Indoor air quality (IAQ) is very important for people’s comfort & health. Most people spend over 90% of their time indoors. Air acts as the vehicle for the movement of contaminants, many of which are potentially harmful to people’s health. The removal of them is very important on medical, social and economic grounds. Poor IAQ also adds to increased housekeeping and machine wear costs.
Poor IAQ can contribute to the development of chronic respiratory diseases as well as causing headaches, dry eyes, nasal congestion, nausea & fatigue. Apart from natural contaminants such as earth, sand, vegetation, mould spores, bacteria and pollens, many of the contaminants in the air today, particularly around industrial areas and in cities, arise from man made activities. Therefore the quality of the outdoor air should be a consideration of the level of filtration required inside a building.
The HVAC system requirement is to supply an air flow of sufficient volume, temperature & cleanliness.
Many HVAC system failures are caused by dust & dirt. Regularly changed, clean filtration is vital to having the system work as efficiently as possible. By trapping and removing the dust particles from the airflow, it prevents accumulation elsewhere. Up to 60% of an average facility’s energy use comes from the HVAC system, clean filtration therefore contributes toward both energy savings and corresponding reductions in greenhouse gas emissions.
The size of contaminants & particles are usually described in microns. One micron equates to 1/25,000th of an inch or 1/1,000th of a mm. To give a perspective, a human hair has a diameter of about 75 microns. The smallest particle seen by the naked eye is about 10 microns. Particles of about 10 micron+ tend to settle whilst smaller particles tend to remain suspended and soil surfaces they come into contact with.
Outdoor air may contain 5 million suspended particles per cubic foot.
A typical office building air contains about 500,000+ particles of 0.5 microns & larger per cubic foot of air.
A class 1000 cleanroom is designed to limit to 1000 particles of 0.3 microns per cubic foot of air.
G1-G4 (EU1-EU4) filters are measured by Arrestance. This is a filter’s ability to capture dust & describes how well a filter removes larger particles; it’s also called the gravimetric test. The dust holding capacity is the amount, by weight, a filter will hold without exceeding its final resistance. Since larger particles make up most of the weight of the sample dust, a filter can remove a fairly large percentage of these but have little effect of capturing smaller particles in the sample. The air velocity also affects dust holding capacity; a lower velocity generally gives a higher dust holding capacity.
F5-F9 (EU5-EU9) filters are measured by the dust spot efficiency. This describes a filter’s ability to remove larger particles from standard ambient air. The ambient air is passed through the test filter with special filters upstream & downstream, with a light meter used to measure the discolouration. It is based on the effects of air pollution soiling. The difficulty is maintaining a uniformity of ‘ambient atmospheric air’ so over many test runs an average efficiency rating is given.
H13 (EU13) filter media is tested using hot DOP vapour which condenses at a uniform 0.3 microns and is injected into the airstream in front of the test filter, with upstream & downstream particle concentrations measured by light scattering instrumentation. Cold smoke DOP testing is also carried out on the finished HEPA filter to scan the filter for pinhole leaks.