AIR FILTRATION

AIR FILTRATION

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THEORY AND BACKGROUND

Airborne Particle Characteristics

Airborne contaminants pose a constant threat to our environment. Man and nature both foul the air with particulate and gaseous pollutants. Despite efforts to control emission of these pollutants, the air around most large cities still contains billions of particles per cubic foot of air. A great many of these are dangerous to plant and animal life. Clean air is dependent upon a reduction of these particulate levels.

The following information is provided to give a basic understanding of the characteristics of particulate matter.

Particulate Matter

For our purposes , particles are defined as bodies with:

1. Definite physical boundaries in all directions.
2. Diameters ranging from 0.001 micron to 100 microns.
3. Liquid or solid phase material characteristics.

A micron, or micrometer, is a measure of length in the metric system. One micron equals one-millionth (1/1,000,000 or 0.000001) of a meter. In English units one micron equals 1/25,400 inch.

EXAMPLES:

1. 1 inch = 25,400 microns (or 0.000039411 = 1 micron).
2. Eye of needle (1/32 inch) = 749 microns.
3. The dot of an "i" (1/64 inch) = 397 microns.

Particle Visibility

The ability to see an individual particle depends on the eye itself, the intensity and quality of light, the background and the type of particle. The particles seen on furniture or floating in a ray of sunshine are in the range of 50 microns or larger, although 10 micron particles can be seen under favorable conditions. A beam of light is visible due to the light scattering effects produced by a multitude of particles present in the air. In this manual, 10 microns has been chosen as a conservative dividing line between larger. visible and smaller, invisible particles.

Particles smaller than 10 microns are visible under a microscope. Electron microscopes can resolve particles down to 0.001 micron. Particles as small as 0.01 micron are demonstrably removable by an electronic air cleaner although theoretically it will remove particles down to 0.001 micron.

The majority of invisible particles are 3 microns in diameter and smaller. If these smaller particles are present in vast numbers, they are usually visible as a pollutant due to their light scattering quality. For instance, a wisp of cigarette smoke is actually composed of tiny particles (0.01 to 1 micron) which are too small to be seen individually. As soon as the smoke particles disperse, they are no longer visible.

Visible particles (less than 10% of the total airborne particles by count) tend to settle on horizontal surfaces of furniture, floors and shelves where they can be removed by a dust cloth, mop or vacuum cleaner. Invisible particles can deposit on vertical as well as horizontal surfaces. In industry, this contributes to the soil and grime that collects on walls, windows, machinery and clothing, forming potential health hazards as well as maintenance problems.

Particle Weight and Density

In ambient air, 99% of airborne particles by count are less than 1 micron but contribute only 20% of the total particle weight. The remaining weight comes from a rather small number of particles up to 100 microns in size. Industrial processes generate particulate, adding to material already suspended in the air. The "smoke" they generate usually consists of high concentrations of particles less than 10 microns (typically, 60% are less than 2 microns).

Standard filter media can remove particles above 10 microns very effectively. As particle size decreases to 5 microns, 2 microns and on into the sub-micron range, mechanical particulate removal systems become increasingly expensive to operate at high efficiency. It is in this range that the electronic air cleaner performs best, yielding high collection efficiency at a very low expenditure of energy.

Particle Settling Factor

The rate at which particles of the same density (same weight per volume) settle out of the air is an important factor affecting the performance of air cleaning equipment. In a room with an eight foot ceiling, the time necessary for particles to settle out of the air can be dramatic.

Particle Settling Times

Many 10 micron and larger particles settle out of the air before they reach the air cleaner. About 5 to 10% (75 to 90% by weight) settle in the rooms and never reach the air cleaner. Particles less than 1 micron have masses so small that gravity is seemingly neutralized. Their settling velocities are so low that they are easily affected by air movement from hot working machinery and plant circulation systems. These particles are also subject to Brownian motion, i.e., erratic movement of particles in a fluid (in this case, air). Brownian effects become dominant on particles less than 0.3 micron in size, where their random motion keeps them almost indefinitely suspended in the air. By continuously recirculating plant air through an air cleaner, or series of cleaners, a high efficiency of small particle removal can be attained. By capturing the particles at the generation source, an even higher efficiency can be achieved, usually with less air cleaner capacity.

Respirable Fraction

Industrial hygienists are concerned about airborne particulates and their effect on the welfare of workers. The human body is a marvelous filter mechanism, but vulnerable to heavy concentrations of small particles. Some particles are particularly dangerous to the human anatomy since they can become trapped for long periods of time, or even permanently.

Some particle size ranges have been identified as areas of special attention. Particles 10 microns and below fall into the "inhalable fraction" range, i.e., those particles small enough to pass through the body's standard filtration mechanisms and deposit. Particles below about 2.5 microns constitute the "respirable fraction", i.e., that percentage of particles which can be trapped in the human lung or even find their way into the blood stream. Potential chemical reactivity, surface reactivity and immunological effects make these particulates extremely dangerous. Two stage electrostatic precipitators, with peak efficiencies in the inhalable and respirable fraction range, are useful tools in industrial particulate emission control.

Particulate Adhesion and Soiling Effects (Housekeeping)

Particles under 1 micron in size are suspended in the air until they deposit on some type of surface. They deposit on vertical and underside surfaces such as walls and ceilings, as easily as they do on floors. Once deposited, they are imbedded or attached to that surface by molecular adhesion so that manual cleaning is the only way to remove them. Examples of this include oily residue on machinery, discoloration and dirt build-up on walls and light fixtures, and thick layers of powdery residue in welding shops.

The Electronic Air Cleaner

The electronic air cleaner is technically referred to as an electrostatic precipitator. An electrostatic precipitator is a device which ionizes, or charges, then collects, particulate suspended in a gas stream. The term "electrostatic" is used even though the mechanism of removal is dynamic rather than static. This term evolved because of the close relationship between the physical behavior of the device and the general field of static electricity that exists in it. Credit is given to two modem day pioneers, F. G. Cottrell in the United States and Sir Oliver Lodge in Great Britain, for developing the single stage precipitator for particulate removal in industries such as blast furnaces, reverberatory furnaces and power plants. In 1933, Dr. Gaylord Penney developed the two-stage electrostatic precipitator-the type most commonly used today in commercial and industrial process applications.

Theory of Electrostatic Particle Attraction

Common electrical phenomena, such as a comb attracting bits of paper or dry clothes that cling to the body, (occurrences normally ascribed to static electricity), illustrate the attractive force employed in electronic air cleaners. Under normal conditions, particles in the air tend not to exhibit visible attraction to one another because they are electrically "neutral," carrying little, if any, charge.

In the example of the comb and bits of paper the rubbing of the comb changes the electrical balance, or polarity," and causes the comb to attract the paper. In the same manner, an electronic air cleaner actively alters the electrical balance of particles in the air by imparting a high positive charge to those particles. The particles' tendency to be repelled by other positive surfaces and to be attracted by negative surfaces is radically intensified. These phenomena occur because the materials involved have different electrostatic charges, or polarities, with respect to one another. However, none of the above exhibit a visible attraction for any of the others without special treatment. They are usually electrically neutral because each one tends to be balanced electrically and has very little, if any, charge. Rubbing the objects disturbs the electrical balance and creates a slight mutual attraction between them.

How Electronic Air Cleaners Work

A two-stage electrostatic precipitator is constructed in two sections-a charging, or ionizing, section and a collecting section. The charging section contains a series of fine wires suspended between metal plates; the collecting section is a series of parallel, flat metal plates spaced about a quarter inch apart. The entrained particles are first given an electrical charge by the ionizer. They are then collected on plates which have an opposite charge in the collection cell.

The Ionizing Section

When observed in darkness, a pale violet glow appears around the fine wires of the ionizer when it is operating. This is a visible indication of a corona discharge in the air immediately adjacent to the wires. It is in the area of this discharge that the electric charges are produced for the particles to be collected. The intense electrical activity which occurs in this area may be explained as it applies to the charging of particles in an electronic air cleaner. According to electrostatic theory, when a continuous DC voltage is applied to a fine wire suspended between grounded metal plates (a large surface in relation to the wire), a nonuniform electrostatic field is formed in the inter-electrode space (on both sides of the wire between the grounded plates). The field is said to be non-uniform because it is very strong near the wire, decreasing rapidly, as distance from the wire increases, to a relatively low value at the surface of the grounded plates. By increasing the voltage on the wire, field strength is proportionately increased. Eventually, depending upon wire size, wire shape and distance from wire to plate, corona starting conditions are reached and air (gas molecules) near the wire is forced to undergo an electrical change

A molecule is the smallest portion of any substance that can exist and still retain the chemical characteristics of the substance. Each molecule includes protons that carry a positive electrical charge and electrons that carry a negative electrical charge. The negative charges of the electrons are all of the same value and in an electrically neutral molecule their sum equals the sum of the positive charges of the protons in the molecule. But if one or more of the electrons is knocked out of the molecule, for instance by collision with a foreign electron, the molecule is left with a surplus of positive charge and then is called a positive ion. If the positive ion is in an electrostatic field, it will be propelled toward the negative side of the field. The freed electron will be propelled toward the positive side of the field. The propelling force will be proportional to the gradient of field intensity. "Free" electrons (those not attached to atoms) exist everywhere, even in a vacuum. Within the non-uniform electrostatic field in an electronic air cleaner, free electrons are accelerated toward the positively charged wire. The velocity becomes very great as they pass through the increasingly higher fie!d intensity in approaching the wire. On their way, many free electrons strike air molecules and knock other electrons out of them. The dislodged electrons then accelerate toward the positive wire, in turn knocking more electrons free from other molecules. In this way a vast number of positive ions are created and they move rapidly toward the grounded plates. The electrons attracted to the wire tend to neutralize the positive charge on the wire, but are prevented from doing so by the continuously supplied current from a highvoltage power supply. In the process, electrons pass through the wire and the power supply circuit to the negatively charged plates, where they again combine with positive ions and prevent the neutralizing of the negative charge on the grounded plates.

Dense clouds of charged air molecules, or ions, are diffused and accelerated away from the wire (toward the grounded plates) by electrostatic field and molecular forces. A dense cloud of air ions exists in the interelectrode space across the inlet to the electronic air cleaner. These ions attach to particulate and cause the particulate to be removed by the field. The disruption of the molecules in the process of creating the positive ions causes energy to be radiated. Some of this energy is in the visible light spectrum producing a visible corona around the wire.

The Collecting Section

While some collecting occurs in the ionizer of a two-stage electronic air cleaner, most takes place in the separate collecting section, or second stage. The collecting section is comprised of a series of flat metal plates set parallel to the airflow through the air cleaner. Their spacing is a design consideration that varies somewhat from air cleaner to air cleaner, but is usually about a quarter-inch. To make the collector work, a high voltage DC source is applied to every other plate. The alternate plates are grounded so that there is a high voltage difference between plates.

The following examines the collecting process with reference to just one set of two adjacent collecting plates. (This applies to a series of plates as well.) A uniform electric force field is produced between the two plates when a voltage is applied to them, creating a uniform distribution of electrons (negative charge) on the surface of one plate opposite an equal and uniformly distributed deiciency of electrons (positive charge) on the other. The voltage gradient is uniform throughout this field.

A single, positively charged particle entering such a field is acted upon by a force, the sum of all the attracting and repelling forces due to the interaction of the uniformly distributed charges on the plates and the charge on the particle itself. These forces accelerate the particle toward the negative (grounded) plate. Likewise, a negatively charged particle is forced toward the positive plate. As a design consideration, it is important to note that the amount of force on the particle depends on the amount of charge imparted on the particle in the ionizing section, the voltage applied to the cell plates, and the space between the plates. These relationships, along with the velocity of the airstream itself, account for significant differences in efficiencies between various types and brands of electronic air cleaners.

Because of the uniform characteristics, the amount of force on the particle is the same whether the particle is near the negative plate, positive plate or anywhere in between. If no other force is acting on the particle, it is accelerated (constant rate of increase in velocity) in the direction of the negative plate. The force on a small particle can be in excess of 1,000 times the force of gravity.

Other forces also act on this particle as it passes between the collecting plates in an air cleaner. Among them are the resistance of the airstream, the repelling and attracting forces between it and other particles, gravity, inertia and others. Any or all of these forces may affect the movement of the particle toward the collecting plates. Although the actual paths of particles in the collector vary considerably, the component of force (the electrostatic field force) toward the plates is great enough in relation to the downstream component to transport the particles along a nearly diagonal path to the collector plates.

The air cleaner designer uses this information to increase the effectiveness of the collecting section. He might lengthen the plates (in the direction of airflow) so that the particles have more time to migrate to them. He might increase the voltage, decrease the space between plates, decrease velocity or increase the charge on the particles by strengthening the electrostatic field in the charging section.

Other Air Cleaner Components

To maintain maximum efficiency, the air entering the cleaner must be equally distributed across the ionizer and collecting cell. This is accomplished with pre and afterfilters. The prefilter diffuses the air across the ionizer as it enters the air cleaner. It traps larger particles that could short out the active components and allows the precipitator to collect the small to submicron particles to which it is best suited. The afterfilter also aids in equalizing air distribution.

Measuring Efficiency

Finding an accurate test procedure and evaluation method has occupied filter manufacturers for over 20 years. The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) Technical Advisory Committee on Air Cleaning has been establishing test procedures since the early 1930's.

Lack of standardization has resulted in line after line of filtering products which are almost perfect according to the manufacturer's own ratings. From published efficiency ratings, it is often only possible to find differences among filters in the last 1/10 of 1%. In recent years, there has been a more realistic approach to advertised ratings of air cleaning devices due, in part, to the emergence of electronic air cleaners and other new filter products.

Ozone

Ozone is a pungent, colorless, toxic, unstable form of oxygen. Its chemical symbol is O3 It is formed in nature as well as by artificial means. It is usually produced by the discharge of electricity (lightning) in ordinary air or by subjecting air or oxygen to ultraviolet radiation. Normally, it is present in low concentrations wherever there is oxygen. The ozone layer in the stratosphere at an altitude of about 35 to 40 miles has concentrations of 10 to 20 parts of ozone per million (PPM) of air. Normal down drafts and other atmospheric disturbances bring some of this ozone down to the surface of the earth where concentrations seldom exceed a few parts per million.

Ozone generators are devices that produce ozone as a primary function. Electrostatic precipitators, copy machines and arc welders are devices that produce some ozone as a by-product of their intended function.
Ozone can be injurious to health when reaching certain levels. It can have undesirable physiological effects on the central nervous system, heart and vision. The predominant physiological effect is that of irritation to the lungs resulting in pulmonary edema. On the positive side, ozone acts as a deodorizing agent for objectionable odors. It readily oxidizes organic matter and has a variety of uses such as sterilization of water, bleaching and control of fungi in cold storage rooms.

Like many substances, ozone has advantages and disadvantages depending on its intended use and concentrations. Concentrations of ozone, as related to adverse health effects, influence allowable exposure time. High levels of ozone can be tolerated for a short period of time or low levels of ozone for a long period of time.

OSHA (Occupational Safety and Health Administration) has established 0. 1 parts per million (PPM) by volume of air, 0.2 Mg/M3, as the maximum allowable safe concentration of ozone for an 8 hour industrial exposure.

Ozone, being a form of oxygen, mixes with and decomposes in air. Decomposition is more rapid in higher humidity. The amount of ozone, therefore, that will be in the air depends upon the rate of generation versus the rate of decomposition. Another factor affecting the amount of ozone is the dilution of the air. The above statement applies only to an airtight room. The average building today completely exchanges the air 1 to 4 times an hour depending on the insulation, tightness of construction and make-up air systems.

Independent laboratory tests show that some air cleaner generate ozone at an average rate of 0.8 parts per 100 million (PPHM), or 0.008 parts per million (PPM) or less, a figure well below allowable concentrations.