Low-Flow Fume Hoods:
Baffles and Vortices
A Summer Research Project funded by the DOE ERULF program by:
Mentor: Geoffrey Bell, P.E.
Ernest Orlando Lawrence Berkeley National Labs
Environmental Energy Technologies Division, "A"pplications Team
This summer I worked at LBNL as a summer undergraduate research fellow.
I joined Geoffrey Bell of the Environmental Energies Technology division's
Applications Team, in his efforts to develop a working alpha prototype
of a new laboratory fume hood design. What's new about our hood is
the way we approach the task of keeping a technician safe from potentially
toxic fumes emitted within the hood. Conventional designs tend to
rely on a single airflow pattern: suction out the top of the hood.
Ours, invented by visiting LBNL fellow Dr. Helmut Feustel, utilizes a “push-pull”
approach to containment and evacuation with the help of supply fans at
the top and bottom of the face. These fans direct room air into the
hood, creating a protective “curtain” of room air to prevent toxic fumes
from reaching the worker's breathing zone. The supply fans are small
and allow a significant decrease in power required of much larger exhaust
fan. This leads to a decrease in volume of room air flowing through
the face of the hood. Since face velocity is currently the standard
by which most fume hoods’ performance are evaluated, our task has been
to not only optimize the design parameters (fan speed, screen shape, baffle
configuration) and work towards a patent, but also to identify/create a
new standard of testing that allows for variations in approach such as
ours. When the project is completed and a finalized version of the
fume hood is ready to be manufactured and installed into new and existing
labs, we anticipate that our work will have produced a device that utilizes
30% of the power required by conventional fume hoods--significant savings
for environmental and economic concerns, given the fact that most fume
hoods are never turned off in a lab.
This was the basic structure of our project team. Although we had individually assigned task areas, our project was much more cohesive than a simple set of exclusive delegated assignments. For example, Mike's work with plenum designs and screen profiles directly interfaced with mine on baffle arrangements and fan speeds, for we were both working towards the same goal: optimization of airflow throughout the hood. On occasion we would even be lucky enough to have Jeff descend from his cubicle and try his hand at some sawing or drilling. This project was definitely an interactive team effort; Geoffrey presided over all aspects, providing direction and support by splitting time between office-based research in building 90 and hands-on empirical testing of the hood in the workshop of building 63.
You could say that I had a "baffle"ing experience at the lab this summer! Not only did I have to deal with the frustration of working with a silly, bickering Australian who can barely speak English, but I also did extensive work experimenting with various baffle, airfoil, and fan speed configurations in an effort to obtain the best airflow patterns through the hood. The tasks set out for me at the beginning of the summer were essentially twofold: analyze the effect of the airflow vortices produced within the hood, and perform the aforementioned optimizations based on my conclusions regarding the vortex issue. I was also involved in other aspects of the project. I spent a great deal of time working on random tasks that were needed to keep the project moving along, including plenum modifications, presentations and reports for the various consultants and guests we had coming in to view our project, and making preparations for ASHRAE-110 testing of the hood. The variety of my responsibilities this summer meant that I had an opportunity to learn and perform many functions: empirical testing and working with various crafting tools, interaction with a Computational Fluid Dynamics (CFD) modeling program for theoretical predictions of airflow, and Auto CAD diagram production.
Here's one of several CFD plots I used to help me predict airflow patterns within the hood. The multi-colored lines indicate velocity vectors through a two-dimensional cross-section of our fume hood. Fluent is a very powerful program--it allows you to specify all the dimensions and geometry of your device, and then input entry air locations and speeds. The result is an invaluable tool in predicting the behavior of your device--I found it to be quite accurate in predicting the two vortices shown in the plot above. It was also nice to be able to experiment with different baffle setups and fan configurations without actually having to crawl inside the hood and tear it apart. Unfortunately, I didn't have the time this summer to learn how to actually work the program on my own, but it was still really helpful to have access to it as a tool.
Contrary to my expectations coming into the lab this summer, a lot of the research I performed was highly qualitative in nature. It's true that our project falls under the category of "engineering" or more specifically "energy efficient applications," but the truth is that fluid dynamics and the study of airflow is definitively empirical. Although there are a few equations and formulas which help predict how a fluid will behave, the vast dependency on the specifics of your setup, apparatus, geometries, etc. means that most of what I learned was through playing around with cardboard, tape, and other hands-on work within the hood. Additionally, my goal was to find an "optimum setup" of the baffles and airfoils. There were a few quantitative aspects to my research, such as determining the velocity of air (in feet per minute, or fpm) flowing through the inlets and throughout the hood, or the amount of sulfur hexaflouride (in parts per million, or ppm) detected at the nose of our mannequin during the ASHRAE-110 testing, but my findings in those areas served more as a guide rail from which I had to branch off and make conclusions on performance based on my own judgment. A few of my findings: