Lighting with integrity

It is wrong to think of cleanroom luminaires as merely sealed metal boxes with lamps inside; they have to be considered an integral cleanroom component.

This thought process must begin at the concept stage, if they are to meet the exacting standards demanded by the industry. The quality of materials and construction of the luminaire, the type of seals employed and the chemicals that will be used for cleaning must all be taken into account. In addition, it is important to consider cost of ownership issues, such as energy consumption and maintenance.

Suiting the ceiling Consideration must firstly be given to choosing a luminaire and a fixing system that suits the ceiling system. As this could be a modular grid system with either exposed tee grid or spring tee grids, plasterboard or prefabricated panels of up to 170mm in thickness, choosing the right fixing system is critical to maintaining the integrity of the room, both after validation and during operational maintenance. If a back access luminaire is chosen and is to be mounted into a thick panel ceiling with the cut aperture uncapped and/or unstrengthened, the panels may relax or compress slightly and consequently leave gaps to the void where the luminaire seal cannot fully engage with the ceiling. It is therefore vitally important to have a sturdy and adjustable bracket system that can be easily and routinely checked and tightened. The use of push-to-fit or spring fixings on diffuser frames may not accommodate ceiling irregularities with sufficient pressure to create a proper seal; it makes more sense to use screw fixings to the frame, while positive jacking bracket systems become vital to ensure even compression of all seals around the luminaire. The quality and type of materials used for gaskets is also important: the luminaire must maintain the room's integrity while remaining user-friendly for routine maintenance. Some systems are reliant on mastic sealants between luminaire and ceiling, but this can cause problems in three areas. First, the diffuser frame can be difficult to remove during maintenance, while the removal of old residue and reapplication of a new mastic seal is often time-consuming, thus increasing downtime; second there is a danger of contamination from fragments of dried mastic that could enter the cleanroom space during maintenance; and third, and perhaps most importantly, the drying and cracking of sealants can leave the room open to contamination. A well-designed quality gasket seal using closed-cell construction will avoid these problems and prevent passage of contaminants from the ceiling void to the room or vice versa. Of the various rubber gaskets available, thermoplastic rubber offers the highest level of integrity and durability, leading to a long and trouble-free life. As the diffuser frame is the only part of the luminaire that will be inside the cleanroom, the choice of materials for its construction is of the utmost importance. Steel is sometimes offered as an alternative to aluminium due to its lower manufacturing costs, but it does not offer the same rigidity as an extruded aluminium section, and if the paint coating were to be damaged, exposing the steel to both cleaning solutions and atmosphere, it is more likely to shed contaminants into the cleanroom, increasing the risk of contaminating the process.

Accessing the luminaire An extract from ISPE's (International Society for Pharmaceutical Engineering) Baseline Guide says that: "stainless steel or aluminium fixtures may be considered appropriate as they are non-shedding and resistant to environments which may be corrosive". It is important to apply a high-quality, hard paint finish to the frame (excluding stainless steel); 120µm, baked to 180°C would be appropriate. With the front of the luminaire in place and sealed with a gasket, the assembly should provide ingress protection to IP65. However, the integrity of the ceiling is only as good as the IP rating of the luminaire body once the front assembly has been removed. Purely from a maintenance point of view, all cleanrooms could be designed using walk-on ceilings with access to the luminaires from above to eliminate the need for maintenance staff within the space. Where back access luminaires are used they should be designed in such a way as to allow all maintainable components to be easily accessible from the ceiling void. Features such as rapidly detachable and fully inter-changeable gear trays using plug and socket connections will ensure easier and quicker maintenance. Where a walk-on ceiling is not to be employed, the body of the luminaire should offer the appropriate level of protection, dependent on the classification required. The type of diffuser material to be used in the luminaire is another important consideration. The nature of the diffuser will control the light distribution in the space and may be either a clear material below a Cat2 louvre, where computers or VDT's are widely used, or a prismatic material that refracts the light to provide an even distribution. With conventional luminaires, diffusers are usually either polycarbonate TP(a) or acrylic TP(b), depending on the relevant fire and building regulations. In a cleanroom environment the type of material used is important for reasons in addition to the above: the performance and life cycle costs of the installation can be greatly affected by the wrong choice of diffuser material; and time should be given to discover which cleaning chemicals are to be used because of the potential for adverse reactions with thermoplastics. Research by Whitecroft Lighting has shown that many cleaning chemicals used in cleanrooms are aggressive to thermoplastics, shortening the life of the diffusers, which leads to more frequent replacement and greater disruption. The higher the classification of the room the stronger the chemicals used for cleaning, but choosing the wrong diffuser material could necessitate the replacement of all diffusers in as little as two to three years. An interesting development in this area comes from a relatively new plastic material for diffusers called PETG (glycol modified terephthalate), also known as glycolised polyester – a modified form of PET (polyethylene terephthalate). The addition of glycol modifiers makes PETG less susceptible to premature ageing or discolouration. Unlike polycarbonate PETG has a closed cell structure. It also has better light transmission qualities. In the context of cleanroom lighting, PETG is more resistant to chemical degradation than polycarbonate and is more resistant to discolouration due to the high levels of ultraviolet (UV) light present in fluorescent lighting. However, all plastic diffusers will degrade and discolour with time if exposed to UV light, making it important that they are replaceable with minimum disruption. In high classification cleanrooms with rigorous clean down regimes, it will often be worth considering glass diffusers as glass is chemically inert and will not degrade with exposure to UV light. The initial capital outlay may be slightly higher, and the additional weight has structural implications for the ceiling, but glass is a better long-term solution. An integral part of the maintenance regime is the cleaning of the luminaires. This should be made as easy as possible. Ideally there will be no gaps between the diffuser and the frame or the frame and the ceiling, and no cracks or crevices that can harbour dirt and contaminants. If a surface screw is used to fix the frame, it should be large enough to clean, making a large slotted screw a good solution. Semi-recessed or crosshead screws are difficult to clean and can harbour contaminants. Fully recessed screws covered with an IP65 rated flush cap are the best solution, being easy to clean and aesthetically pleasing. Two of the key drivers to achieving low life cycle costs for lighting are lower energy consumption and extended lamp life. The T5 lamp and control gear package offers a higher lumen output per circuit watt of energy consumed than the T8 lamp, and high colour rendering (Ra 85). T5 lamps have a life of 20,000 hours to 50% failure, extending lamp replacement intervals and lowering maintenance costs. Most new commercial and hospital developments are lit using T5 lamp technology, and in write-up and office areas lit with T5 lamps, it makes sense to light the cleanrooms in the same way. The lamp reaches its maximum output at 35°C, which is ideal for the higher running temperatures in sealed luminaires, while the optimum working tempera-ture of the T8 is 25°C. Typically a T5 cleanroom luminaire can be 10-15% more efficient than its T8 equivalent. The improved efficiency of T5 luminaires means that higher light outputs can be achieved from standard luminaire sizes, or the same overall light output can be obtained using fewer luminaires. This is beneficial as the use of smaller or fewer luminaires can increase flexibility at the design stage. There are many points that factor into the equation for achieving the best long-term lighting solution for a cleanroom application. The cost of the lighting needs to be balanced against the indirect costs of using an inappropriate system, such as product contamination and loss of production time, and for these reasons it makes sense to team up with a lighting supplier that has an in-depth knowledge of this specialised area and the expertise to guide you through the selection process.