Clean Room Ventilation

DESIGN of clean spaces or clean-rooms covers much more than traditional temperature and humidity control. Other factors may include control of particle, microbial, electrostatic discharge (ESD), molecular, and gaseous contamination; airflow pattern control; pressurization; sound and vibration control; industrial engineering aspects; and manufacturing equipment layout. The objective of good cleanroom design is to control these variables while maintaining reasonable installation and operating costs.




Airborne Particles And Particle Control

Airborne particles occur in nature as pollen, bacteria, miscellaneous living and dead organisms, and windblown dust and sea spray. Industry generates particles from combustion, chemical vapors, and friction in manufacturing equipment. People in the workspace are a prime source of particles (e.g., skin flakes, hair, clothing lint, cosmetics, respiratory emissions, bacteria from perspiration).

These airborne particles vary from 0.001 ?m to several hundred micrometers. Particles larger than 5 ?m tend to settle by gravity. In many manufacturing processes, these airborne particles are viewed as a source of contamination.

Particle Sources in Clean Spaces
In general, particle sources, with respect to the clean space, are grouped into two categories: external and internal.

External Sources.

Externally sourced particles enter the clean space from the outside, normally via infiltration through doors, windows, and wall penetrations for pipes, ducts, etc. The largest external source is usually outside makeup air entering through the air conditioning. In an operating cleanroom, external particle sources normally have little effect on overall cleanroom particle concentration because HEPA filters clean the supply air.

However, the particle concentration in clean spaces at rest relates directly to ambient particle concentrations. External sources are controlled primarily by air filtration, room pressurization, and sealing space penetrations. Internal Sources. People, cleanroom surface shedding, process equipment, and the manufacturing process itself generate particles in the clean space.

Cleanroom personnel can be the largest source of internal particles, generating several thousand to several million particles per minute in a cleanroom.

Personnel-generated particles are controlled with new cleanroom garments, proper gowning procedures, and airflow designed to continually shower workers with clean air. As personnel work in the cleanroom, their movements may re-entrain airborne particles from other sources.

Other activities, such as writing, may also cause higher particle concentrations. Particle concentrations in the cleanroom may be used to define cleanroom class, but actual particle deposition on the product is of greater concern.
The sciences of aerosols, filter theory, and fluid motions are the primary sources of understanding contamination control.

Cleanroom designers may not be able to control or prevent internal particle generation completely, but they may anticipate internal sources and design control mechanisms and airflow patterns to limit their effect on the product.

Fig. 1 ISO Air Cleanliness Class Limits

Fibrous Air Filters

Proper air filtration prevents most externally generated particles from entering the cleanroom. High-efficiency air filters come in two
types: high-efficiency particulate air (HEPA) filters and ultra-low-penetration air (ULPA) filters. HEPA and ULPA filters use glass fiber paper technology; laminates and nonglass media for special applications also have been developed. HEPA and ULPA filters are usually constructed in a minipleat form with either aluminum, coated string, filter paper, or hot-melt adhesives as pleating separators. Filters may vary from 25 to 300 mm in depth; available media area increases with deeper filters and closer pleat spacing.

Theories and models verified by empirical data indicate that interception and diffusion are the dominant capture mechanisms for HEPA filters. Fibrous filters have their lowest removal efficiency at the most penetrating particle size (MPPS), which is determined by filter fiber diameter, volume fraction or packing density, and air velocity. For most HEPA filters, the MPPS is 0.1 to 0.3 ?m. Thus, HEPA and ULPA filters have rated efficiencies based on 0.3  and 0.12 ?m particle sizes, respectively.

Air Pattern Control

Air turbulence in the clean space is strongly influenced by air supply and return configurations, foot traffic, and process equipment layout. Selecting air pattern configurations is the first step of good cleanroom design. User requirements for cleanliness level, process equipment layout, available space for installation of air pattern control equipment (i.e., air handlers, clean workstations, environmental control components, etc.), and project financial considerations all influence the final air pattern design selection. Numerous air pattern configurations are used, but they fall into two general categories: unidirectional airflow (often mistakenly called laminar flow) and nonunidirectional airflow (commonly called turbulent).

Nonunidirectional Airflow

Nonunidirectional airflow has either multiple-pass circulating characteristics or nonparallel flow. Variations are based primarily on the location of supply air inlets and outlets and air filter locations. Examples of unidirectional and nonunidirectional airflow of pharmaceutical
cleanroom systems are shown in Figures 2 and 3. Airflow is typically supplied to the space through supply diffusers with HEPA filters (Figure 2) or through supply diffusers with HEPA filters in the ductwork or air handler (Figure 3). In a mixed unidirectional and nonunidirectional system, outside air is prefiltered in the supply and then HEPA-filtered at workstations in the clean space (see the left side of Figure 3). Nonunidirectional airflow may provide satisfactory contamination control for cleanliness levels of ISO 14644-1 Classes 6 through 8. Attaining desired cleanliness classes with designs similar to Figures 2 and 3 presupposes that the major space contamination is from makeup air and that contamination is removed in air-handler or ductwork filter housings or through HEPA filter supply devices.
When internally generated particles are of primary concern, clean workstations are provided in the clean space.

Unidirectional Airflow

Unidirectional airflow, though not truly laminar, is characterized as air flowing in a single pass in a single direction through a cleanroom with generally parallel streamlines. Ideally, flow streamlines would be uninterrupted; although personnel and equipment in 16.4 2003 ASHRAE Applications Handbook (SI) the airstream distort the streamlines, a state of constant velocity is approximated. Most particles that encounter an obstruction in unidirectional airflow continue around it as the airstream reestablishes itself downstream of the obstruction. Air patterns are optimized, and air turbulence is minimized in unidirectional airflow.

In a unidirectional flow room, air is typically introduced through the ceiling HEPA or ULPA filters and returned through a raised access floor or at the base of sidewalls. Because air enters from the entire ceiling area, this configuration produces nominally parallel airflow. In a horizontal flow cleanroom, air enters one wall and returns on the opposite wall. A downflow cleanroom has a ceiling with HEPA filters. A lower class number requires more HEPA filters; for an ISO Class 5 or better room, the entire ceiling usually requires HEPA filtration. Ideally, a grated or perforated floor serves as the exhaust.

This type of floor is inappropriate in pharmaceutical cleanrooms, which typically have solid floors and low-level wall returns. In a downflow cleanroom, a uniform shower of air bathes the entire room in a downward flow of ultraclean air. Contamination generated in the space will not move laterally against the downward flow of air (it is swept down and out through the floor) or contribute to a buildup of contamination in the room.

Care must be taken in design, selection, and installation to seal a HEPA or ULPA filter ceiling. Properly sealed filters in the ceiling can provide the cleanest air presently available in a workroom. In a horizontal flow cleanroom, the supply wall consists entirely  of HEPA or ULPA filters supplying air at approximately 0.45 m/s or less across the entire section of the room. The air then exits through the return wall at the opposite end of the room and recirculates.

Aswith the downflow room, this design removes contamination generated in the space and minimizes cross-contamination perpendicular to the airflow. However, a major limitation to this design is that downstream air becomes contaminated. The air leaving the filter wall is the cleanest; it then becomes contaminated by the process as it flows past the first workstation. The process activities can be oriented to have the most critical operations at the clean end of the room, with progressively less critical operations located toward the return air, or dirty end of the room. ISO 14644-1 does not specify velocity requirements, so the actual velocity is as specified by the owner or the owner’s agent.

The Institute of Environmental Sciences and Technology (IEST) published recommended air change rates for various cleanliness classes.

These recommended ranges should be reviewed by the owner; however, the basis for the ranges is not known. Acceptable cleanliness has been demonstrated at lower air change rates, suggesting that results are depend more on filter efficiency and coverage than on air changes. Careful testing should be performed to ensure that required cleanliness levels are maintained.

Other reduced-air-volume designs may use a mixture of high- and lowpressure- drop HEPA filters, reduced coverage in high-traffic areas, or lower velocities in personnel corridor areas. Unidirectional airflow systems have a predictable airflow path that airborne particles tend to follow. Without good filtration practices, unidirectional airflow only indicates a predictable path for particles. However, superior cleanroom performance may be obtained with a good understanding of unidirectional airflow. This airflow remains parallel to below the normal work surface height of 760 to 915 mm.

However, flow deteriorates when it encounters obstacles such as process equipment and work benches, or over excessive distances. Personnel movement also degrades flow, resulting in a cleanroom with areas of good unidirectional airflow and areas of turbulent airflow. Turbulent zones have countercurrents of air with high velocities, reverse flow, or no flow at all (stagnancy). Countercurrents can produce stagnant zones where small particles may cluster and settle onto surfaces or product; they may also lift particles from contaminated surfaces and deposit them on product surfaces.

Cleanroom mockups may help designers avoid turbulent airflow zones and countercurrents. Smoke, neutral-buoyancy helium-filled soap bubbles, and nitrogen vapor fogs can make air streamlines visible within the cleanroom mockup.