by Matt Strong, PE, LEED AP
Cleanrooms present unique design challenges because they must meet specific requirements that go beyond what is encountered in a typical commercial space. When specifying HVAC equipment in a critical environment such as a cleanroom, engineers must consider the key issues of air filtration, humidity control, and pressurization. While a specifying engineer may have experience with these issues in the context of a traditional commercial application, the considerations are substantially different in a cleanroom.
When designing and specifying equipment for a cleanroom, it is important to think of it as a ‘building within a building’—one that must have a completely separate HVAC system. Cleanrooms neither share exterior walls with the main building nor have direct outside access. These highly specialized spaces also have a differential pressure to the interior of the building, which must be continuously maintained to mitigate infiltration of particles from adjacent, less clean spaces into the cleanroom.
Compared with, for example, an office building, cleanrooms use higher volumes of outside air, and have tighter temperature and humidity tolerances. Further, there are very specific guidelines and standards that must be adhered to within these spaces, which are carefully defined by international standards recognized worldwide.
ISO 14644, Cleanrooms and Associated Controlled Environments Package, defines these spaces as rooms in which the amount of airborne particles is tightly controlled. The standard includes several classifications of cleanroom, based on the amount of particles allowed, as well as rules for construction of the space, and guidelines to control temperature, humidity, and pressure within the cleanroom according to its intended use.
Cleanroom classifications are based on the number of particles that is equal to and greater than 0.5 per micrometer (µm) in one cubic foot of air. There are nine classifications, with the tightest starting at 0.01 and increasing by a power of 10 to the highest—one million particles greater than 0.5 µm. The standard of cleanliness required depends on the task performed in the room, so the more susceptible the product is to contamination, the higher the classification of the room. ISO 14644 also determines the frequency and type of air quality tests required (e.g. particle count, air pressure, or air flow) to be in compliance.
The utility matrix (UM) is a crucial document to the specification process of a cleanroom. Developed directly by the cleanroom owner, the UM contains the tool layout and process flow required for the room. The completed UM and tool list form the baseline for design and enumerate all the utilities to be used by the tool set. Specifiers take information from the finalized UM to design the specific HVAC-related systems, including:
- exhaust air treatment and distribution;
- make-up air; and
- cleanroom recirculation air cooling and filtration.
The make-up air (MUA) and exhaust systems have an important impact on the size of the HVAC equipment needed to maintain the cleanroom. The MUA requirements dictate the amount of chilled and heating water required from the central plant, which affects the overall size and number of chillers and boilers. Likewise, the exhaust air quantity affects the heat removal from the cleanroom, which in turn has an impact on the size of the sensible cooling coils and recirculation air system.
Specifiers use the exhaust flow rates given in the UM to determine the MUA quantities needed to cover the air exhausted from the building plus surplus to pressurize the building. This is based on the following formula: