Essential Airflow Requirements for Fume Hoods That Users Must Ensure

A fume hood is very essential for limiting exposure to toxic or hazardous fumes, dust, and vapours. Such equipment for air handling requires enough airflow. Otherwise, it can't safely discharge the chemical contaminants from the air. Fume hoods or fume cupboards can't work alone. It depends on the fundamental dynamics and air availability of a laboratory room. 

The method and principle of work of a fume hood

An exhaust blower in the hood pulls the room's air to get the airflow for it. This airflow passes through and into not only the exhaust system but the hood itself. The air volume which passes through the fume hood generally equals the multiplication of the overall sash opening and the average face velocity. If the sash opening is 7.5 square feet and the hood requires 100 FPM (feet per minute), the air volume rate of the hood will be (100 x 7.5) = 750 CFM (cubic feet per minute). 

However, this is only applicable to a Constant Air Volume or CAV. A Variable Air Volume or VAV involves another formula. For VAV, the air volume rate changes according to the sash movement. 

The system incorporates such logic in order to reduce the costs of air-conditioning while maintaining safety. 

Emphasizing the airflow requirements

CFM of air must remain less for a laboratory room. For a non-ac laboratory, the less CFM of air should enter through the doors or windows. In the case of an air-conditioned lab room, the supply air ducts will pass the forced flow. The fume hood can't remove the contaminated air continuously without the laboratory being under negative pressure. 

If the incoming CFM exceeds the exhausted CFM, the laboratory room will go under positive pressure. Then, the air contaminants would be stuck inside the room. But the incoming CFM can't be too low either, or the fume hood lacks enough air. Starving of air interrupts it from functioning correctly and driving the contaminants out of the lab. 

So, this is how air handling is done in a laboratory with the help of fume hoods. The design of the overall air handling system depends mainly on the fume hoods' types, numbers, sizes, diversity and locations. 

Certain international standards like ASHRAE 110, EN 14175, and SEFA govern the installation process and design.

Ensuring the containment efficiency

The airflow should be without turbulence to methodically remove the air contaminants from the lab. In terms of airflow, there are other requirements to ensure that even a few contaminants don't remain inside. The suction area of a fume hood should be kept totally without any turbulence. Therefore, you have to block any direct airflow from fans or air vents. 

Additionally, prevent any kind of man or material movement directly in front of the fume hood. Even minor turbulence, which is often neglected, can significantly hamper a fume hood's containment efficiency.

Besides, an apparatus within a fume hood should be placed at a gap of six inches, at least, behind the airfoil. You need to keep the rear baffle area free from obstructions. Heat load restrictions are another essential factor. You have to ensure that the airflow within the fume hood and in its vicinity remain laminar. Otherwise, a fume hood can't work effectively and with ease.

How to track the airflow?

There are airflow monitors to track and display the face velocity for a fume hood. The monitor will show you whether the airflow is safe or unsafe.

For the best functioning

A fume hood, along with the AFMs, requires regular audits, calibration, testing and certification to function at its best. Anyways, you have to maintain the same for the room air supply system also.

Activities that should be avoided

The users need appropriate guidance on the airflow requirements of fume hoods before installing and using them. Some of the don'ts regarding the use of fume hoods are the following:-

• Don't leave the sash open

• Don't hold something like a paper strip to check the status of the airflow

• Don't put your head inside the fume hood at any cost

Final words

The advanced fume hoods with the latest technology are more efficient and safer. Tools like computational fluid have added to its efficiency. Overall, an appropriate use can make the fume hood deliver the most impressive performance. With this, the laboratory personnel will get a safer and more comfortable environment to work in.