COOLING TOWER

Cooling towers are heat removal devices used to transfer process waste heat to the atmisphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wer-bulb air tempature or in the case of "Close Circuit Dry Cooling Towers" rely solely on air to cool the working fluid to near the dry buld air temperature. Common applications include cooling the circulating water used in oil refineries, chemical plant, power stations and building cooling. The towers vary in size from small roof-top units to very large hyperboloid structures (as in Image 1) that can be up to 200 metres tall and 100 metres in diameter, or rectangular structures (as in Image 2) that can be over 40 metres tall and 80 metres long. Smaller towers are normally factory-built, while larger ones are constructed on site. They are often associated with nuclear power plants in popular culture.

HVAC

An HVAC cooling tower is a subcategory rejecting heat from a chiller. Water-cooled chillers are normally more energy efficient than air-cooled chillers due to heat rejection to tower water at or near wet-bulb temperatures. Air-cooled chillers must reject heat at the dry-bulb temperature, and thus have a lower average reverse-Carnot cycle effectiveness. Large office buildings, hospitals, and schools typically use one or more cooling towers as part of their air conditioning systems. Generally, industrial cooling towers are much larger than HVAC towers.

HVAC use of a cooling tower pairs the cooling tower with a water-cooled chiller or water-cooled condenser.A ton of air-conditioning is the removal of 12,000 Btu/hour (3517 W). The equivalent ton on the cooling tower side actually rejects about 15,000 Btu/hour (4396 W) due to the heat-equivalent of the energy needed to drive the chiller's compressor. This equivalent ton is defined as the heat rejection in cooling 3 U.S. gallons/minute (1,500 pound/hour) of water 10 °F (5.56 °C), which amounts to 15,000 Btu/hour, or a chiller coefficient performance (COP) of 4.0. This COP is equivalent to an energy efficiency ratio (EER) of 13.65.

Cooling towers are also used in HVAC systems that have multiple water source heat pumps that share a common piping "water loop". In this type of system, the water circulating inside the "water loop" removes heat from the condenser of the heat pumps whenever the heat pumps are working in the cooling mode, then the cooling tower is used to remove heat from the water loop and reject it to the atlmosphere. When the heat pumps are working in heating mode, the condensers draw heat out of the loop water and reject it into the space to be heated.

Heat transfer methods

Mechanical draft crossflow cooling tower used in an HVAC application

With respect to the heat transfer mechanism employed, the main types are:

• Wet cooling towers or simply open circuit cooling towers operate on the principle of evaporation. The working fluid and the evaporated fluid (usually H₂O) are one and the same.
• Dry Cooling Towers operate by heat transfer through a surface that separates the working fluid from ambient air, such as in a tube to air heat exchanger, utilizing convective heat transfer. They do not use evaporation.
• Fluid coolers or Closed Circuit Cooling Towers are hybrids that pass the working fluid through a tube bundle, upon which clean water is sprayed and a fan-induced draft applied. The resulting heat transfer performance is much closer to that of a wet cooling tower, with the advantage provided by a dry cooler of protecting the working fluid from environmental exposure and contamination.

In a wet cooling tower (or Open Circuit Cooling Tower), the warm water can be cooled to a temperature lower than the ambient air dry-bulb temperature, if the air is relatively dry. (see: dew point and psychrometrics). As ambient air is drawn past a flow of water, an small portion of the water evaporate, the energy required by that portion of the water to evaporate is taken from the remaining mass of water reducing his temperature (aproximately by 970 BTU for each pound of evaporated water). Evaporation results in saturated air conditions, lowering the temperature of the water process by the tower to a value close to wet bulb air temperature, which is lower than the ambient dry bulb air temperature, the difference determined by the humidity of the ambient air.

To achieve better performance (more cooling), a medium called fill is used to increase the surface area and the time of contact between the air and water flows. Splash fill consists of material placed to interrupt the water flow causing splashing. Film fill is composed of thin sheets of material (usually PVC) upon which the water flows. Both methods create increased surface area and time of contact between the fluid (water) and the gas (air).

Air flow generation methods

A forced draft cooling tower

With respect to drawing air through the tower, there are three types of cooling towers:

• Natural Draft, which utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. Warm moist air is less dense than drier air at the same pressure. This moist air buoyancy produces a current of air through the tower.
• Mechanical draft, which uses power driven fan motors to force or draw air through the tower.
  • Induced draft: A mechanical draft tower with a fan at the discharge which pulls air through tower. The fan induces hot moist air out the discharge. This produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake. This fan/fin arrangement is also known as draw-through. (see Image 2, 3)
  • Forced draft: A mechanical draft tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design. The forced draft benefit is its ability to work with high static pressure. They can be installed in more confined spaces and even in some indoor situations. This fan/fill geometry is also known as blow-through. (see Image 4)
• Fan assisted natural draft. A hybrid type that appears like a natural draft though airflow is assisted by a fan.

Hyperboloid (a.k.a. hyperbolic) cooling towers (Image 1) have become the design standard for all natural-draft cooling towers because of their structural strength and minimum usage of material. The hyperboloid shape also aids in accelerating the upward convective air flow, improving cooling efficiency. They are popularly associated with nuclear power plant. However, this association is misleading, as the same kind of cooling towers are often used at large coal-fired power plants as well. Similarly, not all nuclear power plants have cooling towers, instead cooling their heat exchangers with lake, river or ocean water.

SANITARY LANDFILL

A landfill, also known as a dump, rubbish dump or both, Rubbish Landfill Dump (and historically as a midden), is a site for the disposal of waste materials by burial and is the oldest form of waste treatment. Historically, landfills have been the most common methods of organized waste disposal and remain so in many places around the world.

Operations

A section of a landfill located in Barclay, Ontario. This landfill is one of several landfills used by Dryden, Ontario.

Typically, in non hazardous waste landfills, in order to meet predefined specifications, techniques are applied by which the wastes are:

1. Confined to as small an area as possible.
2. Compacted to reduce their volume.
3. Covered (usually daily) with layers of soil.

During landfill operations the waste collection vehicles are weighed at a weighbridge on arrival and their load is inspected for wastes that do not accord with the landfill’s waste acceptance criteria. Afterward, the waste collection vehicles use the existing road network on their way to the tipping face or working front where they unload their load. After loads are deposited, compactors or dozers are used to spread and compact the waste on the working face. Before leaving the landfill boundaries, the waste collection vehicles pass through the wheel cleaning facility. If necessary, they return to the weighbridge in order to be weighed without their load. Through the weighing process, the daily incoming waste tonnage can be calculated and listed in databases. In addition to trucks, some landfills may be equipped to handle railroad containers. The use of 'rail-haul' permits landfills to be located at more remote sites, without the problems associated with many truck trips.

Typically, in the working face, the compacted waste is covered with soil daily. Alternative waste-cover materials are several sprayed-on foam products and temporary blankets. Blankets can be lifted into place with tracked excavators and then removed the following day prior to waste placement. Chipped wood and chemically 'fixed' bio-solids may also be used as an alternate daily cover. The space that is occupied daily by the compacted waste and the cover material is called a daily cell. Waste compaction is critical to extending the life of the landfill. Factors such as waste compressibility, waste layer thickness and the number of passes of the compactor over the waste affect the waste densities.

Impacts

Landfill operation. Note that the area being filled is a single, well-defined "cell" and that a rubberized landfill liner is in place (exposed on the left) to prevent contamination by leachates migrating downward through the underlying geological formation.

A large number of adverse impacts may occur from landfill operations. These impacts can vary: fatal accidents ( scavengers buried under waste piles);infastructure damage (e.g., damage to access roads by heavy vehicles); pollution of the local environment (such as contamination of groundwater and/or aquifers by leakage and residual soil contamination during landfill usage, as well as after landfill closure); offgassing of methane generated by decaying organic wastes (methane is a greenhouse gas many times more potent than carbon dioxide, and can itself be a danger to inhabitants of an area); harbouring of disease vectors such as rats and flies, particularly from improperly operated landfills, which are common in Third-World countries; injuries to wildlife; and simple nuisance problems (e.g., dust, odour, vermine, or noise pollutin).

Environmental noise and dust are generated from vehicles accessing a landfill as well as from working face operations. These impacts are best to intercept at the planning stage where access routes and landfill geometrics can be used to mitigate such issues. Vector control is also important, but can be managed reasonably well with the daily cover protocols.

Most modern landfills in industrialized countries are operated with controls to attempt to manage problems such as these. Analyses of common landfill operational problems are available.

Some local authorities have found it difficult to locate new landfills. Communities may charge a fee or levy in order to discourage waste and/or recover the costs of site operations. Many landfills are publicly funded, but some are commercial businesses, operated for profit.

INCINERATION

Incineration is a waste treatment process that involves the combustion of organic substances contained in waste materials.^[1] Incineration and other high temperature waste treatment systems are described as "thermanl treatment". Incineration of waste materials converts the waste into ash, flue gas, and heat. The ash is mostly formed by the inorganics constituents of the waste, and may take the form of solid lumps or particulates carried by the flue gas. The flue gases must be cleaned of gaseous and particulate pollutants before they are dispersed into the atmosphere. In some cases, the heat generated by incineration can be used to generate electric power.

Specialized incineration

Furniture factory sawdust incinerators need much attention as these have to handle resin powder and many flammable substances. Controlled combustion, burn back prevention systems are essential as dust when suspended resembles the fire catch phenomenon of any liquid petroleum gas.

Use of heat

The heat produced by an incinerator can be used to generate steam which may then be used to drive a turbine in order to produce electricity. The typical amount of net energy that can be produced per tonne municipal waste is about 2/3 MWh of electricity and 2 MWh of district heating. Thus, incinerating about 600 metric tons (660 short tons) per day of waste will produce about 400 MWh of electrical energy per day (17 MW of electrical power continuously for 24 hours) and 1200 MWh of district heating energy each day.

Pollution

Incineration has a number of outputs such as the ash and the emission to the atmosphere of flue gas. Before the flue gas cleaning system, the flue gases may contain significant amounts of particulate matter,heavy metals, dioxins, furans, sulfur dioxide, and hydrochloric acid.

In a study from 1994, Delaware Solid Waste Authority found that, for same amount of produced energy, incineration plants emitted fewer particles, hydrocarbons and less SO₂, HCl, CO and NO_x than coal-fired power plants, but more than natural gas fired power plants. According to Germany's ministry of environment, waste incinerators reduce the amount of some atmospheric pollutants by substituting power produced by coal-fired plants with power from waste-fired plants.

Solid outputs

Operation of an incinerator aboard an aircraft carrier

Incineration produces fly ash and bottom ash just as is the case when coal is combusted. The total amount of ash produced by municipal solid waste incineration ranges from 4 to 10 % by volume and 15-20 % by weight of the original quantity of waste, and the fly ash amounts to about 10-20 % of the total ash. The fly ash, by far, constitutes more of a potential health hazard than does the bottom ash because the fly ash often contain high concentrations of heavy metals such as lead, cadmium, copper and zinc as well as small amounts of dioxins and furans. The bottom ash seldom contain significant levels of heavy metals. In testing over the past decade, no ash from an incineration plant in the USA has ever been determined to be a hazardous waste. At present although some historic samples tested by the incinerator operators' group would meet the being ecotoxic criteria at present the EA say "we have agreed" to regard incinerator bottom ash as "non-hazardous" until the testing programme is complete.

[]Other pollution issues

Odor pollution can be a problem with old-style incinerators, but odors and dust are extremely well controlled in newer incineration plants. They receive and store the waste in an enclosed area with a negative pressure with the airflow being routed through the boiler which prevents unpleasant odors from escaping into the atmosphere. However, not all plants are implemented this way, resulting in inconveniences in the locality.

An issue that affects community relationships is the increased road traffic of waste collection vehicles to transport municipal waste to the incinerator. Due to this reason, most incinerators are located in industrial areas. This problem can be avoided to an extent through the transport of waste by rail from transfer stations.

SMOG

Smog is a type of air pollution ; the word "smog" is a portmateau of smoke and fog. Modern smog is a type of air pollution derived from vehicular emission from internal combustion engine and industrial fumes that react in the atmosphere with sunlight to form secondary pollutants that also combine with the primary emissions to form photochemical fog. Smog is also caused by large amounts of coal burning in an area caused by a mixture of smoke and sulfur dioxide.

In the 1950s a new type of smog, known as photochemical smog, was first described. The compound when sunlight hits various pollutants in the air and forms a mix of inimical chemicals that can be very dangerous. A photochemical smog is the chemical reaction of sunlight, nitro oxide (NO_x) and volatile organic compound (VOCs) in the atmosphere, which leaves airborne particles (called particulate matter) and ground level zone^]

Nitrogen oxides are released by nitrogen and oxygen in the air reacting together under high temperature such as in the exhaust of fossil fuel-burning engines in cars, trucks, coal power plants, and industrial manufacturing factories. VOCs are released from man-made sources such as gasoline (petrol), paints, solvents, pestisides, and biogenic sources, such as pine and citrus tree emissions.

Health effects

Highland Park Optimist Club wearing smog-gas masks at banquet, Los Angeles, circa 1954

Smog is a serious problem in many cities and continues to harm human health. Ground level-zone, sulfur dioxide, nitrogen dioxide and carbon monoxide are especially harmful for senior citizens, children, and people with heart and lung conditions such as emphysema, bronchitis, and asthma. It can inflame breathing passages, decrease the lungs' working capacity, cause shortness of breath, pain when inhaling deeply, wheezing, and coughing. It can cause eye and nose irritation and it dries out the protective membranes of the nose and throat and interferes with the body's ability to fight infection, increasing susceptibility to illness. Hospital admissions and respiratory deaths often increase during periods when ozone levels are high.

The U.S. EPA has developed an Air Quality index to help explain air pollution levels to the general public. 8 hour average ozone concentrations of 85 to 104 ppbv are described as "Unhealthy for Sensitive Groups", 105 ppbv to 124 ppbv as "unhealthy" and 125 ppb to 404 ppb as "very unhealthy". The "very unhealthy" range for some other pollutants are: 355 μg m⁻³ - 424 μg m⁻³ for PM10; 15.5 ppm - 30.4ppm for CO and 0.65 ppm - 1.24 ppm for NO₂.

The Ontario Medical Association announced that smog is responsible for an estimated 9,500 premature deaths in the province each year.

A 20-year American Cancer Society study found that cumulative exposure also increases the likelihood of premature death from a respiratory disease, implying the 8-hour standard may be insufficient.

Areas affected

Beijing air on a day after rain (left) and a smoggy day (right)

Smog can form in almost any climate where industries or cities release large amounts of air pollution, such as smoke or gases. However, it is worse during periods of warmer, sunnier weather when the upper air is warm enough to inhibit vertical circulation. It is especially prevalent in geologic basins encircled by hills or mountains. It often stays for an extended period of time over densely populated cities or urban areas, such as London, Atlanta, Houston, Phoenix, Las Vegas, New Delhi, New york,Cairo, Los Angeles, Sacramento, Sao Paulo, Mexico city, Santiago of Chile, Toronto, Milan, Athens, Beijing, Shanghai, Manila, Hong Kong, Seoul, the Randstad or Ruhr Area and can build up to dangerous levels.

TEMPERATURE INVERSION

What is a temperature inversion?

A temperature inversion is a thin layer of the atmosphere where the normal decrease in temperature with height switches to the temperature increasing with height. An inversion acts like a lid, keeping normal convective overturning of the atmosphere from penetrating through the inversion. This can cause several weather-related effects. One is the trapping of pollutants below the inversion, allowing them to build up. If the sky is very hazy, or is sunsets are very red, there is likely an inversion somewhere in the lower atmosphere. This happens more frequently in high pressure zones, where the gradual sinking of air in the high pressure dome typically causes an inversion to form at the base of a sinking layer of air. Another effect that an inversion has is to make clouds just below the inversion to spread out and take on a flattened appearance. For instance, marine stratocumulus clouds over cold ocean waters; or the tops of thunderstorms when they reach the base of the stratosphere, which also forms a temperature inversion. Still another effect is to prevent thunderstorms from forming. Even in an air mass that is hot and humid in the lowest layers, thunderstorms will be prevented if an inversion in the lower atmosphere is keeping this air from rising. The conceptual opposite of a temperature inversion is an unstable air layer. Normal atmospheric conditions Usually, within the lower atmosphere (the trophosphere) the air near the surface of the Earth is warmer than the air above it, largely because the atmosphere is heated from below as solar radiation warms the Earth's surface, which in turn then warms the layer of the atmosphere directly above it e.g. by thermals (convective heat transfer). How and why inversions occur Height (y-axis) versus Temperature (x-axis) under normal atmospheric conditions (black line). When the layer from 6–8 kilometers (designated A-B) is descended dry adiabatically, the result is the inversion seen near the ground at 1–2 kilometers (C-D). Under certain conditions, the normal vertical temperature gradient is inverted such that the air is colder near the surface of the Earth. This can occur when, for example, a warmer, less dense air mass moves over a cooler, denser air mass. This type of inversion occurs in the vicinity of warm fronts, and also in areas of oceanic upwelling such as along the California coast. With sufficient humidity in the cooler layer, fog is typically present below the inversion cap. An inversion is also produced whenever radiation from the surface of the earth exceeds the amount of radiation received from the sun, which commonly occurs at night, or during the winter when the angle of the sun is very low in the sky. This effect is virtually confined to land regions as the ocean retains heat far longer. In the polar regions during winter, inversions are nearly always present over land. A warmer air mass moving over a cooler one can "shut off" any convection which may be present in the cooler air mass. This is known as a capping inversion. However, if this cap is broken, either by extreme convection overcoming the cap, or by the lifting effect of a front or a mountain range, the sudden release of bottled-up convective energy — like the bursting of a balloon — can result in severe thunderstorms. Such capping inversions typically precede the development of tornadoes in the midwestern United States. In this instance, the "cooler" layer is actually quite warm, but is still denser and usually cooler than the lower part of the inversion layer capping it. Subsidence inversion An inversion can develop aloft as a result of air gradually sinking over a wide area and being warmed by adiabatic compression, usually associated with subtropical high pressure areas. A stable marine layes may then develop over the ocean as a result. As this layer moves over progressively warmer waters, however, turbulence within the marine layer can gradually lift the inversion layer to higher altitudes, and eventually, even pierce it, producing thunderstorms, and under the right circumstances, leading to tropical cyclones. The accumulated smog and dust under the inversion quickly taints the sky reddish, easily seen on sunny days. Consequences of a thermal inversion Fata Morgana of a ship is due to an inversion Winter smoke in Shanghai with a clear border-layer for the vertical air-spread. Temperature inversion in Bratislava, viewing the top of Nový Most bridge With the ceasing of convection, which is normally present in the atmosphere, a number of phenomena are associated with a temperature inversion. The air becomes stiller, hence the air becomes murky because dust and pollutants are no longer lifted from the surface. This can become a problem in cities where many pollutants exist. Inversion effects occur frequently in big cities such as Mumbai, India, Los Angeles, California; Mexico City; Sao Paolo, Brazil; Santiago, Chile; and Tehran, Iran but also in smaller cities like Oslo, Norway; Prague, Czech Republic; Ljubljana, Slovenia; Salt Lake City, Utah;Logan, Utah; Vancouver, British Colombia; Chiang Mai and Boise, Idaho which are closely surrounded by hills and mountains that together with the inversion effect bottle-caps the air in the city. During a severe inversion, trapped air pollutants form a brownish haze that can cause respiratory problems. The Great Smog oof 1952, one of the most serious examples of such an inversion, occurred in London and was blamed for thousands of deaths. Sometimes the inversion layer is higher so that the cumulus clouds can condense but then they spread out under the inversion layer. This cuts out sunlight to the ground and prevents new thermals from forming. A period of cloudiness is followed by sunny weather as the clouds disperse. This cycle can occur more than once in a day. The index of refraction of air decreases as the air temperature increases, a side effect of hotter air being less dense. Normally this results in distant objects being shortened vertically, an effect that is easy to see at sunset (where the sun is "squished" into an oval). In an inversion the normal pattern is reversed, and distant objects are instead stretched out or appear to be above the horizon. This leads to the interesting optical effects of Fata Morgana or mirage.