Architects engineers and chefs are well aware of exhaust hoods required for stoves in residential kitchens as well as exhaust and sprinkler hoods required for commercial kitchens. These hoods have the ability to exhaust suck in unwanted smoke from cooking appliances and exhaust it to the outside air.
Laboratory workers are well aware of fume hoods as indoor environmental protection devices. Fume hoods are devices enclosed on three sides as well as the top and bottom. Fume hood units are normally set against rear walls and fitted with infills above to cover up the exhaust ductwork.
Fume hoods encapsulate vapors, dust and gases produced in the hood. Contaminants are then mechanically exhausted to the outside air by the user. Laboratory fume hoods protect workers with a clear sliding window, called a sash. The sash prevents harm from fires and minor explosions inside of the hood.
When to Use a Chemical Fume Hood
Appropriate exhaust hood use involves the following below:
- Flammable chemicals
- Any Chemicals with a national fire Protection Association rating of 3 or 4
- Explosive and reactive materials
- Materials that may splatter
- Carcinogens and other hazardous substances
- Volatile substances such as chloroform and formaldehyde
- Toxic gases such as CO, F2 and NH3
Biohazardous materials such as microbes and spores, will require the use of biological safety cabinets.Chemical fume hoodsare not equipped with HEPA filters. They only protect workers from chemicals and gases. On the contrary, biological safety cabinets remove particles, but not vapors and gases which are removed with fume hoods.
Types of Fume Hoods
Fume hoods are installed with exhaust ducts or they may operate as a ductless unit. The following below describe models of exhaust duct units.
- Constant Air Volume (CAV)
- Non Bypass CAV Hoods
Safety is increased as the hood is closed. Many CAV hoods specify a maximum height for sash opening position. A disadvantage to a conventional CAV laboratory fume hood is the disturbed instrumentation and forced room contaminates as air pressure increases from the closed sash.
- Bypass CAV Hoods
These fume hoods exhaust the same amount of air at all times, regardless of how high or low the sash is raised. The air velocity changes at the front of the hood as the sash is lowered and raised.
- Low Flow/High performance CAV Hoods
These newer type hoods often contain sash stops or horizontal sliding sashes to limit capacity. Other features include air flow sensors (controls air baffles) and newer aerodynamic designs. The higher cost may be returned in energy savings resulting from face velocities as low as 60 fpm.
- Variable Air Volume (VAV) Hoods
These newer models modulate air flow regulated by sash height, and maintain 100 fpm at all times. Optimum face velocity is between 80 to 120 linear feet per minute. Lower velocities may not sufficiently remove particles. Velocities higher than 120 fpm may produce eddy currents and backflows that retard contaminant removal. VAV hoods are equipped with alarms that activate when malfunctions occur.
- Canopy Fume Hoods
Similar to range hoods in kitchens, these units only encompass a hood without a sash or enclosure. This laboratory fume hood will remove non-toxic smoke, steam, heat and odors.
Why Not Use a Ductless Hood?
Mostly engaged for educational use, ductless hoods are mobile and easy to install. However, the lack of ductwork means that contaminant removal relies totally on filters. Contaminates may build up on the filter. A maintenance schedule must be set up, usually by a contractor. The contaminants are disposed of as hazardous waste. Some university research organizations have banned the use of ductless chemical fume hoods.
Fume hoods are optimized as part of a larger HVAC system in a laboratory. They remove large volumes of conditioned air. Owners should have knowledgeable staff personnel on call before attempting any adjustments outside of sash height and baffle control.