Exhaust Hoods - Critical But Costly

Exhaust hoods protect operators from breathing harmful fumes by capturing, containing, and exhausting hazardous gases created in laboratory experiments or industrial processes.  These box-like structures, often mounted at tabletop level, offer users protection with a movable, window-like front “face” called a sash.  Fans draw fumes out the tops of the hoods.  (See Fig. 1)

Fume hoods typically exhaust large volumes of air at great expense.  The energy to filter, move, cool or heat, and in some cases scrub (clean) this air is one of the largest loads in most lab facilities.  A six-foot-wide hood typically exhausts 1200 cubic feet per minute (cfm), 24 hours per day, and consumes three-times more energy than an average house.  The annual operating cost of U.S. fume hoods is approximately $3.2 billion, with a corresponding peak electrical demand of 5,000 megawatts.  This equates to the electrical output of about 20 electric power plants (at 250 megawatts each), plus nearly 200 trillion cubic feet of natural gas burned each year, for the associated cooling and heating of outside make-up air.  Consequently, greenhouse-gas emission caused by operating this typical hood is equivalent to six automobiles.

Standard Hoods

Design Characteristics Dictate High Exhaust

Fume hood exhaust induces airflow through the fume hood’s “face.”  The generally accepted “face velocity” is 100 feet/minute; a high airflow rate causing large exhaust flows.  Interestingly, increasing face velocity does not necessarily improve containment (See Fig. 2).  Instead, errant eddy currents and vortexes can be induced around hood users as air flows into the hood, reducing containment effectiveness.

Fume hoods frequently operate 24 hours/day.  Since many laboratories have multiple hoods, they typically dictate a lab’s required airflow and thus the supply and exhaust systems’ capacity.  The result is larger fans, chillers, boilers, and ducts compared to systems having less exhaust.  Consequently, fume hoods are a major factor in making a typical laboratory four to five times more energy intensive than a typical commercial space.

State-Of-The-Art System Limitations

Most state-of-the-art, energy-efficient fume hood systems require several interactive features and diligent users.  Sophisticated controls, for each hood and for supply and exhaust air streams, communicate with a variable air volume (VAV) system to provide a constant face velocity and a pressure differential between the laboratory and adjacent space.  These controls add significantly to the system’s first cost and complexity.

In a VAV system, energy savings occur only if a hood’s sash is less than fully open, which reduces exhaust flow while maintaining a constant face velocity.  Each hood user must operate the sash properly to ensure that the system achieves full energy savings potential.  Also, when sizing air distribution and conditioning equipment, many designers assume worst-case conditions¾all sashes fully open¾requiring larger ducts, fans, and central plants than if assuming some sashes are partly closed.