Fume Hoods and other Local Exhaust Ventilation Equipment

Common Fume Hood Types

Conventional or Constant Volume (CV)

The systems volumetric exhaust flow (Q) remains constant. Closing or opening sashes causes proportional increases or decreases in the face velocity (V) at the access opening area (A). Q=VA .

In automated systems, sash position sensors or switches can be used to control the ventilation system flow and compensate for this effect. Q follows A.

Bypass

Similar to a CV hood but bypass openings or dampers are added to open/close in opposition to the change in area of the sash opening.

This is not an even offset but it helps attenuate the face velocity changes and limits extreme velocities as the sash approaches completely closed positioning.

Note: Local controllers such as timers and on/off switches have been used for energy savings on older systems, these should all be disabled and replaced.

Constant Volume (CV) Conventional Fume Hoods

This type of hood can experience potentially turbulent flow and/or containment problems when sash positions other than the approved working height are used, unless they are integrated into an HVAC controls system that adjusts exhaust flow based on the hood sash position.

Bypass Fume Hoods

These have been designed to minimize face velocity changes due to sash repositioning. This is especially important for the extreme increases that would be experienced as the sash approaches fully closed.

The design goal is typically to limit increases to < 3.5 times that at the working sash height (< 350 feet per minute (fpm) maximum).

Face Velocity Differences
(CV vs. Bypass Type Fume Hoods)

Click to Enlarge

Other Fume Hood Types

Auxiliary

Unconditioned supply air is admitted into the front of the hood at the sash face to limit the amount of conditioned laboratory air that must be exhausted.

This “auxiliary” air accounts for up to 70% of the total volume exhausted from the fume hood.

Very small laboratory spaces and overtaxed laboratory designs have attempted to use these hoods to address static pressure problems and energy usage.

Since this design exhausts less conditioned room air, it saves energy but performance is degraded.

Ductless

Employ filter beds to remove select contaminants before recirculating air back into the room.

These are becoming more sophisticated and reliable but in a research environment such as ours, utilizing many different types of chemicals, they are almost never safe or practical.

Auxiliary Air Fume Hoods

NFPA requires introduction of auxiliary air to be exterior to the hood sash, older models (such as shown here) introduced this air just inside the sash. These hoods have proven to have serious performance problems. The National Research Council recommends that these hoods not be purchased and that existing ones be replaced or disabled ASAP.

Existing installations must have the auxiliary supply disabled. Supply ducts should be permanently disconnected and/or capped to keep them from being inadvertently returned to service and to prevent outside air entrainment via static pressure differences between outdoors and the laboratory.

Fume Hood Configurations

Bench Top

The hood is placed upon appropriate working height countertops or base cabinets. These are the most common hood configurations found in research laboratories.

Design features such as slightly recessed working surfaces to contain spills, air foils at the front edge of internal hood countertops to minimize airflow turbulence, and integral cup sinks for permitted liquid waste disposal are typical.

Floor

These are similar to Bench Top units but twice as tall with two sliding sashes and the working surface is the floor itself.

These are commonly called “walk-in” hoods but that is a serious misnomer, they should never be entered during use!

A common application is for dispensing or bulking solvents to/from large drums. This use requires grounding and bonding systems to control static discharge induced fires in flammable vapor environments and these devices are often found with this type of configuration.

Fume Hood Configurations

Distillation

These are very similar to Floor models but the horizontal working surface is typically only one to two feet above the floor level. The back wall inside the hood has a latticework grid or “monkey bars” installed that is used to attach distillation equipment.

They should only be open during initial setup and adjustments. Many of these have odd sash configurations that could limit the physical protection and containment normally afforded by standard Bench Top designs. Users should know how the ASHRAE-110 and annual testing is performed and operate the hoods accordingly.

Demonstration

Also referred to as “California” hoods, these are really just ventilated cabinets designed to provide some containment during demonstrations.

They help remove non-hazardous particulate, aerosol, or vapors while allowing maximum ability to view the operation.

They are not inherently capable of providing the same level of worker protection, are not performance tested to the same standards, and should never be used in place of a Bench Top hood.

Fume Hood Systems

To understand the capabilities of your particular fume hood you must know not only about its type and configuration but also how the ventilation system serving your environment affects performance.

There are three primary types of ventilation systems used in conjunction with fume hoods and laboratories at FSU:

Constant Volume (CV)
Two Position (sash open or closed) Constant Volume (CV₂)
Variable Air Volume (VAV)

CV Systems

Individually Exhausted

Each fume hood is connected to its own rooftop exhaust fan.

The loss of an exhaust fan only impacts the hoods served and the laboratory where they are located.

Contaminants are not diluted before they are exhausted.

Combined Plenum

The individual ducts from several fume hoods attach to a larger duct and are exhausted by the same rooftop fan(s).

Individual hood flows have to be balanced by design (duct sizing) or through the use of dampers in the exhaust ducts serving each device.

Since it is uncommon for multiple hoods to be used at the same time, contaminants are much more diluted before being exhausted.

CV₂ Systems

Hoods used with CV₂ systems have a sash position switch that provides input to the Building Automation System (BAS) in order to reduce flow and save energy when the hood is not in use.

When the sash is open (~2 inches from fully closed for switch to activate), the flow volume is set to a programmed operational flow rate that will provide a 100 fpm velocity at the maximum working sash height (~18 inches).

When the sash is completely closed, the flow volume is reduced to the programmed low flow rate. This was based on either the volume required to maintain 25cfm/ft² of the inner fume hood working surface (to prevent exceeding the lower explosive limits (LEL) of highly flammable contents) or the amount of exhaust volume required to maintain the laboratory supply/exhaust balance, whichever is greater. Note: New lower minimums for LEL control have been demonstrated to be effective recently and may soon be adopted.

VAV Systems

Hoods used in VAV systems have installed sash position sensors that proportionally change the flow volume by controlling a damper placed in the exhaust duct serving that hood.

The goal is to maintain a constant face velocity as the sashes are repositioned.

These systems can improve safety and save energy but are more complex.

System response to establish a new equilibrium can take about 30 seconds. Always reposition sashes slowly and allow time for the system response to complete.

This needs to be done within the scope of a BAS (Building Automation System) so that flow rates in other hoods and laboratory environments can be maintained at their programmed levels.

This type of system requires a greater initial investment.

CV or Bypass Hoods - installed in VAV Systems

Hoods used in Variable Air Volume (VAV) ventilation systems use sensors to monitor sash position and control flow dampers in the exhaust duct to maintain a more constant face velocity regardless of sash position.

Bypass hoods used in VAV systems must have the bypass areas restricted or blocked in order to allow proper operation in this type of installation.

Flow Volume: CV₂ vs. VAV System Designs
(4 foot hood CFM approximations shown as an example)

Click to Enlarge

Proper Fume Hood Operation

Avoid opening and closing fume hood sashes rapidly, making swift arm or body movements, walking by hoods in use, or any similar event that could perturb the delicate air flow at the sash.

Place chemical sources and apparatus at least 6 inches inside the hood and center them as much as possible. Quantitative studies have demonstrated breathing zone contamination concentrations about 300 times higher when the source is located closer to the sash near the front of the hood.

Avoid creating air flow dead spaces in the hood. Give consideration to separating and elevating bulky equipment and prevent blocking baffles.

Avoid using large objects within the hood that can block airflow that may reduce hood efficiency.

Vertical Sliding Sashes

This is the most common sash design.

The maximum safe operating height will be clearly marked on either side of the sash track. This mark is often associated with a physical sash stop and should never be exceeded during use.

The fully opened height should only be used during non-hazardous operations, such as analytical setup or equipment installation.

Horizontal Sliding Sashes

Place horizontal sashes so they serve as a physical barrier between your body and the hood contents for protection, and limit the size of the opening to the greatest practical extent for maximum containment and safety.

Other Sash Considerations

Combination Sash Hoods offer the user a choice of either the horizontal or vertical operating positions for maximum flexibility, but this added benefit is offset by a slight deficit in containment performance.

These hoods should never be used with vertical and horizontal panels opened at the same time!

All Fume Hoods that are not in use need to be closed, this enhances safety and can conserve significant amounts of energy in modern designs!

ASHRAE-110 Testing

This is a quantitative testing standard, published in 1995, that uses a neutrally buoyant tracer gas (i.e. SF₆) and smoke to assess fume hood performance characteristics under a prescribed set of static and dynamic operating conditions.

This test must be done at the end of any new installation or renovation projects while the project team is still available, to prove performance goals were achieved before the hoods are used and warranties may still be in effect.

The testing needs to be scheduled following any operations that may impact hood performance such as HVAC Test and Balancing or component replacement/reconfiguration.

Quantitative data obtained during this testing serves as a baseline for future face velocity verifications; observed deviations from the baseline values warrants system inspection and repair and/or repeated ASHRAE-110 evaluation.

Adjunct LEV Equipment

This includes direct exhausting equipment such as snorkels and canopy hoods.

These devices are not routinely tested to verify performance and are not generally designed for work with hazardous materials.

They should only be used for non-hazardous operations or to control hazardous emissions for operations that could not otherwise be contained after very specific engineering and industrial hygiene process evaluations have been performed.

Snorkels

These are used to remove nuisance heat, dust, or odors directly at the source, before it mixes with general laboratory air.

To be effective, the suction nozzle needs to be located directly above any heat or lighter than air source or, have the center of the suction nozzle placed within ½ duct diameters from other emission sources.

The flow volume of the source must be less than 70% of the snorkel exhaust flow (a 200 cfm snorkel should only be used for emissions that are flowing less than 140 cfm).

Canopy Hoods

May be used to remove nuisance heat, dust, or odors near the point of emission.

These should only be installed after engineering review of the HVAC system for the specific area, equipment, and operation served. They should never be installed for general use or without this specific knowledge being available to the design team because this will most likely result in ineffective performance and a significant waste of energy.

Other Specialty Devices

Portable or semi-installed ventilation devices such as glove boxes, laminar flow hoods (clean benches), and biological safety cabinets are often available in laboratories. Use of these devices needs to be covered by other training that is more specific to the type of work for which they will be used.

For the purposes of this module, it is important to recognize these devices and understand that they are not to be used for general laboratory work and should never be used without additional training.

Biological Safety Cabinets

These are used for work with biological materials that may present as an airborne hazard.

They draw air away from the user and pass it through internal HEPA filtration to remove biological organisms before the air is recirculated (most common) or exhausted from the building.

Laminar Flow Hoods (Clean Benches)

These appear similar to a biological safety cabinet, but they are intended to protect the product from contamination by the user/laboratory environment.

The air that passed through the internal HEPA filtration blows straight from the back of the hood, across the working surface, and towards the user.

They offer no protection to personnel!

Glove Boxes

There are two basic types available in a multitude of models. The types can be distinguished by their function:

Those designed to protect the enclosed materials. These are typically evacuated, conditioned, and filled with an inert gas such as argon.

Those used to protect personnel from the enclosed products. These may be simply sealed or open to the atmosphere via HEPA filtration.

They can be as simple as a small plastic bag with attached gloves or as complex as a large computer monitored and controlled unit.

Complete the following form to receive credit for training.