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Glossary of Terms Environmental Impacts Solid Waste Management Wastewater Management Air Pollution Regulatory Factors Energy Factors Farming Environmental Management US EPA Dioxin Reassessment Dioxin Measurement Units Baghouses vs Precipitators NIMBY Peer Review Desidarata Deteriorata |
Glossary of Termsby John W. Norton, P.E., B.C.E.E.
In any field, it is important to understand the terms of expression used therein. This list of terms should help. If there are other terms not found here which you would have found useful, please suggest them in the feedback feature. This list of terms, below, is not in alphabetical order. The definition of many of the terms lead naturally into other terms. Where this is so, I could not resist leaving them in that order. For ease of looking terms up, the list to the right has been alphabetized. You can return to this list from many places throughout the definitions below by the link TO LIST "top". If you find errors in this list of terms, please send me some feedback so that they may be corrected, or addressed. Solid Waste Mostly the solid debris from households, industries and construction. It does contain some liquids, some gases, and some infectious materials from house bound ill persons. A disposal technique resulting in burial of the waste using an engineered method intended to protect the environment, typically employing plastic liners and drains on the bottom to collect liquids and covers on the "top" to keep rain water out and to keep methane and other gases from escaping. See also "Landfill." The juice from garbage, usually, but not always, in a landfill. May be caused by water washing through and dissolving things or may be part of the original liquids that were in the garbage. Not In My Back Yard. In other words, an attitude that says: "Not near my house!" Test Similar to the EP Tox Test referred to elswhere in this glossary. The Acronym stands for Toxic Characteristics Leaching Procedure. The flat surface on which the arriving trash trucks dump their loads of solid waste at an incinerator. A place, usually a building, where smaller route trucks unload their garbage so that it can be transferred into large, over-the-road semi-tractor trailer trucks or special trains for long trips to the landfill or other disposal point. Integrated Solid Waste Management A management system that uses several techniques such as composting, recycling, incineration, landfilling, and education to dispose of and/or minimize solid waste. The fee charged to dump at an incinerator, transfer station, or landfill. Unwanted appliances, such as washing machines, stoves, etc., very often white. It also includes other such waste metals as fencing, swing sets, buckets, etc. Extraction Procedure Toxicity Test. The test required by USEPA for the testing of waste materials to determine if they are "Hazardous." It was meant to simulate the acid conditions and leaching that would naturally occur in a sanitary landfill. It is generally agreed that the test, as presently configured, does not accurately simulate a sanitary landfill; it is far too aggressive (too acid). The watery bio-solids removed from wastewater during treatment. Does not include the solids that were screened out of the wastewater as it came into the wastewater plant. Sludge from municipal wastewater treatment plants (with good industrial oversight programs to minimize metals) is very good for farm soil. Sludge from industrial water treatment may, or may not, be good for farm soil. Horticulturists would need to make that determination individually because these industrial sludges can be very different. A device which uses fabric to remove particles from flue gas. See also "Bag House." The exhaust gases from a combustion process contained for a time in the flue. Solid particles that literally fly out of the combustion process. Often called "particulate." Wastes defined as "hazardous" by law. Most such waste is only "hazardous" if mis-managed. Such terms can lead to public concern or complacence if not used with care. Regular municipal solid waste can be a "hazard" to health and the environment if left unmanaged. Solid waste from doctor's offices and hospitals. Most states have legal definitions which need to be considered. Such definitions exclude as well as include, so the result is not always as expected. For example, all municipalities have some ill and homebound people which renders all municipal solid waste potentially infectious. Further, most legal definitions of infectious waste exclude the "small quantity generator", ie small doctor's offices [even the offices of large doctors, perhaps], and thus the waste from such offices is mixed in with the regular municipal solid waste. All municipal solid waste is potentially infectious. Always has been. Waste not managed. A type of incineration that requires virtually no pre-processing. The mixed municipal refuse is burned as is -- generally everything that is "no larger than a bread box". Particulate Tiny pieces (particles) which are suspended in the "smoke" or exhaust from any process. See "fly ash." Products of Incomplete Combustion, ie carbon monoxide (CO2) and other compounds. Primary Organic Constituent. A term usually applied to Hazardous Waste combustion situations, not a municipal waste concern. Solid waste has many different densities, all valid. Loose, in a Dumpster container for example, it weighs 100 - 250 lbs. per cubic yard. Squashed into a modern route truck it weighs from 400 lbs. to 800 lbs. per cubic yard. Crushed and compacted into a modern landfill it can weigh more than 1000 lbs per cubic yard. Incinerator ashes in an ash monofill weigh approximately 1 ton per cubic yard. Lime, Hydrated Lime, and Quick Lime These are forms of Calcium bearing minerals which are often used to clean up various environmental situations. The terms are sometimes used carelessly for the different forms. Natural Limestone is mostly calcium carbonate and is often used in powdered form to neutralize acids in water, soils or air. Limestone powder is plowed into some fields by farmers and it is also used to make cement. The different forms listed here have differing amounts of material other than Calcium such as water molecules and carbon. The Hydrated lime is more aggressive than plain limestone, and the Quick lime is very aggressive and more difficult to handle safely. Polychlorinated dibenzodioxins (PCDDs), a class of several chlorinated organic compounds, some of which are quite toxic to laboratory mice and rats, though not necessarily to humans. One should note that incinerators are only a small source of ambient dioxin. At least half of ambient air dioxin comes from natural sources according to Dr. G. W. Gribble, a professor of chemistry at Dartmouth College, in a thoroughly documented paper published in 1994 in the Journal of Environmental Science & Technology. Only about 10% comes from manufacturing processes and incineration. This definition references several things and must be taken in context such as: the co-disposal of incinerator ash with mixed municipal raw garbage in a landfill, or the incineration of sewage sludge in incinerators designed for the burning of municipal solid waste, or the composting of grass and lawn organics with sewage sludge. A natural chemical process which rapidly breaks down organic materials [largely composed of Carbon (C), Oxygen (O), and Hydrogen (H)] into more stable compounds [largely carbon dioxide CO2 and water vapor H2O] while releasing heat energy from the break down of the organic compound. Additional Oxygen is required for combustion. For example: methane (CH4) plus Oxygen (O) yields CO2 and H2O plus heat energy in the home furnace. The largely incombustible residue left after incineration. In American Municipal Incineration, the ash residue is approximately 85% glass, rock, and crockery, and approximately 13 -15% iron and steel. In American Municipal Incineration, very little of this "ash" material resembles typical household fireplace ash. An earthworks designed for the disposal of solid waste. As of the 1990s modern landfills in America were required to have liners to prevent liquids from contaminating the groundwater and liners to prevent landfill gas from escaping from the biologically active trash buried therein. Such liners are made up of several layers of clay, sand (for drainage), and plastic blankets. USEPA has mandated in the 90s that most water from rainfall (as much as possible) be kept out of the landfill in order to minimize the danger to the groundwater posed by leaching liquids escaping out the bottom. USEPA has also required that all landfills have gas extraction systems installed and that the gas be at least flared to minimize its environmental impacts. Wastes in landfills continue to break down from natural biological activity giving off gas which contains mostly carbon dioxide and methane, although organic acid gas is very obviously present. Buildings located on the perimeter of old landfills often give clear evidence of acid gas attack. Steel doors and door frames show corrosion very quickly in many cases. Occupants of such buildings sometimes feel the effects of such acid gases each summer as the ground warms up and biological activity increases. This author has witnessed such conditions. In Dayton, Ohio, the EPA tested and documented at the Cardington Road Landfill that most known organic compounds are emitted in some quantity from a closed landfill. These particular tests were done at least 10 years after the landfill was closed. Many of the organic gases detected in these landfill gas emissions are known carcinogens. USEPA has required as of the 90s that all new landfills and some recently closed landfills have gas extraction systems installed and that the gas be processed (at least flared) to minimize its environmental impacts. There is, however, no monitoring required for this uncontrolled flaring which has many products of incomplete combustion in its exhaust. Recycling is the recovery for sale of materials from the waste stream. Typical materials to recover include plastics, paper, glass, steel, and aluminum. Recycling work costs money, however, and the value of the recovered material should justify the effort of recovery. The recovered materials from municipal solid waste are typically dirtied in some manner from food, paints, or other wastes. Imperfect separation causes their value to be less than for new raw materials in many cases. Recycling plastic beverage containers into new beverage containers, for example, has proved to be quite difficult due to Food and Drug Administration rules for cleanliness and the weakened material strength in the plastic itself. Curb side recycling is the process of separately collecting fairly clean materials from the residential curb side in concert with collection of the regular mixed trash. All recycled material requires reprocessing in some fashion such as repulping used paper to make new, or remelting plastic to make new. While often thought by the general public to be environmentally clean trash disposal, recycling has its own emissions: extra truck (often diesel) exhaust, hot water requirements, wastewater from cleaning, exhaust emissions from melting and smelting. Some recycling has been around for many years because it has been economically justifiable: junk yards in every community are recycling steel and auto parts. Dayton, Ohio, has had three large firms recycling office paper and cardboard for over 30 years. Unfortunately, such recycling efforts are often unwelcome because of their ragged appearance; the margins of profit are not all that large. The controlled organic digestion of materials such as grass clippings, food wastes, and lawn debris. Composting can be used to manage the disposal of a large part of the American waste stream; grass and lawn debris in 1990 constituted 20% of American solid waste. Composting can be done in the residential yard in conjunction with gardening, or it can be done with separate collection and a large central processing facility. If the product is to be sold, it needs to be very uniform and "pure" with no bits of plastic or metals to cause appearance problems. In Holland, for example, municipal waste compost was used widely on farm fields where it seemed to be doing well from a crop production standpoint. Unfortunately, now those fields that have received several annual applications of such compost are beginning to look like fields of plastic debris. I have reviewed many composting processes and found the key to clean compost to be keeping the plastic out in the first place; it cannot be completely removed once it is in the compost. The composting process has its own emissions, mostly carbon dioxide and water vapor (like organic combustion), but also a plethora of organic gases and acids which, to be fair, need further study. The daily quantity of municipal solid waste and trash. The control of this stream of material is the key to financing disposal facilities. Many states are now receiving a stream of waste from other states which do not have adequate disposal facilities within their own borders. Waste stream control, however, has been found to be a violation of US Constitutional guarantees against "Unfair Restraint of Free Trade." The public is generally unaware that the US Constitution provides for such control, however, under Rules to be established by the US Congress. Congress has failed to establish such rules because landfill and trucking lobbys have been so effective. Ironically, in 1992 the Congress failed to institute such rules by one vote from Virginia, which is now inundated with the waste stream coming from New York and New Jersey. In 1992, Virginia thought it had 100 years of landfill space available. Now it appears to have less than 20 years of space because of the stream from out of state. The laws and ordinances used to direct Solid Waste to a certain facility for disposal. Unlike a wastewater plant which is tied to its intended customers with piping, a solid waste facility has no physical control to direct the "stream" of waste into the processing plant once it is constructed. Thus, in order to be financed, solid waste plants need to have other guarantees that they will receive the solid waste they are intended to process. Recent US Supreme Court Rulings, however, have found local and state flow control ordinances to be in conflict with the free trade provisions of the US Constitution. Only when rules are adopted by the US Congress to address this matter, may some form of flow control become legal once again. Gases which exhaust from power plants, other combustion processes, and landfills contain substantial amounts of various chemical compounds which go into the atmosphere and combine with natural moisture to form various acids which fall as acid rain. New York rainfall appears to have a higher level of acidity when it lands on the earth than rain which falls downwind of the industrial Midwest. In many northeastern American states, such as New York, the natural alkalinity of the ground is not sufficient to neutralize acid rain water. Here in Ohio, however, natural limestone based soil is sufficient to neutralize such acid rainfall. In New York State, the lakes are actually becoming more and more acidified. Some are so acidified that aquatic life has ceased to exist. There is substantial disagreement among the technical community about the true source of this acid rain. Some experts believe that it results from natural carbon dioxide gases which escape from many sources (including people); others claim it is largely from power plants. Carbon dioxide, chlorine, sulfur dioxide, and fluorine are all gases which can form acids. Sulfur dioxide, which is emitted in large quantity from many power plants burning coal or oil, is thought to be the worst culprit. Fortunately, hardly any sulfur dioxide originates during the municipal solid waste incineration process. Carbon dioxide, chlorine and fluorine are the acid producing gases which emits in detectable quantities from the incineration process. Due to the ability of these gases to combine quickly with water, a wet scrubber can provide neutralization for much of the acid in such gases. The addition of lime, ammonia, or caustic to the conditioning chamber or scrubber water can neutralize the acid contained therein. Lime slurry can be injected into a hot gas stream to neutralize acid gases. Neutralization of much of the acid before it reaches air pollution control devices can help protect the devices from corrosion problems. The small particles which make smoke visable. All gases are clear in small quantities like an exhaust gas would contain. If smoke is visible, it is due to particulate only. Physical or chemical evidence given off to the environment by a process as a result of its activity. Any process gives off emissions. People give off heat and carbon dioxide as a result of being alive. People also reject various liquids and solids after they have gleaned the useful content for sustaining their bodily functions. Automobiles give off exhaust gases that contain additional carbon dioxide as a result of the internal combustion in their engines. Automobiles also give off carbon monoxide and other acid and organic gases as a result of their combustion process. Diesel engines give off many such gases including dioxin. Incinerators give off somewhat similar exhaust gases, although modern incinerators emit only very very small amounts of such bad gases as carbon monoxide, organics, and dioxins due to their very complete combustion processes. The two most important emissions from automobiles, diesels, and incinerators are acid gases and particulates; both of these are reduced to nearly undetectable levels from incinerators by the emission control devices included with a modern facility. These devices include bag houses, electrostatic precipitators, and scrubbers. The phrase used to describe the Environmental Protection Agency Policy with regard to multiple stacks on a single facility. The policy recognizes that the aggregate emission from the stacks is more important than the specific emission from each individual stack. Therefore, a facility may choose to "super clean" some of its stacks while leaving some others a little "dirty", so that the average emission from all stacks will fall within acceptable emission standards. In air or groundwater; a feather (plume) shaped emission of gases or water. The term is usually used to refer to a pollutant in air or ground water. A gas washing device which operates at a relative low pressure drop--between 4 inches and 14 inches of water column. Scrubbers work by causing the pollution laden gases to zigzag sharply through a wetted, washed labyrinth. The solid particles (smoke), which are heavier than the gas which carries them, tend to travel in a straight line, thereby impacting and sticking to the wetted surface. Such scrubbers are also effective in removing gaseous pollutants such as chlorine and fluorine. Gas removal is accomplished by simple surface contact and chemical reaction. The water in scrubbers is typically recycled to minimize the cost for fresh water and sewage disposal. The recycled water is processed to remove the captured pollutants such as smoke particles and acid. A gas washing device which employs Venturi passages to produce severe impact between smoke particles and wash water usually requires a substantially higher energy input--greater than 14 inches of water column--but can effect the removal of much smaller particles called "particulates." Particles greater than one micron in size can be removed rather effectively with a simple Zigzag device such as a packed tower, but to remove sub-micron sized particulate, a Venturi type scrubber must be employed. As the gas stream and injected water pass through the Venturi neck, they are compressed, sped to a high velocity, decompressed suddenly, and slowed down suddenly. This quick series of actions causes the tiny particulates to gather condensed moisture, gain weight, and impact the water droplets. The scrubber water can then be processed to concentrate the particulates for removal and neutralize acids. Scrubbers are not usually used with municipal incineration systems, today. Most are in industrialized countries where the level of affluence is adequate to justify the use of electrostatic precipitators or baghouses, either of which may also have some acid gas neutralization included in the process. A low energy scrubber employing Zigzag devices usually tumbled randomly into the reaction vessel. Water is sprayed continuously over the "top" of the tumbled packing while the dirty gas is passed in the opposite direction for cleaning. The packing is typically fist-sized plastic or ceramic devices designed to maximize the washable surfaces in order to optimize the chance for collisions with particulates and their removal by washing. Vessels which produce a whirlpool separation effect in passing gases. Large particles can be separated from a gas or liquid stream with such a device. Large, crude cyclones are used outside sawmills and lumber yards to remove saw dust from woodworking machines. Multiple cyclones are frequently employed on coal fired plants to remove the rather uniform and heavy coal, fly ash. Devices which use the same principal as a home vacuum cleaner filter bag. Typically it is a rectangular vessel with a number of tube-shaped filter bags through which the gas stream is pushed or pulled. Baghouses are quite effective on particles which are greater than one micron in size. Sub-micron particulate can quite easily plug (or blind) filter bags. When used on sub-micron particulate, some provision has to be made to prevent plugging. A typical method is to entrain larger "target particulate" in the gas stream to both attract and agglomerate the sub-micron particulate, and to bridge the passages in the filter bag to prevent plugging. One negative inherent in the operation of a baghouse is its energy consumption; for similar cleaning limitations, a baghouse will typically require a pressure drop between ten and 25 times higher than an electrostatic precipitator. A measure of heat; specifically the amount of heat required to raise the temperature of 1 pound of water 1oF. Approximately 1000 BTU's are needed to make 1 pound of steam vapor. American trash, with all of its paper and plastic, with very little food waste, has about 5000 BTUs per pound. Coal has about 11,000 BTUs per pound. By burning American trash, it is possible to make large amount of energy. The trash in other counties is often loaded with high moisture from food waste from home processing of raw food. High moisture makes for poor fuel. Refuse Derived Fuel is manufactured from garbage by shredding and sorting to select material with fairly consistent fuel characteristics: size, weight, and high BTU content. The process was also meant to recover recyclable materials, but the materials usually prove to be so contaminated with food, paint, other materials, etc. that recovery was too expensive to justify. The main value in most modern trash is in its energy content. A large, refractory lined, steel tube which can rotate and tumble hot material, often heated inside by open flame. Used to calcine cement in a cement mill, dry grains for processing, or burn garbage which has a relatively high moisture content. Special masonry, clay or cement like material designed to resist heat damage, used to line and protect furnaces, incinerators, rotary kilns, etc. May be in brick form or castible slurry, or semi-solid clay like material which is pounded into place (known as "plastic" refractory.) This material protects the steel shell from the intense heat in the same way a fireplace protects the wooden walls of a house from the fire. Containers specially designed to hold a fire intended to stabilize the waste content and reduce its volume. Incinerators are lined with refractory which protects the walls from the intense heat housed therein. Incinerators must have a way to get the waste inside and a way to feed air or oxygen to the fire inside. There also has to be a way to get the ashes out of the fire box. The incinerator fire can sterilize and stabilize the materials being burned by literally killing the bad germs and chemically reducing the material to a more chemically and biologically stable condition. For example, baby diapers are thoroughly destroyed in a modern incinerator where all the fluids are boiled off and sterilized by the intense heat and the solids (including the diaper) are converted from carbon compounds to carbon dioxide and water vapor. Before being incinerated, the dirty diapers have obvious problems such as odors, germs, and bulk. In a modern incinerator, plastics make excellent fuel with low ash remains (none in the case of most popular plastics) and perfectly complete products of combustion in the exhaust gases (carbon dioxide and water vapor.) Popular belief has the opposite, but wrong, opinion. Where did such an extremely opposite view originate? From house fire prevention safety education which correctly states that smoke from poorly burned plastics in a house fire can kill people instantly. Plastics burning in a house fire cause smoke which rises slightly and can be avoided by laying low to the floor, while escaping. How can the smoke from plastics in a house fire be so different from smoke from an incinerator? The incinerator fire has perfect combustion conditions, while the house fire has very poor conditions. In a house fire the plastic is melted and the organic compounds are broken down into partially complete products of combustion, toxic smoke. If combustion conditions are bad enough (too cool, lacking air, wet fuel), the house fire will finally go out, saving the house. In modern incineration, the combustion conditions are optimized: enough air, heat and time are allowed for the plastic fuel to completely oxidize; it goes from a carbon bearing organic compound (plastic) to clean carbon dioxide and water vapor, no black or toxic smoke. Many waste combustion situations generate significant quantities of energy that can be captured with a boiler and put to good use. A typical pound of American municipal waste contains from 4500 to 6000 BTUs of energy. Thus, a typical ton of American refuse contains about 10 million BTUs, or enough to heat 100 homes for an hour during the coldest day of the year in Northern Michigan. Most modern incinerators include an energy recovery system as part of the air pollution control device, since the hot gases have to be cooled before they can be cleaned up. Many full scale attempts have been made throughout America to recover the valuable materials in the daily flow of solid waste. Every time any waste handling and processing scheme has been tried, it has proved to be true that 95% of the value contained in the trash is to be gleaned from the energy content, not the materials such as paper and plastics which are usually too dirty to be economically reprocessed for use. A heat exchanger which in its simplest form raises the heat content of water (frequently above the boiling point) while at the same time cooling a gas stream which is passing through the device. It accomplishes this cooling through the action of evaporation and contact cooling in the same fashion as a prequench chamber or a conditioning tower. However, in the case of a boiler, the steam is kept separate from the gas stream which is cooled. This allows some additional benefits such as the sale of steam and the reduction in volume of the gas stream which exits the boiler unit. Boilers can also be used to boil other fluids, such as Freon or cool other heat bearing streams. The term "waste heat boiler" is used simply to mean that the boiler is installed on a gas stream containing heat energy which would be wasted if it were not captured by the boiler and transferred to steam. Boilers are used in most modern municipal incinerators to lower the temperature of the exhaust gas stream before it goes on to the electrostatic precipitators or baghouses for cleaning. A device (the turbine) which uses the kinetic energy of steam to turn a shaft which is coupled to an electric generator which makes electricity for sale or use in a plant. Open flames used to burn off and destroy combustible gases. Occasionally, fuel is added to insure complete combustion of semi-flammable gases. These can often be seen at landfills burning off the gases which the landfills continually generate. A construction project in which the selected contractor performs all aspects from start to finish; including details of the design. It's finished when the contractor calls in the buyer, turns it on ("with the key") and hands the "key" over. The manager of a construction project and all its aspects. The general contractor subcontracts many of the items of the project to different contractors who specialize in that type of work: roofing, concrete, steel erection, machinery placement, etc. A device which produces a sharp drop in gas temperature by the injection of an appropriate amount of water (or other fluid.) The temperature drop results from a combination of evaporative cooling and surface to surface heat transmission. Evaporative cooling accounts for most of the cooling of the gas above 212oF, and surface to surface contact cooling accounts for most of the temperature drop below 212oF. A prequench chamber is commonly used in front of another piece of air pollution equipment which requires a lowered inlet gas temperature. Many processes use a prequench chamber to drop gas temperatures from high (perhaps 1800oF) down to lower temperatures (perhaps 150oF) before the gas enters a low energy wet scrubber for cleaning. Prequench chambers employ relatively small vessels and are used when crude control of temperature is acceptable. Temperature feed-back loops are seldom used with prequench chambers. To accomplish the desired temperature drop, water feeding valves and nozzles are manually set---usually at a rate which exceeds the amount actually required. Conditioning Tower (or Chamber ) Similar to a Prequench Chamber except that the vessel is usually much larger (5 to 10 times) and the temperature within the chamber is controlled to a much finer degree, usually with a feedback control loop which monitors the gas exit temperature. In this type unit, the valves and nozzles which inject the water are controlled by a temperature sensing device at the exit of the vessel. In some cases, many valves are manually set and fine tuning is done with temperature control feedback to only a few nozzles. In addition to temperature conditioning of a gas stream, conditioning towers can include chemical injections which can produce desired results with regard to pH control, or other chemical concerns. Many incinerators use a conditioning tower to lower the gas temperature from approximately 1800oF down to 500oF or 550oF, the temperature required for an electrostatic precipitator installation, or baghouse. The temperature is adjusted in this type unit by the action of evaporation or surface contact cooling in the same fashion as a prequench chamber. Moisture content (relative humidity) can also be adjusted in a conditioning chamber. Certain humidity levels are beneficial to electrostatic precipitator operation. Process Control Feedback Loops In some processes, measurements are made of various physical qualities of a passing stream such as temperature, pH, humidity, etc. and the measurement is used as a basis for adjusting process devices such as valves, nozzles, etc. This feedback of information can be either manually or automatically used to control processes. Gas conditioning towers typically measure the exit temperature and use that measurement to automatically adjust valves which control the flow of water to the injection nozzles. A device which employs the concept that quickly moving streams can carry more, larger, particles than slow moving streams. A wide spot in the processing train which produces a slow down of the passing gas; thereby allowing the heavier particulates to fall out. The process of encouraging very small particles to join with each other to make larger particles. Chemical additives can be added to the gas stream to encourage agglomeration, although some agglomeration usually occurs naturally as a gas is allowed to stabilize. Pressure and decompression can also be used to assist this process. Encouraging impact among particles will assist in the agglomeration process also. A machine used to move gases, such as air. Some fans push with pressure, while some pull by creating a vacuum. Fans are grouped by type. Axial fans are the airplane propeller type, similar to the portable home room cooling fans with a propeller. By adding a shroud around the propeller, the air can be directed as desired. The second most common fan is the squirrel cage type which operates like a centrifugal pump. Most home furnaces use squirrel cage fans. The forced draft and induced draft fans at municipal incinerators are usually squirrel cage types. They consist of a paddle wheel device which spins causing the air to move away from the axle (through the paddle wheel vains and outward). A shroud on this type fan directs the moving gas. Combustion gas moved by a fan pulling with a vacuum. In a combustion device, leaks into the unit are likely to occur in areas where induced draft is present. Combustion air moved by the addition of pressure. Leaks in a combustion unit will flow outward from areas where forced draft is present. Vertical tube used to raise gases to some height where they can disperse freely. Tubes used to pass gases from one process to the next. Ducts may be round or rectangular. The choice is based on engineering mechanics and economics. A device with evenly spaced static conductors which are charged strongly negative and positive. When gases are passed between the conductors, particulates (smoke particles) are attracted to the conductors. The conductors (plates and wires) are shaken as often as necessary to remove the gathered debris (perhaps every few hours) or washed off, if it is a "wet" electrostatic precipitator. Electrostatic precipitators can be designed in such a fashion that the static charges do not short out when a damp gas stream is passed through the device or if the surfaces are sprayed continuously with some fluid for the purpose of cleaning the device. In all other respects electrostatic precipitators are similar, wet or dry. The static charge is developed with a system of transformers and rectifiers which maintain the charge just below the level at which sparks would jump from plate to plate. The distance between conductors range from 3" to 6" depending upon the manufacturer's philosophy of design. The first electrostatic precipitator was demonstrated around 1910. They are in wide use now throughout industry for dust and smoke collecting. |
List of Terms
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Disclaimer: The author's intent is offering the best environmental balance advice available; the user is free to use it, although no guarantee can be implied. All user circumstances are unique and require individual analysis. |
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