A non-chemical method of air disinfection, it is an ideal way of ensuring all the air entering the cleanroom through the filters is free from any microbial contamination
Cleanrooms used in the semiconductor and pharmaceutical industries have to be many thousands of times cleaner than hospital operating rooms.
Various safeguards are in place to ensure this, one of which is the ventilation system, which must be strictly monitored to prevent microbial contamination of the ducts or air filters.
As they often operate in conditions of high humidity, however, ventilation ducts and even filters are often breeding grounds for microorganisms.
The problem It is well-known that HVAC systems can become colonised by microorganisms, particularly bacteria and fungi.
What is less well-known is that Hepa (high efficiency particulate arrestance) filters for cleanrooms, which are supposed to prevent contamination, can also harbour these microorganisms.
The large surface area of these filters, combined with a constant through-flow of nutrient-laden air, provides an ideal breeding ground for bacteria and fungi. These microorganisms grow through the filter media, blocking it and causing pressure to drop, resulting in reduced efficiency. Through-growth of fungi can also result in the release of spores on the cleanroom side of the filter, compromising the integrity of the cleanroom.
In addition, Hepa filters do not stop viruses. Research at a number of hospitals in the USA and Holland revealed extensive fungal colonisation of both the primary and secondary filter media.
In several cases the filters had only been in use for a few weeks.
The implications of these results are very significant.
Control of moisture levels in HVAC systems and Hepa filters is of primary importance in regulating microbial colonisation.
In reality this is not always possible, as these systems usually operate within varying environmental conditions, with high humidity and moisture levels sometimes being necessary. In these cases, steps must be taken to prevent the filters becoming contaminated.
A method of controlling microbial contamination which is gaining increasing acceptance is ultraviolet (UV) disinfection.
UV disinfects all incoming and recirculated air to the filters, killing any fungal spores, bacteria or viruses present.
It is a low maintenance, environmentally friendly technology which effectively provides an additional microbiological barrier to the existing filter technology.
A further source of contamination is through the water supply - UV water treatment systems are available which can eliminate the possibility of microbial transfer to the HVAC system from this source.
How UV works.
UV is part of the electromagnetic spectrum between visible light and X-rays.
The specific portion of the UV spectrum between 220-280nm (UVC) has a germicidal effect, with peak effectiveness around 260nm.
At these wavelengths UV kills microorganisms by destroying their DNA.
It is effective against all known microorganisms, including bacteria, fungi and their spores, with a kill rate greater than 99.99%.
A basic UV system consists of an arc tube (or UV lamp) enclosed within a protective quartz sleeve.
High levels of short-wave UV are generated when an electrical current passes between two electrodes at either end of the arc tube, irradiating the surrounding air.
UV systems for the treatment of cleanroom air filters must satisfy a number of strict criteria.
Firstly, they have to cope with the high number of air changes required (typically every four minutes).
For this reason, systems based on medium pressure, high intensity lamps should be used, as opposed to low pressure, low intensity lamps, which can be useful for other applications.
Rigorous independent tests recently carried out have shown that the high intensity UV generated by medium pressure systems provides effective disinfection against all known pathogenic and spoilage microorganisms - not just fungi and bacteria - even though the air is rapidly recirculating and exposure times are short.
Secondly, they should require very little maintenance while being in virtually continuous use.
All Hanovia systems are simple to use and the controls can be integrated into the existing HVAC control interface.
The only regular maintenance required is changing the UV arc-tube every six months or so, an operation that can be carried out by on-site maintenance staff and which only takes a few minutes.
Finally, UV systems should produce tangible results.
As already mentioned, it effectively provides an additional barrier in addition to the existing filters.
By destroying all air-borne microbial contaminants, it can extend filter life by several months, with obvious cost benefits.
Improvements, both in terms of cleanroom air quality and in staff health should also be noticeable, with less contamination and a lowered risk of infection.
Conclusion. Maintaining the integrity of the cleanroom is a constant battle. There are many ways this integrity can be breached, and as many methods of prevention.
Most operators assume that with proper Hepa air filtration systems in place the air supply is safe - this is clearly not the case, with some filters, especially those operating in high moisture conditions, becoming colonised by fungi and other microorganisms.
The implications of this are obvious and steps need to be taken to keep the filters clean.
UV has been shown to be very effective at achieving this.
A non-chemical method of air disinfection, it is an ideal way of ensuring all the air entering the cleanroom through the filters is free from any microbial contamination.
Maintenance requirements are negligible and the benefits, apart from preserving the integrity of the cleanroom, include a lower risk of air-borne infection for cleanroom staff and greatly extended filter life.
