Fume Hood Airflow Requirements
A fume hood needs adequate airflow to remove, contain and safely discharge airborne chemical contaminants. The fume hood is essentially air handling equipment. It doesn’t work in isolation but depends on the overall laboratory room air availability and dynamics.
Airflow in the hood is achieved by an exhaust blower which pulls the air from the laboratory room into and through the hood and the exhaust system. Air volume passing through a fume hood is generally equal to the area of the sash opening multiplied by the average velocity (face velocity) desired. For example, if 100 feet per minute (fpm) is required and the hood has a sash opening of 7.5 square feet, then the hood’s air handling volume rate is 750 (7.5 x 100) cubic feet per minute (CFM). This is true for a constant air volume (CAV) fume hood. For a variable air volume fume hood (VAV) the air volume rate varies, as per the sash movement. This is driven by system logic, the objective of which is to achieve air-conditioning cost reduction without compromising safety.
In any case, a little less volume rate (CFM) of air must enter the laboratory room by way of either through door/ window openings (non-AC lab) or forced flow through supply air ducts (AC labs). Little less air, because we need the laboratory to be under negative pressure to drive out the contaminated air constantly.
If the incoming CFM is more than the exhausted CFM, the lab will be under positive pressure which would mean the contaminants will remain trapped inside. Also, if incoming CFM is too less the fume hood might starve of air to function effectively and drive out the contaminants. This is the principle around which fume hoods and laboratory air handling systems are designed. The design depends upon the fume hood numbers, types, sizes, locations, diversity, etc. The design and installation are governed by international standards such as EN 14175, ASHRAE 110, SEFA, etc.
There are some other airflow requirements too. The objective is to keep laboratory airflow free of any turbulence (theoretically laminar) so that the contaminants are methodically driven out without lingering inside. Therefore, there mustn’t be any turbulence in the suction area of the fume hood. This is achieved by blocking any direct airflow (air vents/fans) in the vicinity, restricting undue man and material movement in front of the fume hood. Small turbulence in the vicinity of a fume hood might lead to a large effect on containment efficiency.
Similarly, within a fume hood, any apparatus should be located a minimum of six inches behind the airfoil. The rear baffle area must be free of obstructions. Also, Heat loads must be restricted. In short, all necessary precautions must be taken to ensure laminar airflow within and in the vicinity of a fume hood.
Air Flow Monitors mounted on the fume hood keep tracking and displaying the face velocity. They display safe/ unsafe status. They must not be tinkered with. Fume hoods and AFMs require frequent audits, testing, calibration, and certification. The same holds good for room air supply and control system.
Users must be sensitized and trained about fume hood airflow requirements. Unsafe practices such as leaving the sash open, putting the head inside the hood for any reason, checking airflow status by holding a paper strip are strictly prohibited.
As modern technology enables the fume hoods to be safer and more efficient, modern engineering tools like computational fluid dynamics are also emerging. The objective always is to keep the fume hood delivering its optimum performance and ensure a comfortable, safer, and energy-efficient laboratory environment.