Face velocity is the average amount of air that is pulled through the face of a hood. If these velocities are too low then chemical fumes can escape the hood and go back into the lab. If the velocities are too high, energy costs will be high as you exhaust large amounts of conditioned air from the room. In other words, if too much air is being pulled into the machine (high volumetric flow) it becomes extremely inefficient. If too little air is being pulled into the machine (low volumetric flow), then it can be dangerous for those around the equipment. Airspeed is everything when discussing face velocity. Today we are going to discuss fume hood face velocity requirements, so you can keep your equipment safe and efficient.
Face Velocity Requirements
A traditional hood should be between 60 and 100 FPM according to many agencies. This may increase to 120 feet per minute if you’re working with highly toxic chemicals. The appropriate velocities are not determined by manufactures but instead are generally dictated by local code. Because different fume hoods can operate at different face velocities, selecting the right hood for the applications should be considered to meet local requirements.
Note: Faster flow doesn’t mean better. In fact, it can actually mean less efficient and create more dangerous condition due to turbulent air.
Average Face Velocity Calculations
Average velocity is calculated by dividing the sash opening into an imaginary grid pattern. Each square of this grid should be approximately 1ftsq. Then take air measurements (velocity readings) at the center point in each of these imaginary squares. You can do this using a velometer, anemometer or similar device. Each one of these readings should be for approximately 10 seconds (the longer the better). Record the reading in a notepad and repeat until you have measured from all squares. Once complete analyze the measurements and look for outliers.
If any measurement is 20% over or under the standard then you may have a problem. Such readings indicate the possibility of turbulent or nonlaminar airflow. Smoke tests will help confirm whether this is problematic. This is according to the National Research Council (US) Committee on Prudent Practices in the Laboratory.
Analyzing The Results
If velocities are found to be greater than .8 m/s then the fume hood face velocity is too fast and should be reduced. If velocities are found to be below .4 m/s then the face velocity is too low and should be increased.
Smoke Tests: Visualizing Fume Hood Airflow
Start by acquiring a smoke pen, puffer, smoke stick, or other smoke source. Take the smoke device and move it around the bottom of the hood. Then along all inside edges and ensure the smoke is being exhausted.
Next place the smoke device just outside the fume hood and determine if the outer environment is affecting the airflow negatively. For additional information on how to perform a full smoke test check out NEBB’s Procedural Standards For Fume Hood Performance Testing.
Additional Safety Checks
Air is supplied to laboratories through air supply diffusers that are usually located on the ceiling. These devices make up for the air that is being exhausted out of the building. If large quantities of air are being exhausted, then high quantities of air are needed for makeup. And if air currents from the diffusers reach the face of the chemical hood, you could have a problem. At a high enough velocity, they can cause fumes to be blown into the lab. This is why it is critically important to ensure diffusers are balanced regularly and moved or replaced if velocities become too high.
Foot Traffic Analysis
The average human walks at a 3mph pace and produces a vortex of air behind them as they walk. This vortex, although small, can cause chemical fumes to be pulled out from the fume hood into the laboratory. That is why it is important to analyze foot traffic. Ensure it is not being directed in front of fume hoods while they are in operation. This is especially important if your face velocity is on the lower end of the acceptable FPM range.
ASHRAE 110 Fume Hood Tests
ASHRAE 110 is the preferred method for testing the performance of laboratory fume hoods. It is important to note, that this certification does not directly affect safety. Instead, this standard gives guidelines to meet when performing quantitative tests for fume hoods. These tests include: **As Manufactured (AM): **The fume hood is built, assembled, and tested by the manufacturer at their facility. As Installed (AI): The fume hood is installed at the location of the customer. This ventilation system is tested and balanced with no fumes in the fume hood's final location. As Used (AU): The fume hood is tested and balanced after it has been in use by the customer.
Have Questions About Fume Hood Face Velocity Requirements?
We can help. Our Lab Safety team performs annual inspections, routine maintenance, service, and repair on all fume hood classes. Annual inspections and routine maintenance commonly include:
Tracer Gas Contaminants
Using a dispersion device, we release a tracer gas like sulfur hexafluoride inside the fume hood. The performance of the hood is determined by measuring the concentration of the tracer gas the breathing zone.
A fume hood's effectiveness is dependent on the environment of the fume hood. If there are drafts in this space, they can create turbulent air pockets and the draw the contaminants from the hoods. Cross-draft testing mitigates this risk.