Issues to Consider When Specifying a Cleanroom Air Handling Unit
There are thousands of details to consider when specifying an air handling unit. Some of the issues–such as governmental regulations, cost, performance, and construction considerations–will be discussed in this article.
By Carl J. Opatnry, P.E.
Specifying an air handling system for cleanroom applications is probably one of the most complex decisions any specifying engineer or facilities planner can face. In addition to the traditional cost and maintenance issues, the very strict operational parameters that are necessary in a process-driven environment, such as those which might be found in a pharmaceutical or computer chip manufacturing facility, also must be considered. Uptime is critical in these 24-hour per day operational facilities.
Recognizing the thousands of details that will impact the equipment specification decision is a primary concern. Although the list of considerations is lengthy, your air handling unit supplier should be able to assist you with this task, providing engineering and design advice to meet your specific air moving and air treatment needs. The key thing to remember is that the most economical time to solve a problem is before the air handling unit has been constructed.
One of the first issues to consider is governmental regulations and building codes, including federal, state and local regulations. In addition to OSHA specifications, space utilization plans may require a cleanrooms air handling unit to meet FDA requirements or EPA exhaust air regulations. Depending on the location of the facility, certain seismic statutes may be applicable. Indoor air quality (IAQ) has become increasingly regulated, particularly in habitation environments, and local and state IAQ mandates must be considered.
Air quality issues can be addressed in a number of ways. For example, mechanical or chemical air filtration may be appropriate depending on space utilization. Prefilters, afterfilters, HEPAs and non-bacterial media are appropriate forms of mechanical filtration for certain applications, while odor control, carbon or permanganates might serve chemical filtration requirements. Exhaust air standards could require the use of air scrubbers, incinerators or additional mechanical or chemical filtration.
Cost is obviously another consideration but, like many other aspects of building specification, the incremental capital costs for well-designed and constructed air handling equipment can be justified by the long-term benefits.
It is not unusual for custom-designed air handling units to deliver twice to three times the service life of off-the-shelf units. Reasons for this are sometimes application-specific. For instance, a custom cleanroom air handling unit that is designed and built to meet a specific and rigid set of service and performance parameters to support a process-driven environment must feature premium quality design and componentry. As cleanroom air handling unit downtime must be avoided, these custom units are usually built to provide increased durability and reliability.
An off-the-shelf modular unit, however, might be designed to allow great variance in performance (humidity and temperature control, for instance) as well as to support a habitational people-occupied environment. People-occupied spaces usually permit much greater variation on issues such as downtime and temperature/humidity parameters than process-driven space. Accordingly, quality and service life may not be overriding issues.
Pre- or post-installation attempts to modify incorrectly applied or incorrectly designed units to meet cleanroom specifications will almost always cost more in the long run and such may never perform as well as planned. Operating costs, including maintenance and service, will likely exceed those associated with a properly engineered and custom-built unit designed to meet a specific cleanroom need.
Costs associated with installation, maintenance and service, as well as operational expenses, including energy consumption and unit efficiency, must also be considered. High efficiency or energy recovery systems that recoup energy before it is released into the outside air, such as rotary energy wheels, plate exchangers, run-around coils and return air mixing systems, will likely prove more efficient in the long term. Cleanrooms requiring once-through air supply are ideal for energy recovery system considerations. As always, operational and maintenance costs must be weighed against up-front equipment costs.
Beyond air quality and cost issues, specific performance parameters must be determined. In particular, temperature, humidity, sound, vibration, cleanliness and operating environment requirements will most impact the specifying decision.
As opposed to habitation-only environments, process-driven environments like cleanrooms are likely to dictate strict temperature and humidity requirements. In many cases, temperature and humidity set points and acceptable ranges must be maintained over long periods of time or the integrity of the environment and the process it houses will be compromised.
Several methods are available for heating cleanrooms, including hot water, steam, gas and electric, while cooling can be accomplished with chilled water, chilled glycol or refrigerant. Again, the specific cleanroom application may favor one system over another. A skilled air handling unit supplier should be able to recommend the most efficient and appropriate system for any application.
Humidification can be accomplished through the use of steam injectors, atomizers or saturated media. When evaluating humidification sources, special care must be taken to assure the cleanliness of the humidification process and to evaluate the impact of the humidification on surrounding components. Proper location of the humidification device within the air handler is critical to its effectiveness and efficiency. Materials of construction must be carefully evaluated.
Dehumidification is another cleanroom concern. Typically, cleanroom air handling units use either a desiccant system, which employs an absorptive or adsorptive material to pull moisture from the air, or a low-temperature glycol coil system. The use of desiccant systems requires awareness of reactivation temperatures and modes and knowledge of the compatibility of the desiccant material within the process environment. Low-temperature coil systems, while effective, can be costly, complex and cumbersome to maintain.
Once temperature and humidity parameters have been determined, the question of air volume must be addressed. Typically, cleanroom spaces are positively pressurized, negative pressurization of the cleanroom space is also often desired in biocontainment facilities, in which sensitive biological organisms may pose a threat to the environment outside the cleanroom.
Within the confines of room pressurization schemes, allowable pressure and volume variations must be determined. Operational cycles of the cleanroom, such as higher hood or exhaust air usage, may permit pressure and air volume turndowns that can save energy. These may be false savings, however, if the process environment will lose its integrity with a decrease in air pressure or volume.
Concerns about fan sound and vibration must also be factored into the specifying decision. Proper fab and appropriate RPM considerations must be evaluated to avoid inappropriate vibration or harmonics. To assure that noise will not interfere with the cleanroom environment or process, fan configuration, either belt-driven or direct-drive, must be taken into consideration. Additionally, vane control or variable frequency drive control may be employed to address air volume needs in conjunction with meeting sound or vibration limits.
Properly engineered units address all noise and vibration concerns at the source. Among the most commonly employed methods for reducing vibration are spring-base, inertia-base and kinetic designs that isolate the air handling unit vibrations from the facility. Floating floors, snubbers and fan flex isolation designs may also be appropriate for certain applications. Sound attenuators should isolate the fan and be designed to attenuate specific, objectionable octave bands.
In addition to particle control, special consideration should be given to units that will serve process environments that may contain bacteria, corrosives, toxic chemicals or volatile or explosive agents. Issues such as indoor or exhaust air quality, safety, unit longevity and serviceability need to be analyzed. Vapor- and air-tight construction methods may be advisable under these circumstances.
Space limitations may affect the physical size of the air handling unit, its weight or access to its interior. Therefore, component service or replacement needs must be factored into the design of the unit. An air handling unit manufacturer should be able to design the unit with built-in service assistance devices, including telescoping beams, chain falls or trolleys that will accommodate equipment service and replacement. In addition the location of service doors and access panels must be carefully determined, particularly if the unit is to be situated in an unusually tight space.
The physical layout of the air handling system will be impacted by the available mechanical space. An air handling unit supplier should be able to guarantee results from a unit in any reasonable configuration including “T”, “U”, “L”, or “I” shapes. Vertical or horizontal air flow patterns should be equally viable. The air handling unit can be designed in either a draw-through or a blow-through configuration, and one may be better than another for a given cleanroom process.
The process environment may present specific concerns that should be addressed with air- and vapor-tight construction methods. Once it has been filtered, humidified, cooled or heated, air serving a cleanroom becomes very expensive air. Any infiltration or outfiltration of the air through the air handling unit walls could compromise unit performance and cost significant operational dollars.
Purchasers of custom-designed air handling units may also wish to consider specifying “no-through-metal” construction, which features barriers between the unit panel walls that help to prevent surface condensation on the air handling unit. In addition to concerns about the effects of moisture and condensation on the unit exterior, moisture can adversely affect the internal components of the air handling unit itself, increasing maintenance costs and shortening service life.
It is crucial to avoid any equipment design characteristic that could result in pools of standing water inside the air handler. Standing water can serve as a breeding ground for dangerous bacteria and fungus. As the purpose of the air handler is to distribute and retrieve air from building spaces, standing water can provide a frighteningly efficient means for the spread of disease and contaminants. Custom air handling units are likely to employ graded floors, containment sumps, sloped or curbed sections and a variety of other engineering features to prevent standing water concerns.
Regardless of your cleanroom application, materials of construction and cleanability should be high on the list of considerations. Build-clean and ship-clean protocol, in which measures are taken to assure that no contaminants are introduced to the unit during construction or shipping, should be considered for any cleanroom application. Corrosion-resistant construction materials, such as aluminum, stainless steel or coated metals, may be advisable depending on the specific process to be undertaken in the cleanroom.
Interior construction should provide easy access to all equipment and control while featuring simple, easily cleaned surfaces throughout. Additionally, vermin-resistant construction methods should be considered, particularly for cleanroom units that service biogenetic or agricultural research environments.
Finally, the internal componentry of the air handler should be chosen with all of the previous design and performance parameters in mind. Numerous fan designs are available for various applications, and each type should be considered in regard to volumetric capacity, efficiency, operational costs, spatial dimensions, maintenance, sound and vibration.
Likewise, the choice of coils for heating, cooling and dehumidification must be made with care. Coils of copper, stainless steel and aluminum may prove appropriate for some environments but not others. In some cases, coated coils may provide better services.
Managing the complex specification process
Specifying a cleanroom air handling unit is an enormous job that requires the consideration of literally thousand of complex issues related to determining and attaining the correct environment for the specific process involved. In addition to thorough exploration and understanding of the process to be accommodated within the cleanroom, environmental, operational and geographical parameters of operation must also be weighed. Process- and environment-specific peculiarities and requirements must be fully factored in to the specification decision.
The key to a workable and efficient cleanroom air handling system lies in a disciplined approach to the research and specification process. Professional and experienced air handling unit manufacturers can assist the consulting engineer and the end-user with the many intricate details to assure that nothing is overlooked. This involvement should occur as early in the design process as possible.
Carl J. Opatnry, P.E., is an engineering manager at Air Enterprises, Inc. (Akron, OH).
The components required to build a make-up air handling unit for a cleanroom application are likely to be complex and extensive. As a result, the finished unit may be very large, as is illustrated by this rooftop unit.
Air handling needs for a specific cleanroom application may dictate that corrosion-resistant materials, such as aluminum (pictured here), stainless steel or other coated metals, be used throughout the unit.
The architectural constraints of any building design will require that air handling units be configured to meet available space. Units may need to be configured in “L,” “T,” “I,” “U” or even double-decker shapes, like the air handling unit pictured here.
Special unit service or maintenance access needs, such as the corrosion-resistant stairway included on this step-up unit, may be necessary inclusions for units that must meet certain operational parameters.