There are five main hood construction types (although custom designs are available). These include:
Conventional Hoods represent the original, simplest hood design style. With a conventional hood the volume of air exhausted is constant, regardless of sash height.
Bypass Hoods have an added engineering feature and are considered a step up from conventional hoods. An air bypass incorporated above the sash provides an additional source of room air when the sash is closed.
Auxiliary air Hoods have attached dedicated ducts to supply outside air to the face of the bypass hood. The main advantage of an auxiliary air hood is the energy savings realized by reducing the amount of heated or air conditioned room air exhausted by the hood.
Variable air volume (VAV) Hoods are the most sophisticated hood types, requiring technically proficient design, installation and maintenance. The primary characteristic of VAV hoods is their ability to maintain a constant face velocity as sash height changes.
Ductless Fume Hoods have a conventional hood design but are self-contained to recirculate air back into the lab after filtration occurs. These hoods use either High Efficiency Particulate Air (HEPA) filters or Activated Carbon Filtration (ACF) technology to remove contaminants from the hood air.
The efficiency of a fume hood is measured by its hood face velocity and required air flow.
Hood Face Velocity is a measurement of air flow speed across the imaginary plane running between the bottom of the sash to the work surface. Generally measured in feet per minute (fpm), the greater the hood face velocity, the more quickly toxins and other vapors can be flushed from the system.
Required Airflow is related to hood face velocity in that it is a measurement of the amount of air flow required to achieve a laminar flow velocity of 100 feet per minute.
General purpose fume hoods usually function with a hood face velocity of 100 fpm. Stronger fume hoods are available to work with specific chemicals and technologies. Two commonly sought-out types include radioisotope and perchloric acid hoods.
Radioisotope Hood Systems are ideally made from welded stainless steel to ensure against absorption of radioactive materials. In order to comply with licensing requirements, radioisotope hoods require a face velocity of 125 fpm.
Perchloric Acid Hoods have wash-down capabilities to prevent the buildup of explosive perchlorate salts within the exhaust systems.
General: Factors to consider when selecting a fume hood:
A facility designed for intensive chemical use should have at least 2.5 linear feet of hood space per student.
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