Specifying HVAC equipment in critical environments

December 5, 2016

medtronics-cleanroom2
All images courtesy C1S Group

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:

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:

MUA=EXH+10%

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Figure 1: An example of cleanroom pressure cascade.

Make-up air and exhaust systems work together
MUA performs three crucial functions in the critical environment. It:

The clean spaces will typically be positively pressurized with respect to surrounding spaces. On the other hand, rooms that contain any gases and chemicals used in the manufacturing that occurs in the facility will have negative pressure. Additionally, the clean areas will see a pressure cascade going from highest pressure in the cleanest zones to lower pressure in adjacent areas. For example, the main cleanroom will have a pressure of +25 Pa (0.10 in.), and the next space will drop to +20 Pa (0.08 in.). The adjacent gowning room will be pressurized at +12 Pa (0.05 in.), and general building areas will be at +7 Pa (0.03 in.). The pressure cascade ensures particulates will not travel in the air stream from a lower classification of clean space into a higher one (Figure 1).

The MUA system is also responsible for controlling humidity levels throughout the clean spaces (which have much narrower tolerances than are found in traditional buildings), in order to prevent static electricity and keep the air free of particles. Within clean environments, relative humidity (RH) is maintained at levels between 40 to 50 percent—with variations as little as one to three percent allowed in the most stringent environments. Humidity is controlled using the standard methods of adding moisture to the air stream, or applying extra cooling with a chilled water coil and then reheating the air to dry it out. An important difference is cleanrooms must use deionized water when adding moisture to prevent introducing particulates into the air from the water.

The exhaust air system serves to remove air that has warmed above the required temperature of the clean space, and that may contain particulate or other contaminates. Specifiers determine the exhaust quantities by carefully analyzing the tool, codes, and emergency requirements. There is typically little diversity regarding the exhaust system equipment, since these components are normally required to be operating at all times.

The exhaust system is critical not only to maintaining conditions inside the facility, but also to protecting the outside environment. The nature of the processes performed in the clean space means acid and caustic scrubbers, as well as volatile organic compound (VOC) abatement systems, will need to be added to exhaust systems to treat the air prior to discharge to the environment. Further, the exhaust system in a hazardous production material (HPM) facility must be designed and installed to remove any potentially hazardous fumes that may escape from fabrication or support equipment. Consequently, specifiers must have a clear understanding of the processes performed in the cleanroom and the related byproducts that must be removed from the exhaust air.

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Figure 2: The typical requirements for recirculation airflow are depicted within the above table, from ISO Class 1 to 100k.

Recirculation and filtration create a clean space
The driving factor in cleanroom recirculation is the continuous dilution and removal of any unwanted particles in the cleanroom envelope. Therefore, the cleanroom recirculation air system must be designed to provide sufficient clean and conditioned air to the space for maintaining the cleanroom classification during full room operation (Figure 2).

While the MUA units handle the moisture removal and pressure control in the clean space, sensible cooling coils are utilized within the cleanroom recirculation air system to ‘trim’ the temperature to meet the tight tolerances. In cleanrooms, the temperature range is between 20 and 26 C (68 and 78 F), with stringent rooms allowing a variance of only 0.5 to 1.7 C (1 to 3 F).

There are three viable options for recirculation air systems, each with unique advantages and disadvantages. These technologies are:

RAHUs are generally a lower-cost option for recirculation air systems. They are typically located on a fan deck or perimeter of a cleanroom, which provides maintenance accessibility from non-clean space. Disadvantages of RAHUs include high noise levels (i.e. rarely below 70 dB[A]) and higher energy consumption. These large units also take up critical facility space—whether in an adjacent mechanical room or at the fan deck level (in which case, it raises the building height).

VLF fan towers rely on laminar airflow to control particulate contamination through the vertical profile of the cleanroom. (Laminar airflow refers to air moving at the same speed and in the same direction, with little or no cross-over air streams that can randomly deposit particles.) Usually located on the perimeter of a cleanroom, VLF fan units fit in the same area required for sensible coils and a return chase, allowing for maintenance accessibility either from the subfloor beneath the cleanroom or from an adjacent chase.

VLF fan units are usually a lower-cost option for large, open, ballroom-style cleanrooms with filter coverage greater than 50 percent. Used in conjunction with variable frequency drives (VFDs) equipped with large, premium-efficiency motors, they have lower energy usage. While installing small, mini-VLF fan towers of around 340 m3/min (12,000 cfm) is relatively easy, large VLFs greater than 2000 m3/min (75,000 cfm) can be more difficult. Like RAHUs, these systems can be noisy, with sound levels of 65 dB(A).

VLF fan units also require a more costly and time-intensive gel ceiling grid—there is a U-channel around the perimeter of the filter opening and silicon-based gel poured into the grid. The associated ultra-low particulate air (ULPA) filter has a ‘knife’ edge that seats down into the gel, creating an airtight seal. The gel grid is typically only used in a pressure plenum system where it is critical to ensure air does not leak into the cleanroom.

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Figure 3: This table lists cleanroom filter requirements for each cleanroom class.

Since the gel grid system installation is tricky, more owners prefer to use the easy-to-install gasketed ceiling with filter fan units sitting directly atop the gasket. Since filter fan units are located on the cleanroom ceiling grid in the plenum space, these systems do not take up facility floor space and offer maintenance access from below or above. With noise levels as low as 55 dB(A), these units offer quiet operation as well.

Units using the latest DC motor models have proven to be highly reliable, and they also have very low energy usage. Filter fan units have a negative-pressure plenum that forces any leaks up into the plenum instead of the cleanroom; they typically use a gasketed ceiling grid consisting of a closed-cell foam gasket located around the perimeter and atop the filter opening. The units are very cost-competitive when coverage is less than 50 percent, but typically a high quantity of units is required for greater coverage or for large cleanrooms.

medtronics-cleanroom3
Process utilities for this cleanroom include service lines for gases used at the workstations.

Airflow within the clean space is another consideration for specifiying the recirculation system. Based on the needs and use of the room, the flow must be enough to allow for the prescribed recirculation, but not so fast it stirs up particles. Additionally, designers should consider the impact of air flowing over people in the room and depositing particles on the work product when determining the locations of the supply and returns within the space. Overall, the goal is to provide a gentle blanket of air in the cleanroom that provides comfort and cleanliness without causing disruption that adds particulate to the air.

The cleanliness of the supply air delivered to the cleanroom is primarily determined by the ceiling filter coverage, as well as the efficiency of the filters. For the initial intake of outside air, a pre-filter is used to remove larger particles. Within the cleanroom, ULPA and high-efficiency particulate air (HEPA) filters remove the smaller particles. HEPA filters are 99.97 to 99.995 percent efficient at 0.3 µm MPPS, while ULPA filters are rated 99.9995 to 99.9999995 percent efficient at 0.12 µm MPPS. The pre-filter, which should be changed according to contaminant loading, also serves to extend the life of the more expensive HEPA/ULPA filters. For each cleanroom classification, there is a related filter efficiency recommendation that should be followed (Figure 3).

Building automation provides control
The building automation system (BAS) is critical to the proper functioning of the cleanroom HVAC systems, so specifiers should possess a strong grasp of control systems in order to write the detailed sequences required for the cleanroom BAS. Today’s advanced direct digital control (DDC) systems greatly simplify the monitoring and control of cleanroom conditions—no longer does an employee have to walk around, manually checking gauges and documenting the room pressures. Rather, the MUA air-handlers are equipped with variable frequency drives controlled by the BAS, making precise regulation of air pressure automatic. The BAS monitors and stores data related to pressure, temperature, and humidity; it also activates alarms when conditions are beyond accepted tolerances.

The BAS not only controls conditions within the cleanroom, but reports on them as well—a critical step for the continued success of the cleanroom. Certification of the room and the products produced within it depend on meeting design and cleanroom classification criteria. The BAS provides detailed records documenting the conditions in the cleanroom consistently meet these standards—this way, the room or products do not lose certification.

Conclusion
Before designing and specifying systems for a cleanroom HVAC system, the engineer must first understand the classification of the room and what activities will be performed there, as these will influence the type of HVAC systems required. As these rooms have precise tolerances for everything from temperature and humidity to air pressure, precision is required in the design of the HVAC systems. Fortunately, the requirements laid out in the utility matrix, as well as the ISO classification, provide a road map for what the HVAC system must accomplish. This will guide the engineer toward successful design and specification.

Matt Strong, PE, LEED AP, is president of C1S Group, which is a full-service professional engineering and construction firm based in Dallas. Strong has more than 26 years of experience in the design and installation of mechanical and electrical systems. He can be reached via e–mail at matt.strong@c1sinc.com[1].

Endnotes:
  1. matt.strong@c1sinc.com: mailto:matt.strong@c1sinc.com

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