Construction And Installation

Laboratory hoods shall not have a user-controlled on/off switch. Exhaust fans shall run continuously without direct local control from laboratories.

 

The switch could be inadvertently turned off if it is located in the laboratory.

 

UC Practice

 

New fume hoods shall be from a manufacturer acceptable to the University and specifically approved by the area Industrial Hygienist. All fume hood designs shall demonstrate containment of tracer gas less than 4.0AM0.01 and 4.0AI0.05 when tested according to ASHRAE Test Standard 110-1995.

 

ANSI Z9.5

 

Variable air volume (VAV) hoods should be used, unless there are sound reasons to not use VAV hoods (e.g., if there are only a few hoods or dedicated single-ducted hoods). In those cases where VAV hoods cannot be used, CAV hoods with bypass air openings shall be used.

 

All hoods shall be equipped with sash stops on vertical rising sashes allowing the sash height to be set at 18 inches during routine use, unless there are sound reasons to use another sash height. Hoods equipped with sash stops should be furnished with an alarm to indicate openings in excess of the design sash opening area.

 

ANSI Z9.5 3.1.1.1

 

Where CAV hoods are used, the bypass air opening shall not be uncovered until the sash has been lowered to 2/3 of the full opening height. The opening shall progressively and proportionally uncover as the sash is lowered to its lowest point. The face velocity at the lowest sash opening should not exceed three times the nominal face velocity with the sash open to the operating height. The hood exhaust volume shall remain essentially unchanged (<5% change) when the sash is fully closed.

 

AIHA Z9.5 3.2.1

 

New hoods may be mounted on a chemical storage cabinet.

 

UC Practice

 

Underhood storage units shall comply with the guidance of the 'Hazardous Materials Storage Cabinets' section of this design manual.

 

Interior fume hood surfaces shall be rigid and safe, and be constructed of corrosion-resistant, nonporous, noncombustible materials appropriate for the intended use.

 

The interiors of hoods shall have smooth and impermeable interior surfaces with rounded corners. Interior surfaces should be free of cracks and crevices to provide for easy cleaning.

 

Laboratory hood work surfaces shall be provided with a means of containing minor spills. Generally, a 3/8-inch indentation in the work surface provides adequate containment. Any openings in the work surface, for example, chases for routing hoses to underhood vacuum pumps, shall extend above the top of the work surface containment and be caulked liquid-tight to the work surface.

 

Hoods shall have airfoils and sidewall designs that reduce leakage, and airflow eddies at the front edge of the work area. Airfoils shall not interfere with the hood’s ability to meet the criteria of performance testing specified by the University.

 

The airfoil at the front of the hood floor assures a good sweep of air across the work surface toward the back of the hood. This minimizes the generation of turbulence or eddy currents at the entrance to the hood.

 

UC Practice

AIHA Z9.5 3.1

 

The rear and top interior of the hood shall be furnished with baffles to provide at least two, preferably three, slots. Baffles should be continuous across the back of the fume hood. Externally adjustable baffles shall not be used.

 

This is in order to attain a reasonably uniform face velocity under various conditions of hood use.

 

UC Practice

NFPA 45, Chapter 6-8.1.2

 

All hoods shall be equipped with a quantitative air flow indicator, and should have an alarm to alert users to high- and low-exhaust flow. The flow-measuring device shall be capable of indicating airflows at the design flow and ±20% of the design flow. The means of alarm or warning chosen should be provided in a manner readily visible or audible to the hood user. The alarm should warn when the flow is 20% low, and that is 80% of the set-point value. The choice of audible vs. visible alarms shall be made considering the potential needs of a physically disabled user.

 

Excessive face velocity (greater than approximately 125 fpm) can create turbulence and instability within the fume hood. Low face velocity can cause the fume hood to spill under use conditions. The flow monitor and selected should be easily checked for calibration and, if necessary, adjusted. Manufacturer’s procedures should be obtained and followed concerning calibration of the flow indicators and alarms.

 

Mandatory -  UC Policy

Title 8 CCR 5154.1

8 CCR 5154.1(e)(3)

NFPA 45, Chapter 6-8.7.1

 

Light fixtures should be of the fluorescent type, and replaceable from outside the hood. Light fixtures should be displaced or covered by a transparent, impact-resistant, vapor-tight shield to prevent vapor contact. Hood lighting shall be provided by UL listed fixtures. If located within the hood interior, the fixtures shall meet the requirements of NFPA 70 (National Electrical Code) sections appropriate for hazardous atmospheres.

 

NFPA 45, Chapter 3-6

24 CCR California Electrical Code

NFPA 70 National Electrical Code

 

The valves, electrical outlets, and switches for utilities serving hoods shall be placed at readily accessible locations outside the hood. Shutoff valves shall be clearly labeled. Plumbing (e.g., vacuum lines) should exit the sides of the fume hood and not the benchtop.

 

UC Practice

ANSI Z9.5 3.1

NFPA 45, Chapter 6-8.5.1

 

Hood electrical switches shall have indicator lights.

 

Hoods shall have an individually trapped sink or cup sink, when needed. Backflow preventers or vacuum breakers shall be used to protect domestic water supplies, in accordance with local policies.

 

Drying ovens shall not be placed under fume hoods

 

UC Practice

 

Supply or auxiliary air hoods are not permitted.

 

It is very difficult to keep the air supply and exhaust of supply hoods properly balanced. In addition, the supply air is not tempered, causing discomfort for those working in the hot or cold air stream. As a result, the supply vent is often either shut or blocked off, which eliminates any potential benefit of this type of hood. Finally, the presence and movement of the user's body in the stream of supply air creates turbulence that degrades the performance of the hood.

 

UC Practice

 

Portable, nonducted fume hoods are not permitted.

 

Portable hoods often do not meet the regulatory airflow requirements. Filters/sorbent beds used with these units shall be changed frequently, and vary in filtration effectiveness from chemical to chemical. Contaminants adsorbed on carbon will also tend to desorb with the passage of time, and the lifetimes of the sorbent beds can not be readily predicted when used for multiple contaminants. Experience has demonstrated that an OSHA compliance officer may require quarterly monitoring of hood exhaust to demonstrate the effectiveness of the filtration in the given application and the corresponding protection of the workers occupying the space. These hoods are often misused.

 

UC Practice

 

Floor-mounted hoods are used when the vertical working space of a bench hood is inadequate for the work or apparatus to be contained in the hood. The base of the hood shall provide for the containment of spills by means of a base contiguous with the sidewalls, and a vertical lip of at least 1 in. (2.54 cm) or equivalent. The lip can be replaced by a ramp to allow wheeled carts to enter the hood. The hood must be furnished with distribution ductwork or interior baffles to provide uniform face velocity. Doors and panels on the lower portion shall be capable of being opened for the installation of apparatus.

 

ANSI Z9.5 2.3.5