Simply stated, filtration and air cleaning remove unwanted material from an air stream. For HVAC applications, this involves air filtration and, in some cases, air cleaning (for gas and vapor removal). The collection mechanisms for particulate filtration and air-cleaning systems are very different. The following description of the principles governing filtration and air cleaning briefly provides an understanding of the most important factors you should consider when selecting or enhancing your filtration system. A more detailed discussion of air-filtration principles can be found in the National Air Filtration Association’s (NAFA) Guide to Air Filtration [NAFA 2001a] and the ASHRAE Handbook: HVAC Systems and Equipment [ASHRAE 2000].
Particulate Air Filtration
Particulate air filters are classified as either mechanical filters or electrostatic filters (electrostatically enhanced filters). Although there are many important performance differences between the two types of filters, both are fibrous media and used extensively in HVAC systems to remove particles, including biological materials, from the air. A fibrous filter is an assembly of fibers that are randomly laid perpendicular to the airflow (Figure 2). The fibers may range in size from less than 1 µm to greater than 50 µm in diameter. Filter packing density may range from 1% to 30%. Fibers are made from cotton, fiberglass, polyester, polypropylene, or numerous other materials [Davies 1973].
Fibrous filters of different designs are used for various applications. Flat-panel filters contain all of the media in the same plane. This design keeps the filter face velocity and the media velocity roughly the same. When pleated filters are used, additional filter media are added to reduce the air velocity through the filter media. This enables the filter to increase collection efficiency for a given pressure drop. Pleated filters can run the range of efficiencies from a minimum efficiency reporting value (MERV) of 6 up to and including high-efficiency particulate air (HEPA) filters. With pocket filters, air flows through small pockets or bags constructed of the filter media. These filters can consist of a single bag or have multiple pockets, and an increased number of pockets increases the filter media surface area. As in pleated filters, the increased surface area of the pocket filter reduces the velocity of the airflow through the filter media, allowing increased collection efficiency for a given pressure drop. Renewable filters are typically low-efficiency media that are held on rollers. As the filter loads, the media are advanced or indexed, providing the HVAC system with a new filter [Spengler et al. 2000].
Figure 2. Scanning electron microscope image of a polyester-glass fiber filter.
Four different collection mechanisms govern particulate air filter performance: inertial impaction, interception, diffusion, and electrostatic attraction (Figure 3). The first three of these mechanisms apply mainly to mechanical filters and are influenced by particle size.
- Impaction occurs when a particle traveling in the air stream and passing around a fiber, deviates from the air stream (due to particle inertia) and collides with a fiber.
- Interception occurs when a large particle, because of its size, collides with a fiber in the filter that the air stream is passing through.
- Diffusion occurs when the random (Brownian) motion of a particle causes that particle to contact a fiber.
- Electrostatic attraction, the fourth mechanism, plays a very minor role in mechanical filtration. After fiber contact is made, smaller particles are retained on the fibers by a weak electrostatic force.
Figure 3. Four primary filter collection mechanisms.
Impaction and interception are the dominant collection mechanisms for particles greater than 0.2 µm, and diffusion is dominant for particles less than 0.2 µm. The combined effect of these three collection mechanisms results in the classic collection efficiency curve, shown in Figure 4.
Figure 4. Fractional collection efficiency versus particle diameter for a mechanical filter.* The minimum filter efficiency will shift based upon the type of filter and flow velocity. (Note the dip for the most penetrating particle size and dominant collection mechanisms based upon particle size.)
*This figure is adapted from Lee et al. [1980].
Electrostatic filters contain electrostatically enhanced fibers, which actually attract the particles to the fibers, in addition to retaining them. Electrostatic filters rely on charged fibers to dramatically increase collection efficiency for a given pressure drop across the filter.
| Electrostatically enhanced filters are different from electrostatic precipitators, also known as electronic air cleaners. Electrostatic precipitators require power and charged plates to attract and capture particles |
As mechanical filters load with particles over time, their collection efficiency and pressure drop typically increase. Eventually, the increased pressure drop significantly inhibits airflow, and the filters must be replaced. For this reason, pressure drop across mechanical filters is often monitored because it indicates when to replace filters.
Conversely, electrostatic filters, which are composed of polarized fibers, may lose their collection efficiency over time or when exposed to certain chemicals, aerosols, or high relative humidities. Pressure drop in an electrostatic filter generally increases at a slower rate than it does in a mechanical filter of similar efficiency. Thus, unlike the mechanical filter, pressure drop for the electrostatic filter is a poor indicator of the need to change filters. When selecting an HVAC filter, you should keep these differences between mechanical and electrostatic filters in mind because they will have an impact on your filter’s performance (collection efficiency over time), as well as on maintenance requirements (change-out schedules).
Air filters are commonly described and rated based upon their collection efficiency, pressure drop (or airflow resistance), and particulate- holding capacity. Two filter test methods are currently used in the United States:
- American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard 52.1-1992
- ASHRAE Standard 52.2-1999
Standard 52.1-1992 measures arrestance, dust spot efficiency, and dust holding capacity. Arrestance means a filter’s ability to capture a mass fraction of coarse test dust and is suited for describing low and medium-efficiency filters. Be aware that arrestance values may be high, even for low-efficiency filters, and do not adequately indicate the effectiveness of certain filters for CBR protection. Dust spot efficiency measures a filter’s ability to remove large particles, those that tend to soil building interiors. Dust holding capacity is a measure of the total amount of dust a filter is able to hold during a dustloading test.
ASHRAE Standard 52.2-1999 measures particle size efficiency (PSE). This newer standard is a more descriptive test, which quantifies filtration efficiency in different particle size ranges for a clean and incrementally loaded filter to provide a composite efficiency value. It gives a better determination of a filter’s effectiveness to capture solid particulate as opposed to liquid aerosols. The 1999 standard rates particle-size efficiency results as a MERV between 1 and 20. A higher MERV indicates a more efficient filter. In addition, Standard 52.2 provides a table (see Table 1) showing minimum PSE in three size ranges for each of the MERV numbers, 1 through 16. Thus, if you know the size of your contaminant, you can identify an appropriate filter that has the desired PSE for that particular particle size. Figure 5 shows actual test results for a MERV 9 filter and the corresponding filter collection efficiency increase due to loading.
Figure 5. ASHRAE Standard 52.2 test data for a MERV 9 filter showing how collection efficiency increases as the filter loads.
Table 1. Comparison of ASHRAE Standard 52.1 and 52.2
|
ASHRAE 52.2 |
ASHRAE 52.1 |
Particle
size
range, µm |
Applications |
|
|
Particle size range |
Test |
|
MERV |
3 to 10 µm |
1 to 3 µm |
.3 to 1 µm |
Arrestance |
Dust spot |
1 |
<20% |
— |
— |
<65% |
<20% |
>10 |
residential
light
pollen,
dust mites |
2 |
<20% |
— |
— |
65-70% |
<20% |
3 |
<20% |
— |
— |
70-75% |
<20% |
4 |
<20% |
— |
— |
>75% |
<20% |
5 |
20-35% |
— |
— |
80-85% |
<20% |
3.0-10 |
industrial,
dust,
molds,
spores |
6 |
35-50% |
— |
— |
>90% |
<20% |
7 |
50-70% |
— |
— |
>90% |
20-25% |
8 |
>70% |
— |
— |
>95% |
25-30% |
9 |
>85% |
<50% |
— |
>95% |
40-45% |
1.0-3.0 |
industrial,
Legionella,
dust |
10 |
>85% |
50-65% |
— |
>95% |
50-55% |
11 |
>85% |
65-80% |
— |
>98% |
60-65% |
12 |
>90% |
>80% |
— |
>98% |
70-75% |
13 |
>90% |
>90% |
<75% |
>98% |
80-90% |
0.3-1.0 |
hospitals,
smoke
removal,
bacteria |
14 |
>90% |
>90% |
75-85% |
>98% |
90-95% |
15 |
>90% |
>90% |
85-95% |
>98% |
~95% |
16 |
>95% |
>95% |
>95% |
>98% |
>95% |
17 |
— |
— |
≥99.97% |
— |
— |
<0.3 |
clean rooms,
surgery,
chem-bio,
viruses |
18 |
— |
— |
≥99.99% |
— |
— |
19 |
— |
— |
≥99.999% |
— |
— |
20 |
— |
— |
≥99.9999% |
— |
— |
Gas-Phase Air Cleaning
Some HVAC systems may be equipped with sorbent filters, designed to remove pollutant gases and vapors from the building environment. Sorbents use one of two mechanisms for capturing and controlling gas-phase air contaminants—physical adsorption and chemisorption. Both capture mechanisms remove specific types of gas-phase contaminants from indoor air. Unlike particulate filters, sorbents cover a wide range of highly porous materials (Figure 6), varying from simple clays and carbons to complexly engineered polymers. Many sorbents—not including those that are chemically active—can be regenerated by application of heat or other processes.
Figure 6. Scanning electron microscope image of activated carbon pores.
Understanding the precise removal mechanism for gases and vapors is often difficult due to the nature of the adsorbent and the processes involved. While knowledge of adsorption equilibrium helps in understanding vapor protection, sorbent performance depends on such properties as mass transfer, chemical reaction rates, and chemical reaction capacity. A more thorough discussion of gas-phase aircleaning principles is provided in Appendix C of this document. Some of the most important parameters of gas-phase air cleaning include the following:
- BREAKTHROUGH CONCENTRATION: the downstream contaminant concentration, above which the sorbent is considered to be performing inadequately. Breakthrough concentration indicates the agent has broken through the sorbent, which is no longer giving the intended protection. This parameter is a function of loading history, relative humidity, and other factors.
- BREAKTHROUGH TIME: the elapsed time between the initial contact of the toxic agent at a reported challenge concentration on the upstream surface of the sorbent bed, and the breakthrough concentration on the downstream side of the sorbent bed.
- CHALLENGE CONCENTRATION: the airborne concentration of the hazardous agent entering the sorbent.
- RESIDENCE TIME: the length of time that the hazardous agent spends in contact with the sorbent. This term is generally used in the context of superficial residence time, which is calculated on the basis of the adsorbent bed volume and the volumetric flow rate.
- MASS TRANSFER ZONE OR CRITICAL BED DEPTH: interchangeably used terms, which refer to the adsorbent bed depth required to reduce the chemical vapor challenge to the breakthrough concentration. When applied to the challenge chemicals that are removed by chemical reaction, mass transfer is not a precise descriptor, but is often used in that context. The portion of the adsorbent bed not included in the mass transfer zone is often termed the capacity zone.
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