The use of chemicals and hazardous compounds separates a lab from other types of building space. Although not all labs are alike and different labs contain various hazard levels and uses, a host of criteria—including worker safety and comfort, energy efficiency, and overall suitability for a specific application—must be considered in a lab renovation.
Fume hoods
A fume hood is intended to ensure the safety of all lab workers by limiting their exposure to hazardous fumes, vapor, and dust. There are two main types of fume hoods: ductless and ducted.
A ductless hood is a standalone, self-contained workstation. Air is drawn from the front of the hood and passes through carbon and/or HEPA filters to remove fumes, vapors, and particulates in the air before it is recirculated back into the lab. The flow of air into the hood and the face velocity protect the user from exposure to fumes and vapor.
A ducted hood is attached directly to a building exhaust system and draws room air into it. Air streams that enter the hood prevent chemicals inside the air from escaping. Air in the hood is then exhausted, drawing vapors, gases, and particulates out.
Ductless versus ducted hoods
In a lab renovation, the obvious benefits for selecting a ductless hood over a ducted hood are the reduction in lab ventilation energy costs and the savings in the cost of ductwork installation and wiring. In addition to these benefits, there are other features that lab designers and planners must consider to ensure that their choice of hood meets certain safety and worker comfort requirements, reduction in energy consumption, and overall hood suitability for a specific application.
1. Safety
Safety regulations, guidelines, and standards have been published by many industry groups including the U.S. Occupational Safety and Health Administration (OSHA); the Building Officials and Code Administrators International, Inc. (BOCA); the American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc. (ASHRAE); and the American National Standards Institute (ANSI).
Ductless hoods
A ductless hood does not have any requirement for make-up air as the air recirculates through the hood and back into the lab. Therefore, safe operation of a ductless hood does not depend on its location since it can be placed in air-starved areas.
A ductless hood is generally equipped with features such as safety controllers that monitor airflow and filter conditions, real-time gas sensor technology to ensure protection from exposure to toxic chemical fumes and vapors, visible and audible alarms to alert users of filter saturation, and airflow sensors.
Ducted hoods
A ducted hood draws in room air. Room air currents have a large impact on the ability of a ducted hood to contain air. Therefore, a ducted hood must be placed away from walkways to minimize air being drawn out of the hood. It must also be placed away from doors to minimize the air from entering the lab and upsetting the air flow of the hood.
2. Worker comfort
Safety must be balanced with maintaining worker comfort. Worker comfort involves maintaining the appropriate temperatures and air velocities in the lab while ensuring the lab equipment is organized to allow lab personnel to perform their tasks with minimal effort.
Worker comfort is affected by the functions that will be performed, the chemicals and materials that will be used in the hood, and the level of hazard and risk. Above all, worker comfort is also affected by the hood location.
Ductless hoods
The location of a ductless is flexible. Since it has no make-up air requirement, it can be located in hard-to-vent areas or in air-starved labs. Its portability adds to worker comfort, as it can be shared among several labs or can be stored when not in use.
Ducted hoods
The flexibility of location of a ducted hood is limited by its basic configuration. A benchtop hood is generally physically placed on the lab bench and a floor-mounted hood is mounted on the floor.
3. Energy efficiency
Conditioning, supplying, and exhausting the large volume of air used in a lab results consumes a sizeable amount of energy.
Ductless hoods
Since a ductless hood is a standalone, self-contained workstation, it has no make-up air requirements and no requirement to be connected to any HVAC and plumbing system. With no ductwork involved, and heated or cooled air is not exhausted from the lab, there is a savings in installation costs and energy and operating costs with a ductless hood.
Ducted hoods
A ducted hood is attached directly to a building exhaust system and requires that the HVAC and plumbing system complement the hood mechanical system. Installation costs are higher with a ducted hood since ductwork must be installed. Operating costs are higher than a ductless hood since heated or cooled air must be exhausted from the lab.
4. Applications
Neither a ductless nor a ducted hood are “one size fits all.”
Ductless hoods
Selecting the right ductless hood depends on the application and the chemicals being trapped. There are different classes of ductless hoods, and some limit the applications that can be performed in them. Some ductless hoods are only light-duty or process-specific or can be used with a limited number or a large number of chemicals. Ductless hoods that can be used with a wide range of chemicals may see their filter life shortened by the large chemical volumes, thus requiring that a limited number of different chemicals be used and with a moderate chemical exposure time.
There are also different types of chemical-specific filters with a chemical trapping capacity for different chemical families. For example, HEPA filters efficiently filter particulates for chemical powders and particulates; activated carbon filters chemically adsorb organic vapors; acid-sulfur carbon filters neutralize acids and sulfur-containing compounds; ammonia/amine carbon filters neutralize ammonia/amine compounds; formaldehyde carbon filters neutralize formaldehyde; mixed-bed carbon filters neutralize acids and sulfur-containing compounds, ammonia and amine compounds, and formaldehyde-containing compounds; and radioisotope carbon filters neutralize radioisotopes.
To ensure that the chemical-specific filters remain efficient, monitoring the airflow and chemical saturation point of the filter is key so that the filters can be discarded and replaced. Likewise, HEPA filters should be checked for leaks and periodically changed to maintain high filtration efficiency.
Ducted hoods
Selecting the right ducted hood depends on the air volume capacity in the lab and the existing mechanical and HVAC infrastructure.
There are two general types of ducted hoods: constant air volume and variable air volume. A constant air volume hood is designed to exhaust the same level of air volume. In these labs, any air that is not exhausted through the hood must be exhausted through the room exhaust to maintain lab air balance. A constant air volume hood may be the best choice in a lab with a constant air volume supply.
A variable air volume hood is designed to exhaust only the air required to maintain face velocity at the desired setpoint. When a hood user follows good sash management, a variable air volume hood has the potential to reduce energy costs even more than a constant air volume hood.
Energy costs may be reduced if a ducted hood is equipped with a variable air volume system. However, there are drawbacks with a variable air volume system: it is more expensive to purchase up front and requires consistent monitoring and maintenance to ensure it is working properly.
Conclusion
Although the obvious benefits for selecting a ductless hood over a ducted hood are the reduction in lab ventilation energy costs and the savings in the cost of ductwork installation and wiring, a ducted hood may be a better option if a ductless hood or the chemical-specific or HEPA filters are not be suitable for a specific application, or the cost of replacing the filters is prohibitive. For information on ducted and ductless hoods, visit https://www.aircleansystems.com/.
Lina Genovesi, Ph.D., JD, is a technical, regulatory, and business writer based in Princeton, NJ, U.S.A.; e-mail: [email protected]; www.linagenovesi.com