During the pandemic we've been seeing the perpetuation of myths surrounding personal air filtration (AKA facemasks). As much as we'd enjoy poking fun at urban legends, we'll stay focused on air filtration for gas turbines.
Due to the global COVID-19 pandemic, we’ve been bombarded with information about personal air filters (facemasks), how to apply (wear) them, multistage filtration (doubling masks) and filter ratings (N95). As professional turbomachinery engineers, we’re rather amazed at some of the myths perpetuated about air filtration. But as much as we’d like to poke fun at urban legends, we’ll stay focused on air filtration for gas turbines.
The difference between a facemask and a gas turbine filter looks staggering: While a person inhales about two cubic meters of air in an hour, a typical mid-sized gas turbine will ingest over 200,000 cubic meters in the same period. But there are some critical similarities. What are they?
Well…air-particles are air-particles, leaks are leaks, plugging is plugging, and there are liquids the filter must deal with.
Fouling and degradation in gas turbines are to a large degree driven by the quality of ingested air and the fuel supply. The effect of contaminants from the ambient air on a gas turbine (or a human being), which can be controlled by a properly designed/operated/applied air filtration system, is determined by the contaminant’s particulate constituent, size, and wetness. Solid particles in the air can consist of or contain a number of substances — salts, biological matter (pollen, insects, bacteria, etc.), or various types of dirt and sand — that can create significant problems for engine health if ingested without a filter. Salt in combination with sulfur, for example, can cause accelerated hot corrosion.
Most industrial filters effectively remove air particles above a couple of microns in size. However, even the best filters can’t protect the gas turbine if air can bypass these filters. Open man doors or leaking seams in the filter housing, incorrectly installed filters, or leaking gaskets let unfiltered air enter the engine, which obliterates the benefits of an otherwise efficient air filtration system.
An air filter’s efficiency can be evaluated by mass, volume, or particle size filtration effectiveness. A metric that determines filter efficiency by particle size is the most meaningful. Thus, most air filters are rated by their ability to keep particles of different sizes from entering the engine. Since modern filtration systems have no problem keeping larger particles (5-10 microns or larger) from penetrating the gas turbine compressor, particle erosion is normally not a problem. It’s usually smaller particles that cause compressor fouling and potentially transport contaminants that can cause degradation and hot corrosion in the hot section of the engine.
One of the characteristics of a filter is the “most penetrating particle size,” the particle size for which the filter has the least efficiency. An N95 filter, for example, can remove at least 95% of particles in the size range of its most penetrating particle size, which is about 0.1 to 0.3 microns. For larger particles, the filtration efficiency is much higher — 99.5% for particles above 0.75 microns. Of course, an N95 respirator only achieves these values with a good face seal.
It’s also important to know that filtration efficiency of a given system is reduced for higher flow velocities through the filter. Efficiency often increases if the filter already has some collected particles and is nearing saturation.
Then there are liquids and liquid droplets. Unfortunately, the ability of a filter to keep liquids out is not directly related to its particle removal efficiency. Liquids, such as water, often can seep through most filter materials. In the process, they can dissolve solid minerals previously captured by the filter and pass them through. Liquid droplet penetration can be prevented either by separating the liquid droplets from the airstream using demister vanes or by stopping them from entering the filter material by making its surface hydrophobic. The latter method can be used for filters that have a high efficiency for small particles. The former method requires high flow velocities, which compromises its ability to work effectively with filters that have a high efficiency for solid particles.
Why are we talking about this? Air filters can be very effective in protecting a gas turbine from particle penetration. But small particle effectiveness is limited. They always let a small percentage through. The difference between a highly efficient and a not-so-efficient filter can lead to reduced fouling and degradation rates (as expressed by the time between required engine compressor cleaning) by orders of magnitude. But the positive impact is wiped out if there are leaks in the inlet system — just like your N95 facemask is mostly useless if you’re not wearing it over your nose and mouth.