Most people think of air filters as being a lot like nets. A big hole in a net allows air to go through while larger objects such as leaves and twigs are caught in the big holes. The smaller holes in a net catch even larger objects than that. However, as the size of the objects that are caught in a net’s holes decrease, the size of the holes in the net increase. So, in the end, even the smallest objects (such as dust and other small particles in the air) go through the big holes in a net. This is not the case for most air filters.
There are very large gaps between the fibres of a filter. In fact, the gaps are much larger than the size of the particles that a filter is designed to capture. So how do filters capture so much house dust? It is because of three physical mechanisms that act in the filter. The first two are impaction and interception.
Impaction- This is a term used to describe larger particles that do not have enough mass to change direction with the air as it follows around a fibre. The particle continues in a straight line and collides with the end of the fibre. Interception- These are medium size particles that follow the airflow around a fibre very closely. These particles often will only graze the end of the fibre and become embedded in the filter fibre. There is more on HVAC Williamsport PA at //qualityairmechanical.com/hvac.
Diffusion is the method by which the tiniest particles are caught by the air filter. These are the particles less than 0.1 micrometers in size. Because these particles are so light, gas molecules in the air, like oxygen and nitrogen, hit them from all sides. This causes the smallest of particles to travel in a very random path. In fact, this is called Brownian motion. The particles, in travelling in this random path, are more likely to run into a fibre in the filter than not. Therefore, the smallest of particles can actually catch themselves in the filter.
As mentioned previously, how MERV ratings are calculated tests the ability of air filters to measure their performance at a variety of particle sizes. The various ways that particles interact with the fibres in a filter peak at different sizes and thus a good filter will have a high rating in all three categories of interception, impaction, and diffusion.
The accumulation of a season’s worth of house dust within an air filter to reach its “loading” point is typically sooner than most people would anticipate. A climate similar to central Pennsylvania with long furnace operating hours during the cold months of winter means that a quality air filter can fill quickly. HVAC design engineers that service Williamsport PA locations recommend that their customers check their furnace air filters on a regular basis, every 4 to 6 weeks during the heating season. Typically, this would be every 3 months for most households.
