Tougher product quality requirements call for production in clean or ultra-clean rooms. Even the smallest of impurities in the air can adversely affect production processes and lead to high scrap rates. The air supply in such production environments needs to be almost 100% free of dust and germs, and fans play an important role in this. However, they should be capable of more than simply “moving air”. Besides the required air performance, compact dimensions, low noise emission, materials suitable for clean room use, appropriate controls, networking capabilities and energy-efficient 24/7 operation are among the crucial criteria.
Clean-room conditions are mandatory for many of today’s production processes, not only in the semiconductor industry. Examples include optics and lasers, aerospace, biosciences, medical research and treatment, the production of foodstuffs and pharmaceuticals, and nanotechnology. Clean-room technology prevents contamination from compromising production processes and damaging the sensitive products (product protection). In addition, some production processes involve substances that may not be released into the outside air. Here, too, clean-room technology is necessary to prevent toxic or other reactive substances from leaving the protected area and endangering the environment or human health (personal safety).
Energy-saving clean-room operation
Clean rooms impose special requirements on their ventilation systems (Fig. 1), including sufficient air throughput and pressure, exact regulation of temperature and humidity, and consistent air quality. This applies regardless of the air circulation method (Fig. 2) or the room size. These requirements can be met by so-called fan filter units (FFU), which combine filter systems and fans and are designed for ceiling installation, particularly in large clean rooms (Fig. 3).
The operation of clean rooms with FFUs is the most economical means of supplying extremely clean air. At the core of these FFUs are extremely compact, modular built-in fans with a very flat design that need little space for installation. Energy efficiency is becoming an increasingly important issue as air conditioning and ventilation consume by far the most energy in clean-room operation due to their high air exchange rates. Energy costs and efficiency were once matters of secondary importance, but this point of view is now being reconsidered.
Rising energy prices and the public interest in lowering greenhouse gas emissions have prompted all leading companies to focus on energy efficiency. Currently, FFU manufacturers have to guarantee overall efficiency of at least 50% in clean-room applications. Not only are fans with a highly compact design called for, they also have to work efficiently. This makes GreenTech EC technology especially interesting for use in clean rooms, since FFUs equipped with it achieve efficiencies of more than 50%, impossible with conventional fan technology. GreenTech EC fans can also meet strict requirements in terms of noise emission. In particular, RadiCal® fans are 6-7 dB(A) quieter than the previous market standard, a considerable improvement in both clean-room noise levels and worker comfort.
Motor and fan specialist ebm-papst Mulfingen offers a range of EC centrifugal fans specially designed for use in fan filter units and meeting all mandatory requirements for this application (Fig. 4). They are available in diameters of 310, 355 and 400 mm with air performance of 1170, 1750, and 2330 m³/h at a back pressure of up to 250-300 Pa. These fans are very compact thanks to their external rotor design, and with heights from 190 to 275 mm, they are also relatively flat, so they can be easily integrated in the equally compact FFUs.
Easy control and networking
Thanks to their high efficiency, EC motors also generate little waste heat released into the circulated air, so the production environment is subject to less thermal stress, which reduces the demand on the cooling systems considerably. The speed of fans with EC technology can also be controlled smoothly and easily, so their air performance can always be adjusted to current needs. In terms of economy, this also makes sense as EC motors have a significantly higher efficiency in partial-load operation than motors based on other control concepts. After all, oversized air volumes will not necessarily lead to better product quality, but will inevitably result in unnecessarily high operating expenses. The right setting for the application is crucial.
For demand-based operation, the EC fans can be controlled via an analogue 0-10 V signal or a digital RS-485 interface. The latter lends itself best to clean-room use, where several thousand fans are often in service in the FFUs. In such cases, the networking capabilities with Modbus or ebmBUS have proven very effective in practice. From configuration during commissioning to service, fault diagnosis and maintenance, making technical adjustments is considerably easier if the fans can be accessed from a central PC. The drives can be tied into the higher-level building management control system via the bus interface. The motor speeds can be smoothly adjusted via this interface, which provides a very convenient means of adjusting performance to meet demand.
The air performance of the FFUs can be perfectly matched to the clean room’s ambient and operating conditions. If a fan in the filter ceiling fails, it can be compensated for by increasing the speeds of the neighbouring FFUs. Such a strategy for dealing with failures provides a high degree of operational reliability by maintaining the correct air flow. From configuration and programming to alarm management, ebm-papst’s FFU control software (Fig. 5), which is suitable for use with Modbus and ebmBUS, simplifies systems engineering tasks.
During operation, the user also benefits from other features of EC technology. The fans are very quiet; their drives have faultless noise properties throughout their speed range so they can meet the stringent noise suppression requirements applying to production in clean rooms. This low noise emission is mainly due to the aerodynamically optimised centrifugal impellers.
Metal or plastic centrifugal fans
Whenever fans are used in clean rooms, their impellers are usually made of metal. There are always reservations when it comes to using plastics in the ventilation systems of clean rooms. The additives needed in plastics production are suspected of outgassing and endangering clean-room processes. The situation is completely different for applications where contamination by metals must be ruled out, such as wafer production. Here, fans made of plastic are clearly a blessing. To rule out risks to clean-room production that are associated with plastic particles in the circulated air, ebm-papst carefully analysed the issue of outgassing.
The RadiCal® range of fans has been rigorously tested for substances prohibited in clean-room operation; these substances have been evaluated as per the semiconductor industry’s ITRS standard. RadiCal® impellers from ebm-papst passed the relevant tests with flying colours. According to the test results, the plastics used are non-hazardous. The emission level of potential contaminants is well below the permitted limits. In clean-room applications, too, users can now enjoy the full benefit of the innovative design of these plastic impellers. After all, plastic offers a considerably greater range of design options than sheet metals. The overall design of the fan results in an improved air flow through the impeller, significantly increasing the fan’s aerodynamic efficiency. The rounded inlet contours on both the cover plates and the base plates of the impellers play a part in this, too. Moreover, the interaction between the inlet nozzle and the impeller inlet has been improved significantly.
These EC fans, in combination with the aerodynamically optimised plastic impellers or the proven aluminium blades, are energy-efficient, compact and adjustable components for fan filter units, well suited to the requirements of clean-room applications in every respect. Their performance has been proven around the world, including clean rooms in which flat screens are produced. For good reason, the market leaders from Korea and Taiwan chose GreenTech EC technology from ebm-papst for the production of their big flat-screen products.