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Optimizing Laboratory Fume Hood Selection

Mon, 03/01/2010 - 5:50am
Lindsay Hock

 

Fume hoods have been present within society and science since the 17th century. However, throughout the years fume hoods have changed to accommodate user’s needs and laboratory applications. While the first fume hoods were fixed and far from flexible, with modern and mobility needs, fume hoods have now become flexible in design, more efficient, energy-saving, and also now have many types from benchtop or conventional fume hoods, radioisotope fume hoods, perchloric fume hoods, ductless fume hoods, and more. At the Fall Laboratory Design Conference 2009, Victor Cardona, AIA, and Steve Hackman, AIA, LEED AP, gave a speech entitled “Lab Basics: Optimizing Casework and Fume Hood Selection,” which covered the history of fume hoods and the improvements they have made within laboratory settings.

As a simple definition, a laboratory fume hood is a safety device that is designed to carry undesirable effluents away from laboratory personnel and hopefully out of the laboratory, when connected to a properly designed ventilation system. However, while it is the exhaust system that provides safety for the environment and the air that users breathe, the design features of this standard laboratory equipment are what help ensure user safety. Aspects of the technology include sashes that provide a barrier between the user and the experiment, hood monitors that measure face velocity or exhaust flow verification, and various airfoils and baffles intended to enhance containment and safety.

Besides for being known for their safety attributes, fume hoods are also known for their wide variety of types and designs. The benchtop model of a fume hood, as the name suggests, sits on a bench top or above a storage cabinet or a pedestal. A conventional fume hood used for a wide variety of chemical procedures and is appropriate for use with small to moderate quantities of low to highly toxic materials with low particle mass. The conventional rule of thumb has long been operation at 80-100 FPM face velocity. And, while these are the most commonly found within laboratory settings, ductless fume hoods (which will be covered later) are gaining traction. In addition, there are several other types of standard fume hoods—each unique to the materials they are made from, the applications they deal with, and even the flexibility of the equipment.

One of the most important concepts when selecting fume hoods is figuring out the appropriate exhaust options. Choices need to be made as to whether air will be exhausted, for instance. Will the lab need a constant air volume (commonly abbreviated as CV) exhaust system or a variable air volume (VAV) exhaust system? Will each hood have a dedicated exhaust (mandated by code in some jurisdictions), or will exhaust be ganged in a manifolded system. What type of monitor will be used to verify safe operations? Building systems choices and operations are also crucial—particularly air balancing, since laboratory fume hoods exhaust a large amount of conditioned air from the facility.

While it may seem like a fume hood would function the same anywhere in the lab, many factors actually come into play. The location of a hood affects its efficiency. If located in a high-traffic area, fume hoods could experience turbulence and cause contaminants to be drawn outside the hood, affecting essentially all users of the laboratory and diminishing safety. An improper relationship between hood location and air diffuser location can also cause this effect. In complex environments, computer fluid dynamic (CFD) modeling can provide excellent information about the possible effects of various placement choices.

Choosing between a CV or VAV is also an important decision. As the name implies, CV exhaust pulls air at a constant volume no matter where the sash is positioned and without changing fan speeds. VAV exhaust systems work with sash positioning controls to let the HVAC system know how much the sash is being opened. Depending on the sash position, the controls will let the system know to reduce or increase the volume of air by either modifying the fan speed or a damper position. Needless to say there are also many guidelines, such as SEFA, ASHRAE, and others, that must be noted when selecting the equipment.

While safety is paramount in any laboratory setting, flexibility has become one of the most noted and need aspects of the lab. Ductless fume hoods are one tool for meeting flexibility needs of laboratories. The standard definition of a ductless fume hood is an enclosure designed for the handling of contaminants/products that generate pollutants, which limit the propagation toward the operator/user and other personnel of such contaminated air pollutants. Typically the hood is not attached to any external conduit or exhaust system, making it flexible in nature. The ductless hood’s internal ventilation system forces the contaminated air to travel through a molecular filter and re-circulates within the room after purification. Like all fume hoods, ductless fume hoods come in classes, which each have certain specifications attached. No ductless hood is appropriate to every use.

The last improvement seen generally within new fume hood technology is the energy-saving trends. Fume hoods cut back on lab energy use, especially the noted high performance (low velocity) fume hood. Such hoods are designed to operate with up to 50% less exhaust air volume than a conventional hood, and to provide safe containment levels at a face velocity of 60 FPM or less, as verified by an ASHRAE 110 tracer gas test. Hoods should pass the test even with sashes fully opened vertical sash. The hoods can be made available in either bench-top or floor-mounted hood types, and are an energy-saving option.

As a short run-through in the improvements of fume hoods throughout the years, one can see that this form of safety equipment has now become a flexible, safe, energy-saving staple for all laboratories. Expect further advances in both filtering (ductless) technology and high-performance ducted design.

If you enjoyed the content from Victor Cordona and Steve Hackman’s session from the Fall Laboratory Design Conference 2009, you may be interested in the following sessions at Laboratory Design Conference 2011:

  •  Is "Carbon-Neutral" the Best Gauge of Sustainability? - Chris Leary and Mark M. Maguire, KlingStubbins
  • Choosing Enclosures for Efficiency (Ducted and Ductless Hoods and Other Exhaust Devices) - Victor Cardona and Adam Denmark, SmithGroup
  • Trends of Biohazard Labs in Nanotech Research Cleanroom Facilities - Aldo Beltran, M+W Group

Click here for details  

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