Publish Time: 2023-02-03 Origin: Site
Most fume hoods used for industrial purposes are ducted.A wide variety of ducted fume hoods exist.In most designs, conditioned (i.e., heated or cooled) air is drawn from the laboratory space into a fume hood and then ducted into the outside atmosphere.Fume hoods are just one part of a laboratory ventilation system.Air handling units serving non-laboratory areas are kept isolated from laboratory units since recirculation of laboratory air to the rest of the facility is not permitted.To improve indoor air quality, some labs also use single-pass air handling systems, in which heated or cooled air is used only once before being discharged.Many laboratories continue to use return air systems to laboratory areas to minimize energy and operating costs while still providing adequate ventilation rates for acceptable working conditions.Fume Hoods Used to Exhaust Hazardous Levels of Contaminants.To reduce laboratory ventilation energy costs, variable air volume (VAV) systems are used, which reduce the amount of air that is expelled when the fume hood windows are closed.This product is often enhanced with an automatic sash closing device that closes the hood sash when the user steps away from the hood face.The result is that they will run with minimum displacement as long as no one is actually working in front of them.Since a typical fume hood in the U.S.climate uses 3.5 times the energy of a home,reducing or minimizing exhaust emissions is strategic to reduce facility energy costs and minimize impact on facility infrastructure and the environment.Particular attention must be paid to the location of exhaust emissions to reduce public safety risks and to avoid ingestion of exhaust into the building air supply system.
Auxiliary air
This method is outdated technology.The premise is to bring the outdoor air without air conditioning directly to the front of the range hood, so that it is the air that is exhausted outside.This doesn't work well when the climate changes, as it dumps cold or hot, humid air on the user, making work very uncomfortable or affecting programs under the hood.The system also uses additional plumbing, which can be expensive.
Constant air volume (CAV)
In a 2010 survey of 247 laboratory professionals, Lab Manager Magazine found that approximately 43% of fume hoods were traditional CAV hoods.
Non-bypass CAV
Closing the sash on a non-bypass CAV hood will increase the face velocity (inflow velocity or "pull"),a function of the total volume divided by the sash opening area.Therefore, the performance (from a safety point of view) of a conventional hood is mainly dependent on the sash position, with the safety increased as the hood is closed.To address this issue, many conventional CAV hoods specify a maximum height to which the hood can be opened to maintain a safe level of airflow.A major disadvantage of traditional CAV hoods is that when the sash is closed, the velocity increases to the point where it interferes with instruments and precision equipment, cools hot plates, reacts slowly and/or creates turbulence that forces pollutants into the room.
Bypass CAV
Bypass CAV fume hoods (sometimes called conventional fume hoods) were developed to overcome the high velocity issues affecting conventional fume hoods.These hoods allow air to be pulled from above through the "bypass" opening when the sash is closed.The bypass is positioned such that when the user closes the sash, the bypass opening becomes larger.The amount of air passing through the hood remains constant regardless of where the sash is located and without changing the fan speed.Therefore, the energy consumed by the CAV hood (or rather, the energy consumed by the building HVAC system and the energy consumed by the hood exhaust fan) remains constant or nearly constant regardless of sash position.
Low flow/high performance bypass CAV
"High performance" or "low flow" bypass CAV enclosures are the newest types of bypass CAV enclosures, often featuring improved sealing, safety, and energy savings. Low-flow/high-performance CAV hoods typically feature one or more of the following: sash stops or horizontally sliding sash to limit opening; sash position and airflow sensors that can control mechanical dampers; Air Curtain Barrier; refined aerodynamic design and variable dual baffle system maintain laminar (undisturbed, turbulent-free) flow over the hood. Although the initial cost of high-performance range hoods is typically higher than traditional bypass range hoods, the improved containment and flow characteristics allow these range hoods to operate at face velocities as low as 60 fpm, which can translate to $2,000 per year or more energy efficient depending on hood size and sash settings.
Reduced air volume (RAV)
Reduced air volume range hoods (a variant of low flow/high performance range hoods) incorporate a bypass block to partially close the bypass, reducing air volume and saving energy.Often, stops are used in conjunction with sash stops to limit the height of the sash opening, ensuring a safe face velocity during normal operation while reducing the air volume of the range hood.By reducing air volume, the RAV hood can run with a smaller blower, another cost-saving advantage.Since RAV hoods restrict sash movement and reduce air flow, these hoods are less versatile and can only be used for certain tasks.Another downside of the RAV hood is that the user can theoretically cover or disengage the sash stop.If this happens, face velocity could drop to unsafe levels.To deal with this, operators must be trained not to go over the sash stop when in use and only do so when loading or cleaning the hood.