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Hybrid Filters

2008-06-21T10:32:00.002+08:00

(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

2008-06-21T10:30:00.000+08:00

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

2008-06-21T10:29:00.002+08:00

(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

2008-06-21T10:22:00.003+08:00

( 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

2008-06-21T10:20:00.000+08:00

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

2008-06-17T16:56:00.002+08:00

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
Region	Sulphur Dioxide	PM10	Ozone	Carbon Monoxide	Nitrogen Dioxide⁺	PSI*	Air Quality Descriptor	Responsible Pollutant
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?

2008-05-05T06:17:00.003+08:00

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.

Airborne Pollutants

2008-04-21T22:12:00.001+08:00

In order to remove airborne pollutants from your home, you must first educate yourself on which pollutants cause the most harm, and how these make their way inside to where they can affect you. Some of the most common, and most harmful are listed below:
· Environmental Tobacco Smoke (ETS), commonly referred to as Secondhand Smoke
· Mold, Bacteria, Animal Dander, Pollen
· Dust
· Combustion Products, such as Carbon Monoxide and Nitrogen Dioxide
· Cleaning Agents
· Beauty Products
· Hobby Products
Secondhand smoke has been proven to contribute to cancer, respiratory infections, and Asthma. According to the American Lung Association, "Secondhand smoke is responsible for between 150,000 and 300,000 lower respiratory tract infections in infants and children under 18 months of age, resulting in between 7,500 and 15,000 hospitalizations each year, and causes 1,900 to 2,700 sudden infant death syndrome (SIDS) deaths in the United States annually." 1
In recent years, action has been taken in some states to eliminate Secondhand Smoke in public buildings. For example, Delaware, New York, Massachusetts, and Rhode Island prohibit smoking in all private workplaces, restaurants, and bars. Connecticut, Vermont, and Maine prohibit smoking in all restaurants and bars. 2 However, this is not the case in all areas, and these smoking bans do not apply to private homes. If there are smokers in your home, this will affect everyone else in the household. It is strongly advisable to eliminate this source of indoor air pollution.
Biological air pollutants, such as mold, bacteria, animal dander, and pollen can also find their way into your home. Mold and mildew can grow in areas with too much moisture buildup. If rooms in your home have higher than 50% humidity, you are at risk. Animal dander is present in your home if you keep indoor pets, such as cats or dogs. The dander is created from the saliva these animals use to clean themselves. Indoor pets are also likely to shed, and airborne fur, hair, and related particles can cause allergic reactions in some people.
Combustion products, such as Carbon Monoxide and Nitrogen Dioxide are colorless and odorless, but pose a health risk if they are allowed to build up in the air. Carbon Monoxide and Nitrogen Dioxide are produced by any activity in the home that involves combustion, such as the use of a gas stove.
Any household cleaning agents, beauty products, or hobby products containing spray chemicals will wind up polluting your indoor air. The effect this has on what you breathe is intensified when these products are used in confined, inadequately ventilated areas in your home.
For more information on the potential pollutants that exist in your home, take the American Lung Association's Clean Air Assessment.

Air Devices Used in Hospital

2008-04-21T22:08:00.000+08:00

An air cleaning device has been developed, with which a room in a general hospital ward can be converted into a biological clean room. Measurements inside the unit showed that the particle concentration (particle diameter .GEQ.0.5.MU.m) was reduced from 240,000/ft'3' to 100/ft'3' after operation. However, when an air conditioner was used simultaneously, it decreased to only 3,000/ft'3', so two layers of a medium efficiency particulate filter were installed over the exit of the air conditioner, which improved the particle concentration to 1,000/ft'3'. No airborne microbes were detected inside the unit. This result shows that the device is effective for the prevention of infection during the myelo-suppressive period of chemotherapy. A major advantage is that it can be easily installed without any construction.

Pollutant Destruction

2008-04-03T18:42:00.000+08:00

Some air cleaners use ultraviolet (UV) light technology intended to destroy pollutants in indoor air. These air cleaners are called ultraviolet germicidal irradiation (UVGI) cleaners and photocatalytic oxidation (PCO) cleaners. Ozone generators that are sold as air cleaners intentionally produce ozone gas, a lung irritant, to destroy pollutants.
UVGI cleaners use ultraviolet radiation from UV lamps that may destroy biological pollutants such as viruses, bacteria, allergens, and molds that are airborne or growing on HVAC surfaces (e.g., found on cooling coils, drain pans, or ductwork). If used, they should be applied with, but not as a replacement for, filtration systems.
PCO cleaners use a UV lamp along with a substance, called a catalyst, that reacts with the light. They are intended to destroy gaseous pollutants by converting them into harmless products, but are not designed to remove particulate pollutants.
Ozone generators use UV light or an electrical discharge to intentionally produce ozone. Ozone is a lung irritant that can cause adverse health effects. At concentrations that do not exceed public health standards, ozone has little effect in removing most indoor air contaminants. Thus, ozone generators are not always safe and effective in controlling indoor air pollutants. Consumers should instead use methods proven to be both safe and effective to reduce pollutant concentrations, which include eliminating or controlling pollutant sources and increasing outdoor air ventilation.

Negative effects of air cleaning devices

2008-04-03T01:13:00.001+08:00

Ozone production

Some air cleaners (called ozone generators) use an electrical charge to generate ozone. Although ozone (also referred to as trivalent oxygen or saturated oxygen) is a necessary part of the upper atmosphere (10-30 miles above us), in the part of the atmosphere we breathe, ozone is a potent lung irritant. It can have damaging health effects, especially for persons with asthma and other lung diseases, children and the elderly. It is produced directly by ozone generators and indirectly by ion generators and some other electronic air cleaners. This will be further discussed.

Diagrams

2008-04-03T00:49:00.002+08:00

Figure 1.1 Shows an example of an electronic cleaning device

Mechanical filters

2008-04-03T00:21:00.003+08:00

Mechanical filters used for air cleaning can actually be further classified into two major types.

Flat or panel filters generally consist either of a low packing density of coarse glass fibers, animal hair, vegetable fibers, or synthetic fibers often coated with a viscous substance (e.g., oil) to act as an adhesive for particulate material, or slit and expanded aluminum. (A flat filter in use in many homes is the typical furnace filter installed in central heating and/or air-conditioning systems.) Flat filters may efficiently collect large particles, but remove only a small percentage of respirable size particles.

Flat filters may also be made of "electret" media, consisting of a permanently-charged plastic film or fiber. Particles in the air are attracted to the charged material.
Pleated or extended surface filters generally attain greater efficiency for capture of respirable size particles than flat filters. Their greater surface area allows the use of smaller fibers and an increase in packing density of the filter without a large drop in air flow rate.

Some newer systems on the market are referred to as "hybrid" devices. They contain two or more of the particle removal devices discussed below. For example, one or more types of mechanical filters may be combined with an electrostatic precipitator or an ion generator.

Gaseous Pollutant Removal

2008-03-30T08:58:00.001+08:00

Gas-phase air filters remove gases and odors by using a material called a sorbent, such as activated carbon, which adsorbs the pollutants. These filters are typically intended to remove one or more gaseous pollutants from the airstream that passes through them. Because gas-phase filters are specific to one or a limited number of gaseous pollutants, they will not reduce concentrations of pollutants for which they were not designed. Some air cleaning devices with gas-phase filters may remove a portion of the gaseous pollutants and some of the related hazards, at least on a temporary basis. However, none are expected to remove all of the gaseous pollutants present in the air of a typical home. For example, carbon monoxide is a dangerous gaseous pollutant that is produced whenever any fuel such as gas, oil, kerosene, wood, or charcoal is burned, and it is not readily captured using currently available residential gas-phase filtration products.

Particle Removal

2008-03-21T19:06:00.001+08:00

Two types of air cleaning devices can remove particles from the air -- mechanical air filters and electronic air cleaners. Mechanical air filters remove particles by capturing them on filter materials. High efficiency particulate air (HEPA) filters are in this category. Electronic air cleaners such as electrostatic precipitators use a process called electrostatic attraction to trap charged particles. They draw air through an ionization section where particles obtain an electrical charge. The charged particles then accumulate on a series of flat plates called a collector that is oppositely charged. Ion generators, or ionizers, disperse charged ions into the air, similar to the electronic air cleaners but without a collector. These ions attach to airborne particles, giving them a charge so that they attach to nearby surfaces such as walls or furniture, or attach to one another and settle faster.

Air cleaning devices can help to improve indoor environment

2008-03-14T08:26:00.001+08:00

Indoor air pollution is among the top five environmental health risks. The best way to address this risk is to control or eliminate the sources of pollutants, and to ventilate a home with clean outdoor air. The ventilation method may, however, be limited by weather conditions or undesirable levels of contaminants in outdoor air. If these measures are insufficient, an air cleaning device may be useful. While air cleaning devices may help to control the levels of airborne allergens, particles, or, in some cases, gaseous pollutants in a home, they may not decrease adverse health effects from indoor air pollutants.

(Reference:http://www.epa.gov/iaq/pubs/airclean.html)

2008-02-28T23:32:00.000+08:00

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