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All openings and potential apertures for air penetration must be considered when designing a cleanroom so that the HVAC system can maintain the desired negative or positive pressure.
Traditionally, room pressurization is considered a heating-ventilation-air conditioning (HVAC) issue. It is well known that air pressure is a matter of balancing the quantity of air supplied into a room against the air exhausted. By regulating the relationship between the two, either a negative or positive pressure can be developed and maintained. What is often overlooked, however, is the integrity of the “container” that holds the pressure. If the room is not tight enough, air leakage will result, and the desired pressure will not be maintained. As the phrase “container integrity” implies, any penetration of the interior surface of the room is a potential problem. This article addresses how to handle some of the most common causes of pressurization failures.
To be functional, it is necessary for every room to have a door. These wall penetrations are intentional and planned. In fact, for a door to work, air leakage is a necessity. If a swinging door is airtight, then the pressure on the door will either prevent it from opening or, depending on the direction of air flow, it will never close. The amount of air passing through a door, however, can be calculated. It is standard design practice to integrate this anticipated leakage into the capacity of the ventilation system within a cleanroom. While it is always necessary to consult with an engineer, the usual practice is to provide seals on the sides and top of the door while leaving the bottom of the door (the undercut) open. This approach provides a great deal of control over air flow and the influence of doors on the air system.
To facilitate the movement of materials in and out of GMP spaces, large openings are required. In the past, this was achieved using bi-parting, swinging doors. Today, it has become commonplace to use high-speed, fabric roll-up doors instead. Roll-up doors are complex devices with many parts, and they present difficulties in achieving predictable levels of leakage. Manufacturers have responded to this need, and today some manufacturers make fabric doors that are pressure-tested and purpose-built for GMP installations. Having a known leakage value allows an air system to be designed with adequate capacity. Only those doors that have a tested leakage value should be used.
Ceilings and walls
It is in the room enclosure itself, however, where difficulties regarding pressurization usually arise. The most common type of commercial ceiling is a 2 x 4-ft. lay-in panel. Such a ceiling can be acceptable in less critical GMP spaces, if a vinyl-faced, cleanroom tile is used and is placed in a gasketed suspension grid. Gypsum wallboard ceilings, however, are obviously more monolithic and present significantly less opportunity for leakage. In contrast, every 2 x 4-ft. ceiling panel has 12 linear feet of perimeter, and every inch is a potential location for a leak. A gasketed grid begins to address this situation, but it is still necessary to ensure that the gasket is fully engaged in all locations. A heavy, gypsum wallboard ceiling tile is a good choice, and the use of hold-down clips provides even more reassurance, especially when the room is positively pressured and the ceiling tiles are being lifted by the pressurized air. In any case, for more demanding applications, a solid gypsum wallboard ceiling should be used.
Regardless of the type of ceiling specified, light fixtures need to be appropriately selected, including consideration for room pressurization. Fixtures that are not intended for GMP application have numerous holes, which create opportunities for air to pass through the housing and be exchanged with air in the plenum above. Even with a sealed GMP fixture installed in a gypsum ceiling, leakage can occur at the perimeter where the housing meets the gypsum board. Ideally, this joint is caulked. This detail of sealing devices directly to the gypsum wallboard should be applied to all penetrations, including process piping and equipment. Even when these are installed with an escutcheon plate, which provides protection around the joint, it is important to also caulk around the plate to create a seal tight enough to resist the passage of air.
It is not just ceiling penetrations that can compromise a room’s containment. In a typical facility, there are probably more penetrations in the walls than in the ceiling. Besides equipment and process piping, common penetrations from devices located on the walls include electric and data outlets, fire and security devices, light switches, human-machine interfaces, and utility drops of all sorts. Power and data outlets are perhaps the greatest obstacle to achieving a tightly sealed room. There is no such thing as a GMP back-box, and conventional back-boxes are designed with perforated tabs that are removed to accommodate a multitude of wiring configurations. In addition, there are no seals where the wiring enters the box. The result is that, under pressure, a large quantity of air can pass through these boxes. Retrofitting them after they are installed and wired is a difficult task. One approach is to install a gasket under the face plate. However, one must be sure that the socket or switch that protrudes from the faceplate is, in fact, sealed. The most comprehensive strategy is to specify waterproof devices. The gaskets and seals that prevent the intrusion of water also prevent excessive air leakage.
Room pressurization is about more than just balancing the HVAC system. The quality and integrity of the room enclosure must also be considered. Achieving a monolithic enclosure is not as easy as it might seem at first. The materials of construction need to be considered, and the various types of penetrations must be identified and understood. For each type of penetration, an appropriate strategy has to be developed. In some cases, a special product or accessory can be specified, but in many cases, the only way to maintain a tight seal is to field caulk. To do this effectively, knowing where holes are found is paramount. By taking the time to investigate and address these common conditions, room air leakage can be controlled and a room can be successfully commissioned for the production of pharmaceuticals.
About the Author
Eric Bohn is partner at Jacobs Wyper Architects, 1232 Chancellor St., Philadelphia, PA 19107, tel: 215.985.0400, www.jacobswyper.com.
Article DetailsPharmaceutical Technology
Vol. 39, No. 12
Pages: 45, 47
When referring to this article, please cite it as E. Bohn, "Preventing Room Pressurization Failures," Pharmaceutical Technology 39 (12) 2015.