Care and Maintenance of Filters in Cleanrooms and Air Handling Units
Contamination control depends on reliable filters and ventilation systems. A malfunctioning or faulty filter can introduce contaminants into a controlled environment resulting in significant risks or damages. Filters need to remove impurities from the process stream but avoid transferring impurities that could settle on a device or products surface and a well-designed system with suitably selected filters should be relatively hassle free if properly maintained.
The filters themselves must be manufactured and packaged in a controlled environment to ensure that they don’t introduce contamination when fitted. Replacing with low quality or inadequate filters can be costly in the long run as they affect the overall system performance. Custom filters can be particularly expensive and may take weeks or months to be delivered, so adequate planning is an essential. Following good protocols will keep filters working optimally and minimize premature change-outs, with the associated expenses for parts and labour as well as disruption and potential contamination.
Preventive maintenance is obviously the first step to ensure that filters function properly, and that the amount of time and money spent on them is minimised.
Tips for an efficient system
- HEPA filters involve special considerations in the filtration system design so that they can function as intended. Pre-filters and final filters should be installed upstream of any HEPA filters to protect and extend the life of the HEPA.
- Pressure drop is another important consideration, since this affects the amount of energy required to keep filters operating effectively. Filters with a lower pressure drop will require less energy, and will therefore reduce operating costs.
When to change filters?
ISO 14644-2 provides guidance on how to monitor and test filters. Rather than following a specific time frame, maintenance staff must observe their filters and change them as soon as is necessary.
HEPA filters for example can typically last between five and seven years before they need to be replaced, but rules of thumb can be misleading. Following a rigid schedule can be dangerous or expensive as the lifespan of the filter is dependent on what they are filtering, the load placed on them and the airflow and pressure which they are exposed to, in cleanrooms other factors such as humidity and temperature also play a role in the filters useful life.
Best practice is to monitor the differential pressure and the air flow rate or air velocity on a quarterly or bi-annual basis using a calibrated Velometer and a Magnehelic pressure gauge fitted to the plenum housing. As a rule of thumb, the pressure should not exceed 250 Pa to avoid damage to the HEPA filter.
Supported pleat filters are often used as pre-filters for higher efficiency filters or they may be used to shield heating and cooling coils from becoming coated with dust. Pre-filters may need to be changed up to six times per year in extremely dusty environments.
Selecting the right kind of filter, and taking proper care of it once it’s installed, will be the difference between an easy road and a long, expensive one.
Guidelines for filter selection
FOOD MANUFACTURING/PROCESSING AND AREA AIR FILTRATION LEVEL/GRADES
|APPLICATION||FINAL FILTRATION LEVEL||FILTER SYSTEM MAKE-UP|
|Covered product / General office||M5/ M6 and up to F7||Single filter use acceptable if dust loadings are low|
|Food Process Hall (General process areas defined as non “high care/high risk”)||F7||G4/M5 pre-filter panels or pocket type followed by F7 rigid bag filter|
|“High Care/ High Risk” Food Production||F8 – H11||G4/M5 pre-filter panels or pocket type followed by F8/F9 pocket filter, if required this can be followed by H11 rigid bag final filter in leakproof framing system|
|High Risk direct product contact||H11||G4/M5 pre-filter panels or pocket type followed by F8/F9 pocket filter followed by H11 rigid bag final filter in leakproof framing system|