©2018 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 60, no. 2, February 2018.
By Wei Sun, P.E., Fellow ASHRAE
Wei Sun, P.E., is president of Engsysco with offices in Ann Arbor and Farmington Hills, Mich. He is an ASHRAE Distinguished Lecturer and past chair of ASHRAE TC 9.11. He served as IEST society president in 2016 – 17 and as ISO/TC 209 standards U.S. delegate.
An airlock is a transitional space that typically has two doors in series to separate a controlled environment (such as cleanroom, lab, operating room, or isolation room) from a corridor, or vice versa. The two doors should be interlocked to avoid being opened at the same time. An airlock area is often ventilated with filtered supply air, with return or exhaust air to exit. However, the relative air volume for supply, return or exhaust could vary based on the type of airlock. Poor airlock design concepts may cause undesired migration of chemical fumes, particles, or microbiological agents, which can be toxic, harmful, or infectious, into protective areas, or cause contaminated products or processes from excessive airborne contaminants inside the controlled environments.
Types of Airlocks
The most stringent airlocks are often installed for biosafety level 3 and 4 (BSL-3/4) labs and cleanrooms; similar, but less stringent mechanisms can be found in health-care facilities such as anterooms in isolation or operating room suites and vestibules in commercial buildings. Table 1 indicates common practices and functions among vestibules, anterooms and airlocks. In the rest of this discussion, we will use “airlock” as a general term to include both anteroom and vestibule.
Airlocks can be categorized into “cascading,” “bubble,” “sink” and “dual compartment” types; each type exhibits different airflow directions and pressure differences between the controlled environment, airlock, and corridor. Figure 1 shows common types of these airlocks, though alternative pressure relationships can be used in specific applications.
Door Operation and Pressure Differential Loss
As illustrated in Figure 1, airlock spaces are typically designed to maintain certain pressure relationships with respect to controlled environments and corridors. However, when a door is being opened, the room’s air leakage through the door’s opening suddenly becomes much higher than the leakage through minor cracks when doors are closed. This is because airlock rooms usually do not have sufficient surplus supply air to maintain the required pressure differential needed to prevent reverse flow due to air turbulence during door operation. Even if a room is equipped with a fast-response (1 to 3 seconds) VAV air supply system to provide extra surplus air, it is not fast enough to prevent an airborne cross contamination.
Quantitative Measure of Contaminant Migration Across Barrier
To identify the performance and effectiveness of various barriers (airlock, single door, mini-environment, or isolation chamber, etc.) to contain contaminant migration, it is necessary to establish a quantitative measure so various “contaminant migration barriers” can be compared or optimized for better performance. ASHRAE Research Project RP-1431 established a new terminology called contaminant migration rate, or contamination rate (CR), for cleanroom applications.
Figure 1
Table 1
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