Air Filter Design Parameters for Cleanroom Facilities
A cleanroom is defined by ISO14644-1 as a room in which the concentration of airborne particles is controlled, and which is constructed and used in a manner to minimize the introduction, generation, and retention of particles inside the room and in which other relevant parameters, e.g., temperature, humidity, and pressure, are controlled as necessary.
Air; whether it is from outside or re-circulated within the area, acts as a vehicle for airborne contaminants brought in by the movement of people, material, etc. Since many of these airborne contaminants are harmful (either to products or people working in such environments) and so their removal and control becomes absolutely necessary.
Sources of Contamination
The airborne contamination level of a cleanroom is largely dependent on the particle generating activities in the room, and these contaminants are generated from five basic sources (1) the facilities (including HVAC), (2) people, (3) tools, (4) Air Borne Contaminants and (5) the product being manufactured.
Cleanroom Classification
Cleanroom specifications for particulate matter (such as dust) are defined according to the maximum allowable particle size (diameter), and also according to the maximum allowable number of particles per unit volume. For non-particulate contaminants, the maximum allowable density in terms of microbes per cubic meter, or molecules per cubic meter, is specified.
ISO Classification of Cleanrooms
ISO 14644-1 and ISO 14698 are non-governmental standards developed by the International Organization for Standardization (ISO). The former applies to clean rooms in general (see table below); the latter to cleanrooms where biocontamination may be an issue.
US FED STD 209E
US FED-STD-209E was a United States federal standard. It was officially cancelled by the General Services Administration on November 29, 2001, but is still widely used.
EU GMP classification
EU GMP guidelines are more stringent than others, requiring cleanrooms to meet particle counts at operation (during manufacturing process) and at rest (when manufacturing process is not carried out, but room AHU is on)
Key Elements of Cleanroom Design
Four basic components define a controlled environment:
1) Cleanroom Architecture – Materials of construction and finishes are important in establishing cleanliness levels and are important in minimizing the internal generation of contaminants from the surfaces.
2) The HVAC System – The integrity of the cleanroom environment is created by the pressure differential compared with adjacent areas through heating, ventilation and air-conditioning system. The HVAC system requirements include:
- Supplying airflow in sufficient volume and cleanliness to support the cleanliness rating of the room.
- Introducing air in a manner to prevent stagnant areas where particles could accumulate.
- Filtering the outside and re-circulated air across high efficiency particulate air (HEPA) filters.
- Conditioning the air to meet the cleanroom temperature and humidity requirements.
- Ensuring enough conditioned makeup air to maintain the specified positive pressurization.
3) Interaction Technology – Interaction technology includes two elements: (1) the movement of materials into the area and the movement of people and (2) maintenance and cleaning. Administrative instructions, procedures and actions are necessary to be made about the logistics, operation strategies, maintenance and cleaning.
4) Monitoring systems – Monitoring systems include a means of indicating that the cleanroom is functioning properly. The variables monitored are the pressure differential between the outside environment and the cleanroom, temperature, humidity and, in some cases, noise and vibrations. Control data should be recorded on a routine basis.
The use of High Efficiency filters:
The use of high efficiency particulate air (HEPA) filters having filtration efficiency of 99.97% down to 0.3 microns is a distinguishing feature of cleanrooms. The HEPA filters for stringent cleanrooms are normally located at the terminal end and in most cases provide 100% ceiling coverage.
FILTRATION SYSTEM
Any air introduced in the controlled zone needs to be filtered. Air filtration involves the separation of “particles” from airstreams. Their removal method is almost as diverse as the size ranges of the particulates generated. Understanding separation techniques requires an exact definition of what particles are. As particles become very small, they cease to behave so much like particles as they do gas phase molecules. It is difficult to tell whether such small particles are actually suspended in air (particles) or diffused throughout it (gas or vapor). The bottom boundary where particles act as true particles is about 0.01 micron. The normal theory of separation does not apply to particles below this size and removing them from air requires techniques reserved for gaseous materials. Particles above 0.01 micron are usually considered to be filterable.
All air entering a cleanroom must be treated by one or more filters. High-efficiency particulate air (HEPA) and ultra-low penetration air (ULPA) filters are the most common filters used in cleanroom applications.
Filtration Principles Relevant to HEPA Filters
1) Interception occurs when a large particle, because of its size, collides with a fibre in the filter that the air stream is passing through. In this method, particles are small enough to follow the air stream. The particles come in contact with the fibres and remain “stuck” to the fibres because of a weak molecular connection known as ‘Van-der-Waals’ Forces.
2) Diffusion occurs when the random (Brownian) motion of a particle causes that particle to contact a fibre. Diffusion works with very small particles and works in HEPA and ULPA filters. The particles are so small that they move in a random motion causing the particle to acquire a vibration mode. Because of this vibration mode, the particles have a good chance of coming in contact with the fibres. The smaller the particle, the stronger this effect is. For large particles, over one micron in diameter, this filtration mechanism has virtually no effect.
The most critical areas lie between interception and diffusion. There are other principles of Filtration such as Impaction and interception which are the dominant collection mechanisms for particles larger than 1 μm. Diffusion is dominant for particles smaller than 1 μm.
Filter Installation and Design Considerations
HEPA & ULPA filters used in most stringent cleanrooms are generally built-in ceiling and can be installed in groups housed in a proprietary modular pressure plenum system. They can also be installed in single filter housings, individually ducted, suspended in an inverted “T” grid support system, and sealed to prevent unfiltered bypass air from entering the cleanroom. Cleanroom design conventionally follows the following guidelines for filter coverage.
Installing HEPA/ULPA filters directly in the ceiling of the cleanroom is driven by the desire to minimize, if not eliminate, dust-collecting surfaces, such as the inside of ductwork, between the downstream face of the filter and the cleanroom. Remote mounting of HEPA filters is common in Less Stringent applications since the number of particles that can be contributed by ductwork downstream of the HEPA filters is small as a proportion of the amount that can be tolerated. An exception would be where a standard air– conditioning system with no cleanliness classification is being upgraded to support a cleanroom intended to carry a cleanliness rating per Federal Standard 209 or ISO Standard 14644. In that case, all ductwork downstream of the filter should be thoroughly cleaned.
The average HEPA filter, properly installed, and with frequent changes of the prefilter, should last from five to eight years. There are always unusual cases: filter used to capture hazardous particles or pathogenic organisms should, of course, be changed when they become unsafe for use. Otherwise, the resistance of the filter as indicated on a monometer or the air flow measured with a velometer is indications of need for a change.
Terminal Filters
These filters are available in two types of constructions: (1) Box type and (2) Flanged type.
- Box type filters are more suitable for housing within the ceiling slab cut out where removal of filter is from above. Whenever filter removal is not from above e.g. in case of filter being mounted in false ceiling, flanged type of filters is used.
- With flanged type of filters, additional housing is also required to facilitate the mounting of filters and transfer the load to false ceiling members. These housings can also be provided with an alternate arrangement to transfer the filter load to ceiling slab.
Face Velocity across HEPA/ULPA Filters
The face velocity of ceiling mounted filters generally can be as high as 150 fpm and as low as 50 fpm depending on the design of the system. Since the system supporting the filters, such as the inverted “T” grid, may occupy as much as 20% of the ceiling area, a 100-fpm filter-face velocity translates into an 80 fpm average velocity at the work surface within the cleanroom. The typical ceiling mounted clean filter is designed for a pressure drop on the order of 0.5 inch wg. at a face velocity of 100 fpm.
Cabinet fans or air handlers with HEPA filter racks on the discharge side are frequently used in Less Stringent applications. The HEPA filters used in these applications are generally high velocity filters, based on 500 fpm filter-face velocity, with a pressure drop significantly higher than those used in ceiling installation. A clean 2 ft x 2 ft high– velocity HEPA filter can have a 1 inch wg. pressure drop at 500 fpm.
Pre-filters to HEPA Filters
In order to prolong the service life of HEPA filters, pre-filters are recommended to filter out majority of particles above 1 micron. Pre-filters are normally mounted in a separate plenum with access door after supply air fan discharge at an appropriate location.
Pre-filters are available in various sizes with 6” and 12” thickness and with pressure drop in the range of 0.2” to 0.25” w.c. However, dust holding capacity of these filters is poor. The applications which require a filtration system with good dust holding capacity, bag type filters with fiberglass scrim cloth media are recommended. These give efficiencies ranging from 85% (down to 20 microns) to 99.97% (down to 5 microns).
AIRFLOW
Airflow is usually specified either as average air velocity within the room or as air changes per hour.
Cleanroom Industry Design Thumb Rule
* Recommendations are not based on a clear consensus on an optimum ACR/ air velocity.
Face velocity
The velocity of the air is often determined by the degree of contamination control we wish to achieve–as a general rule, cleaner rooms require more air velocity than rooms that are less clean. Supply air volume is also highest in Class 1, and decreases as the requirement for cleanliness decreases.
For years, a value of 90 fpm (0.46 m/s) ±20% has been used to specify the airflow in the cleanest of cleanrooms. The primary objective is to maintain airflow in parallel flow streams that has two purposes: first, it needs to dilute particle concentrations that may have formed in the room due to personnel or process activity and second, to carry away particles or contaminants generated within the room. Although, higher air velocity is advantageous in particle removal/settlement, this will also result in over sizing of equipment that may be very energy inefficient.
Set velocity of 90 FPM! Is it Mandatory Requirement?
There is nothing called set velocity; the 90 fpm velocity is just a widely accepted practice. There is no scientific or statutory basis for this guideline. The figure 90 fpm velocity is purely derived from past practices over two decades and has become a common industry practice. In recent years, companies have experimented with lower velocities and have found that airflow velocity specifications ranging from 70 to 100 fpm (0.35 to 0.51 m/s) ± 20% could be successful, depending on the activities and equipment within the room. For example, in an empty room with no obstructions to the airflow, even the air velocities @70 FPM shall remove contamination effectively. There is no single value of average velocity or air change rate accepted by the industry for a given clean-room classification. In general, the higher values are used in rooms with a greater level of personnel activity or particle-generating process equipment. The lower value is used in rooms with fewer, more sedentary, personnel and/or equipment with less particle-generating potential.
Airflow based on Air change rate (ACR)
Air change rate is a measure of how quickly the air in an interior space is replaced by outside (or conditioned) air. For example, if the amount of air that enters and exits in one hour equals the total volume of the cleanroom, the space is said to undergo one air change per hour. Air flow rate is measured in appropriate units such as cubic feet per minute (CFM) and is given by
Air flow rate = Air changes x Volume of space/ 60
Air change rate is an indication of the air-tightness of a room, but it is difficult to pin down because it depends significantly on how the house is used, as well as the wind and temperature differentials it experiences during the year. Even if the rate were determined with some precision, which is established with a blower-door test, there is no assurance that value would apply under other conditions. The air change per hour criterion is most commonly used in cleanrooms of less stringent cleanliness. Intermediate cleanrooms are usually designed with hourly air change rates between 20 and 100, while less stringent cleanrooms have hourly air change rates up to 15. The designer selects a value based on his experience and understanding of the particle-generating potential of the process.
Higher ACR equate to higher airflows and more energy use. In most cleanrooms, human occupants are the primary source of contamination. Once a cleanroom is vacated, lower air changes per hour to maintain cleanliness are possible allowing for setback of the air handling systems.
Air Flow Pattern
Airflow pattern have evolved into three major types:
1) Unidirectional flow (also referred to as “laminar flow”), where the air streamlines are essentially parallel to one another.
2) Non-unidirectional flow (also referred to as “turbulent flow”), where air streamlines are other than parallel to one another.
3) Mixed flow, where air streamlines may be parallel in one part of the cleanroom and not parallel in other parts.
Unidirectional (Laminar) Airflow System Designs
Stringent cleanrooms with classification rating 100 and below are almost invariably designed for unidirectional airflow. A Laminar airflow system contains three basic elements – a blower, a high efficiency air filter, and a plenum. There may be variations on this idea – many blowers, many filters, and very large plenums, but all have the same basics.
Typically, laminar flow is achieved by supplying air through HEPA/ULPA filters, ensuring 100% ceiling coverage. The air moves vertically downward laterally from the ceiling to a return air plenum on a raised floor. This approach allows the contamination generated by the process or surroundings to drift to the floor void. The particles are finally captured by the vacuum pump in the floor void or sucked back for recirculation through the HEPA filters in the ceiling.
In the scheme above, the class-100 room is shown with 100% HEPA ceiling coverage. The make-up air handler (MAH) is a fresh air unit that provides the room pressurization. This unit feeds to two re-circulation air handlers (RAH) that supply air into the cleanroom.
The key characteristics of unidirectional air flow system are as follows:
- Unidirectional airflow system is designed for an air velocity of 60 to 90 FPM. This air velocity is sufficient to keep the contaminants directed downwards and remove particles before they settle onto surfaces.
- For wider rooms (>16ft), it is best to provide raised floor return so that the airflow tends to remain parallel (or within 180 degrees of parallel). Where the clean space is fairly narrow, of the order of 14 to 16 ft (4.2 to 4.8 m) from wall to wall, the raised floor can be eliminated in favour of low sidewall return grilles. The air will move vertically downward to within 2 to 3 ft (0.6 to 0.9 m) of the floor before splitting and moving toward the sidewall returns.
- Unidirectional (Laminar) airflow tends to become turbulent if it encounters obstacles such as people, process equipment and workbenches. Placing these obstructions in a manner that prevents dead air spaces from developing will minimize turbulence. Use of workstations with perforated tabletops will allow the air to pass through them uninterrupted. Equipment shall also be raised on a platform (plinth) where possible to allow free air flows beneath it.
- In unidirectional arrangement, HEPA filter banks must be “pinhole” tight and checked for any pinhole leaks in the media, sealants, frame gaskets, and supporting frames.
5. In some designs, the supply air can be projected upwards from floor void and is drawn into a ceiling void. This arrangement is preferred in applications where the localized hardware or equipment has high heat dissipation. The conventional supply airflow from ceiling may not be directional enough to cool the equipment that results in hot spots.
NON-UNIDIRECTIONAL AIRFLOW
This method is often used in intermediate cleanroom classification 1000 and above. Here, the air streamlines are random with no definable pattern.
The airflow is typically supplied through terminal HEPA diffusers installed in the ceiling in a pattern that provides fairly uniform coverage. The HEPA filters are sometimes installed straight in the ductwork or the air handler itself. The return is usually through the sidewall grilles uniformly distributed around the periphery of the room.
MIXED FLOW APPROACH
The mixed-flow approach is used where critical and non-critical processes are in the same clean space. Zones are created by adjusting the filter pattern in the ceiling; in a stringent area, more filters are installed in the ceiling and in less critical areas, fewer filters are installed. Supply air may have to be canalized downward over the critical zone before it diffuses to the general space. Depending on clean-room ceiling height, a 2 ft high Plexiglas shield, or even a flexible plastic curtain draped to within 12 to 18 in of the floor, can be used, to separate different zones of cleanliness.
Return air patterns are adjusted by appropriately locating return grilles to accommodate the varying filtered air quantities and to prevent cross contamination. A raised floor with air return plenum would be more effective.
AIR DISTRIBUTION STRATEGIES
Numerous air-management concepts have been devised over the years to supply and re-circulate air in cleanrooms. Two common design strategies for air handling system are (1) Single Pass System or Once Thru System and (2) Re-circulated System. The choice depends on number of factors such as; the type of product being handled, the process operation, the process equipment design, toxicity of the product being produced and impact on energy use.
Single Pass System or Once-thru Air System
Filtered air enters the room and is not re-circulated. All the air is exhausted outdoors. The system is used for cleanroom processes demanding 100% makeup air . As an example, when the potential of releasing dust or aerosolized materials exists, “once-through” HVAC system is recommended.
Recirculation Air System Types
Re-circulated systems are the most popular design for the reasons of economy of scale, size and energy conservation.
Filtered air enters the room, exits through plenum walls and is re-circulated through a sealed plenum using motorized fan modules with HEPA filters. There are two fundamental recirculation system configurations: (1) Centralized recirculation air-handling units (RAHs) and (2) Ceiling distributed fan-filter units (FFU).
1) Centralized Re-circulating Air Handling Units (RAHs)
The centralized air-handling system typically uses custom rooftop or package air handlers for makeup air. The makeup air handler (MAH) supplies pre-filtered air for pressurization. The outside air is forced into a pressurized plenum which also draws re-circulation air from indoor spaces. The pressurized plenum ceiling is provided with HEPA filtration ceiling to distribute air in unidirectional path. The schematic below depicts a standard cleanroom module using a conventional vane-axial fan distributing air into pressurized plenum.
Lot many variants of the recirculation air system are possible. In the arrangement above, the re-circulated air is not further treated or conditioned (for temperature, humidity or dust control). Simply the large volume of indoor air is re-circulated by axial fans through the ceiling HEPA filters.
If the indoor process generates significant dust and temperature rise, it is recommended to use re-circulation package air-handler (RAH) units instead. The re-circulating air handlers consist of centrifugal fans and have additional provision of sensible cooling coil and pre-filters to minimize dust loading of ceiling HEPA filters.
2) Ceiling Distributed Fan-filter units
Fan-filter units typically consist of a centrifugal plug fan driven by a motor, controller and a HEPA/ULPA filter enclosed in a box, which fits into common cleanroom ceiling grids, typically 2 x 4 ft or 4 x 4 ft.
The air is supplied to the room via terminal Fan filter units using “spider leg” ducting. Each branch leg of spider ducting is connected to the fan filter neck and the units are simply gasketed in the ceiling. These are generally not used in pressurized plenum arrangement, which requires gel-track ceiling.
(All above information & Data are derived from Book records of Filtration Standards & Other related standards, research papers and its related webpages)