Views: 0 Author: Site Editor Publish Time: 2023-03-09 Origin: Site
Most fume hoods for industrial purposes are ducted.A large variety of ducted fume hoods exist.In most designs,conditioned (i.e. heated or cooled) air is drawn from the lab space into the fume hood and then dispersed via ducts into the outside atmosphere.The fume hood is only one part of the lab ventilation system.Because recirculation of lab air to the rest of the facility is not permitted, air handling units serving the non-laboratory areas are kept segregated from the laboratory units.To improve indoor air quality, some laboratories also utilize single-pass air handling systems,wherein air that is heated or cooled is used only once prior to discharge. Many laboratories continue to use return air systems to the laboratory areas to minimize energy and running costs,while still providing adequate ventilation rates for acceptable working conditions.The fume hoods serve to evacuate hazardous levels of contaminant.To reduce lab ventilation energy costs, variable air volume (VAV) systems are employed, which reduce the volume of the air exhausted as the fume hood sash is closed.This product is often enhanced by an automatic sash closing device, which will close the fume hood sash when the user leaves the fume hood face.The result is that the hoods are operating at the minimum exhaust volume whenever no one is actually working in front of them.
Since the typical fume hood in US climates uses 3.5 times as much energy as a home, the reduction or minimization of exhaust volume is strategic in reducing facility energy costs as well as minimizing the impact on the facility infrastructure and the environment. Particular attention must be paid to the exhaust discharge location, to reduce risks to public safety and to avoid drawing exhaust air back into the building air supply system.
This method is outdated technology.The premise was to bring non-conditioned outside air directly in front of the hood so that this was the air exhausted to the outside.This method does not work well when the climate changes as it pours frigid or hot and humid air over the user making it very uncomfortable to work or affecting the procedure inside the hood.This system also uses additional ductwork which can be costly.
Constant air volume (CAV)
In a survey of 247 lab professionals conducted in 2010, Lab Manager Magazine found that approximately 43% of fume hoods are conventional CAV fume hoods.
Closing the sash on a non-bypass CAV hood will increase face velocity (inflow velocity or "pull"), which is a function of the total volume divided by the area of the sash opening.Thus, a conventional hood's performance (from a safety perspective) depends primarily on sash position, with safety increasing as the hood is drawn closed.To address this issue, many conventional CAV hoods specify a maximum height that the fume hood can be open in order to maintain safe airflow levels.A major drawback of conventional CAV hoods is that when the sash is closed, velocities can increase to the point where they disturb instrumentation and delicate apparatuses, cool hot plates, slow reactions, and/or create turbulence that can force contaminants into the room.
Bypass CAV hoods (which are sometimes also referred to as conventional hoods) were developed to overcome the high velocity issues that affect conventional fume hoods.These hood allows air to be pulled through a "bypass" opening from above as the sash closes.The bypass is located so that as the user closes the sash, the bypass opening gets larger.The air going through the hood maintains a constant volume no matter where the sash is positioned and without changing fan speeds. As a result, the energy consumed by CAV fume hoods (or rather, the energy consumed by the building HVAC system and the energy consumed by the hood's exhaust fan) remains constant, or near 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.