Air Purification Defined

Air quality

As HVAC professionals we are regularly faced with issues related to the quality of indoor air and the adequacy of ventilation, but the HVAC system is not normally the original the source of the problem.
Health hazards related to the inferior quality of indoor air have been documented for centuries, but it wasn't until the late 1970's that these topics were given much attention. Even though the awareness of these hazards has grown tremendously over the last two decades there is still a substantial portion of the population that is surprised to find out of the hazards that exist.

What causes poor indoor air quality? There are two primary reasons. The first is increased dependence on artificial products which contaminate the indoor air. (1) Many of these are VOC’s (Volatile Organic Compounds) (2) The second is the increased focus on energy conservation leading to buildings with "tight" construction, which limits the introduction of air from the outdoors. (3) Why is that a problem? Most of the western population spends about 90% of their time indoors. (4)
It get’s better.

A sick building
The phenomenon of "sick building syndrome" (SBS) is used to describe a situation in which the occupants of a building experience acute health or comfort related effects that seem to be linked directly to the time spent in the building. No specific illness or cause can be identified but a pattern of symptoms (eye, nose, and throat irritation; dry mucous membranes, headaches, dizziness, and other symptoms) that are related to problems with air quality has led to increased emphasis on air quality in building codes and standards. (5)

Sources of Problems with Indoor Air Quality
There are hundreds of potentially harmful substances that can be problematic when they are concentrated in indoor air. The sources of these substances range from occupants (e.g., exhaled carbon dioxide), to construction materials (e.g., formaldehyde in wood products), to biological contaminants (e.g., pollens, mold) and many other potential toxins that can affect overall health.

Some of the criteria to be considered.
• Lack of outdoor air supply caused by tight construction practices
• Use of construction materials that contribute to contamination
• Improperly located air intake and exhaust discharge allowing intake of contaminated air
• Poor pressure balance creating areas of negative pressure that draws in contaminants
• Inadequate or poorly maintained air handling equipment, air cleaners, and filtration equipment.
• Inadequate operation of ventilation equipment

Considerations for HVAC Professionals

Issues with indoor air quality can usually be addressed by a combination of strategies.

o The most important strategy for improving the quality of indoor air is bringing in fresh air from the outdoors to dilute indoor pollutants. Code requirements specify ventilation rates, with unique requirements for particularly sensitive operations such as hospitals or nursing homes.
o A well designed, operated, and maintained ventilation system is key to good indoor air quality. Buildings where this may be an issue include those with limited mechanical ventilation systems and those that reduce the amount of outdoor air brought into the heating or cooling systems.
o Balancing the fresh air intake and the exhaust of indoor air to allow a slight positive pressure in the building is generaly desirable, but the balance must be such that intake of outdoor air is greater than exhaust of indoor air.
o Air cleaners and filters must be regularly inspected and properly maintained to perform properly overtime. It may be necessary to educate the customer/building owner regarding this requirement.

Filtration
Filtration is a helpful strategy to ensuring good indoor air quality. Several problems with indoor air quality have to do with the presence of particles of varying size in the air. These particles can include dust, pollen, fibers, mold spores, and other contaminants. People vary in their sensitivity to particle contamination, but concern has grown about the potential of these contaminants to cause problems with allergies, asthma, and other health problems.

o Air filters extract particles from the airstream. A variety of filters are available on the market today. Many are inexpensive and may not be very efficient. The two most common types of filtration include the following:
(1) Mechanical filters that trap particles in some type of filter medium, such as cotton, fiberglass, or other material
(2) Electronic or electrostatic that attract and hold particles via electrical charge

Mechanical filtration
Filter manufacturers commonly use polyester fiber to make web formations used for mechanical air filtration. Polyester is widely used in commercial, industrial or residential HVAC applications. Polyester is sometimes blended with cotton or other fibers to produce a wide range of performance characteristics. Polypropylene, which has a lower temperature tolerance, is used in some cases to enhance chemical resistance.

o Tiny synthetic fibers, also known as micro-fibers are typically used in many types of HEPA (High Efficiency Particulate Air) filters.
o Many in-duct filters for residential forced air heating and air conditioning systems are made from spun fiberglass fibers. These filters are typically inexpensive, disposable, and come in assorted sizes and densities.
o Lower density filters allow for higher airflow, but they do not remove as much dust. Higher density filters remove more particles but are more expensive and offer more resistance to the air.

A lot of claims are made about the efficiency of air filters, but until recently, it was difficult to evaluate these claims objectively. The American Society of Heating Refrigeration and Air Conditioning Engineers(ASHRAE) and the air filter industry have come up with a standardized rating system, which provides a Minimum Efficiency Reporting Value (MERV) rating foreach type of air filter. (6)

The MERV system rates filters on a scale from 1 to 16, with a rating of 1 indicating the least effective filtration, and a rating of 16 indicating the most effective. Some MERV ratings for typical types of air filters include the following:

o Disposable Fiberglass Media: MERV 1 - MERV 4
o Pleated Media Air Filters (30% ASHRAE): MERV 10 -MERV 11
o Pleated Media Air Filters (65% ASHRAE): MERV 13(note that 65% ASHRAE is about 20% effective on less than 1 micron particles)
o Pleated Media Air Filters (95% ASHRAE): MERV 14
o It is recommended to use a MERV 6 or higher air filter to lower the amounts of pollen, mold and dust that reaches the wet cooling coils in air conditioning systems. Wet coils with lots of pollen and dust can cause mold growth.

HEPA Filters
A High Efficiency Particulate Air (HEPA) filter can remove at least 99.97% of dust, pollen, mold, bacteria and any airborne particles with a size of 0.3 micrometers at 85 liters per minute. The 0.3 diameter specification responds to the most penetrating particle size (MPPS).
It is assumed that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through, but that is an incorrect assumption. Just as in membrane filters, large particles that are as wide as the largest opening or distance between fibers cannot pass through them at all. But HEPA filters are designed to target much smaller pollutants. The particles passing through a HEPA filter are primarily trapped (they stick to a fiber) by one of the following three mechanisms:
o Interception: Particles following a line of flow in the airstream come within one radius of a fiber and adhere to it.
o Impaction: Larger particles are not able to avoid fibers and are forced to embed in one of them directly; this increases with decreasing fiber separation and higher air flow velocity.
o Diffusion: This mechanism results from the collision of the smallest particles with gas molecules, primarily those below 0.1 micron in diameter.

HEPA filters do not have MERV ratings, because they exceed the ASHRAE standards. HEPA filters are the only filters that are tested and certified to meet a specific efficiency at a specific particle size. HEPA filters must meet a minimum efficiency of 99.97% at 0.3 microns. The downside of the high efficiency of HEPA filters is that they can load up with contaminants and block air-flow. They are often used with a prefilter system to trap larger particles before they can reach and clog the HEPA filter. (7)

Electrostatic Air Cleaners

An electrostatic air cleaner, or electrostatic precipitator(ESP) is a particulate collection device that removes particles from a flowing gas, such as air by using the force of an induced electrostatic charge. Electrostatic air cleaners are highly efficient filtration devices that minimally impede the flow of gases through the device, and can easily remove fine particulate matter such as dust and smoke from the airstream.

The most basic electrostatic precipitator, or air cleaner, contains a row of thin wires which is followed by a stack of large flat metal plates. The air stream flows through the spaces between the wires, and it is then passed through the stack of plates. A high voltage power supply is used to transfer the electrons from the plates onto the wires. This process will result in a negative charge of thousands of volts on the wires, compared to the positive charge on the plates. While the particulate matter is carried past the strong negative charge on the wires, it picks up the negative charge and it becomes ionized. The ionized particles are then moved past the positively charged plates. This results in the ionized particles becoming strongly attracted to the positively charged plates. Once the particles are in contact with the positive plate, they give up their electrons and become positively charged like the plate, and thus start acting as part of the collector.

Due to this mechanism, electrostatic precipitators can tolerate large amounts of waste buildup on the collection plates and still operate effectively, since the waste itself helps collect more waste from the airstream.

Precipitator failure usually only occurs once a very heavy buildup of waste material has formed on the plates. The buildup can become heavy enough to block airflow or can become thick enough to bridge across insulating gaps and short out the high voltage power supply. This typically does not damage the power supply, but effectively stops further electrostatic precipitation.

Wet Electrostatic Air Cleaners spray moisture into the incoming air flow which helps collect the extremely fine particles by reducing the electrical resistance of the incoming dry material. A wet electrostatic air cleaner merges the functions of a wet scrubber with an electrostatic precipitator to make a self-washing, self-cleaning device that is also high voltage.
There are a few negative aspects of these highly efficient filters. One is that they are somewhat messy to clean and they produce ozone which some people and pets are sensitive to. However, electrostatic precipitators offer benefits over other air purifications technologies, such as HEPA filtration, which require expensive filters, and can become saturated with many harmful forms of bacteria.

(1) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2873021
(2) https://www.epa.gov/indoor-air-quality-iaq/volatile-organic-compounds-impact-indoor-air-quality
(3) https://www.epa.gov/indoor-air-quality-iaq/fundamentals-indoor-air-quality-buildings
(4) https://www.epa.gov/indoor-air-quality-iaq/inside-story-guide-indoor-air-quality
(5) (5) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2796751/
(6) https://www.nafahq.org/understanding-merv/
(7) https://www.energy.gov/ehss/high-efficiency-particulate-air-hepa-filter-test-facility-ftf