Air Filtration and Ultraviolet Light Treatment
We take in over 20,000 breaths per day, and in those breaths inhale over 350,000,000 measurable particles. 40,000,000 people in the U.S. suffer from allergies and asthma. Millions are spent on treatment, and asthma cases are up over 50 percent in the past ten years. Over 30,000,000 school days were lost due to allergy related illnesses. As a result, homeowners are looking for answers to their individual indoor air quality problems. Air filtration and ultraviolet light treatment systems address these issues by collecting air particles and killing bacteria that may become trapped in the evaporator coils of an HVAC system.
Types of Air Filtration
Residential whole house electronic air cleaners
These air cleaners are sold and installed by trained HVAC technicians. Most are put in the return or cold-air section of a heating and air conditioning system; some are installed in a central return grille. As the central heating or air conditioning fan operates air is circulated through the ductwork and electronic air cleaner. The electronic air cleaner uses two stage electrostatic precipitation technologies to clean the air: ionization and then collection. These systems can clean airborne particles down to .01 micron in size.
Electronic Air Cleaner Appliance Units
Table Top and Console models are also available through retailers and direct marketers which use the same type of two stage electrostatic precipitation technology. These appliance units are simply plugged into a 120 volt outlet and depend on a self-contained fan package that circulates air and cleans air in a smaller area.
Residential Media Air Filters
Central in duct residential media air cleaners are installed by heating and air conditioning technicians. These use a simple dense material to mechanically filter air as it flows through a house. Media is replaced as needed; normally once or twice annually, and effectively filter particles down to one micron in size.
Media Appliance Units
Table Top and Console models also are available through retailers and direct marketers which use a paper type of filter. These appliance units are simply plugged into a 120 volt outlet and depend on a self-contained fan package that circulates air and cleans air in a smaller area.
Residential Panel Type Filters
Typically either one or two inches in depth; panel type filters are normally installed on the return air side of the residential air handling system, either in a technician-supplied rack or in a return air grille. This type of filter is designed for basic system protection and only captures the largest of airborne particles. Available through many sources including retail, these filters are effective on particles down to five microns in size.
Indoor air can be five times more contaminated than outside air. There are many different types of filtration devices for diverse physical applications, user needs, and serviceability.
Electronic air cleaning technology was developed in the 1950s and refined over the past 60 years. Electronic air cleaners and hybrid air cleaners use electrostatic precipitation to charge and collect airborne particles down to .01 micron in size.
Media air cleaners use a filter or membrane to clean the air down to one micron in size (basically visible dust). As the filter accumulates dirty particles it becomes more efficient but also more resistant to the air flow. Most media air cleaners’ filters or membranes are replaced, while others are washable.
Your heating and air conditioning professional can best determine the type and location of your filter. In addition to elevated health benefits of cleaner air, there is also the potential for energy savings. A cleaner indoor coil, heat exchanger and fan package will help maintain the engineered efficiency of the residential heating and air conditioning system.
Costs are determined by the type of air filter that best meets your needs. A tabletop portable unit can cost as little as $20; a sophisticated central hybrid system may be as much as $1,500 including installation. Portable units’ costs are determined by not only the efficiency but also the area of coverage. Installed system costs are based on the price of the unit and the installation and ductwork transition necessary to maximize the performance.
Installation and Maintenance
The placement of UV lights in the HVAC system is generally in either the return air duct near the unit, or preferably, mounted near the evaporator coil and drain pan.
The dark, damp nature of the typical evaporator coil is conducive to the growth of bacteria, fungi and mold. During periods when the cooling system is not operating, the warm air provides an additional stimulus for growth. Because these contaminants thrive in these conditions, they will multiply and spread throughout the evaporator coil, causing degradation in capacity, efficiency, and air flow. Therefore, the preferred location for UV lights is at the coil and drain pan. Evaporator coils containing fungi and mold could cause a musty smell throughout the building, while bacteria growth commonly results in the “dirty socks” smell.
It is recommended that UV lights are always “on,” and not cycled with the fan or the HVAC system. The most opportune time for growth of fungi and mold is when the air is still.
In applications with deep coils already exhibiting biological growth, the addition of UV lights will be effective. By first killing the organisms on the outside of the coil, they are removed by condensation, allowing the UV light to penetrate even further into the structure of the coil, continuing to clean it deeper and deeper.
Using UV light at the evaporator coil and drain pan should be made with consideration to the effects of UV light on certain man-made materials, such as plastics used in wiring insulation. Some manufacturers use UV-resistant materials. Although UV resistant materials may exhibit some fading or chalking, the properties of the material remain sound. If non-UV resistant materials are used and exposed to the light, they should be shielded using foil tape, sheet metal, or conduit. Although filters are usually not in a location where they are exposed to UV light when the bulbs are positioned at the evaporator coil, non-fiberglass media filters would be damaged by UV light.
When the UV lights are placed in the return air duct, the germicidal effectiveness is limited. Efficacy of UV lights for a specific biological contaminant is a function of lamp output, intensity (based on distance) and the UV “dosage” required to kill that organism. In a “fly by” application (where the UV light is placed in the return air duct) and the offending organisms are travelling at several hundred feet per minute, the required exposure time for germicidal effectiveness may be too long to achieve the desired kill rate. Another disadvantage of positioning the UV light in the return air duct would be the exposure of the filter to UV light, possibly causing the filter material to prematurely degrade.
Typical Maintenance Considerations
UV bulbs are generally maintenance-free, although periodic replacement is required. Over time, bulbs will lose intensity, resulting in a lower germicidal capability. Many UV bulb manufacturers recommend annual replacement. Refer to the manufacturer’s replacement guideline.
During service of the unit, or replacement of the bulbs, the power must be disconnected. Exposure to UV light can cause serious temporary eye and skin irritation. Never expose unprotected eyes or skin to the UV light. Bulbs should not be handled while “on” or recently turned off, as they are hot and can cause burns. Allow sufficient time for the bulb to cool before handling or performing service in the unit near the bulbs.
When replacing bulbs, use gloves to avoid transferring oils from fingers, which can result in etching of the glass in the lamp, weakening the structure. Rubbing alcohol and a clean cloth can be used to clean the bulb if necessary, only after the bulb has been turned off and thoroughly cooled.
In applications where the UV light accessory is placed in the return air duct upstream of the filter, dust may accumulate on the bulb. If so, the bulb can be cleaned using rubbing alcohol and a clean cloth as described above.
UV bulbs contain a small amount of mercury. If a lamp breaks during installation or replacement, the area should be cleaned thoroughly, and proper disposal procedures should be used. Undamaged lamps, when taken out of service, should be recycled like normal fluorescent bulbs.
Many furnaces, fan coils, rooftop units, and commercial air handlers are designed with an internal slot or rack for the system’s filter. Residential high efficiency filters are often designed and installed as an external accessory to the furnace or fan coil. Specific installation instructions for various filter systems, whether internal or external to the unit, are provided by the HVAC unit’s manufacturer; however the following guidelines should be considered:
- Higher efficiency is achievable by sealing the filter housing and filter fit within the housing.
- The face area of the filter should be at a right angle to the airflow. Dead air spaces should be avoided. Air should be distributed evenly over the entire face of the filter. If necessary, use turning vanes, baffles, or air blenders.
- The filter should be selected with ample capacity for both dust load and air flow.
- The filter system should be selected to meet the needs of the operating conditions, including the degree of cleanliness required, dust volume in the system or environment, maximum pressure drop, air flow, temperature, humidity, and maintenance.
- Where filters are mounted in housings external to the unit, duct connections to and from the filter should change size gradually to ensure the entire filter face is subject to the airflow.
- Sufficient service space upstream and downstream of the filter will facilitate inspection, cleaning, and replacement. Service access for filter removal is also required.
- Sheet metal ducts should be as airtight as possible, especially on the supply side, downstream of the filter.
Safety ordinances should be considered during selection of the filtration system. Some local regulations may prohibit the use of combustible filtering media in some applications. Filters that are capable of collecting bioaerosols may require special handling.
Return Filter Grilles
In some HVAC system installations, where large central returns are used, and where the unit is not accessible to facilitate an easy filter change, return grilles designed to accommodate filters can be utilized. The return air filter grille can be installed in a wall or in the ceiling, providing easy access to change the filter.
Typical Maintenance Considerations
Although there is no reliable method for precisely calculating filter life, the following factors can be useful in making a reasonable determination:
Presence of lint in the filtered air
In flat panel filters, large amounts of captured lint will “surface load” the filter, reducing its life. On an extended surface/pleated filter, the presence of lint may extend the filter life by providing an increased bed depth.
Variations in particle size and concentration
The volume of airborne particles generated by interior dust-producing activities, and from outdoor air has an impact on filter life. Although smaller particles may constitute a larger percentage of the number of the total particles in a given volume of air, the smaller particles will load the filter slower than larger particles. Further, the larger particles have more mass and will tend to stay airborne for longer periods in turbulent air than in quiet air. Therefore, moving more air for longer periods will increase the likelihood of capturing more of the larger particles, having a significant impact on filter life.
Although it is understandable that changing the total volume of air being filtered will have an impact on the life of the filter, the air flow rate also plays a role. Operating a filter with lower-than-rated airflow decreases the face velocity, decreasing the impingement process. While this would extend the life of the filter, it would result in fewer captured particles. In contrast, when airflow is increased, the impingement process will be enhanced, resulting in a shorter life as more particles would be captured, loading the filter.
Reliable filter operation can be affected by high temperatures, vibration, corrosion, wetting, and excessive air velocity and filter resistance. These factors may cause either gradual or immediate filter failure; therefore, it is important to understand the operating conditions of the filter so that the proper system can be specified and operating conditions maintained through inspection and service.
In addition to the cost of the filters, there are other costs associated with filter cleaning or replacement. For replaceable filters, maintenance costs include labor to order and bring replacements to the system, labor to change the filter, inspection of frames and gaskets, and removal/disposal costs.
Where roll filter media is used, once the entire roll has been exhausted, it must be removed and then disposed of, along with replacing the roll onto the empty spool.
Electronic air cleaners may have removable collection cells for cleaning, or may have wash-in-place elements. The cost of labor and cleaning solutions should be considered.
Regardless of the type of filter used, a regular schedule should be established to inspect, clean or replace filters.
Regardless of the filter type, a certain amount of energy is required to overcome the filter’s resistance to airflow (pressure drop). With more emphasis on energy efficiency in today’s homes and commercial buildings, operating costs for the HVAC system, including blower watts, is a major consideration. In general, the lower the efficiency of the filter, the lower the pressure drop, resulting in low or negligible blower watts required to move the air through the filter. Higher efficiency filters tend to have higher pressure drops, requiring more energy to overcome the resistance through the filter, having a greater impact on energy costs in the form of watts used by the blower motor.
To determine the amount of energy required to overcome the filter’s pressure drop, first calculate the required horsepower using the following formula:
Hp = (CFM x Static Pressure of the filter) / 6358
Then convert horsepower to theoretical kilowatts using the following formula:
Hp = Kilowatts = 0.001173 (CFM)(SP)
Then, to convert theoretical kilowatts to actual kilowatts (if, for example, fan efficiency is 68 percent, drive efficiency is 99 percent and motor efficiency is 86 percent):
Actual kilowatts = Theoretical kilowatts x 1 / (.67 x .99 x .86)
Finally, multiply actual kilowatts, by the number of run hours to get kilowatt hours (kWh).
(Source: NAFA Guide to Air Filtration, Third Edition)
A similar formula to determine energy use required by the system to overcome the filter’s pressure drop is:
P = power consumed by system in kW
Q = air flow, m3/s
Em, Ed, Eb = fractional efficiency of motor, drive and blower at Q
R = filter resistance, Pa
(Source: The HVAC Handbook)
Note that the formulas above do not factor in energy input for electronic or powered filters.
Common Abbreviations and Definitions
µm : micrometer or micron, one-millionth of a meter
Aerosols: Solid and liquid airborne particles, typically ranging in size from 0.001 to 100 µm.
air cleaning: Removal of gases or vapors from the air.
air filtration: Removal of aerosol contaminants from the air.
airborne contaminants: Gases, vapors, or aerosols
arrestance: Ability of a filter to capture a mass fraction of coarse test dust.
ASHRAE: American Society of Heating, Refrigerating, and Air-Conditioning Engineers
ASTM: American Society for Testing and Materials
Cfm: cubic feet per minute
challenge concentration: Airborne concentration of the hazardous agent entering the sorbent.
chemisorption: Sorbent capture mechanism dependent on chemically active medium (involves electron transfer).
collection efficiency: Fraction of entering particles that are retained by the filter (based on particle count or mass).
composite efficiency value: Descriptive rating value for a clean filter to incrementally load different particle sizes.
diffusion: Particle colliding with a fiber due to random (Brownian) motion.
dust spot efficiency: Measurement of a filter’s ability to remove large particles (the staining portion of atmospheric dust).
dust holding capacity: Measurement of the total amount of dust a filter is able to hold during a dust-loading test.
electrostatic attraction: Small particles attracted to fibers, and after being contacted, retained there by a weak electrostatic force.
electrostatic filter: A filter that uses electrostatically enhanced fibers to attract and retain particles.
EPA: Environmental Protection Agency
filter bypass: Airflow around a filter or through an unintended path.
filter face velocity: Air stream velocity just prior to entering the filter.
filter performance: A description of a filter’s collection efficiency, pressure drop, and dust-holding capacity over time.
Fpm: feet per minute
gas: Formless fluids which tend to occupy an entire space uniformly at ordinary temperatures.
gas-phase filter: Composed of sorbent medium, e.g., natural zeolite, aluminaactivated carbon, specialty carbons, synthetic zeolite, polymers.
HEPA: high-efficiency particulate air
impaction: Particle colliding with a fiber due to particle inertia.
interception: Particle colliding with a fiber due to particle size.
large particle: Particles greater than 1 micrometer in diameter.
life-cycle cost: Sum of all filter costs from initial investment to disposal and replacement, including energy and maintenance costs.
mechanical filter collection mechanism: Governs particulate air filter performance.
m/s: meters per second
m2: square meters
m3/min: cubic meters per minute
MERV: minimum efficiency reporting value
Nm: nanometers, one-billionth of a meter
OSHA: Occupational Safety and Health Administration
particulate filter: Collects aerosols only—mechanically or electrostatically.
fibrous: Assembly of fibers randomly laid perpendicular to airflow.
high-efficiency: Primarily used to collect particles <1 micrometer.
low-efficiency: Primarily used to collect particles >1 micrometer.
mechanical: Cotton, fiberglass, polyester, polypropylene, or numerous other fiber materials that collect particles.
polarized: Contains electrostatically enhanced fibers. particulate filter design: Flat-panel filter, pleated filter, pocket filter, renewable filter (see Particulate Air Filtration).
Ppm: parts per million
pressure drop: The difference in static pressure measured at two location in a ventilation system. A measure of airflow resistance through a filter.
PSE: particle size efficiency
sorbent: Porous medium that collects gases and vapors only.
vapor: The gaseous form of substances that are normally solid or liquid at ambient temperatures.
> Standards and Guidelines: ANSI/AHRI Standard 680, Performance Rating of Residential Air Filter Equipment