Not all HEPA filters are the same

When it comes to HEPA filtration, there are two main options to select from: a filter made from Microfiber glass (glass fiber) or PTFE membrane. Both can provide HEPA level filtration and filter out fine particles from the gas turbine inlet. To achieve this, both use fine media but they are constructed and perform very differently.

PTFE is a simple polymer composed of carbon and fluorine. Expanded polytetrafluoroethylene (ePTFE) filter membranes consist of a single, very thin layer of finer media that creates a ‘sieving’, surface filtration effect. While highly efficient at capturing fine particles, this means the membrane has a relatively low filtration surface area. Any moisture droplets that become trapped, therefore, can quickly cause a complete blockage of the inlet filter. In real-world installations, the complete blocking of these filters can occur in as little as three weeks after installation.

Microfiber glass filters use a media layer that is ten times thicker than ePTFE, greatly increasing the filtration surface area. Rather than using the ePTFE model of a thin layer of finer pores, it uses its depth to capture particles as they travel inside its matrix. Even if moisture droplets block pores within the media, the volume of pores means the media will take much longer to become completely blocked. Any deterioration will happen slower than with ePTFE membranes; extending the life of the filter and giving operators plenty of warning that cleaning or replacement is required. It is this predictable performance that, for the time being, continues to make microfiber glass the preferred choice in heavy industrial applications.

Robustness of filter design

Many filters are designed for use in HVAC or laboratory-style conditions. To withstand the rigors of a power plant, a filter needs to be designed to handle a tougher environment and greater operating pressures.

Particular areas of weakness to consider when reviewing a filter’s construction include the gasket that seals the filter to its holding frame. Any joins in this gasket may be potential break points. The protection provided to the media pleat packs, i.e. does it have supporting mesh downstream to enable the pleats to cope with the increased working pressures in this application? For cartridge filters, the way the media pleats are glued to hold the pleats is important as any glue beads formed in the assembly process if not done correctly may break off the filter once it is installed. The overall materials of construction, including the compatibility of any adhesives also needs to be appropriate for the site conditions to avoid brittleness or cracking and to ensure the filter frame does not become warped over time or under the wider extremes of operating temperature.

Optimizing a filtration solution

Selecting a filtration solution will depend on a number of factors beyond filter media and unit design. The best solution may not be the most expensive and, ultimately, it is how the turbine performs that shows the suitability of a filter for its application. The level of output and heat rate of the turbine and how this varies over time are the best forms of data in establishing the effectiveness of a filtration solution. If an operator sees unacceptable increases in heat rate and decreases in output, then the filtration solution should be re-evaluated and other types considered.

Although testing standards continue to be researched and developed as environmental factors become more clearly understood, one of the problems operators face today is that the standard efficiency rating of the filter will not necessarily equate to the performance of the installed system. Recent comparative studies carried out by a major gas turbine OEM on the performance of equivalent ePTFE membrane and microfiber glass HEPA filter products covered overall efficiency, pressure loss, hydrophobic performance, wet performance and the dust holding capacity of each technology. The tests showed the microfiber glass filters to produce equal or better performance than the ePTFE equivalents, and at a lower cost.

This article is not trying to say microfiber glass media is better than ePTFE. CLARCOR manufactures ePTFE membranes which are successfully installed in many other filtration applications and future developments may well deliver improved resilience to moisture and hydrocarbons of this media type. As the technology stands today, however, the unpredictable response ePTFE membranes have to the wide-ranging air contaminants found at gas turbine installations means CLARCOR does not recommend them for use on turbine installations.

Summary

Filter test standards and materials for use in gas turbine filtration solutions continue to develop as we learn more about environmental impacts. What is clear is that operators need to take a wider view of filter and turbine performance rather than considering only standard efficiency ratings.

Both microfiber glass and ePTFE membranes can provide HEPA filtration levels. While ePTFE membranes can deliver a typical lifespan of two years, however, this is still much shorter than microfiber glass equivalents for power plant gas turbine installations. Where moisture levels are high or there is likelihood of fog or mist weather events, the thicker microfiber glass media will give a more predictable response. ePTFE solutions will require close monitoring and quick change out to protect machinery if they suddenly become blocked.

As technology and understanding develops, it is recommended that power plant operators consult filter manufacturers to understand the options available to them. By working with filter suppliers that appreciate and understand the varying needs of different environments, a turbine can be better protected and this may not be the most expensive option. By eliminating unnecessary maintenance or shutdowns and improving turbine performance, the right solution can give a very quick return on investment.

Both microfiber glass and ePTFE membranes can provide HEPA filtration levels. While ePTFE membranes can deliver a typical lifespan of two years, however, this is still much shorter than microfiber glass equivalents for power plant gas turbine installations. Where moisture levels are high or there is likelihood of fog or mist weather events, the thicker microfiber glass media will give a more predictable response. ePTFE solutions will require close monitoring and quick change out to protect machinery if they suddenly become blocked.

As technology and understanding develops, it is recommended that power plant operators consult filter manufacturers to understand the options available to them. By working with filter suppliers that appreciate and understand the varying needs of different environments, a turbine can be better protected and this may not be the most expensive option. By eliminating unnecessary maintenance or shutdowns and improving turbine performance, the right solution can give a very quick return on investment.

Author:
Steve Hiner is chief engineer of Gas Turbine Inlet Systems at CLARCOR Industrial Air.

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https://www.power-eng.com/2017/05/22/not-all-hepa-filters-are-the-same/