In Australia, the use of gel seal HEPA filters has become the “default” choice. This is driven by the inherent performance advantages of the gel seal; a soft sealing material that results in filter installations that are easy to seal and require low clamping pressure compared to typical foam gasket systems.
Polyurethane gels may be used, however the general chemical and environmental stability of Silicone gels have gained in popularity within HEPA applications.
In general, gels are formed by carefully mixing measured parts of the co-polymer and adding (pouring) it to the HEPA channel. The nature of these lightly cross-linked systems is that they are unforgiving to errors in processing – with significant variations in the final physical characteristics. However, if reasonable care is exercised in manufacturing, stable gels that give excellent characteristics and performance result.
An experienced installer is required for mechanical sealed housings, as it is difficult and quite time consuming to obtain an acceptable seal. Gel sealed housings allow for a more rapid and reliable change over, which is practical for many hospital applications.
Misguided attempts to retro-fit mechanical seal to gel seal housings (or the reverse) to existing housings of one type are often costly and time ineffective. If there is an application specific reason for using one particular sealing type, that needs to be determined prior to installation – or if change is needed, replace the entire housing and filter style.
These two gel types provide a physically compliant, air tight seal between the HEPA filter frame and its housing. Both contain similar characteristics in terms of elasticity, surface tack and softness and will perform the basic job of sealing HEPA filters to the air delivery system. However there is a wider acceptance of Silicone gels in general HEPA applications.
Silicone gels are produced by several major corporations and are included in a range of electrical, electronic, vibration damping and sealing applications. They are generally more resistant to chemical breakdown and enable higher usage temperatures; but are also relatively high in cost, and may suffer cure inhibition when applied directly to some incompatible substrates.
Polyurethane gels are not commonly used by many HEPA filter manufacturers. Although they are relatively low cost and offer fairly good chemical resistance, commonly cited disadvantages include high moisture sensitivity of the unreacted components resulting in a limited raw material shelf-life, less resistance to chemical breakdown and incompatibility with certain substrates that may result in cure inhibition.
Multiple reports of issues with limited batches of HEPAs from multiple vendors have been documented over the years. Some of these include:
These issues may be divided into some basic categories:
As a trivial case, a filter may be loaded to excess, perhaps by uncleaned ductwork after construction, so the fully compliant filter gel is pushed out by excessive static across the filter. This case is simply one of the filter being exhausted – not a failure.
Gel chemistry is fragile and unforgiving to errors in processing. Manufacturing related process problems include liquefaction, unacceptably large variability of final gel properties, cure inhibition (wet spots), and gel dripping.
These problems may be associated with using a gel that is past its shelf life, is not metered and/or mixed properly, is not stored properly, or is applied to an incompatible or contaminated substrate. Numerous vendors have occasional examples over the years, generally over a small batch of filters. Once identified, corrective action can be swiftly taken, and affected filters discarded.
Installation problems most likely include insufficient penetration of the knife edge into the gel OR cutting of the gel by excessive penetration. Tearing or gel damage can result if a HEPA is pushed onto the knife edge then moved aggressively to enable seating.
Exceeding the elastic limits of the gel will result in splitting and cutting of the gel. Below the elastic limits, the gel will “snap back” and return to nearly its original state when the filter is removed (“self-healing”). Self-healing is somewhat misleading as once the gel is cut or fractured, it can stick back together and form a seal, but it will not reform chemical bonds.
Finally, incorrect (excessive) air flow rates or mis-designed ductwork can raise a host of issues that impact overall performance. These range and scope of these issues is well dealt with in other publications.
There is some evidence to suggest that a range of chemicals and environmental conditions may contribute to premature gel failure. Oxidizing chemicals (e.g. bleaches, sulphites, SLS’s, SLR’s) used in detergents and disinfection solutions, when sprayed aggressively around HEPAs and HEPA modules, contamination of the gel surfaces can occur. Powerful oxidizers, acids, solvents etc. will compromise the materials. It is possible that if materials are at the limit of their performance specification –chemical attacks could be significant.
Some test oils have been speculated as an issue, most modern non-PAO test oils have proven reliable. Excessive loads of oil or inferior oils may create chemical attack or excessive pressure spikes that damage the filter. There is some literature evidence that this chemical damage has occurred with PAO test oils.
The issue of gel dripping can be caused by a combination of multiple factors which include; the original choice of gel, the quality of the components, the reliability of the mixing process and the exposure of the gel to oxidizing chemicals or other detrimental environmental stressors. A significant excess exposure to oil or elevated temperature may exacerbate this problem.
In addition to batch testing of HEPAs for arrestance and pressure drops, the ultimate assurance of filter performance is NATA certified testing on site. Irrespective of how rigorously filters have been tested in the factories, these fragile items may suffer considerable damage in shipping or storage. Thus, the yearly certification testing is the most cost effective way to assure performance at a site.
In some applications (such as TGA sites) further “traceability” may be required for documentation purposes. While this is available at an increased cost – it does not relieve the importance and necessity for site testing and NATA certification to assure performance. If a filter fails its primary arrestance test – it should be repaired or replaced at the first opportunity.
To be operated in a cost effective manner, pre-filtration should be considered to protect and extend the life of the HEPA filters. HEPA filters require adequate protection from general dust, lint and contamination by inexpensive, disposable filters to stop premature loading and replacement. In general, the higher the rating of the pre-filters, the longer the protected HEPA will last.
On a supply side application, a typical filter arrangement may be as follows:
Without the sacrificial pre-filters, the HEPA filters are likely to clog rapidly, resulting in a service life of months instead of three to five years. When good practices are followed, useful, energy efficient, high performance particle filtration should be achieved for 3 to 4 years of operation of the HEPA filter.
The upstream pre-filters will be changed at far more frequent intervals, of 3 to 9 months. The pre-filters are “sacrificed” to capture small to large particles, and will generally be changed out to maintain energy efficiency and assure no particle “break-through” occurs due to overpressure that has affected the structural integrity of the pre filter.
Airepure recommend replacing gel seal HEPA filters when contaminated or the loaded pressure is reached, but no longer than 5 years from manufacture date due to possible degradation of the gel.
Gel seals, silicone based or even polyurethane, offer reliable and versatile sealing solutions for most HEPA applications. If due care is applied to the factors described above, the failure rate for these units is low. However, despite the simple appearance of the HEPA filter, it is a complex assembly of parts, with a highly cross-linked polymer that requires careful manufacturing, transport, storage and installation to achieve its design purpose.
Written by Dr Allan Heckenberg (PhD.) and Shannon Roger (B.Ed.) for Airepure Australia 2016. Rev 1118 Airepure Australia Pty Ltd 2018.
Airepure Australia offer a range of products, services and consulting expertise that can assist you with your compliance to ACHS, DHS VIC Guidelines (and equivalent for QLD, WA and NSW), ISO/IEC 17025:2005 Requirements, AS/NZS 2243.3:2010 and AS/NZS 2243.8:2014. Airepure is a leading national air filtration company providing unique, powerful and integrated air filtration solutions, ranging from basic HVAC filtration and odour control right through to high end HEPA/ULPA filtration and airborne containment technologies. For more information, visit www.airepure.com.au or call 1300 886 353.
Devine, S. “Technical Bulletin: A Brief Discussion of Gel” 2012