Understanding (H)EPA filtration – Benefits of using multi stage filtration vs. single stage filtration

By Pertti Sundberg, Your Friendly Filter Guy at Eagle Filters

There is a huge variety of products sold under (H)EPA grade filtration – If you read through this short description you will know everything you need to know about (H)EPA filtration in Combustion Turbine Inlet air Filtration.

Let′s start with the basics - what is (H)EPA:

EN1822 standard is giving the filtration efficiencies for various (H)EPA grade filters:

E10 is designed to filter out at least 85% of the Most Penetrating particle size, typically 0,2-0,4 um in size. E11 is taking out at least 95% and E12 at least 99,5%.

Great figures – but what does that mean in practise?

If you operate a GE′s 7FA Gas turbine with 5000 annual operating hours in a clean area fulfilling the WHO recommended air quality for PM10 particles being 20 ug/m3 you will get 126 kg of air borne particles to your Filtration system. If you are in an industrial location you will have to multiply the PM10 particle amount by multiplier of 2 to 5.

7FA even in clean environment with 5000OH per year is sucking into the Inlet air duct 126kg of dust every year

Without any filtration your Compressor would get that amount of air borne particles and you would have horrible wear&tear and loss of performance. To capture most of the dust particles you need sn efficient filtration system.

With the (H)EPA grade filtration your 7FA GT compressor is getting the following dust loading:

E10 – 126 kg reduced to 4,9 kg per year by capturing 121,1 kg per year

E11 – 126 kg reduced to 1,5 kg per year by capturing 124,5 kg per year

E12 – 126 kg reduced to 0,4 kg per year by capturing 125,6 kg per year

So if you want to protect your Gas turbine blading from degradation it′s better to leave out the E10 (H)EPA from the discussion due to low performance as 4,9 kg is way too much to be accepted if you really want to protect your GT performance.

But even the best (H)EPA filters have one limitation - they are designed to take out the smallest particles and don′t have a good dust loading capability – ie. they block easily if used as standalone filter.

And you don′t want to use the most expensive filter to take out the large particles that can be captures with much cheaper Pre- and Fine filters - that is why we always prefer staging the filtration:

• 1st stage - Pre-filters taking out the large particles while capturing almost 60% of incoming particles by weight and reducing the humidity in the air (assuming hydrophobic material is used)
• 2nd stage – Fine filters reducing the amount of particles in the air by capturing 25% of entire incoming particles by weight thus reducing the dust loading on the 3rd stage
• 3rd stage – (H)EPA filters capturing about 15% of the incoming particles by weight

By staging the filtration into three stages, we are also improving the overall filtration efficiency of the system – Prefilter and Fine filter will take out majority of the particles and by having an E11 as final stage we are still able to get close to E12 grade filtration results - with better service life performance than by having only E12 filters in place.

If we put M5/F8/E11 three-stage filters to the same 7FA we looked at earlier we are able to reduce the dust going to the compressor to 0,7 kg per year and by using high performing Eagle Filters′ E11+ as 3rd stage the dust loading is further reduced to 0,1 kg per year – giving even better filtration result than E12 filter alone would give!

As a summary - by staging the (H)EPA filtration, we are able to:

• Increase the overall filtration efficiency
• Reduce the overall cost of the filtration
• Extend the service life of the (H)EPA filters

and eliminate the Degradation and need of On-line or Off-line washing!

And in most cases we are able to improve the filtration to (H)EPA without modifications to the existing filterhouse structures !

If you are still not sure what would be the best filtration grade for your Gas Turbine, you can always reach out to Your Friendly Filter Guy!