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Issue : October-December 2003

Historical Developments in
Air Cleaning & Clean Rooms

By B. Bhattacharjee
Director, Bhabha Atomic Research Centre
Mumbai

B. Bhattacharjee is a chemical engineer with a masterís degree and a post-graduate training programme in Nuclear Science and Technology. A member of the Atomic Energy Commission, he was conferred the National Award ìPadma Shriî for his contributions to Nuclear Science and Technology.

The technologies involved in air cleaning and clean rooms play an important role in almost all the major activities in the nuclear fuel cycle, starting from fuel fabrication at the front-end to the nuclear reactor and then to the reprocessing and waste management facilities at the back-end. This technology is just as important in pharmacy/ radio-pharmacy, bio-safety, healthcare, electronics and for special applications in other sectors like defence, research and space technology.

The concept of clean environment and its management was first proposed about a century ago in the medical facilities when doctors felt the need for controlling infections of wounds due to bacteria and other micro-organisms. Although pioneer microbiologists and surgeons of those days tried to formulate the outline of a clean zone concept, it was not until the early sixties when the first serious attempt to develop an organised clean room with controlled environmental conditions in respect of temperature, humidity, air borne particles count, air changes per hour, illumination levels etc. was made.

The role of artificial dynamic ventilation was studied for the first time by the famous American engineer Willis Whitfield and his team at the Sandia Laboratories, who realized the concept of uni-dimensional or laminar air flow to create the so-called Piston Effect of Aerodynamics, for quick and effective removal of micro contaminants from clean space. The maturity of HEPA filters technology at around the same time made it possible to realize this concept and these filters quickly made their way into the air cleaning and clean room technology.

Clean rooms have come a long way since then and today we see diverse requirements of modern clean rooms in many spheres of activities, as we advance both in R&D and in the commercial sector. Their role in advanced technological areas like bio and genetic engineering, space and nuclear applications, advanced material sciences etc., is extremely important. In fact, clean rooms and air cleaning are almost mandatory in pharma, food and agro, micro-electronics and semi-conductors and healthcare segments.

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Classification of Clean Rooms

The perception of clean rooms have seen a sea change since the first clean room standards were formulated in the early sixties. While the class of cleanliness levels of Class 1,000 or Class 100 were thought to be stringent in those times, today Class 10 or even Class 1 is a practical proposition, with the latest standards specifying classes better than Class 1. The gradual degradation of our atmosphere by way of pollutions on one hand and the stringent quality control requirements on the other have put a great burden on engineers to build and maintain high class modern clean rooms. In fact, additional parameters like the levels of noise and vibration, ionising and electromagnetic radiation, etc., have also been included for controlled environment in clean rooms for certain special applications in the areas of material science, nuclear and space applications, etc.

The latest International Standards Organisation (ISO) standard on clean rooms is a step in this direction, which gives guidelines for classification of ultra pure clean rooms. International Standards Organisation has introduced ISO classification numbers for clean rooms from ISO-1 to ISO-9 and has released 2 standards ISO-14644-1 and ISO-14644- 2, wherein the objectives has been, to combine the best features of the different base documents available from Europe, Japan and the USA and this is a very welcome measure from the point of view of standardization.

These standards have also clearly spelt out the mandatory tests along with the optional tests that are necessary to demonstrate the continuing compliance of clean room standards.

How Clean Rooms achieve control?

Before we proceed further, it would be appropriate at this stage to have a look at the five means (5 P’s) that are available for the control of air borne particulate matter.

• Preventing entry of particulate matter. This is accomplished by filtration of the air entering into the clean room.
• Purging of particulate matter. The air handling system changes the air in the room and thus removes particulate matter generated within the room.
• Prohibiting the generation of particulate matter. Clean room clothing is made of “limited linting” fabric, which will not produce excessive quantities of lint as it is flexed. Similarly, room appointments, floors etc., are chosen for this resistance to particle generation.
• Protecting the product from impact and settling of particulate matter. The low level of particulate matter in clean rooms contains a majority of the smaller size particles, which settle out very slowly. These smaller particles have a very long “life” in the air.
• Providing an area for the cleaning of parts and personnel. Everything entering the clean room is cleaned, so that as little contamination as possible is added to the room atmosphere by transfer from dirty objects.

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Developments in Clean Room technology

There have been a series of major technological developments that have greatly assisted in achieving the daunting challenges posed to clean room industries for realization of their ever-increasing goals.

HEPA filters
Development of HEPA filters (high efficiency particulate air filters) in the 1940s, which formed the heart of the clean rooms, in filtering the air borne particle contamination to levels of 0.3 microns at 99.97% efficiency, were originally developed for nuclear applications, which were later on considered as an industry accepted standard.

Test procedures were formulated using forward light scattering photometer for measuring performance of the HEPA filters after installation. As 0.3 micron size range were the most difficult to remove from the air stream at that time, this size of the particle was taken as the base for formulating the standards.

ULPA filters
Growing market demand from advanced science and technology calls for improved efficiencies and that led to a new term in the clean room industry, i.e., ULPA filters (ultra low penetration air filters) which provide a minimum of 99.999% efficiency (0.001% maximum penetration) on 0.3 micron particles for achieving better cleanliness classes and cleaner working environments. These are used for ultra-clean rooms, where contamination levels have to be controlled at levels better than that which can be achieved with conventional HEPA filters.

Boron free ULPA filters / Metal free HEPA
Boron free ULPA filters of 99.9997% efficiency for particles down to 0.12 micron size for Class 10 and Class1 clean rooms are specially used in electronic/semiconductors/ wafer manufacturing industries, where tolerance to contamination level above 0.12 micron is also very critical and not permitted. These are special filters specifically manufactured depending on the application and which are rated for much higher efficiencies to avoid product contamination.

Similarly, metal free HEPA filters were developed by this centre for application in ultra trace analysis of materials.

Minipleat separatorless HEPA/ULPA filters
A separatorless filter pack made up of folds (or pleats) of micro glass fibre media supported by micro fibre glass ribbons has been introduced in the late 90s, specially for Class 10 and better clean rooms, which has several advantages over conventional HEPA filters, such as:

• Higher filtering surface area for the same size of filters
• Lower pressure drop, thereby saving of energy
• Elimination of aluminium separators, which are likely to oxidise rapidly in the ionic conditions and corrode, shedding particles on the downstream, and reducing the filtration efficiency
• Feasibility of elimination of the protective grilles on the downstream side of the filter
• Lighter in weight

These filters are slowly finding their place in many critical applications replacing the conventional HEPA filters.

VLSI laser tested filters
As a result of the need for super clean environments, a new standard of air cleanliness has emerged. The traditional standard of defining classes of clean rooms in terms of 0.5 microns particles is no longer satisfactory. Particles as small as 1,200 Angstroms (0.12 microns) must be removed with an efficiency of 99.99995% to provide clean rooms of the proper class to mass produce VLSI (very large scale integrated circuits) with minimum rejection, in the manufacturing of silicon wafers, micro chips, disc drives and other semi-conductor industries. A laser spectrometer that measures the upstream and downstream particulate concentration with computer calibration has been in use to measure accurately particles of this size.

Advances in materials and sub-systems for Clean Rooms
In parallel, lots of advances have been made in construction materials, clean room fittings and subsystems for new generation of clean rooms that are being presently built. Some salient advances are worth mentioning.

1. HEPA filters. With sub-micron glass fibre filter media formed into a high density paper in extruded, anodized aluminium casing/medium density fibre board casing, fire retardant, easily incinerable, with knife edge type in mini-pleat separatorless construction.
2. ULPA filters. These are boron free, of 99.9997% efficiency for particles down to 0.12 micron size, for Class 10 clean rooms, in anodized aluminium casing and mini-pleat separatorless construction.
3. Wall panels. The wall panels will be extruded aluminium grid sections with honeycomb core of varying thickness (from 6 mm to 50 mm) which will be of modular construction. These wall panels will be of zero out-gassing type, either anodized or powder epoxy coated finish.
4. Ceiling grids and liquid gel for sealing. The ceiling grids are of extruded aluminium, either anodized or powder coated, which can be ceiling suspended and firmly fixed on to the side walls. The ceiling grids will have built in recesses for light fixtures and terminal filters. The liquid gel sealant will make all the joints perfectly leak right. Commercially available ceiling grids consume approximately 18% of ceiling as dead zone as attic. A ceiling grid with attic area as low as 6.2% has been made in BARC.
5. Raised flooring. These can be of die-cast aluminium tiles with perforations for the return passage, installed over a grid work of robust die-cast aluminium pedestals. All the utility and service pipes can be brought into the clean room through these flooring tiles at pre-determined locations. Heavy equipment can also be installed over these flooring tiles at predetermined locations with adequate passage for the return air movement. The floor void beneath the raised floor will act as the return air plenum and has to be designed as a clean room floor.
6. Lighting fixture. For the Class 100 and better clean rooms, the lighting fixtures are of tear-drop type or flush mounting type (recess type), which can be made leak tight with liquid sealant. Flame-proof and explosion-proof type light fixtures are also available, which are wall mounting type, because of their size and weight.
7. Sequential particle counter. These are usually used inside Class 10 and better clean rooms, which can continuously monitor the cleanliness of the room. These will have sensors mounted inside the clean room at pre-determined locations and will be drawing the air sample and sending the particle counts to a PC connected to it, which is installed in a control room.
8. Laser particle counters of portable type for Class 100 and better clean rooms.
9. Ventilation ducting in SS-304 material with plasma welding for leak tightness is the preferred route of fabrication.
10. Air showers. The air showers are of SS-304 construction and are available in various classes such as Class 10, Class 100, with high velocity air outlet nozzles mounted along the two side walls and the ceiling. The air shower will have grated flooring and the return air is taken back to the fan suction through a return air path in the air shower wall.
11. Air handling units. These are of double skin type in SS-304 construction or GI powder coated construction. These can be very silent in operation and are built with pre-insulated PU foam between the outer sheath and the inner sheath. These air handling units can be directly installed on the roof with ducting connected to it at the outlet and the inlet. This eliminates the need for a separate plant room or AHU room.
12. Fan Filter Units. Where a better class of cleanliness is required for a small area within a clean room, these modules are being used, which are economical and fast to build. These modules will have in-built fans, filter section and can be directly fitted in the ceiling grid at the required location.
13. Clean room garments. The personnel working inside a clean room have to strictly follow the clean room protocol and shall use contamination control clothing in non-static and non-linting fabrics. Clean room furnishings should be made of lowparticle shedding and low-static generating materials.
14. Automatic fire detection and extinguishing system with fire fighting equipments. Automatic smoke detection units and fire protection (extinguishing system) involving “INERGEN” gas cylinders (combination of nitrogen, argon and CO2 gases) for quick extinguishing of fire and remote switches for switching off the blower in case of fire emergencies.

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Laminar Clean Rooms

Non-laminar and laminar flow clean rooms (both horizontal and vertical) have been built, generally as per specific requirements from users. However, vertical laminar flow clean rooms have become very popular for most of the critical and specific applications in view of their following advantages:

1. Ease of construction (since modular type wall panels, ceiling grids, raised floors etc., are all readily available in standard sizes for assembly).
2. Economy in constructional and operational costs.
3. Maintaining the required class at working level.
4. Space saving, time saving and ease of assembly.
5. Easy to build, operate and maintain.

Indian scenario

The clean room market has seen considerable growth in India during the past 10-15 years and this demand will remain so, as more and more industries and R&D centres will have prime requirement for this technology in future.

Today, clean rooms in India are not only concerned about particulate and microbial contamination control, but also about a complete environmental regime conducive to the process requirements, in which various parameters are systematically regulated and controlled. Over the years, we have also realised the importance of education and motivation while dealing with the operation and maintenance of clean rooms.

Efforts to develop indigenous technology for production of the basic borosilicate glass fibre, that are needed for manufacture of high efficiency particulate intermedium, have started yielding results by way of production of such fibres on a bench scale at CGCRI, Kolkata. Other developments worth noting are:

• Indigenisation of paper – by UP Twiga, New Delhi
• Ceiling grid with 6.2% attic – developed by BARC
• Horizontal LAF Bank – developed by BARC

However, it is the maintenance of clean rooms during their prolonged use, which is more challenging than design and construction of the same. I observe from the list of topics of the special lectures, that this aspect is not covered in detail. I am sure, this will be taken care of during the proceedings. However, we have complete programmes in place with protocols that are well defined for implementation, routine safety norms and follow up procedures, which are very important for the successful operation of clean rooms.

Comparison between the earlier Clean Rooms and the new generation Clean Rooms, presently built in India

About two decades back there were a very limited number of suppliers who carried out the job of building clean rooms in India. Generally, air conditioning contractors used to take up this job as a part of their total contract. Specialized and skilled personnel were generally not available and expertise available was limited. Most of the components for better-than Class 100 clean rooms had to be imported. For example, HEPA filters with efficiencies of 99.97% down to 0.3 micron (particle size) only were available. The filter media was fully imported and sometimes the entire filter had to be imported. In addition to the above, full scale testing facilities as per standards were not available and had not been set up by the manufacturers. Third party validation of clean rooms could not be carried out in the absence of suitable agencies. Last but not least, the cost of construction was very high.

Today, a considerable number of indigenous vendors are available to do the job for conventional clean rooms of Class 100 and above. Specialised and skilled personnel are available now and expertise for design and construction is available indigenously. Only a few components of better-than Class 100 clean rooms have to be imported and remaining are being developed in-house and manufactured indigenously. Both HEPA as well as ULPA filters with higher efficiencies are being produced in India and a few suppliers are available to manufacture as per customer’s requirements. Separatorless mini-pleat HEPA filters which are being widely used in clean room applications are produced indigenously in glass fibre and ceramic fibre media. Full fledged filter testing facilities are available with many of our manufacturers, both at their factories and for site testing. Agencies are now available indigenously for carrying out third party validation and testing at site with portable instruments. However, they are yet to be formalized. Nowadays, the cost of construction is competitive and comparable with imported units.

However, the quality of the components and the workmanship of the indigenously manufactured items for clean rooms have much scope for improvement in comparison with the imported components and materials.

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ISO Classification of Clean Rooms

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Future actions

Looking into the future of clean rooms, one important aspect that dominates our mind is the economic viability of building high quality clean rooms. Having been personally associated with clean rooms, I feel the need to sincerely address this issue, if we are serious to ensure sustained growth of clean room technology in India. High constructional and operating costs must be reduced so that small manufacturers and entrepreneurs could also gain access to this technology.

This would need detailed studies of the basic process to be carried out inside the clean spaces by a specialized group/ consultants along with studies on, aero-dynamic flow contours vis-a-vis the process activities, particle generation sources and their dispersion paths, containment at source, improved architectural concepts to reduce air movements leading to reduced air volumes, etc.

This also calls for concerted efforts for development and availability of cheaper finishing materials for modular clean rooms. It is only through such detailed studies of all these factors, coupled with the control of effective operation and safety, can we bring down the cost. I feel this is the need of the hour and a challenge to the Indian clean room industry today.

Another aspect I would like to dwell upon is the need to constitute a national body, which shall be responsible for providing guidance for various needs and quality assurance programme for the clean room industry in India. This body could work in various areas like formulation of local codes and standards to suit user requirements, broad-based design specifications, implementation procedures, operation methodologies, maintenance and validation procedures, training and certification of manpower specialized to take up these jobs, conducting regular courses and lectures for various levels of management and overall consultancy to users and manufacturers in this area.

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