An Exhaust Solution for Controlled Environments

Aug. 17, 2005
Mixed-flow impellers offer pharmaceutical plants enhanced safety, better plume dispersal, lower emission concentrations and reduced energy costs.
Most pharmaceutical manufacturers maintain controlled environment work areas for research, pilot production and, occasionally, for general product manufacturing. All of these facilities share an air quality challenge: exhaust from laboratory workstation fume hoods or other procedure/processing areas must be safely discharged into the atmosphere without causing re-entrainment, creating an odor problem or violating environmental regulations.
Rooftop stacks, which can provide an undesirable but recognizable "signature" as to the procedures being conducted within the facility.

Most pharmaceutical manufacturers also face high energy costs for their HVAC systems, and particularly for their conditioned, controlled environment facilities. Energy costs for pharmaceutical research and manufacturing are among the highest such costs for all standard industrial classification (SIC) categories. Exhausting laboratory workstation fume hoods has traditionally been handled by centrifugal belt-driven fans with tall, individually-dedicated stacks or combined multi-stacks on the roof. However, mixed-flow impeller technology, with low-profile roof exhaust systems, has been gaining acceptance over the last few decades as a solution to exhaust cost and safety challenges.Mixed-flow impeller roof fans send their exhaust streams hundreds of feet into the air in a vertical plume, diluting outside air with exhaust gases at the point of discharge. This highly efficient plume dispersal prevents local re-entrainment and, through high-efficiency mixing of the discharge air with the surrounding atmospheric air, eliminates potential odor problems at ground level. Mixed-flow fan systems can also reduce unnecessary energy losses since they can pre-heat (and pre-cool) makeup air before it enters a building. This is achieved by the inclusion of heat transfer run-around-coils which capture exhaust heat and return it in a safe manner to the incoming air stream. This system permits substantial, safe, energy savings for research and manufacturing organizations.Exhaust stream dilution prevents re-entrainmentRoof exhaust systems’ main purpose is to prevent re-entrainment, to ensure healthy indoor air quality (IAQ) and conformity to local legislation. Should re-entrainment occur, there is a real risk to employees at pharmaceutical facilities due to exhaust re-entrainment through building intake vents, doors, windows and other openings. Re-entrainment can be caused by inefficient roof fans, poor design or location of exhaust stacks, position of building air intakes, weather and wind conditions, and a host of other factors.However, the legal consequences of re-entrainment can extend well beyond employers. Building owners, consulting engineers, heating, ventilation, and air conditioning (HVAC) contractors and even architects have been named as defendants in major cases associated with employee illness and IAQ. Research laboratories at most pharmaceutical organizations can range from discrete prototyping facilities through complex biosafety containment level (BSL) 3 or 4 facilities, which require accurate, repeatable control and management over such environmental parameters as temperature, pressure, airflow and humidity—almost always in combination. While all BSL 3 and 4 laboratories have terminal HEPA filters, consideration of roof exhaust re-entrainment cannot be ignored as there is always the potential for filter failure and a highly efficient plume discharge is required during fumigation procedures.Because high containment BSL laboratories present a unique set of problems with regard to re-entrainment and pollution abatement, they are governed by rigid pollution-abatement codes and standards promulgated by organizations that include the American National Standards Institute (ANSI), the American Society of Heating, Refrigeration and Air conditioning Engineers (ASHRAE), OSHA and CDC. In some cases, there are hard-and-fast codes, just guidelines and recommendations.Special exhaust requirements at BSL laboratoriesBiosafety level laboratories of the containment levels 3 and 4 must incorporate special design and engineering features to prevent microorganisms from being discharged into the environment. These features would typically include specially shielded isolation rooms under negative pressure with high efficiency particulate arresting (HEPA), terminal filters, sophisticated control and monitoring systems for managing their environmental parameters; they would also require 100% conditioned “makeup” air to prevent the re-use of ambient air within an enclosed facility.Obviously, exhaust emissions from laboratory workstations at these facilities must be treated carefully, including the discharge of fumigation gases. They may be highly toxic or noxious, or both. Even if the exhaust stream does not present health issues, the public will no longer tolerate annoying odors, while government agencies are continually setting more stringent standards and lowering allowable exposure limits.HEPA filter modules reduce dust emissionsIn pharmaceutical research laboratories and pilot processing areas, a dedicated air supply and exhaust system is critical to safety as well as comfort. The HVAC system is typically independent of all other supply and exhaust systems within the building. Because of increasingly stringent environmental regulations, mixed-flow impeller systems incorporating bag-in/bag-out (BIBO) High Efficiency Particulate Arresting (HEPA) modules and filters are also being used for research, pilot plant and commercial manufacturing environments. The modules are typically matched with application-specific filter media which accommodate a variety of HEPA and ASHRAE filter efficiencies.Mixed-flow impeller systems with HEPA modules and filters are particularly useful at manufacturing sites, since unwanted dust is created during the manufacturing and packaging cycle. Sources for this dust vary from product mixing, tableting, spray coating, drying (which creates particulate fines through airstream abrasion or contact friction) and even packaging. Heat recovery, energy savingsMixed-flow impeller systems can offer substantial energy savings if they are equipped with heat recovery modules consisting of glycol/water-filled coils that extract heat from workstation fume hood exhaust before it is discharged above the roofline. The warm air from the heat exchanger is transferred to the supply-side handler to preheat the conditioned air entering the building, thus reducing the amount of natural gas or fuel oil needed to heat makeup air. This process works the same way for chilled air.By some estimates, mixed-flow impeller exhaust systems with heat exchanger coils save about 3% of energy costs for each 1° F rise in recovered heat; similar, but not quite as dramatic, savings are realized for cooling applications. Systems such as these are most practical when outside air temperatures are below 40° F (5° C) or above 80° F (27° C). On the cooling side, if the outside air temperature is 90° F (32° C) and the chilled indoor air is sent through the heat recovery coils, the makeup air temperature drop is typically 4° to 5° F.Mixed-flow impeller fans typically consume about 25% less energy than conventional centrifugal fans, and offer faster payback periods as well. Typical energy reduction is $0.44 per cubic foot per minute (CFM) at $0.10/kilowatt-hour, providing an approximate two-year return on investment.Up on the rooftopTall exhaust stacks from centrifugal fans usually require complex, expensive mounting hardware (elbows, flex connectors, spring vibration isolators, guy wires, roof curbs, etc.), and often still do not totally prevent re-entrainment of exhaust fumes back into the building or adjacent facilities. In addition, they provide an undesirable but recognizable “signature” as to the procedures being conducted within the facility.The roof mounted, belt-driven fans also require regular maintenance; because of this, they are often housed inside a rooftop “penthouse,” which protects workers from the elements during maintenance; however, these workers might also be subject to exposure from toxic and/or noxious fumes, since the fans’ discharge is always under positive pressure.The low-profile design of mixed-flow impeller roof exhaust systems — they are typically about 15 feet high — eliminates the need for structural reinforcements on the roof. Because they are substantially shorter (and constructed modularly) than taller stacks, installation time and costs are reduced. In many retrofit applications, there is almost no downtime associated with their installation.Under normal conditions, mixed-flow impeller systems are designed to operate continuously for years without maintenance. Direct drive motors have lifetimes of L10 400,000 hours (Box). And the non-stall characteristics of the blade design permits variable-frequency drives to be used for added variable-air-volume (VAV) savings, built-in redundancy, and design flexibility. These fans also operate at lower noise levels than centrifugal fans — particularly in the lower octave bands. When noise is still an issue, accessories such as low-profile acoustical silencer nozzles can be used.Aesthetics can also be an issue. When designing or retrofitting a new roof exhaust system, stack height must be considered for a variety of reasons already mentioned.Low-profile systems can also help to solve the architectural / aesthetic / operational issues associated with high profile stacks. In some areas, for example, no stacks are allowed on rooftops.For facilities looking to upgrade, retrofit or construct new laboratory workstation facilities or process areas, mixed-flow impeller technology exhaust systems represent a practical and cost-effective approach for eliminating re-entrainment, preventing pollution and neighborhood odor, conforming to aesthetic ordinances and standards, and cutting energy costs. Based on current trends, this technology will likely continue to meet the needs of a growing base of pharmaceutical research and manufacturing organizations.About the AuthorJohn Gibson, MCIBSE, is an Associate Director with Scott Wilson, a 3,500-employee global consultancy firm. He heads up the specialist Laboratory Building Services design team, with responsibilities in regulatory compliance audit, concepts design, and overall design guidance. His particular field of excellence is in the pharmaceutical and research laboratory sector .John consulted on projects for Pfizer, Eli Lilly, AstraZeneca and Scherer, and for leading universities such as Cambridge, King’s College, Imperial College, University College and the recently completed Cell and Molecular Sciences research facility for Queen Mary, University of London. He can be reached at [email protected].Characteristics of Mixed-flow Impeller Technology SystemsMixed-flow impeller systems operate on a principle of diluting contaminated exhaust air with unconditioned, outside ambient air via a bypass mixing plenum (see figure, right).
The resultant diluted process air is accelerated through an optimized discharge nozzle/windband, where nearly twice as much fresh air is entrained into the exhaust plume before leaving the fan assembly. Additional fresh air is entrained into the plume after it leaves the fan assembly through natural aspiration. The combination of added mass and high discharge velocity minimizes the risk of contaminated exhaust being re-entrained into building fresh air intakes, doors, windows, or other openings. As an example, a mixed-flow fan moving 80,000 cubic feet per minute (cfm) of combined building and bypass air at an exit velocity of 6,300 feet per minute can send an exhaust air jet plume up to 120 feet high in a 10 mph crosswind. This velocity exceeds ANSI Z9.5 standards by more than twice the minimum recommendation of 3000 fpm. Because up to 170% of outside air is induced into the exhaust airstream, a substantially greater airflow is possible for a given amount of exhaust—providing good dilution capabilities and effective stack heights.Mixed-flow systems are designed to operate continuously with a minimum amount of required maintenance. Direct-drive motor bearings have lifetimes of L10 400,000 hours. (This refers to a “sample” of 100 motors in which the bearings in ten motors, 10%, would fail within a 400,000-hour timeframe. It is a baseline for comparison of motor bearing lifetimes.) Non-stall characteristics of the system’s mixed-flow wheel make it well-suited for constant volume or variable air volume (VAV) applications, along with built-in redundancy, and design flexibility. VAV capabilities are achieved via the bypass mixing plenum or by using variable frequency drives to provide optimum energy savings.

Using CFD to Compare Roof-mounted Exhaust Systems

A Computational Fluid Dynamics (CFD) simulation study aimed to compare the performance efficiencies of mixed-flow roof-mounted exhaust systems with traditional stack designs, in anticipation of an exhaust installment at a major research center — a 50,000-square-foot facility with basement, ground-, first- and second-floor laboratories. The study, conducted by the consulting firm Studies in Flow Motion, in association with CFD simulation specialists Flowsolve (Wimbledon Park, England), encompassed a series of simulations and tests over many months to cover most typical operating conditions (low, medium and high wind speed, prevailing wind directions and seasonal temperatures).

The analysis found that the optimum stack height to achieve adequate plume dispersal under varying weather conditions, would be six meters clear from roof level. This was three meters beyond the existing planning approval. The facility received consent from local planners to increase the height of its stack.

Following the project, the consultant engineers began exploring ways in which stack heights could be reduced while retaining or improving plume dispersal. They added three-meter mixed-flow tri stacks to their CFD studies. Results showed that the mixed-flow impeller exhaust system greatly improved plume dispersal.

About the Author

John Gibson | MCIBSE