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FAQs regarding air inlet filtration technology for gas turbines and generators.
How do I assess filter performance tests?
Different filter tests provide different types of stated filter efficiency. Most of us consider the air that we breathe to be relatively clean when, in fact, each breath contains hundreds of tiny “dust” particles. Most of these particles are so small (less than 25mµ) that they cannot be seen with the naked eye. Because a turbine “breathes” considerably more air than we do, it is much more susceptible to the negative effects of these dust particles.
There are three basic types of filter efficiency that are referenced in filter data reports:
ARRESTANCE -- the ratio of the weight of dust particles captured to the total weight of dust exposed to the filter;
DUST SPOT (or ASHRAE) -- which relates to the relative “staining” characteristics of the dust that passes through a filter;
FRACTIONAL EFFICIENCY -- the discrete measure of the number of particles of a specific size downstream of a filter compared to a measure of the number of particles of the same size upstream of the filter.
How are these measurements important in gas turbine filters?
Although “small” particles in the air outnumber “large” particles by a ratio in the range of a million to one, the large particles account for most of the weight. Large particles (>5mµ) are what cause erosion on turbine blades, and if a filter’s efficiency did not quickly approach 99+% on these particles, that filter would not be used for a turbine application. For this reason, arrestance efficiency is of little significance for turbine filters.
Dust spot efficiency is a significant test because it measures the ability of a filter to remove particles that will stain, and the resulting contaminants affect the amount of cleaning necessary to maintain turbine performance.
Particle Count is a significant test, especially on smaller particulate sizes, because it is a method for comparing different filter element efficiencies on specific particle sizes. This test shows differences in efficiency performance of various types of filter media, and for the most part will show the same efficiency relationship between filters as shown by the Dust Spot.
When Analyzing Filter Test Data...
Not only is it important to recognize that there are different types of efficiency, it is also important to recognize that filter data being compared must be from tests performed at similar airflow rates. That is why Donaldson has invested so heavily in multiple tests of the same filters at different airflow rates: there is little value in looking at a filter tested for a flow rate of 500 cfm on a 26" filter element if the filter elements in your air filtration system are 22" long, operating at a flow rate of 900 cfm.
It is also important to look at all of the data in a filter test. If a filter element has an efficiency of 99.5% on a particular micron size after it has been loaded with 100 grams of dust, the average efficiency from start-up to that point will be significantly less, and the 99.5% efficiency is of little value to a turbine operator unless the filter element is loaded with dust before it is installed in the air filter.
The “Average Efficiency” from start-up to any point on a loading curve should be the only efficiency data that is compared and is determined by the ratio of the area under the efficiency curve to the total area of the graph. This is the same method employed when a filter is rated with an (average) ASHRAE Efficiency.
Finally, it is important to recognize that efficiency data shows how much dust a filter media has captured rather than how much dust has passed through (penetrated) the filter media. It is the comparison of dust penetration that should be of interest to turbine users, because it is the dust that passes through a filter that affects the performance of the turbine. For example, although there doesn’t appear to be much difference between a 94% filter efficiency and a 96% filter efficiency, the 94% efficient filter allows 50% more dust to pass through (penetrate) the filter media.
How big is a micron?
It’s one millionth of a meter or .000039 inch.
How will I know if I need to replace some media in my evaporative cooler?
Over time the wetting media inside an evaporative cooler can deteriorate. If any of the conditions listed becomes evident, it’s time to replace media. (Don’t wait until they’re all evident!)
1. The pressure drop across the evaporative cooler has doubled compare to when it was new.
2. Media edges are crumbling, or the media is mushy.
3. There is water downstream of the evaporative cooler.
4. The media is separating from the steel framework.
We experience an onslaught of cottonwood drifts in the spring and they block up our filters. What can we do?
To protect your filter from temporary or seasonal high concentrations of particulate – such as the cottonwood seeds or things like ash from a local fire or insects – use pre-filter wraps. They prevent the (more expensive) primary filters from clogging up with extra debris. When the season is over, simply throw the wraps away.
How much maintenance should I be doing on the pneumatic system in our GDX?
We recommend checking over the pneumatic system pretty thoroughly every 3 months.
1. Check solenoids for air leaks. (Debris can cause the valve to stick open; check by listening for air leaks.)
2. Check diaphragm valves for leaks by listening.
3. For an in-line air filter: (a) Drain water from reservoir as needed and as your own operational experience dictates. (b) Inspect the filter element and replace if needed.
How should I prepare my evaporative cooler for winter?
It’s easy. When the temperature reaches below 60F/16C, winterize the evaporative cooler in three steps:
1. Shut it off.
2. Drain down the sump.
3. Inspect for damaged media and replacing any pieces over the winter to be ready for the next season of hot weather.
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