Manifolding laboratory and fume hood exhaust systems can have many advantages including first cost savings, maintenance and operation savings, system flexibility and system reliability. However, there are some types of laboratory systems that should not be manifolded into a central exhaust system and have special considerations. Some of these include:
Perchloric Acid Fume Hoods
When perchloric acid is heated above ambient temperature the condensing vapors form organic salts that are both shock sensitive and pose a fire and explosion hazard. Fume hoods specifically designed for perchloric acid are constructed of fiberglass reinforced plastic (FRP) or have a seamless welded type 316 stainless steel interior. The National Fire Protection Association (NFPA) Standard 45 requires the entire exhaust system, from the top of the exhaust stack down into the fume hood, to have wash down nozzles to prevent buildup of perchlorates. NFPA 45 also requires the exhaust fan motor and belts to be outside of the air stream. The CRC Handbook of Laboratory Safety even recommends utilizing an induction type exhaust fan so the impeller is completely out of the air stream.
Radioisotope Fume Hoods
Fume hoods for radioisotope use are required to be constructed with a seamless type 304 stainless steel liner with coved corners for easy cleaning and decontamination. The fume hoods must also be strong enough to support any lead shielding required for the radioactive materials. Typically a high efficiency particulate arresting (HEPA) filter or a charcoal HEPA filter located in a bag-out plenum is required in the exhaust ductwork. The filter plenum should be located as close to the fume hood as possible to limit the amount of contaminated ductwork. Exhaust ductwork is typically constructed of fully welded 304 stainless steel.
Biological Safety Cabinets Without Integral Exhaust Fan
Biological safety cabinets require HEPA filters in the exhaust airstream to prevent biological hazards from entering the exhaust ductwork, laboratory and environment. Some cabinets have an integral exhaust fan that is designed to overcome the pressure drop caused by the HEPA filters. However, some biosafety cabinets do not have integral exhaust fans and are typically segregated from other exhaust systems due to the high pressure drop. The pressure drop of the cabinet exhaust system can also increase as the HEPA filters become loaded.
If you need assistance in understanding more about dedicated laboratory exhaust in your existing HVAC systems or for your next project, please do not hesitate to contact Douglas C. Barnhart, PE at (717) 845-7654 or dcb@ba-inc.com.