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.
Exhaust fan performance, selection
The combination of airflows into fewer shafts allows the connection of multiple control areas into fewer duct risers. In combination with variable airflow systems within the labs, using variable air volume (VAV) fume hoods or other similar methods, a greater amount of turndown in a manifolded fan system can be achieved compared to the single control zone exhaust model. The minimum air change rates of the other connected control areas still allow for a high level of dilution to take full advantage of operational diversity and setback.
While manifolded exhaust provides energy and space-saving benefits, the IMC still prohibits manifolding of incompatible materials, such as perchloric acid and unfiltered radioisotope hoods or biological safety cabinets, or other such situations where a mixture of exhaust airstreams will produce contaminants that exceed the levels stipulated above. Manifolding may not be appropriate where long horizontal distribution is required, particularly for low rise buildings.
To manifold the ductwork, the IMC requires that redundant fans be provided to allow full exhaust airflow should planned or unplanned maintenance necessitate an individual fan shutdown. For systems connecting more than two control areas, this allows a reduction in the number of fans even with the redundancy requirement. By extension, there is potentially less cost and less maintenance, and exposure to hazardous exhaust is reduced for the inspection and maintenance personnel.
Manifolded exhausts can also reduce the friction loss. An 8,000 cfm duct with a velocity of 1000 fpm generates a friction loss of 0.032 in. wc/100 ft. In comparison, a single duct with 24,000 cfm at the same velocity generates a friction loss of 0.016 in. wc/100 ft, half that of the individual exhaust ducts.
A fewer number of larger fans can increase fan efficiency and reduce energy consumption. Consider Table 1, which reports three backward inclined belt-driven utility fan selections: one fan to handle the entire manifolded exhaust system, three fans to handle three different control areas, and three fans assuming a modest 5% increase in static pressure representing the area savings of a common shaft compared to individual shaft partitions. Based on these selections, there is a 13% power reduction at full load between using a single fan compared to three individual fans at the same static pressure, and 17% reduction assuming the economy of space for a single shaft.
While the 2015 International Building Code (IBC) requires redundant fans for hazardous exhaust, it permits the designer the flexibility to choose whether to run the fans in a duty-standby or lead-lag condition. The fan affinity laws in Equation 1 indicate that motor power is proportional to the cube of flow rate, allowing the accrual of energy saving for fans running in parallel. The single fan at half the flow rate is 1.9 hp. To achieve the full exhaust flow, the two fans running in parallel need 3.8 hp, 25% of the theoretical full power of the single fan motor. While actual fan selection and performance may erode the savings, the energy reduction potential is substantial.
Equation 1: Fan affinity law
Power1 / Power2 = (Flowrate1 / Flowrate2)3
The International Energy Conservation Code (IECC) and ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings generally requires energy recovery for air handling systems delivering outside air greater than 70% of the design airflow rate. Exhaust air energy recovery can take many forms, commonly:
- Direct air exchange with or without desiccants such as rotary enthalpy wheels (up to 85% effectiveness)
- Air-to-air plate heat exchanger (up to 65% effectiveness)
- Refrigerant heat pipe (up to 65% effectiveness)
- Runaround loop (up to 65% effectiveness).
However, previous versions of the IMC prohibited the use of energy recovery for hazardous exhaust systems in section 514.2. Recognizing the potential for energy savings due to heat recovery even when VAV exhaust systems are employed, the 2015 IMC includes an exception that no longer prohibits heat recovery, provided the equipment is coil-type heat exchanger, which provides a completely closed loop and separation of supply and exhaust air. The use of membrane-type equipment such as enthalpy wheels comes with a chance of cross-contamination into the supply air side.
While there is a range of energy recovery methods, the intent to require only coil-based solutions is to minimize contamination between the supply and exhaust airstreams. Energy recovery wheels have some small amount of bypass between the airstreams. While many plate heat exchangers have very durable construction, there is leakage that correlates to a pressure differential imparted on the plates between the airstreams. Like plate heat exchangers, refrigerant heat pipes require adjacency of the two airstreams, limiting the configuration of the ductwork or air handling unit. Even when installed in an air handling unit (AHU), there is also casing leakage between the two airstreams given a differential pressure between the casing wall that separates the two.
The system that offers the greatest degree of separation between the airstreams is a runaround coil system, which pumps a fluid between a coil in the outside and exhaust airstreams to transfer the heat between them. When the air upstream of the coil can drop below freezing, glycol must be added to the fluid, reducing the heat transfer coefficient and increasing viscosity and pumping energy, both of which reduce the overall effectiveness of heat recovery.
Control of relative humidity (RH) in labs, while contributing to environmental stability in which research and experimentation is conducted, also leads to increased energy consumption. Increasing space humidity is a recognized method of ignition control, especially in climates such as the northeastern United States (winter) and southern California (easterly winds). NFPA 77: Recommended Practice on Static Electricity suggests a controlled RH range of 30% to 60% as a means to limit static discharge. This is useful, for example, in hazard control for flammable liquid handling, which is common in labs.
Active laboratory humidification is sometimes designed without a particular end-user need and included building-wide to provide flexibility in allowing humidity-sensitive research to be executed anywhere in the building. An alternative planning approach allows certain portions of the building to be humidified. This can be achieved by relegating an AHU with humidification to a specific wing or floor of the building. Alternatively, in-duct humidification can target specific zones or rooms.
A discussion with the end-users can also help to limit the humidification setpoint. The target is often 50% RH, but specific lab processes may allow for a depression in the setpoint. For example, low-sensitivity electronics work may only require up to 30% RH to control electrostatic discharge when coupled with appropriate flooring materials and grounding protocols. The lower the exhaust air RH, the fewer the incidents of condensation generation through heat recovery. Lower space humidity also reduces the chance for condensation generation on cold surfaces (e.g., glazing mullions in extremely cold climates).
Humidification to produce indoor humidity levels at 50% RH compared to 30% RH occurs 50% more often in New York City for 100% outside air systems. Reduction in the area and setpoint required for humidification will decrease the energy required to generate steam, and reduce the infrastructure cost of steam and steam condensate generation and distribution piping.
For extremely cold climates, condensate may be generated in the exhaust airstream, which is especially true for actively humidified labs. Depending on the makeup of the exhaust, the condensate may need to be neutralized prior to draining to the sanitary system, or otherwise captured and properly disposed. The presence of condensate on coils, within ductwork, or on AHU casings, coupled with corrosive chemicals in the exhaust airstream, can increase material deterioration, requiring more frequent maintenance or replacement of components. Compatibility of the airstream materials with corrosive chemicals in the airstream is an important aspect of the design process.
Humidification poses an increase in energy costs and maintenance, including controlled startup and shutdown at the start and end of the humidification season, boiler chemical maintenance, steam trap inspections, and the potential added expense of providing reverse osmosis water for clean steam generation. In addition to the associated piping or local generators, some of the first costs for humidification can include the pressure reducing valve station with all the attendant controls and safety devices, specially trained and qualified staff for higher pressure steam, piping expansion, and anchoring.
There is an ongoing challenge for designers to balance safety and low energy for which there are several conditions and design approaches to consider in every laboratory project. The 2015 IMC provides new and innovative language on manifolding, fan redundancy, and heat recovery for hazardous exhaust. Each of these three provisions may be used to achieve energy savings, which can be substantial if they are implemented in concert. Reduction or elimination of space relative humidity requirements also will save energy and infrastructure cost. NFPA 77 offers humidification as a method of ignition control, which must be considered carefully to define a setpoint that recognizes safety and energy use.
Jon Eisenberg is an associate principal in Arup’s Boston office, and is an expert in industrial and laboratory fire protection and hazard analysis. Jeffrey Huang is an associate in Arup’s New York office, and applies the latest low-energy approaches to the design of laboratories for both commercial and university clients.