NEWS AND BLOG

Early-Stage Considerations in Cleanroom Design

Aerospace and Defense industry cleanrooms are crucial to support the development of advanced aircraft and spacecraft. Manufacturing and assembling the exacting components require environments that eliminate contamination and comply with customer requirements for strictly controlled environmental conditions.

Below are some of the key design decisions that must be considered.

Cleanroom classification. A vital first step is to understand what classification level is required. For aerospace and defense, the standard for cleanroom classifications is ISO 14644-1. Cleanroom classifications are often referred to by the outdated standard, Federal 209E. The stricter the classification, the higher the air change rate (or the number of times the air is removed, filtered, and then returned into the space). Many times, a cleanroom is designed to a stricter ISO classification than presently needed. With an eye to the future, more stringent requirements to operate a cleanroom at a lower classification is simple; retroactively upgrading it to a higher standard is challenging and expensive. Understanding how a program may evolve helps engineers design in the flexibility necessary to change course.

Function of the space. There are times when the classified nature of what the space will be used for means our engineers must rely on the client to provide a general concept of the manufacturing processes to be performed within the space. This general concept provides us the necessary information to understand the operating conditions more closely and if there are areas within the cleanroom that require stricter cleanliness than others. This includes vertical zones in high bay cleanrooms.

Upgrading. Some clients need to improve a room’s requirements as a program evolves. To determine potential upgrades, we discuss how they are going to operate within a higher classification room. Sometimes we are helping clients identify vital clarifications, and other times we are assisting them in making decisions.

Environmental considerations. The next discussion to occur is about indoor environmental conditions. This encompasses temperature and humidity specifications. The program and employee needs help our engineers make recommendations and critical design decisions.

A thorough understanding of environmental considerations allows our team to design the cleanroom to meet the program’s set points and tolerances. Furthermore, it provides us with clear parameters to achieve the ideal indoor conditions for temperature and relative humidity. Frequently, the potential for electrostatic discharge also factors into the relative humidity specifications.

If the program has not defined specific requirements, we will typically design ranges to make the employee comfortable. This discovery process includes discussing how employees gown-up since that factors into personal comfort and often means lower temperature setpoints.

Weather consideration. It is very important to know and understand outdoor weather conditions that are not represented in standard published weather data for the larger geographic area. While the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) weather data tells us certain things, we need to consider many factors for our aerospace and defense clients that do not show up in this data. For example, some facilities are in high and low desert areas. In these locations, we need to take into consideration the unique weather created by localized wind patterns and events—including wide swings in humidity—often within a few-hour period. This geographical phenomenon also causes strong, arid downslope winds that originate inland and affect coastal areas of Southern California and northern Baja California. On occasion, and often enough to be a design concern, these conditions cause single-digit (extremely dry) relative humidity along the coast. As the temperature rises and the humidity drops the risk of wildfires increases, resulting in additional outside or make-up air filtration concerns. These are all factors that need to go into the design discussion.

This type of geographical area also experiences monsoonal weather that does not show up in average weather data. These storms dump an extreme amount of moisture in a short time span, usually during warm to hot temperatures, causing excessive humidity. Some manufacturing operations can handle the extra humidity, while others must go to greater lengths to wring out excess moisture during these high humidity events. This capability is something we design and engineer into cleanrooms.

Tolerances and cost. Knowing absolute outside-range tolerances is essential because tighter tolerances can impact cost. The stricter the environmental and control requirements, the more capacity and capability to be designed into the system.

Pressurization. Another vital consideration in design is pressurization. Pressurization is ventilation technology that controls the migration of air. Cleanrooms require bringing outside air into the space to push air outward and ensuring that particulate does not come in through cracks, gaps, or leaks, and periodically open doors. This requires a comprehensive understanding of code requirements for ventilation and construction details of the building assemblies that contain the cleanroom.

Exhaust requirements. Understanding exhaust requirements allows designers and engineers to right-size HVAC systems. Knowing if the room will require a fume hood or slot hood exhaust that will necessitate makeup air is critical.

Security: Many aerospace and defense clients have security requirements that must be incorporated into the HVAC design. These requirements vary from client to client and program to program. Security includes physical, acoustic, and visible (line-of-sight) factors. Security devices that may be incorporated include security bars, non-conductive breaks, white or pink noise, grounding, double bends (Z-ducts), and acoustic liners.

Airflow methodologies: The type of HVAC system to provide is not simply about providing a chilled water system or a refrigerant-based cooling system. It is about the methodology of how the air is being exchanged within the cleanroom. It is about how the air is being delivered, circulated, and taken out of the space.

We are working with a client right now that has all three of these approaches in one facility.

• • Fully-ducted is where the air is supplied to the space through air handling unit(s) (AHUs) that are ducted all the way to the HEPA filters mounted in the ceiling. The return air is then ducted from the return-air wall plenums back to the AHUs. This is considered a traditional system and requires more ductwork than other methodologies.

• • Negative pressurized plenum system is where fan-filter units (FFUs) do the circulation within the space. The fan filter units return the air through low-wall intakes to a HEPA—or ULPA— filter and recirculate the air to the space. A small AHU provides conditioning of the air (temperature and humidity and maintains positive pressure for the space. There is a significant advantage to this system in terms of initial cost savings as it requires less ductwork as well as smaller or fewer AHUs. A considerable advantage is that the plenum above the ceiling is negatively pressurized and draws air from the cleanroom into the ceiling space. In the event of a leak this prevents unfiltered air from entering the cleanroom space.

• • Positive pressurized plenum system is considered in-between a fully ducted and negative pressure plenum system. This method uses the same AHU equipment to do the major recirculation as a fully-ducted approach. The difference is that air is supplied into the ceiling plenum space and pushed through the HEPA filters into the space by power from the AHU instead of being directly ducted to the HEPA filters. In terms of cost, this method falls between a fully-ducted system and a negative pressure plenum system since it still has large AHUs while having less ductwork.

Each approach has its pros and cons. It is valuable for designers and engineers to talk through these design elements with the client team before finalizing the design.

The most conservative system is the fully-ducted system. Here’s why:

• • The supply air is directly connected from the AHU(s) to the HEPA filters, preventing the possibility of particulate from entering the air stream and being transported into the clean space.

• • The return is directly connected from the wall plenum or duct risers to the AHU(s), where the air is filtered before entering the supply airstream. 

• • This method is the original design for cleanroom HVAC systems and has a proven track record that spans many decades.  

There is no way for something foreign to enter the system other than through a very controlled location (e.g., where you pull the outside air in, doors, and people). However, it is the highest initial cost system. Some programs require a fully-ducted system given its long-proven performance and greatest assurance of airflow control.

Retrofitting a fully-ducted system to achieve a higher cleanroom classification can be difficult and quite expensive and disruptive to system operations. Negative pressurized plenums have one big appeal in that they offer significantly more flexibility for classification upgrades in the future because most of the infrastructure is in place. The classification can be simply increased by adding additional fan-filter units in the ceiling to provide the higher air change rates required. The AHUs providing the conditioning will require evaluation to determine if they have the capacity for the additional cooling required for the new motor heat in the added FFUs. There will be some disruption, but this is minimized. A positive pressure system is adaptive but less so because they utilize large AHUs for circulation that will require additional space and clearance. The decision often comes down to a client’s comfort with the systems.

Confidentiality and design. It is important for designers and engineers to understand what clients are doing in the space and what kind of environment is needed. Due to the confidential nature of clients working in the aerospace and defense industry, this can be a challenge. It is essential for designers and engineers to be good communicators, glean what can be shared, and work with client teams to help them make the best decisions possible.

The preliminary stages of designing a cleanroom require careful consideration. Understanding how the space will be utilized now and in the future is critical for making informed design decisions. Working with an experienced engineering team helps to navigate the complexities of cleanroom classifications and airflow methodologies in order to make smart choices in managing environmental conditions, pressurization, exhaust, and security.

DONNA LORENZEN, PE, LEED AP BDC Manager of Engineering Call 949.451.9960 | Email Donna | View Profile