Cleaning the air and then exhausting it back into the work environment is normally not a recommended practice. However, it can be done if the contaminant concentrations generated are below the PEL and the chemical has good warning properties.

Activated carbon is one type of chemical adsorption medium which can be used for many organic vapors. Other adsorption media are available, depending on the chemical and application. Adsorption filters will eventually accumulate as much contaminant as they are capable of holding. Chemical concentration, humidity, air flow and the chemical's physical properties all play a role in the life of a chemical adsorption filter. Warning properties of the contaminant—the ability to perceive the contaminant at a non-hazardous level—can be one indication of when to change the filter. A safer way to know is to monitor the exhaust air for chemical breakthrough on a routine basis.

The type of collection device used in a fume removal system will depend on the application, the physical properties of the contaminant and the work environment. Three primary types of collection devices are: Cabinet hoods with vertical or horizontal sashes, canopy hoods and local collection hoods which attach directly to a length of ducting.

Cabinet hoods are often used in lab applications because they are effective for a variety of chemical contaminants. The three-sided enclosure is usually made of a chemically resistant material. A horizontal or vertical sash will control air flow through the front opening and can be positioned to offer the best chemical containment. Air is pulled through the front opening and away from the worker. Proper blower size and sash height are important to reduce the chance of turbulence within the hood, which could allow the contaminant to escape. Air currents from the mechanical ventilation system and traffic moving by the hood can all affect the ability of the cabinet to contain and remove the contaminant being generated.

Canopy hoods are wall-mounted or hung from ceilings over the work process and may or may not have side panels. This type of hood works best in applications where the contaminant rises and the worker is not directly under the hood. Cross drafts decrease the effectiveness of these hoods, although side panels can minimize these effects.

Local collection hoods are attached directly to a length of ducting. Hood shape varies depending on the application and the air flow needed to capture the contaminant. These devices are designed for applications where the contaminant is generated at a localized point. Proper positioning of these hoods is critical to their effectiveness.

Definitions

Anemometer—A device which measures air velocity, usually in feet per minute.

Bypass fume hood—A cabinet fume hood constructed such that as the sash closes, air is allowed to bypass the hood face through another opening, usually above the sash. This bypass provides a constant face velocity and hood static pressure.

Capture velocity—The velocity of air induced by a hood to capture emitted contaminants external to the hood.

Ductless fume hood—A fume removal system which utilizes an air purification device and returns exhaust air to the indoor environment.

Dilution ventilation—A form of exposure control which relies on the dilution of airborne contaminants in the workplace air.

Fan curve—A curve relating pressure versus volume flow rate of a given fan at a fixed speed (rpm).

Local exhaust ventilation—A ventilation system which captures and removes emitted contaminants before dilution into the workplace ambient air can occur.

Manometer—A device which measures pressure difference, usually in inches of water gauge.

Static pressure—The pressure developed in a duct by a fan.

Sources for More Information

29 CFR 1910.1000, Air Contaminant Standard

29 CFR 1910.1001, Asbestos Standard

29 CFR 1910.1027, Cadmium Standard

29 CFR 1910.1048, Formaldehyde Standard

29 CFR 1910.1028, Benzene Standard

29 CFR 1910.1025, Lead Standard

29 CFR 1910.1450, Occupational Exposure to Hazardous Chemicals in the Laboratory Standard

29 CFR 1910.94, Ventilation Standard

American National Standards Institute (ANSI)

ANSI Z9.5-2003, Laboratory Ventilation Standard

ANSI Z9.2-2006, Fundamentals Governing the Design and Operation of Local Exhaust Systems

Environmental Protection Agency (EPA)

EZ154 Choosing the Correct Fume Extraction System

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