Cleanroom Variable Airflow Design
Looking at Figure 1 for a typical cleanroom at constant supply airflow, the higher cleanroom particulate levels are associated with personnel entering, leaving, and working in the cleanroom. The area above the particulate level line in Figure 1 represents the times the cleanroom air-handling system is over-ventilating the space. This results in the cleanroom particulate levels dropping substantially below the maximum allowed for a particular IEST 14644-1 cleanroom cleanliness classification or owner-determined particulate level.
In order to save energy while operating a cleanroom, it is important to understand where energy is used. Energy use is going to vary based upon the industry and classification of the cleanroom. In the energy usage breakdown for a wafer manufacturing facility (Table 2), one of the highest non-process energy users is the supply/recirculation air fan. Typically, cleanroom particulate levels are substantially below their maximum established levels. One potential savings opportunity is to vary the cleanroom supply/recirculation airflow where the cleanroom particulate level is maintained just below the established maximum level. Reduced supply/recirculation airflow is an attractive energy-saving measure due to fan laws (which state fan energy savings are the cube of fan airflow reduction) and the resulting reduced air conditioning to offset fan heat.
This article will discuss different cleanroom design variables that need to be evaluated to ensure variable airflow is appropriate for the cleanroom and provides reliable operation. These variables include space pressurization control philosophy, HVAC system design philosophy, particle monitoring system, particle monitoring sampling locations and number required, airflow control devices, HVAC sequence of operation and alarming, cleanroom minimum return airflow, and cleanroom minimum supply airflow.
VERIFYing VARIABLE AIRFLOW
It is critical to verify that regulatory and technical requirements do not prevent implementing variable airflow for a cleanroom. Several situations that could prevent variable airflow include:
• Space thermal load determines space airflow
• Process exhaust determines space airflow
• Air change is required to maintain space below lower explosion limit
• Regulatory agency requires a minimum air change rate
• Process requires a specific supply air discharge velocity
• Airflow is based on meeting exfiltration requirements
• The maximum airflow reduction makes variable airflow uneconomical
SPACE PRESSURIZATION CONTROL PHILOSOPHY
There are two different space pressurization control philosophies used in maintaining a cleanroom space pressure differential relationship with its adjoining spaces: pressure differential and volumetric offset.
The pressure differential control philosophy has one or more pressure differential sensor(s) connected between the cleanroom and adjoining space(s) as illustrated in Figure 2. For a constant airflow cleanroom, the supply airflow is maintained constant, the process exhaust airflow is modulated to meet process exhaust requirements, and the return airflow is modulated to maintain the space pressure differential with the adjoining space(s). The space pressure differential control sequence of operation may include programming to prevent large fluctuations in return airflow due to doors being opened.
For a variable airflow cleanroom, the supply airflow will modulate based upon the particle monitoring system output to ensure the space particulate levels are below the established maximum level, the exhaust airflow is modulated to meet process exhaust requirements, and the return airflow is modulated to maintain the cleanroom space pressurization differential with the adjoining space(s).
Depending upon how the space pressure differential programming is set up, it might require extensive empirical testing and fine-tuning of the pressure differential control programming to get stable space pressurization through all modes and ranges of operation.
The volumetric offset control philosophy has airflow measuring stations on the supply, return, and exhaust air ductwork as illustrated in Figure 3. For a constant airflow cleanroom, the supply airflow is maintained constant with the total combined return airflow and exhaust airflow equal to a set airflow differential with the supply airflow. The exhaust airflow must meet process exhaust requirements, and the return airflow will modulate to maintain a total combined airflow for exhaust and return airflows.
From a control complexity and commissioning prospective, the volumetric offset control philosophy tends to be simpler and more stable than the pressure differential control philosophy.
HVAC SYSTEM DESIGN PHILOSOPHY
There are a number of HVAC system details that need to be included in a cleanroom variable airflow system.
• Two or more spaces can’t be controlled by one set of modulating dampers. Each space must have dedicated supply, return, and exhaust modulating dampers.
• The modulating dampers need to have the same performance characteristics.
• The AHU must be able to meet airflow from all modulating dampers at full open to all modulating dampers at minimum position.
• For an AHU serving a dedicated space, the supply fan and return air fan will be controlled by airflow measuring stations and VFDs, modulating dampers are not needed.
• For an AHU serving multiple spaces, the supply fan and return air fan will be controlled by duct static pressure sensors, and each cleanroom will have its airflow varied by modulating dampers.
• If transitional spaces (corridors, gowning, air locks) have dedicated AHUs, variable airflow may not be economically attractive due to the lower air change rates of these spaces. Also, using constant flow for transitional spaces will improve the cleanroom suite’s overall pressure differential stability.
PARTICLE MONITORING SYSTEM SELECTION
There are two types of real-time particle monitoring systems: the continuous particle monitoring system and the sequential particle monitoring system.
The continuous particle monitoring system has a dedicated particle counter located at each particle monitoring sampling location as illustrated in Figure 4. The counter can have a dedicated vacuum pump or can be serviced from a central vacuum system for drawing air samples.
The continuous particle monitoring system provides continuous detection at each particle counter, which can provide immediate alarming when particle contamination level is above the established maximum level and can also be used for meeting regulatory environmental verification requirements.
The sequential particle monitoring system samples air from a specific location for a minute or two while the other locations are not being monitored. After the minute or two sampling timeframe, the system will sample the next location and subsequently every location being served by the particular sequential particle monitoring system. The system may only monitor a particular monitoring sampling location once or twice an hour.
The sampling outlets at each particle monitoring sampling location has air samples drawn through tubing to a manifold that is connected to a particle counter and vacuum blower as illustrated in Figure 5.
Though continuous particle monitoring systems require each particle counter to be calibrated and requires more maintenance than the sequential particle monitoring system, it has the ability to monitor all sampling locations at all times and react faster to rising space particulate levels.
It is important to understand that most particle counters only measure two particle sizes and not all six particle sizes identified in IEST 14644-1. The particle monitoring air sampling flow rate are usually between 0.1 CFM and 1.0 CFM. Particle measuring systems with four or more particle size measurements are available.
PARTICLE MONITORING SAMPLING LOCATIONS AND NUMBER REQUIRED
The number of particle monitoring sampling locations is based upon the cleanroom cleanliness classification, cleanroom area, process sensitivity, and regulatory requirements. Placing a particle-monitoring sensor in the return air ductwork will only provide an average particulate level for the entire cleanroom and not be able to identify areas in a cleanroom where particulate levels are high. If your cleanroom is subject to regulatory requirements, it is important to verify the number of required particle monitoring sampling locations. The number of these locations is typically based upon the square root of the cleanroom area in square meters.
For critical process cleanrooms, it may be required that the number of particle monitoring sampling locations be the square root of the cleanroom area in square meters. For non-critical process cleanrooms, it may be required that the number of particle monitoring sampling locations be 1/3 or 2/3 of the square root of the cleanroom area in square meters. Using a cleanroom with an 100-square meter area as an example, a critical process cleanroom may need 10 particle monitoring sampling locations. A non-critical process cleanroom may need four to seven particle monitoring sampling locations. It is recommended the designer coordinate number and location of particle monitoring sampling locations with the particle monitoring system manufacturer.
The primary particle monitoring sampling locations should be directly at the workbench level. The secondary sampling locations should be in close proximity to high particle generating processes, entry doors, and high-traffic areas. The particle monitoring sampling locations should be at the work surfaces, not near the ceiling, and not below the work surface.
AIRFLOW CONTROL DEVICES
For constant airflow cleanrooms with no exhaust or constant exhaust airflow, the supply, return, and exhaust airflows may be balanced with static dampers and typically do not require modulating dampers. Variable airflow cleanrooms will require precision modulating dampers on all supply, return, and exhaust airflows. The modulating dampers need to accurately control airflow (+5% accuracy) through the full range of airflow, have a large operating range (10% to 100% of maximum modulating damper airflow), avoid contributing particulate to airstream, don’t allow particulate to leak into the airstream through the modulating damper, don’t corrode when exposed to chemicals within airstream, be pressure independent, have low-speed (5 sec) and high-speed (1 sec) full range actuation actuator options, have controls that can communicate with the BAS, and be specifically designed to control airflow. Commercial-grade variable air volume boxes do not meet the modulating damper performance requirements. There are different types of industrial modulating dampers that meet the performance criteria described above; the venturi valve will be illustrated in this article.
In order to modulate the venturi valve, the actual airflow needs to be measured. Most venturi valve manufacturers determine airflow by damper position and airflow characteristics (+5% accuracy). A more accurate airflow measurement option is to install a separate airflow measuring station (+2% accuracy) in the same ductwork as the venturi valve. It is important to evaluate the venturi airflow range for each cleanroom. For example, a cleanroom has a 3,000 cfm supply air; 2,800 cfm return air; and 200 cfm air exfiltration. Using the venturi valve airflow device (5% accuracy), the supply airflow range is 2,850 cfm to 3,150 cfm, and the return airflow range is 2,660 cfm to 2,940 cfm. It is very possible for the cleanroom to experience negative space air pressurization and contamination. Using the airflow measuring station (2% accuracy), the supply airflow range is 2,940 cfm to 3,060 cfm and the return airflow range is 2,744 cfm to 2,856 cfm. Though the cleanroom would not experience negative space air pressurization, the space air pressurization could be substantially reduced.
HVAC SEQUENCE OF OPERATION AND ALARMING
In a variable airflow system, there are HVAC sequence of operation and alarming function questions that need to be addressed including:
• How often should the supply and return airflows be reset? If the supply and return airflows are allowed to change continuously, it may be difficult to maintain stable space pressurization. It is suggested to start with a 15-min timeframe and check the room particulate level for a week to verify proper performance. If needed, the supply and return airflows can be reset more or less frequently.
• Which particle counter controls the supply and return airflow? Considering most cleanrooms will have more than one sensor, it is important to determine whether the cleanroom airflow will be controlled by averaging all the particle counters, letting the particle counter with the highest particle readings control, or letting the particle counter serving the most contamination sensitive process control.
• How do we prevent cleanrooms with volumetric offset control from going negative? Volumetric offset control systems must be provided with pressure differential sensor(s) between the cleanroom and the adjoining space(s) to reset the supply/return airflow offset when the pressure differential is too low or too high.
• Should the venturi valves have low- or high-speed actuation? For applications that have no exhaust airflow or have on/off constant flow exhaust airflow, low-speed venturi valve actuators should provide good performance. For applications that have variable exhaust airflow (e.g., fume hoods), high-speed venturi valve actuators should provide good performance. There is a substantial cost differential between low- and high-speed actuators.
• Which alarm points should be monitored? The pressure differential sensors should be alarmed when the cleanroom experiences low- or high-pressure differentials. The alarming should have a time delay to prevent nuisance alarms from air-handling system transition, exhaust airflow modulation, or doors opening.
DETERMINE CLEANROOM MINIMUM RETURN AIRFLOW
The cleanroom return air venturi valve(s) must provide accurate airflow control from its maximum rated airflow to its minimum rated airflow. A venturi valve does not provide accurate airflow control from its maximum rated airflow down to fully closed. Venturi valves have a minimum accurate airflow of 5% to 10% of its maximum airflow. For cleanroom return airflow design, the minimum cleanroom airflow must be equal to the venturi valve(s) minimum accurate airflow.
DETERMINE CLEANROOM MINIMUM SUPPLY AIRFLOW
The main function of the cleanroom supply air venturi valve(s) is to modulate to ensure the cleanroom particulate levels are below established maximum level. The cleanroom supply airflow must ensure the space pressurization differential is maintained. For a cleanroom with net air exfiltration, the supply airflow (SAF) is equal to the space exfiltration airflow (EXFIL) plus minimum exhaust airflow (EXH) plus minimum return airflow (RAF) plus particle sampling airflow (PSAF): SAF = EXFIL + EXH + RAF + PSAF.
For a cleanroom with a net air infiltration, the cleanroom minimum supply airflow (SAF) is equal to the minimum exhaust airflow (EXH) plus minimum return airflow (RAF) minus infiltration airflow (INFIL) plus particle sampling airflow (PSAF): SAF = EXH + RAF – INFIL + PSAF.
IMPLEMENTATION COST AND ENERGY SAVINGS
Implementation cost will vary substantially based upon the HVAC system being modified to incorporate variable airflow. All HVAC systems being converted to variable airflow require the installation of a particle monitoring system containing particle counter(s), vacuum pump, and computer. Other implementation costs may include venturi valves, airflow measuring stations, VFDs, additional controls and sensors, BAS software development, additional testing and balancing, and additional commissioning.
Energy savings shall include reduced supply fan energy, return fan energy, and reduced cooling load to offset fan motor heat dissipation. An additional benefit is that HEPA filters will need to be replaced less often, saving HEPA filter, replacement cost and labor, and production downtime costs. Looking at a 1,000-sq-ft ISO 5 cleanroom with 450 ach 75,000 cfm supply air; mechanical system with 5.0 in w.g. total static pressure; and $.10/ kwh energy cost: a 25% reduction in average airflow could save $35,000 to $40,000 per year in energy.
When variable airflow cleanroom HVAC systems are properly designed, they can provide reliable cleanroom particulate levels and space pressurization differential with other spaces while saving substantial fan energy. ES