Rooftop exhaust fans: environmental considerations
The subject of indoor air quality (IAQ) in the plant has received widespread publicity over the past few years. There is increased public awareness of pollution issues partly due to news media coverage of multimillion dollar lawsuits concerning IAQ. These lawsuits resulted from claims that dangerous process exhaust either remained in the work area or roof exhaust gases were being re-entrained into the work area.
Fig. 1. Mixed-flow impeller fans use low-profile exhaust stacks.
Exhaust re-entrainment is caused by:
Inefficient roof exhaust fans
Poor exhaust stack design or location
Improper location of building air intakes
Roof exhaust re-entrainment at production and processing facilities can be toxic, noxious, or odoriferous. The danger to people covers a broad spectrum from mildly annoying, but not harmful, to seriously unhealthy under certain conditions.
Even if exhaust re-entrainment is not a health concern at a facility, generating odoriferous emissions can create problems in the neighborhood. A plant producing them, regardless of toxicity, will ultimately be confronted, either by its neighbors or a regulatory body.
Toxic odors are those that are regulated by the EPA, OSHA, and other government agencies. Nontoxic odors are either completely safe, or safe in the amounts that are likely to be generated.
Tolerance for toxic odors has sharply decreased in recent years, partly because government agencies are continually setting more stringent standards, with allowable exposure limits dropping lower and lower.
Roof exhaust technologies
For many pollution abatement problems, including re-entrainment, toxicity, and odor control, there are two popular process exhaust technologies. Centrifugal roof exhaust fans, usually with a single belt-driven motor, and dedicated, single-stack, mixed-flow impeller exhaust systems.
For all but very serious air pollution control methods, where precipitators, scrubbers, chillers, thermal and catalytic oxidizers, and other more complex equipment or aggressive treatments are required, one of these methods is generally satisfactory.
In the past, centrifugal exhaust fans handled most of these applications, generally with one tall exhaust stack for each fan in a dedicated installation.
Tall exhaust stacks usually require expensive mounting hardware (bases, guy wires, roof curbs, etc.), and often do not prevent re-entrainment of exhaust fumes back into the building or adjacent facilities.
Belt-driven centrifugal fans require periodic maintenance, and often are housed on the roof. This is done to protect workers from the elements during maintenance operations. The people working on these fans may be exposed to toxic and noxious fumes.
Another consideration when retrofitting or designing new roof exhaust systems includes the aesthetics of stack height. The lowest possible profile not only eliminates the smoke stack look and negative connotations perceived by many people, but may also help conformance to applicable ordinances (Fig. 1).
Elimination of tall, unsightly stacks, which are either prohibited by code or undesirable, is an added benefit. Low-profile, mixed-flow impeller fans don’t require structural reinforcements on the roof or complex, expensive mounting/stabilizing hardware such as elbows, flex connectors, or spring vibration isolators.
Typically, mixed-flow impeller systems are virtually maintenance-free; there are no belts, elbows, flex connectors, or spring vibration isolators; and penthouses are not required to accommodate maintenance personnel under adverse conditions.
Mixed-flow impeller technology is a combination of axial, radial, and centrifugal flow technologies. The technology capitalizes on the performance characteristics offered by these fan types by combining them into a unique fan blade that provides constant acceleration ratios through blade passageways.
Mixed-flow technology was originally designed for low-pressure, high-flow applications. With today’s advances in blade aerodynamics, it provides optimum performance in virtually all combinations of low pressure/high flow and high pressure/low flow.
Mixed-flow exhaust fans combine outside air with exhaust discharge in a low-profile stack, sending a nearly vertical plume of exhaust gases up to 350 ft above the building roof line (Fig. 2).
Fig. 2. Mixed-flow fan technology can send an exhaust plume 350 ft in the air.
The exhaust gas/air mixture contains as much as 170% outside air, effectively diluting the exhaust plume into the atmosphere, and eliminating pollution problems. The vertical plume also eliminates re-entrainment possibilities, solving part or all of the IAQ problem that may have been caused by roof exhaust.
Exhaust re-entrainment can also be affected by building location, with regard to adjacent buildings, as well as prevailing wind and weather conditions. As a result, for both renovations and new construction, wind studies have become important with regard to IAQ, since harmful exhaust gases must not be allowed to re-enter a building or adjacent buildings.
Wind studies are performed using equations from the ASHRAE Handbook of Fundamentals and an EPA model, known as SCREEN3 Cavity Wake Calculation . These studies predict exhaust concentrations at ground level as well as rooftop locations, using ASHRAE equations. They compare dilution capabilities of exhaust systems. The dilution criteria require a 3-ppm or lower air intake concentration for a 15 cfm chemical release.
A primary concern, when any wind study is conducted, is whether there is a very large building or hill near the plant.
The wind approaching a plant can be turbulent or disturbed by another structure that may cause reverse flows, where the wind is going backwards from what the rest of the area is experiencing.
While all facilities don’t necessarily require wind studies, they should be performed where any question exists with regard to exhaust stack design, adjacent buildings, and re-entrainment possibilities that will affect IAQ.
Wind studies generally lead into other issues, such as manifolding of exhaust fumes vs. nonmanifolding. Generally ASHRAE screening equations (assuming there isn’t some major, complicated building geometry to invalidate a model) will help determine whether a manifolded system would be more efficient in preventing re-entrainment.
Manifolding exhaust is beneficial because the plume rise of an exhaust is proportional to the diameter of the exhaust and exit velocity. Manifolding sources of exhaust together allows a high exit velocity and a large diameter at the same time (Fig. 3).
Fig. 3. Manifolding exhaust sources enhances overall system efficiency and lowers costs.
Without manifolding, the exit velocity may be high but with a small diameter. Combining exhaust effluent sources provides a higher plume rise, which is important for lowering exhaust concentration (Fig. 4).
The subject of noise generation, while not directly associated with IAQ, has become popular since many people are increasingly aware of unwanted noise. Centrifugal-type, dedicated roof exhaust systems are generally noisier than mixed-flow, impeller-type systems based on a direct cfm comparison basis.
Fig. 4. Effective stack height depends on wind velocity and air volume.
Mixed-flow fans are typically in the mid-to-upper 80% efficiency range vs. the mid-to-upper 50% efficiency range for centrifugal fans, based on total efficiency. Since sound is a function of efficiency, mixed-flow technology is inherently quieter.
Noise generation caused by peripheral blade-tip speeds also plays a role in performance sound levels, and mixed-flow impellers rotate at substantially lower speeds than centrifugal fans for the same amount of work.
Most buildings contain two noise sources; supply fans that provide conditioned air and process exhaust fans mounted on the roof.
Each of these systems is usually independent, and each requires a separate set of standards and criteria with regard to noise generation and minimization.
Exhaust acoustics are considered part of a building’s aesthetics. Acoustical analysis of exhaust and ventilation systems, prior to installation, can minimize the acoustic impact on surrounding areas.
To eliminate possible noise problems when building a new facility or refurbishing an existing one, noise study experts should make determinations as to exhaust system operating noise levels, usually at the property line.
The goal is to determine existing noise levels. Anything above that level will be noticed and could be perceived as a problem. Local codes for permitted noise levels at the property line, which are becoming more stringent, especially at night, must also be considered.
If mixed-flow, impeller-type fans are employed, and noise is still an issue, there are accessories available to reduce sound at the property line. These include acoustical screens, louvers, chevron screen walls, and nozzle silencers that use a combination of sound absorption material as well as special airflow patterns for passive noise abatement.
Intake and exhaust outlets can be equally noisy. Analysis can become complex because shadowing of the building element, not the roof itself, creates an acoustic shadow for people close to the building.
As a result, the company must analyze all elements on the roof (penthouses, other equipment, or parapets) that can create an acoustical shield and possibly eliminate the need for external abatement.
— Edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, firstname.lastname@example.org