Hybrid Filters

(The Effect of Dust Loading on the Fractional Efficiency of a Charged-Fiber Filter)

Hybrid filters incorporate two or more of the filter control technologies discussed above. One such approach uses one or more types of mechanical filters combined with an electrostatic precipitator or an ion generator in an integrated system or single self-contained device.
An example of a hybrid filter is the "electret" media filter which uses permanently charged media fabricated into either flat panel filters or extended media filters. The media filter, made from synthetic fibers, is inherently charged in the manufacturing process and retains a charge which attracts airborne particles that are trapped and retained within the fibers in the conventional methods of impingement and diffusion of other dry-type filters.However, this being a media filter, it presents resistance to airflow which increases as the filter becomes soiled. The filter must, therefore, be replaced periodically. The advantages of an "electret" filter are the filter's relatively low energy cost and their high efficiency when clean. The disadvantages are high maintenance costs due to frequent need to replace filters and efficiency that drops with use. Figure 6 demonstrates the decreased efficiency of an "electret" or charged fiber filter with increased use and dust loading.
Figure 6The Effect of Dust Loading on the Fractional Efficiency of a Charged-Fiber Filter
Source: Hanley JT, Ensor DS, Sparks LE,. Aerosol filtration efficiency of in-duct air cleaners
Ionizing charged-media type filters also exist. In this type of electronic air cleaner, dust in the air is initially charged and then collected on a charged-media filter. Several versions of this type of filter exist. They operate by charging the particles in the air with negative or alternating negative and positive charges, which enhances their deposition in conventional extended-media high-efficiency filters. Theoretically, the ions flow into the occupied space, causing particles to become charged and are then drawn back to the central air handler where they are collected. The claimed advantages of such systems is that they enhance the performance of the particulate filters, reduce particulate counts in the occupied space, and reduce the housekeeping costs of particle soiling in the space. The disadvantage of the technology is that it lacks definitive performance documentation and represents very high initial equipment cost over and above the cost of conventional high-efficiency filters.
Another category of hybrid filters, although not yet available commercially, is electrostatically enhanced filters. In this type of interaction, an electric field is actively superimposed on fibrous, media-based air filters. The principle underlying this technology is electrostatic precipitation superimposed on other capture mechanisms such as impaction, sedimentation, or diffusion. Under experimental conditions, this technology generally leads to increased filtration efficiency, relative to media-based filters alone, especially under low-flow velocity conditions. Experimental data have been obtained for different pollutants such as latex aerosols, dioctylphthalate (DOP) smoke, and two different kinds of laboratory generated dust (Kao, et al., 1987).

High-Efficiency Particulate Air Filters

High-efficiency particulate air (HEPA) filters, formerly called high-efficiency particulate arrestors, are a further extension of extended-surface media filters. HEPA filters were originally developed during World War II to prevent discharge of radioactive particles from nuclear reactor facility exhausts. They have since become a vital technology in industrial, medical, and military clean rooms and have grown in popularity for use in portable residential air cleaners.
A HEPA filter has been traditionally defined as an extended-surface dry-type filter having a minimum particle removal efficiency of 99.97% for all particles of 0.3 micron diameter with higher efficiency for both larger and smaller particles. This rating is determined using a test challenge smoke that consists of particles of 0.3 micron average diameter. To qualify as a "true" HEPA, the filter must allow no more than 3 particles out of 10,000 to penetrate the filtration media. The filtering media of a HEPA filter is made of submicronic glass fibers in a thickness and texture very similar to blotter paper. More recently, filters made in the same physical style using less efficient filter paper are being referred to as HEPA filters or "HEPA-type" filters. Their actual efficiency may be 55% or less at 0.3 microns. While still very good filters when compared to conventional panel type and even extended-media pocket filters, these versions of the original HEPA filter have higher airflow, lower efficiency, and lower cost than their original version. The true HEPA has very high pressure drop performance and both versions require prefiltration for maximum life cycle. Also, HEPA filters are generally not applied to residential HVAC systems due to their size and horsepower requirements. A disadvantage of HEPA filters is that the need for a powerful fan leads to increased energy costs compared to less efficient filtration systems, and replacement filters are generally quite expensive (retail prices range from $50 to $100, depending on size). The major advantages of the original HEPA filters, however, include high efficiency, which actually increases with use, and a long maintenance-free life cycle of up to five years when used with a prefilter. Nelson, et al. (1988) state that: "Because the designation of a filter as HEPA ensures a high degree of efficiency, it should be sought if a mechanical filter is to be used." Additionally, the 1990 review of indoor air pollutants and environmental controls published by the American Thoracic Society (1990) concludes that: "High-efficiency particulate filters (HEPA) are highly efficient in removing particles of a wide range of size. A room-size unit will control particles in that room, and a central unit will remove particles from the air of the building when the ventilation system is operating."

Pleated Filters

(Fractional Efficiency of Several Air Cleaners Over the 0.3 -- 10 µm Diameter Size Range)

One of the most effective ways to increase the particle collection efficiency of mechanical filters is to increase the filter media density using small denier fibers. This causes smaller media penetrations and increases the screening or straining mesh size. However, any increase in filter density significantly increases resistance to airflow, causing decreased airflow through the filter. The most effective approach to overcoming this problem is to extend the surface area by pleating the filter medium. This lowers the airflow velocity through the filter and decreases overall resistance to airflow such that pressure drop is reduced. Additionally, pleating of filter media increases the total area available for filtration and, thus, extends the useful life of the filter. The efficiency of extended-surface (pleated) media filters is much higher than for other dry-type filters. For example, Figure 5 demonstrates the efficiency of a pleated paper filter over the 0.3 - 10 micron diameter size-range compared to a typical flat furnace filter.

Flat Filters

( Fractional Efficiency of Several Air Cleaners
over the 0.01 --10µm Diameter Size Range)

Flat or panel filters usually contain a low packing density fibrous medium that can be dry or coated with a viscous substance such as oil to increase particle adhesion. Dry-type filter media may consist of open-cell foams, non-woven textile cloths, paper-like mats of glass or cellulose fibers, wood fill, animal hair or synthetic fibers. They may also consist of slit and expanded aluminum. Media filters of various materials are available in a wide range of sizes and thicknesses. The typical, low-efficiency furnace filter in many residential HVAC systems is a flat filter, one-half-inch to one-inch thick, that is efficient in collecting large particles, but removes a negligible percentage of smaller, respirable-size particles. Figure 4 demonstrates how the efficiency of the typical furnace filter over the 0.01 - 10 micron diameter size-range compares to other types of air cleaners which are described.

Types of Available Air Cleaners

Air cleaning devices are manufactured by many companies in the United States and they vary widely in design, methods of operation, cost and level of efficiency. Air cleaners can either be incorporated into the central heating, ventilation, and air-conditioning (HVAC) system or moved from room to room as portable units such as small table top units or larger portable room consoles.
Table top Units
Until recently, small, inexpensive, table top appliance-type units have typically been quite popular in terms of unit sales (Clean Air Device Manufacturers Association [CADM], spoken communication). They generally contain small panels of dry, loosely packed, low-density fiber filters upstream of a high-velocity fan. Table top units may also consist of a fan and an electronic or other type of filter. Small table top units generally have limited airflow and inefficient panel filters. Most reviews have shown these table top units to be relatively ineffective (Consumer Reports, 1992 and Fox, 1994). The combination of low filter efficiency and low airflow in these units causes them to provide essentially no cleaning when assessed for impact on the air of the entire room (Nelson, et al., 1988).
Performance tests of 12 table top units were reported by Rodale's New Shelter magazine (1982). Tests were conducted in a 1200 cu.ft. room in which cigarette smoke was mechanically generated. Nine of 12 units reduced smoke levels by less than 24%, compared to a 17% decline that occurred by settling in the absence of the air cleaners. The researchers concluded that effectiveness of table top units in removing smoke particles was marginal at best and differed little from using no device at all. Similar results were reported by Offerman, et al. (1985).
Consumer Reports magazine (1992) also tested nine table top units and found that, because they can move only small amounts of air, they suffice only for a very small room or a portion of a room. Two of the table top units tested were ozone generators. Both of these units were judged "not acceptable" because they produced harmful levels of ozone and did not have automatic controls to limit ozone output.
Room Units
Another major type of residential air cleaner is the larger, yet portable, devices designed to clean the air in a specific size room. Due to their larger and more effective filters or collecting plates, these larger portable room air cleaners are considerably more effective in cleaning the air in a room than the table top units (oonsumer Reports, 1992; Fox, 1994) and have become increasingly popular in the past several years, exceeding the table top units in consumer sales (CADM, spoken communication). Room-size air cleaners are generally utilized when continuous, localized air cleaning is necessary (Consumer Reports, 1992 and Fox, 1994). Most units may be moved from room to room to reduce pollutant concentration levels as needed. Similarly to the table top units, room units also incorporate a variety of air cleaning technologies.
Rodale's New Shelter (1982) reported significantly more effective performances for 15 larger HEPA and/or electronic portable air cleaners. The time required to reduce smoke levels by 95% varied from 26 to 120 minutes.
Offerman, et al. (1985) tested a variety of portable air cleaners using tobacco smoke as a source of particles. The test protocol involved turning cleaners on for a three-to-five hour period following a period of decay and mixing followed by a 6-8 hour period of natural decay. The highest efficiency based on clean air delivery rate (CADR) was observed in the room unit utilizing a HEPA filter.
Room units tested by Consumer Reports (1992) all moved more air than table top units. The highest efficiency for smoke and dust removal was observed in a room unit utilizing electrostatic precipitation.
Portable room air cleaners are much more effective in rooms where all doors and windows are closed (AHAM).
Central Filtration Systems
Air cleaning systems can also be installed in the central heating or air-conditioning systems of a residence or in an HVAC system. These units are commonly referred to as "in-duct" units, although this term is actually a misnomer since they are not located in the distribution ductwork, but rather in unducted return air grilles or ducted return air plenums. The term "central filtration system" will be used in this document. This type of unit provides building-wide air cleaning and, by continuously recirculating building air through the unit, can potentially clean the air throughout the entire air handling system, duct work and rooms. However, with these types of units, the HVAC fan must be in constant operation for air cleaning to occur since the airborne contaminants must be captured and carried back to the centralized filter for capture and retention. Thus, central filtration systems must be operated with the fan "on" for constant air movement through the HVAC system. Generally, residential HVAC systems operate only on intermittent fan to maintain a comfortable indoor temperature.

Air quality in Singapore

Previously, we have touched about air quality as a general topic. Now, we shall focus on the air quality of Singapore, how readings are taken, how we can as citizens improve the air quality in Singpapore, so that we will benefit the people living in Singapore.

How air quality in Singapore is monitored.

The ambient air quality in Singapore is routinely monitored by the National Environment Agency (NEA) though the Telemetri Air quality monitoring and Management System (TAQMMS) . The system comprises remote air monitoring stations linked to a Central Control Station (CSS) via dial-up telephone lines.

The monitoring stations monitor both ambient and roadside air quality. Automatic analysis and equipment are deployed at the monitoring stations to measure the concentrations of major air pollutants such as sulphur dioxide, oxides of Nitrogen, Carbon monoxide, Ozone and respirable suspended particles.

Reporting of the air quality in regions of Singapore namely the North, South, East, West and Central regions of Singapore is done using PSI , Pollutant Standard Index.

he PSI is a measure of the air quality in Singapore and is an index developed by the United States Environmental Protection Agency (USEPA) to provide accurate, timely and easily understandable information about daily levels of air pollution. In Singapore, it is determined by taking the highest concentration of the five key air pollutants viz. PM10 (particulate matter of 10 microns or smaller in size), sulphur dioxide, carbon monoxide, nitrogen dioxide and ozone monitored at NEA's 11 ambient air monitoring stations located strategically in different parts of Singapore.

The PSI value gives an indication of the air quality as shown:

PSI Value	PSI Descriptor
0 - 50	Good
51 - 100	Moderate
101 - 200	Unhealthy
201 - 300	Very Unhealthy
Above 300	Hazardous

Currently, the overall PSI reading is posted daily at 4pm on the NEA website at www.nea.gov.sg/psi/.

Take an example for today :

<24-hr PSI at 4pm, 17 June 2008

<

---

Region 24-hr Sub-Index at 4pm, 17 June 2008 PSI* Air Quality Descriptor Responsible Pollutant Sulphur Dioxide PM10 Ozone Carbon Monoxide Nitrogen Dioxide+ North 8 38 10 8 - 38 Good PM10 South 8 32 14 7 - 32 Good PM10 East 8 34 9 5 - 34 Good PM10 West 24 34 14 8 - 34 Good PM10 Central 5 35 7 7 - 35 Good PM10 Overall Singapore* 24 38 14 8 - 38 Good PM10

The major sources of air pollution in Singapore.

The main sources of air pollution in Singapore are from the burning of fossil fuel for heat generation in industries, electricity generation and transportation.

The sources of air pollution can be grouped into three categories as follows:

• stationary sources such as power stations, oil refineries and industries;
• mobile sources such as motor vehicles; and
• others such as open burning of waste materials and transboundary air pollution

The main air pollutants are sulphur dioxide, oxides of nitrogen, carbon monoxide, ozone, lead, hydrocarbons and particulates.

How to improve the air quality in Singapore.

Other than using air cleaning devices. All Singaporeans can do our part like taking public transport instead of taking private cars. However since another major source of air pollution is the burning of fossil fuels, we can cut down the amount of pollutants emitted into the air by saving electricity and going green. The small action of turning the appliance off and not leave it in stand-by mode can go a long way into making sure that the air quality in Singapore remains good and in healthy level.

Why air cleaning devices are incapable of removing gas pollutants/ odour?

Case Study - Activated Carbon

Sometimes, relatively small quantities of activated carbon will reduce odors in a residence to imperceptible levels. However, because many chemicals produce health effects at levels below those where odors are perceived, removal of odors alone is not an indicator of a healthful environment.

Tests of gaseous pollutant removal by activated carbon have generally been performed using only high concentrations of pollutants, so little information is available on the effectiveness of carbon in removing chemicals present at the low (part per billion, or ppb) concentrations normally found in indoor air.

The ability of carbon to reemit pollutants it has trapped from indoor air is also of concern. The National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS), is currently developing a standard method to be used in evaluating the effectiveness of media used for gaseous pollutant removal. They have reported the results of a study using activated carbon, in which the concentration of toluene in the air flowing into the carbon was varied during the test (from 150 to 0 to 340 to 26 to 0 ppm). The experiment simulates the variations in pollutant levels which would be expected in indoor air situations. They found that toluene initially absorbed by the media was slowly reemitted each time the pollutant level entering the media dropped. The amount of toluene emitted by the media during the 45-hour experiment was approximately equal to that adsorbed.