Designing lab ventilation systems
Engineers should consider the codes and standards, safety, risk mitigation, and potential energy savings when designing laboratory ventilation systems.
- Know the codes and standards that dictate the design of laboratory ventilation systems.
- Discuss the changes in the 2015 International Mechanical Code (IMC) section on hazardous exhaust systems (section 510).
- Understand the potential effects of the code changes on energy use.
Laboratory ventilation systems often are designed to meet the requirements for hazardous exhaust systems. To find a balance between the goals of safety, risk mitigation, and energy savings, designers can take several approaches. Here are a few options to help engineers achieve these goals, which may seem somewhat opposed but can provide some overlapping opportunities for innovative design.
The overarching goal of section 510 of the 2015 International Mechanical Code (IMC) is to provide a safe working environment, with secondary goals to increase the durability and reliability of the exhaust systems conveying hazardous materials. The design mandate is to maintain the concentration of contaminants in the exhaust airflow below 25% of its lower flammability limit (LFL). Dilution, the addition of noncontaminated air into the exhaust airstream, is the main method by which this is achieved. For chemical production facilities, an additional layer of protection is mandated through the inclusion of fire protection within the ductwork system.
A hazardous exhaust system is required when the 25% LFL or the 1% median lethal concentration (LC50) will be exceeded in the absence of any mechanical intervention, commonly assumed to be an active exhaust system. With the changing needs of research, it is cumbersome to continuously monitor chemical quantities to determine when LFL or LC50 levels are exceeded, thus many research institutions generally provide hazardous exhaust systems as defined in the code.
Prior to the 2015 version of the IMC, hazardous exhaust from different control zones was required to be conveyed separately through independent ducts. Separate duct risers within shafts were not permitted to share common shafts; therefore, shaft separations through fire-rated construction were required for each control zone. The 2015 IMC contains a change to section 510.5 that permits hazardous exhaust ducts to be combined or manifolded inside a rated shaft by exception. The result of the code change is a savings in shaft space, a reduction in the number of fire separations, and potential energy savings and system redundancy.
Manifolding laboratory exhaust has been the subject of debate for at least the past 10 years with a long-standing precedent within NFPA 45: Standard on Fire Protection for Laboratories Using Chemicals. The main argument is the desire to add additional laboratory exhaust air from separate control areas to enhance the dilution effect within the exhaust ductwork. This presumes that the other control areas have a lower concentration of hazardous exhaust, which is often true in a research environment with intermittent generation of airborne hazards.
Fewer fans also have the potential to simplify energy recovery through system consolidation. Instead of a large number of energy recovery devices in individual control zone exhaust systems, a fewer number of energy recovery devices can be used.
Dampers in hazardous exhaust ductwork
Fire and smoke dampers are prohibited in hazardous exhaust systems to eliminate the flow restriction when these devices close. Both the IMC and NFPA 45 have accepted the use of steel subduct extensions in lieu of fire dampers for duct penetrations through fire-resistance rated shaft enclosures, as long as there is continuous upward flow to the exhaust outlet outside. The vertical upturn coupled with the negative duct pressure minimizes the migration of potential combustion products into ducts connected to the riser at other floors. This provision also recognizes that the inclusion of such active protection devices has the potential to obstruct airflow if it malfunctions, jeopardizing the requirement to continually exhaust the space. The use of fire dampers also would put those people who inspect and maintain the system at a greater risk of exposure to the contaminants.
The Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) recommends that the subduct be no more than 25% of the riser duct cross-sectional area as a rule of thumb. Generally, the riser duct size should be increased to maintain the desired vapor/gas transport velocity in the free annular zone between the subduct and the riser duct to minimize deposition.
The commentary given in NFPA 45 indicates that the continuous upward flow of exhaust under normal operating conditions is not meant to require the use of a generator. With the use of the subduct extension, the IMC requires continuous airflow upward to the outside, though it is silent on the use of a generator.