3 main steps to fume hood mechanical system selection

By Kasey Fulmer, Product Manager

On November 20, 2014 in Articles

Starting the fume hood purchasing process? Not sure how it will affect your mechanical system? This quick guide will explain a fume hood system’s three important aspects, and what you can do to make your mechanical system as efficient as possible.

1. Step one is to narrow down the type of enclosure you require. What is a fume hood? It is a ventilated enclosure where harmful or toxic fumes or vapors can be handled safely. A fume hood works by drawing in room air through the work area and exhausting that air away from the operator so that inhalation and contact are minimized. This means if your application includes working with biohazards or harmful particulates that need to be caught in a filter and disposed of, there are other enclosures you should consider first (such as Biological Safety Cabinets, Filtered Small Enclosures).
    
2. The next step is to decide what type of mechanical system you need to fit your fume hood application. A mechanical system describes how air is exhausted from the building. This includes the fume hood, blower, and ductwork system to facilitate exhausting harmful fumes. There are also enclosures that do not have a ductwork system; they are ductless enclosures, or filtered fume hoods. This article will only deal with ducted systems. Please refer to the article “5 advantages of going ductless” for information about choosing a ductless system.
   
   The majority of mechanical systems used with ducted fume hoods are either constant air volume (CAV) or variable air volume (VAV).  The CAV systems exhaust a constant volumetric flow of air.  VAV systems may incorporate valves or dampers that close off a portion of the air flow so that when the sash is lowered, the volumetric flow of air is lowered. This means energy savings. Used in conjunction with a high performance hood, the VAV system has the greatest energy savings potential of all systems.
    
3. The final step is to choose a fume hood that operates well with your mechanical system. High performance hoods are the newest generation of fume hoods. They rely on containment-enhancing features to maintain safety while saving energy. Although popular thought is to increase the air speed through the fume hood to better contain, in reality pushing more air into the fume hood may create turbulence, increasing vortexes inside the hood and introducing air pathways that can breach containment. Always refer to the manufacturer’s suggested face velocities and local codes for minimum and maximum face velocity through your fume hood.

Please note, the choice between a CAV or VAV system often depends on duration of hood operation and budget. If the fume hood is operating full time, a high performance hood is highly recommended to take advantage of energy savings. High performance fume hoods can operate at face velocities as low as 60 feet per minute (fpm), thus conserving energy and often providing an alternative to auxiliary-air hoods in air-starved laboratories where room supply air is inadequate.

Fume hoods are part of an entire mechanical system. Simply adding a fume hood to a lab without allowing for the exhausted air will leave you with an air-starved room. This will result in all conditioned air being exhausted from the room, making the lab hotter in the summer and colder in the winter, as well as creating problems in other areas if the fume hood is stealing air from other rooms in the attempt to maintain equilibrium. Consider all aspects of your mechanical system before choosing and purchasing a fume hood so that it can operate safely and correctly.