Hoods for Wet Processing

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Fume Hood Frequently Asked Questions

Selecting the appropriate fume hood or ductless exhaust hood, ensuring operator protection, and maintaining optimum hood performance can be a monumental task. Asking the right questions will help drive your decision-making process toward the right system for your application and support your standard operating procedures in maintaining a safe environment.

Does my work require a fume hood?

Use of a chemical fume hood is required to enclose work involving toxic gasses, reactive or explosive materials, volatile chemicals, carcinogens, flammable chemicals, hazardous substances and processes producing aerosols or nuisance odors.

What’s the difference between a ducted fume hood and a ductless exhaust hood?

Exhaust fume hoods vent air from the work area directly outside of the building by connecting to the facility’s ducted exhaust system. Ductless exhaust, or stand-alone, fume hoods use integral blowers to draw air away from the work area through a set of filters before safely releasing the air back into the ambient lab. Ductless hoods commonly use a two-stage filtration system composed of a charcoal filter for vapor containment and a final HEPA filter for particle containment. Charcoal filters capture chemical vapors with varying degrees of efficiency; review your list of chemicals - along with solvent concentrations and aliquoted volumes - with your fume hood specialist to ensure the appropriate filter is purchased. Laboratory fume hoods are required to safely enclose chemicals, such as methanol, that are not captured efficiently by charcoal filtration. Small-footprint ductless fume hoods fit onto crowded benchtops or into areas of the lab without access to the facility exhaust system. The effectiveness of a ductless exhaust hood is contingent upon regular filter maintenance; talk to your hood specialist or EH&S representative to prepare a standard filter replacement schedule.

What’s the difference between DH I, DH II, and DH III?

These acronyms define the different types of ductless hoods outlined by The Scientific Equipment and Furniture Association (SEFA):

Ductless Hood I (Type I): These light duty exhaust hoods are equipped with filtration only suitable for non-toxic chemicals, nuisance odors and particulates. They are not required to have a warning sensor for chemical breakthroughs.
Ductless Hood II (Type II): This type of fume hood meets all DH I requirements, and can filter manufacturer approved toxic contaminants, making it suitable for toxic or hazardous applications. It also includes a chemical detection sensor and alarm in the event of chemical breakthrough beyond the filter. However this design does not provide secondary containment in the event the primary filter fails.
Ductless Hood III (Type III): This filtered fume hood is equivalent to a DH II with the added benefit of backup secondary filtration that is the same type and efficiency as the primary filter. DH III hoods also include a sensor between the primary and secondary filter, to ensure secondary filtration is available in case the primary filter fails.

Do I need a light duty chemical hood or a high-performance fume hood?

Although the primary purpose of a laboratory fume hood is operator safety, demand has increased for hoods designed to carry lower operating costs and reduced environmental impact. As fume hoods consume an average of 70,000 cubic feet of pre-conditioned air per hour, their operating costs are tied primarily to their air consumption, cited as volumetric rate (CFM) or face velocity (fpm). High-performance fume hoods - also called green ventilation hoods, high efficiency hoods or low-flow hoods – provide the highest level of containment at the lowest operating cost. High-performance fume hoods are designed to safely operate while maintaining a face velocity of 60 fpm, the lowest acceptable ASHRAE performance standard as outlined by SEFA. In addition to reduced face velocities, high efficiency fume hoods can integrate with Variable Air Volume (VAV) building automation systems to ensure minimum allowable room air changes, safe room pressurization and desirable temperature and humidity set points. A conventional light-duty, or by-pass, fume hood carries a lower up-front cost than a high-performance hood, but does not include design features allowing for reduced air volume ventilation or VAV integration. Most light-duty exhaust hoods safely operate at face velocities between 90 – 110 fpm, equal to an increase in air consumption of 30 – 50% annually. To decide between a light-duty chemical hood and a high-performance fume hood, a life cycle cost analysis is required to compare energy cost, ambient climate conditions and usage rate.

Does my work require a benchtop fume hood, walk-in hood or canopy hood?

Bench top fume hoods are mounted on an existing work surface, or casework, to create a negative-pressure environment for fume containment. A walk-in fume hood, or floor-mounted hood, includes no work surface or cabinetry, allowing the user to place large items into the hood at floor level. A canopy fume hood is installed on the wall or suspended from the ceiling; it does not include a hood frame or physical barrier between the fume hood area and the ambient lab. Benchtop fume hoods are designed to enclose singular processes on a crowded benchtop, like small-scale solvent evaporation or ethanol extraction. Walk-in fume hoods can accommodate bulk storage systems, like 50-gallon drums, large processing equipment and heavy items transported by lab carts. Canopy exhaust hoods are designed to vent non-toxic materials like heat, steam and nuisance odors emanating from ovens, steam baths or autoclaves.

Should I purchase a remote blower or dedicated, built-in blower with my lab fume hood?

Built-in blowers, or exhaust fans, are installed within the body of the fume hood and positioned above the work area. Remote exhaust blowers are installed on the building exterior and connect to the fume hood through a duct system. Built-in blowers are easier to install and less expensive, but can be noisy. As integral, built-in blowers can place sections of the ductwork under positive pressure, thus pushing contaminants through duct leaks back into the lab, they are recommended for non-hazardous applications, short duct runs, or mobile labs. Remote blowers, though more expensive and complex to install, can be sized for the specific situation, taking into account the duct diameter and run length and quantity of 90-degree or 45-degree elbow turns. Remote blowers keep the ductwork under constant negative pressure; any leaks in the duct system will draw air into the duct for exhaust from the building. Remote blowers include exhaust stacks and zero-pressure weather caps, ensuring the exhausted air is terminated at least 10 feet above the roofline to prevent fumes from returning to the building through the HVAC system.

Which blower should I choose for a ducted fume hood?

To maintain the safety and containment of your lab hood, selecting the proper motor blower is critical. Exhaust blowers are composed of three materials: steel, fiberglass and polyvinyl chloride (PVC). Review your chemical list with your fume hood specialist; steel blowers are designed for low-to-moderate corrosives, fiberglass blowers resist harsh solvent and acids, and PVC blowers can withstand exposure to high corrosives, such as perchloric acid. To determine the proper blower size, or capacity, you must calculate your fume hood resistance, measured is static pressure loss. Fume hood resistance is calculated by accounting for hood face velocity, volumetric rate and static pressure along with exhaust duct diameter, number of duct turns, operating temperature, and facility altitude. Talk to your laboratory hood specialist to calculate your hood resistance and determine an appropriate blower size. Most blowers can safely maintain airflow between 250 – 3,500 CFM.

If I’m working with flammable chemicals, do I need an explosion-proof hood?

Not necessarily. All fume hood electrical components are mounted on the corner posts, outside of the flammable vapor trail. Additionally, air constantly exhausting through the work area continually dilutes the air inside the fume hood, creating conditions under which few chemicals reach their lower explosive or flammability limits, according to the NFPA (National Fire Protection Association). Explosion-proof fume hoods are required if a flammable chemical’s concentration exceeds 25% of the LEL (Lower Explosion Limit) or LFL (Lower Flammability Limit). Talk to your fume hood specialist to calculate your dilution factor and volumetric air flow rate, along with each chemical’s LEL and LFL, to determine if an explosion-proof hood is required.

How do you organize the work area inside a fume hood?

In general, the work area should only contain supplies and reagents dedicated to ongoing experiments. If you must store equipment - such as balances, centrifuges, hot plates or rotary evaporators - inside the fume hood, please ensure the equipment is located away from the rear air baffles and elevated at least 2” off the work surface. Any equipment, sample racks, or consumables positioned downstream of an experiment may block contaminants from safe exhaust, reducing the effectiveness of the fume hood. All open chemicals must be placed at least 6 inches from the front edge of the fume hood airfoil to prevent eddy currents from pushing the vapors through the front of the hood face.

How can I reduce eddy currents in my work zone?

Eddy currents are sporadic, turbulent airflow patterns generated by disruptions inside the work area or at the hood face. Limiting eddy currents are vital to maintaining containment and operator protection. Keep lab doors and windows closed to prevent extra sources of air from sweeping across the hood face. Restrict personnel traffic near the hood when the shield is raised to prevent cross-drafts. Electrical cords, gas hoses or water lines running into the hood must be inserted underneath the air foil or through petcock connections to ensure the hood shield closes properly.

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Features and Benefits

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Laminar Flow and Fume Hoods FAQs

Laboratory hoods serve a number of critical purposes in the lab environment.

Product protection from particulate contaminants in the lab environment, including microbes

Such hoods, often called laminar flow workstations or clean benches, use a flow of HEPA-filtered air to prevent influx of viable and non-viable particles into the enclosure and to sweep away any contaminants generated by the work process.

Protection of personnel and lab environment from substances generated inside the enclosure

For fume containment, laboratory fume hoods use a ducted ventilation system that maintains negative pressure. Where ducting is impractical, ductless hoods are also available for removal of non-hazardous contaminants (fumes or particles), using either HEPA or activated carbon filters.

Protection of product, operator and environment

Biological safety cabinets combine aspects of both hood types: a laminar flow of micro-filtered air and negative-pressure containment of bio-hazard fumes.

Laminar Flow Hoods

Laminar flow hoods (also called laminar flow clean benches) force air through a HEPA or ULPA filter to create a clean work area free of nearly all contaminating particulates, including bacteria, mold spores and many viruses. These hoods use a vertical or horizontal airflow design. Both provide excellent protection of products from particulates and cross-contamination, but do not protect the operator or environment.

So why choose one over the other? Vertical laminar flow hoods (VLF Hoods) require less floor space, but more overhead clearance. In addition, a sash at the front of the VLF hood provides a barrier between air exiting the hood and the operator’s face. VLF hoods create turbulent air flow when air strikes the work surface, requiring sterile work to be performed above the work surface.

HLF hoods minimize air turbulence (and particle backflow) on the work surface except where equipment disrupts the horizontal airflow. However, they direct the airflow directly at the operator’s face and require more depth to accommodate placement of the fan filter unit at the rear of the hood.

Fume Exhaust Hoods

Laboratory fume hoods draw air from the environment through the opening of the hood, allowing environmental contaminants into the work area. Thus, lab fume hoods do not offer a sterile environment or product protection. Instead, chemical fume hoods are designed to protect the operator from dangerous or irritating fumes and powders generated from work performed inside the hood.

In ducted fume hoods and canopy fume hoods, air passes out of the hood through a duct leading to an in-house ventilation system that contains and/or neutralizes hazardous materials.

In ductless fume hoods, fans draw exhaust air through a series of filters that remove irritating substances before release back into the room. Since these filters are not 100% efficient in removing all substances, ductless fume hoods are not advised in the presence of biohazards; however, they provide a convenient, cost-effective solution for operations involving irritating but non-hazardous fumes in facilities where an in-house HVAC system cannot be accessed.

Biosafety Cabinets

Biological Safety cabinets combine design aspects of both laminar flow and fume hoods. Filtered air is directed downward onto the work surface and then travels into a plenum through openings on the front and sides of the hood. Depending on the classification of the biosafety hood, some percentage of air is recirculated through a HEPA filter while the rest is directed out of the hood through an exhaust system. Thus, biosafety cabinets offer operator, environmental and product protection.

These laboratory safety cabinets (sometimes called IV hoods or compounding hoods) are ideal when preparing sterile medications such as chemotherapeutic agents that may pose a risk to the operator.

BSCs, BSLs, and ISO Classes

When it comes to working with hazardous or potentially infectious material, protecting personnel must be the primary concern. However, offering personnel protection does not have to come at the cost of risking product integrity. Class II BSCs provide protection to both personnel and product simultaneously. They maintain an ISO class 5 clean work area for product manipulation, while ensuring that exhaust air is filtered and ducted out of the environment. Thus, a class II BSC is the ideal solution when working with moderately hazardous material and both personnel and product safety are required.

Recessed Lighting and Optional UV-C Sterilization

Included recessed light fixture simplifies cleaning and minimizes airflow disruptions (shown next to optional UV sterilization bulb).

Touchscreen Control Panel

PLC control panel provides control of fan speed, lights, and motorized shield; digital gauge monitors backpressure and issues alerts when filter(s) need replacement

Motorized Access Shield

Motorized access shield, included on select models, enhances laminar flow and protects operator from chemical fumes and other environmental nuisances.

Integrated Stands Isolates Hood from Work Surface

Built-in stand with dissipative PVC side panels allows VLF Stations to be positioned over any table or processing equipment without transferring hood vibration to work surface (shown integrated with optional modular wet station).

N2 and DI Water Spray Guns

Handy spray guns provide a ready sources of nitrogen or deionized water installed in locations to suit your process requirements (shown: DI water spray gun installed on cleaning station).

Gooseneck Faucet

Angled to dispense water in a focused stream, this faucet is made of chemical-resistant PVC

Adjustable Rear Baffles

Adjust air flow direction, speed and volume by changing position of the easy-to-reach baffle plates at the rear of the station countertop.

Explosion-Proof Light and Fire Sprinklers

Incandescent illuminator lights up work area and is wired per NEC guidelines. It is appropriate for Class I, II and III, Divisions 1 and 2 applications using ignitable mixtures. Sprinkler heads located above each sink add an extra layer of safety when working with flammable chemicals.

Split Perforated Sink for Two Users

The removable perforated stainless steel surface safely guides liquids to the bottom drain and prevents parts from falling into the sink. The center partition allows for separate operations and minimizes splashing or mixing of chemicals from the adjacent sink.

Gas and Solvent Service Lines

Downward spraying petcock nozzles above each sink deliver liquid solvents for rinsing and nitrogen/air for drying. The nozzles extend outward for extra clearance and point down to facilitate cleaning procedures.

Pneumatic Foot Pedals for Hands-Free Operation

Pneumatic foot pedals installed at the base of the stainless steel solvent cleaning station allow users to safely control flow of solvent and nitrogen gas at each sink. Hands-free operation reduces risk of contamination.

Exhaust Duct Connection For Safe Fume Extraction

Stainless steel solvent station is equipped with exhaust duct to safely carry fumes away from workers. This view from the top also shows plumbing that feeds two fire sprinklers, and encased wiring for the explosion-proof light.

Hoods for Wet Processing

HEPA Filtration for Particle-Free Airflow
Isolated Work Surface Enhances High Magnification Viewing
ValuLine™ Vertical Laminar Flow Stations
PLC Touchscreen Panel on Vertical Laminar Flow Hood
Tilt-Up Shield

Operators can create ISO 5 particle conditions within Terra’s laminar flow hoods to protect sensitive materials during processing HEPA Filtration for Particle-Free Airflow Operators can create ISO 5 (or better) particle conditions within Terra’s laminar flow hoods to protect sensitive materials during processing.

Terra’s VLF Station provides HEPA-filtered airflow over a microscope workstation to prevent contamination (shown with optional table, vibration-free platform) | 2001-31C displayed Isolated Work Surface Enhances High Magnification Viewing Stainless steel VLF Station provides HEPA-filtered airflow over a microscope workstation to prevent contamination of slide or optics; detached work surface isolates hood vibration for high-magnification viewing.

ValuLine™ Vertical Laminar Flow Stations accommodate multiple users and bulky process equipment | 2001-70 displayed ValuLine™ Vertical Laminar Flow Stations ValuLine™ Vertical Laminar Flow Stations accommodate multiple users and bulky process equipment (shown: 8-foot wide powder-coated steel models).

Touchscreen PLC panel controls operation of motorized shield, light and filtration system; monitors filter back pressure | 2001-32A-MS displayed PLC Touchscreen Panel on Vertical Laminar Flow Hood Touchscreen PLC panel controls operation of motorized shield, which optimizes laminar air flow and protects operator against chemicals

Tilt-up shield on the ValuLine™ Vertical Laminar Flow Station enhances safety and cleanliness | 2001-69 displayed Tilt-Up Shield Static-dissipative PVC tilt-up shield on the ValuLine™ Vertical Laminar Flow Station enhances unidirectional air flow and protects operator against chemical splashes.

Technical Resources

Terra Universal Hoods Vol 8 #16

This mini-catalog features Terra's offerings for laminar flow hoods, fume hoods and biological safety cabinets, including options from both Terra and Labconco.