Fume Containment Hoods – to duct, or not to duct, that is the question
If Prince Hamlet from William Shakespeare’s play Hamlet was living today as a laboratory scientist, he might be pondering, “To duct, or not to duct, that is the question”. This would be very fitting because an informed decision is needed when it comes time for you to select a ducted or ductless chemical fume hood for your facility.
Background information to consider in the selection process
A laboratory fume hood is a type of ventilation system that primarily functions to provide personnel protection against toxic fumes, vapors and particulates. The secondary function is the physical barrier that protects the laboratory and people by shielding against inhalation exposure, chemical spills, runaway reactions and fires. Each laboratory situation must be evaluated individually to determine the best approach to control fumes and vapors.
To ensure maximum containment, fume hoods should be suitably sited to avoid crosscurrents at the hood face due to pedestrian traffic and doors. Sufficient laboratory supply air must be available to allow hoods to operate at their specified face velocities (the speed at which air is drawn into the hood).
The hood functions by maintaining a relatively negative pressure in the interior of the hood to prevent any contaminant from escaping while drawing air in through the hood opening at a consistent rate. Fume hoods with the proper combination of blower capacity and hood face opening size are designed to maintain a predictable air flow rate through the face opening.
Excessive face velocities can often result in turbulence and reduce containment and insufficient face velocities can also compromise hood performance. In general, a hood’s face velocity is recommended to be between 0.3 m/s (60 fpm) and 0.5 m/s (100 fpm), however it is important to check local safety regulations on the face velocity recommendation before using the fume hoods. Only a meter can accurately gauge a hood’s face velocity to determine if it has dropped to unsafe levels.
A suitable hood face velocity is of importance to the safe and effective operation of a fume hood. Suppliers of laboratory equipment and services have therefore developed testing procedures, such as ASHRAE 110, to help ensure user safety.
It can be difficult for users to verify proper operation of the hood. Odor is not a sufficient indicator, because low levels of hazardous chemical fumes can be undetectable by our sense of smell. Only an air velocity meter (anemometer) can accurately gauge a hood’s face velocity to determine if it has dropped to unsafe levels.
Certifying fume hoods involves three different tests: flow visualization (smoke test), face velocity and containment. After acceptance, annual face velocity tests are sufficient, absent renovations to the HVAC system, laboratory or hood.
Conditioning, supplying and exhausting large volumes of air used in laboratories consumes sizeable quantities of energy. Reducing these energy costs has a direct impact on a company’s bottom line. Laboratories must be designed so that energy efficiency gains do not reduce safety and comfort.
So in regards to selecting the best type of fume hood for your application, a filtered or vented hood, we have spelled out some advantages and disadvantages of each type.
Filtered (Ductless) Fume Hoods
Maintenance Responsibilities of Lab Staff
All filters have a life cycle and need to be replaced over time. The fume hood’s operator manual will explain the timing of replacing them. The lab staff needs to become familiar with the use and maintenance of the hood, the filtration system, and keeping proper records of the face velocity measurements.
Advantages:
1. Location Options
One of the best features of a ductless fume hood is that it offers more options for placement in the laboratory because it is free standing. If the location of the hood will ever need to be changed, the ductless hood is more easily relocated, and may be your best option.
2. Labor Costs
There is little activity required by the lab staff in prepping for a filtered hood. There is also little to no engineering involvement so the cost to set up a ductless fume hood is relatively minimal.
3. Energy Costs
A ductless hood recirculates the filtered air back into the room so there is no loss of conditioned air (heated or cooled) out of the building. The only energy cost for a ductless fume hood is the cost to run the blower fan and lights in the hood. There is no cost to recondition the room air for a comfortable temperature.
Disadvantages:
1. Purchase Price
The purchase price of a ductless hood is generally more expensive than a ducted hood without the labor.
2. Filter Cost
Filters need to be purchased and installed in the hoods. Filters for a ductless fume hood can cost anywhere from a couple hundred dollars to a thousand dollars each and are a recurring cost. Find out ahead of time what the cost of the filter is and the recommended frequency of replacement.
3. Air quality
The filtered air of a ductless fume hood is not 100% fume or vapor free. Because ductless fume hoods utilize activated carbon and formaldehyde filters to adsorb chemical vapors and fumes, the air is immediately recirculated back into the laboratory. Ductless hoods are engineered to capture and chemically bond large volumes of chemicals in their filters.
Vented Fume Hoods
Maintenance Responsibilities of Lab Staff
Lab staff responsibilities are minimized, as no time is spent on ordering, inventorying, and replacing filters. Engineering responsibilities, however, are quite involved. Procedures are performed within the hood and parts are needed for safe ventilation, such as an extract blower and ductwork.
Advantages:
1. Purchase Price
The purchase price of a vented hood is generally less expensive than a filtered hood, however, the cost of the engineer’s time and materials usually drives the total cost higher than a filtered hood.
2. Filter Cost
No filter cost to laboratory.
3. Air Quality
One of the best features of a ducted hood is that whatever fumes are generated in it get exhausted to the outside and diluted with lots of other air in the atmosphere. The laboratory air is not contaminated with fumes using an efficiently working vented hood.
Disadvantages:
1. Location Options
Building codes and building structures will limit the number of locations where a vented fume hood can be installed.
2. Labor Costs
Significant costs will be incurred to have experts in HVAC evaluate the existing mechanical system and determine if the existing HVAC system could support the additional load. Site preparation will be needed before installation, which includes working with ducting materials, controls, and exhaust air-handling systems. Installation will possibly require an HVAC contractor or maintenance staff that is familiar with local building codes, physical requirements of the hood and laboratory layout. The engineering involvement of a ducted fume system is high, once you factor in the labor and infrastructure needed such as; ducts, exhaust fans, mechanical systems, and roof elements. Managing the building’s mechanical system will be the responsibility of the Maintenance Department or the Environmental Health and Safety Department.
3. Energy Costs
A ducted hood continuously throws conditioned air (heated or cooled) out of the building and will cause a higher energy bill than a ductless hood.
Some final thoughts before buying your fume hood
Manufacturers offer cost analysis, where they can look at the chemical you work with in the hood and tell you how long their filters will be able to do their job before they need to be replaced. Ductless hoods are growing in popularity because of energy savings and the green movement. Ducted hoods are not nearly as environmentally friendly as ductless hoods, due to the harmful and/or toxic laboratory air that is dumped into the ambient outdoor air is causing pollution to the environment.
Ducted hoods will require some basic training in the operation of the baffles, sash settings, and adjusting the fan. The blower belt will need to be replaced occasionally also.
In anatomic pathology labs where flammable organic solvents (such as xylene), alcohols, and formaldehyde are used, a carbon and formaldehyde filter are stacked for adequate adsorption. The use of HEPA filters are also an option if particulates are of concern.
If you need assistance deciding whether a filtered enclosure or ducted enclosure is most appropriate for your application, call our customer service and they will assist you in analyzing your application and to recommend the right enclosure for you.
REFERENCES:
- “To be, or not to be, that is the question”. The opening phrase of a soliloquy spoken by Prince Hamlet in the so-called “nunnery scene” of William Shakespeare’s play Hamlet.
- Laboratory Design Handbook. TSI Incorporated
- Understanding High Performance Fume Hoods. Beth Mankameyer. Labconco
- Energy Savings Comparison Between Conventional Fume Hood and Ductless Fume Hood. Esco Technologies, Inc
