Heat Exchanger

Heat exchanger An interchangeable plate heat exchanger Tubular heat exchanger. A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact. [1] They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment.The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. Contents| Flow arrangement Countercurrent (A) and parallel (B) flows * Fig. 1: Shell and tube heat exchanger, single pass (1–1 parallel flow) * Fig. 2: Shell and tube heat exchanger, 2-pass tube side (1–2 crossflow) * Fig. : Shell and tube heat exchanger, 2- pass shell side, 2-pass tube side (2-2 countercurrent) There are three primary classifications of heat exchangers according to their flow arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side. In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is the most efficient, in that it can transfer the most heat from the heat (transfer) medium due to the fact that the average temperature difference along any unit length is greater.See countercurrent exchange. In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger. For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger. The exchanger's performance can also be affected by the addition of fins or corrugations in one o r both directions, which increase surface area and may channel fluid flow or induce turbulence. The driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined.In most simple systems this is the â€Å"log mean temperature difference† (LMTD). Sometimes direct knowledge of the LMTD is not available and the NTU method is used. Types of heat exchangers Shell and tube heat exchanger A Shell and Tube heat exchanger Main article: Shell and tube heat exchanger Shell and tube heat exchangers consist of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required.A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and tube heat exchangers are typically used for high-pressure applications (with pressures greater than 30 bar and temperatures greater than 260  Ã‚ °C). [2] This is because the shell and tube heat exchangers are robust due to their shape. Several thermal design features must be considered when designing the tubes in the shell and tube heat exchangers: * Tube diameter: Using a small tube diameter makes the heat exchanger both economical and compact.However, it is more likely for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used. Thus to determine the tube diameter, the available space, cost and the fouling nature of the fluids must be considered. * Tube thickness: The thickness of the wall of the tubes is usually determined to ensure: * There is enough room for corrosion * That flow-induced vibration has resistance * Axial strength * Availability of spare parts Hoop strength (to withstand internal tub e pressure) * Buckling strength (to withstand overpressure in the shell) * Tube length: heat exchangers are usually cheaper when they have a smaller shell diameter and a long tube length. Thus, typically there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities. However, there are many limitations for this, including space available at the installation site and the need to ensure tubes are available in lengths that are twice the required length (so they can be withdrawn and replaced).Also, long, thin tubes are difficult to take out and replace. * Tube pitch: when designing the tubes, it is practical to ensure that the tube pitch (i. e. , the centre-centre distance of adjoining tubes) is not less than 1. 25 times the tubes' outside diameter. A larger tube pitch leads to a larger overall shell diameter, which leads to a more expensive heat exchanger. * Tube corrugation: this type of tubes, mainly used for the inner tubes, in creases the turbulence of the fluids and the effect is very important in the heat transfer giving a better performance. Tube Layout: refers to how tubes are positioned within the shell. There are four main types of tube layout, which are, triangular (30 °), rotated triangular (60 °), square (90 °) and rotated square (45 °). The triangular patterns are employed to give greater heat transfer as they force the fluid to flow in a more turbulent fashion around the piping. Square patterns are employed where high fouling is experienced and cleaning is more regular. * Baffle Design: baffles are used in shell and tube heat exchangers to direct fluid across the tube bundle.They run perpendicularly to the shell and hold the bundle, preventing the tubes from sagging over a long length. They can also prevent the tubes from vibrating. The most common type of baffle is the segmental baffle. The semicircular segmental baffles are oriented at 180 degrees to the adjacent baffles forcing the f luid to flow upward and downwards between the tube bundle. Baffle spacing is of large thermodynamic concern when designing shell and tube heat exchangers. Baffles must be spaced with consideration for the conversion of pressure drop and heat transfer.For thermo economic optimization it is suggested that the baffles be spaced no closer than 20% of the shell’s inner diameter. Having baffles spaced too closely causes a greater pressure drop because of flow redirection. Consequently having the baffles spaced too far apart means that there may be cooler spots in the corners between baffles. It is also important to ensure the baffles are spaced close enough that the tubes do not sag. The other main type of baffle is the disc and donut baffle, which consists of two concentric baffles. An outer, wider baffle looks like a donut, whilst the inner baffle is shaped like a disk.This type of baffle forces the fluid to pass around each side of the disk then through the donut baffle generati ng a different type of fluid flow. Fixed tube liquid-cooled heat exchangers especially suitable for marine and harsh applications can be assembled with brass shells, copper tubes, brass baffles, and forged brass integral end hubs. [3] (See: Copper in heat exchangers). Conceptual diagram of a plate and frame heat exchanger. A single plate heat exchanger An interchangeable plate heat exchanger applied to the system of a swimming pool Plate heat exchanger Main article: Plate heat exchangerAnother type of heat exchanger is the plate heat exchanger. One is composed of multiple, thin, slightly separated plates that have very large surface areas and fluid flow passages for heat transfer. This stacked-plate arrangement can be more effective, in a given space, than the shell and tube heat exchanger. Advances in gasket and brazing technology have made the plate-type heat exchanger increasingly practical. In HVAC applications, large heat exchangers of this type are called plate-and-frame; when used in open loops, these heat exchangers are normally of the gasket type to allow periodic disassembly, cleaning, and inspection.There are many types of permanently bonded plate heat exchangers, such as dip-brazed, vacuum-brazed, and welded plate varieties, and they are often specified for closed-loop applications such as refrigeration. Plate heat exchangers also differ in the types of plates that are used, and in the configurations of those plates. Some plates may be stamped with â€Å"chevron†, dimpled, or other patterns, where others may have machined fins and/or grooves. Plate and shell heat exchanger A third type of heat exchanger is a plate and shell heat exchanger, which combines plate heat exchanger with shell and tube heat exchanger technologies.The heart of the heat exchanger contains a fully welded circular plate pack made by pressing and cutting round plates and welding them together. Nozzles carry flow in and out of the platepack (the ‘Plate side' flowpat h). The fully welded platepack is assembled into an outer shell that creates a second flowpath ( the ‘Shell side'). Plate and shell technology offers high heat transfer, high pressure, high operating temperature, compact size, low fouling and close approach temperature. In particular, it does completely without gaskets, which provides security against leakage at high pressures and temperatures.Adiabatic wheel heat exchanger A fourth type of heat exchanger uses an intermediate fluid or solid store to hold heat, which is then moved to the other side of the heat exchanger to be released. Two examples of this are adiabatic wheels, which consist of a large wheel with fine threads rotating through the hot and cold fluids, and fluid heat exchangers. Plate fin heat exchanger Main article: Plate fin heat exchanger This type of heat exchanger uses â€Å"sandwiched† passages containing fins to increase the effectiveness of the unit.The designs include crossflow and counterflow cou pled with various fin configurations such as straight fins, offset fins and wavy fins. Plate and fin heat exchangers are usually made of aluminium alloys, which provide high heat transfer efficiency. The material enables the system to operate at a lower temperature and reduce the weight of the equipment. Plate and fin heat exchangers are mostly used for low temperature services such as natural gas, helium and oxygen liquefaction plants, air separation plants and transport industries such as motor and aircraft engines. Advantages of plate and fin heat exchangers: High heat transfer efficiency especially in gas treatment * Larger heat transfer area * Approximately 5 times lighter in weight than that of shell and tube heat exchanger. * Able to withstand high pressure Disadvantages of plate and fin heat exchangers: * Might cause clogging as the pathways are very narrow * Difficult to clean the pathways * Aluminum alloys are susceptible to Mercury Liquid Embrittlement Failure Pillow plat e heat exchanger A pillow plate exchanger is commonly used in the dairy industry for cooling milk in large direct-expansion stainless steel bulk tanks.The pillow plate allows for cooling across nearly the entire surface area of the tank, without gaps that would occur between pipes welded to the exterior of the tank. The pillow plate is constructed using a thin sheet of metal spot-welded to the surface of another thicker sheet of metal. The thin plate is welded in a regular pattern of dots or with a serpentine pattern of weld lines. After welding the enclosed space is pressurized with sufficient force to cause the thin metal to bulge out around the welds, providing a space for heat exchanger liquids to flow, and creating a characteristic appearance of a swelled pillow formed out of metal.Fluid heat exchangers This is a heat exchanger with a gas passing upwards through a shower of fluid (often water), and the fluid is then taken elsewhere before being cooled. This is commonly used for cooling gases whilst also removing certain impurities, thus solving two problems at once. It is widely used in espresso machines as an energy-saving method of cooling super-heated water to use in the extraction of espresso. Waste heat recovery units A Waste Heat Recovery Unit (WHRU) is a heat exchanger that recovers heat from a hot gas stream while transferring it to a working medium, typically water or oils.The hot gas stream can be the exhaust gas from a gas turbine or a diesel engine or a waste gas from industry or refinery. They can also be integrated into a heatcatcher system that allows an ORC[further explanation needed] generator to produce waste heat to power. Dynamic scraped surface heat exchanger Another type of heat exchanger is called â€Å"(dynamic) scraped surface heat exchanger†. This is mainly used for heating or cooling with high-viscosity products, crystallization processes, evaporation and high-fouling applications.Long running times are achieved due to th e continuous scraping of the surface, thus avoiding fouling and achieving a sustainable heat transfer rate during the process. Phase-change heat exchangers Typical kettle reboiler used for industrial distillation towers Typical water-cooled surface condenser In addition to heating up or cooling down fluids in just a single phase, heat exchangers can be used either to heat a liquid to evaporate (or boil) it or used as condensers to cool a vapor and condense it to a liquid. In chemical plants and refineries, reboilers used to heat incoming feed for distillation towers are often heat exchangers. 4][5] Distillation set-ups typically use condensers to condense distillate vapors back into liquid. Power plants that use steam-driven turbines commonly use heat exchangers to boil water into steam. Heat exchangers or similar units for producing steam from water are often called boilers or steam generators. In the nuclear power plants called pressurized water reactors, special large heat exchan gers pass heat from the primary (reactor plant) system to the secondary (steam plant) system, producing steam from water in the process. These are called steam generators.All fossil-fueled and nuclear power plants using steam-driven turbines have surface condensers to convert the exhaust steam from the turbines into condensate (water) for re-use. [6][7] To conserve energy and cooling capacity in chemical and other plants, regenerative heat exchangers can transfer heat from a stream that must be cooled to another stream that must be heated, such as distillate cooling and reboiler feed pre-heating. This term can also refer to heat exchangers that contain a material within their structure that has a change of phase.This is usually a solid to liquid phase due to the small volume difference between these states. This change of phase effectively acts as a buffer because it occurs at a constant temperature but still allows for the heat exchanger to accept additional heat. One example where this has been investigated is for use in high power aircraft electronics. Direct contact heat exchangers Direct contact heat exchangers involve heat transfer between hot and cold streams of two phases in the absence of a separating wall. [8] Thus such heat exchangers can be classified as: * Gas – liquid * Immiscible liquid – liquid Solid-liquid or solid – gas Most direct contact heat exchangers fall under the Gas- Liquid category, where heat is transferred between a gas and liquid in the form of drops, films or sprays. [2] Such types of heat exchangers are used predominantly in air conditioning, humidification, industrial hot water heating, water cooling and condensing plants. [9] Phases[10]| Continuous phase| Driving force| Change of phase| Examples| Gas – Liquid| Gas| Gravity| No| Spray columns, packed columns| | | | Yes| Cooling towers, falling droplet evaporators| | | Forced| No| Spray coolers/quenchers| | Liquid flow| Yes| Spray condensers/evaporati on, jet condensers| | Liquid| Gravity| No| Bubble columns, perforated tray columns| | | | Yes| Bubble column condensers| | | Forced| No| Gas spargers| | | Gas flow| Yes| Direct contact evaporators, submerged combustion| HVAC air coils One of the widest uses of heat exchangers is for air conditioning of buildings and vehicles. This class of heat exchangers is commonly called air coils, or just coils due to their often-serpentine internal tubing. Liquid-to-air, or air-to-liquid HVAC coils are typically of modified crossflow arrangement. In vehicles, heat coils are often called heater cores.On the liquid side of these heat exchangers, the common fluids are water, a water-glycol solution, steam, or a refrigerant. For heating coils, hot water and steam are the most common, and this heated fluid is supplied by boilers, for example. For cooling coils, chilled water and refrigerant are most common. Chilled water is supplied from a chiller that is potentially located very far away, but refri gerant must come from a nearby condensing unit. When a refrigerant is used, the cooling coil is the evaporator in the vapor-compression refrigeration cycle. HVAC coils that use this direct-expansion of refrigerants are commonly called DX coils.Some DX coils are â€Å"microchannel† type. [11] On the air side of HVAC coils a significant difference exists between those used for heating, and those for cooling. Due to psychrometrics, air that is cooled often has moisture condensing out of it, except with extremely dry air flows. Heating some air increases that airflow's capacity to hold water. So heating coils need not consider moisture condensation on their air-side, but cooling coils must be adequately designed and selected to handle their particular latent (moisture) as well as the sensible (cooling) loads.The water that is removed is called condensate. For many climates, water or steam HVAC coils can be exposed to freezing conditions. Because water expands upon freezing, these somewhat expensive and difficult to replace thin-walled heat exchangers can easily be damaged or destroyed by just one freeze. As such, freeze protection of coils is a major concern of HVAC designers, installers, and operators. The introduction of indentations placed within the heat exchange fins controlled condensation, allowing water molecules to remain in the cooled air.This invention allowed for refrigeration without icing of the cooling mechanism. [12] The heat exchangers in direct-combustion furnaces, typical in many residences, are not ‘coils'. They are, instead, gas-to-air heat exchangers that are typically made of stamped steel sheet metal. The combustion products pass on one side of these heat exchangers, and air to heat on the other. A cracked heat exchanger is therefore a dangerous situation that requires immediate attention because combustion products may enter living space. Spiral heat exchangers Schematic drawing of a spiral heat exchanger.A spiral heat exchang er (SHE), may refer to a helical (coiled) tube configuration, more generally, the term refers to a pair of flat surfaces that are coiled to form the two channels in a counter-flow arrangement. Each of the two channels has one long curved path. A pair of fluid ports are connected tangentially to the outer arms of the spiral, and axial ports are common, but optional. [13] The main advantage of the SHE is its highly efficient use of space. This attribute is often leveraged and partially reallocated to gain other improvements in performance, according to well known tradeoffs in heat exchanger design. A notable tradeoff is capital cost vs operating cost. ) A compact SHE may be used to have a smaller footprint and thus lower all-around capital costs, or an over-sized SHE may be used to have less pressure drop, less pumping energy, higher thermal efficiency, and lower energy costs. Construction The distance between the sheets in the spiral channels are maintained by using spacer studs that were welded prior to rolling. Once the main spiral pack has been rolled, alternate top and bottom edges are welded and each end closed by a gasketed flat or conical cover bolted to the body.This ensures no mixing of the two fluids occurs. Any leakage is from the periphery cover to the atmosphere, or to a passage that contains the same fluid. [14] Self cleaning SHEs are often used in the heating of fluids that contain solids and thus tend to foul the inside of the heat exchanger. The low pressure drop lets the SHE handle fouling more easily. The SHE uses a â€Å"self cleaning† mechanism, whereby fouled surfaces cause a localized increase in fluid velocity, thus increasing the drag (or fluid friction) on the fouled surface, thus helping to dislodge the blockage and keep the heat exchanger clean. The internal walls that make up the heat transfer surface are often rather thick, which makes the SHE very robust, and able to last a long time in demanding environments. † They are also easily cleaned, opening out like an oven where any build up of foulant can be removed by pressure washing. Self-Cleaning Water filters are used to keep the system clean and running without the need to shut down or replace cartridges and bags. Flow arrangements Concurrent and countercurrent flow. There are three main types of flows in a spiral heat exchanger: 1. Counter-current Flow: Fluids flow in opposite directions.These are used for liquid-liquid, condensing and gas cooling applications. Units are usually mounted vertically when condensing vapour and mounted horizontally when handling high concentrations of solids. 2. Spiral Flow/Cross Flow: One fluid is in spiral flow and the other in a cross flow. Spiral flow passages are welded at each side for this type of spiral heat exchanger. This type of flow is suitable for handling low density gas, which passes through the cross flow, avoiding pressure loss. It can be used for liquid-liquid applications if one liquid has a cons iderably greater flow rate than the other. . Distributed Vapour/Spiral flow: This design is that of a condenser, and is usually mounted vertically. It is designed to cater for the sub-cooling of both condensate and non-condensables. The coolant moves in a spiral and leaves via the top. Hot gases that enter leave as condensate via the bottom outlet. Applications The SHE is good for applications such as pasteurization, digester heating, heat recovery, pre-heating (see: recuperator), and effluent cooling. For sludge treatment, SHEs are generally smaller than other types of heat exchangers. [citation needed] SelectionDue to the many variables involved, selecting optimal heat exchangers is challenging. Hand calculations are possible, but many iterations are typically needed. As such, heat exchangers are most often selected via computer programs, either by system designers, who are typically engineers, or by equipment vendors. To select an appropriate heat exchanger, the system designers (or equipment vendors) would firstly consider the design limitations for each heat exchanger type. Though cost is often the primary criterion, several other selection criteria are important: * High/low pressure limits * Thermal performance Temperature ranges * Product mix (liquid/liquid, particulates or high-solids liquid) * Pressure drops across the exchanger * Fluid flow capacity * Cleanability, maintenance and repair * Materials required for construction * Ability and ease of future expansion * Material selection, such as copper, aluminum, carbon steel, stainless steel, nickel alloys, and titanium. Small-diameter coil technologies are becoming more popular in modern air conditioning and refrigeration systems because they have better rates of heat transfer than conventional sized condenser and evaporator coils with round copper tubes and aluminium or opper fin that have been the standard in the HVAC industry. Small diameter coils can withstand the higher pressures required by the new generation of environmentally friendlier refrigerants. Two small diameter coil technologies are currently available for air conditioning and refrigeration products: copper microgroove[15] and brazed aluminium microchannel. [16] Choosing the right heat exchanger (HX) requires some knowledge of the different heat exchanger types, as well as the environment where the unit must operate.Typically in the manufacturing industry, several differing types of heat exchangers are used for just the one process or system to derive the final product. For example, a kettle HX for pre-heating, a double pipe HX for the ‘carrier’ fluid and a plate and frame HX for final cooling. With sufficient knowledge of heat exchanger types and operating requirements, an appropriate selection can be made to optimise the process. [17] Monitoring and maintenance Online monitoring of commercial heat exchangers is done by tracking the overall heat transfer coefficient. The overall heat transfer coeffi cient tends to decline over time due to fouling.U=Q/A? Tlm By periodically calculating the overall heat transfer coefficient from exchanger flow rates and temperatures, the owner of the heat exchanger can estimate when cleaning the heat exchanger is economically attractive. Integrity inspection of plate and tubular heat exchanger can be tested in situ by the conductivity or helium gas methods. These methods confirm the integrity of the plates or tubes to prevent any cross contamination and the condition of the gaskets. Mechanical integrity monitoring of heat exchanger tubes may be conducted through Nondestructive methods such as eddy current testing.Fouling Main article: Fouling A heat exchanger in a steam power station contaminated with macrofouling. Fouling occurs when impurities deposit on the heat exchange surface. Deposition of these impurities can decrease heat transfer effectiveness significantly over time and are caused by: * Low wall shear stress * Low fluid velocities * Hi gh fluid velocities * Reaction product solid precipitation * Precipitation of dissolved impurities due to elevated wall temperatures The rate of heat exchanger fouling is determined by the rate of particle deposition less re-entrainment/suppression.This model was originally proposed in 1959 by Kern and Seaton. Crude Oil Exchanger Fouling. In commercial crude oil refining, crude oil is heated from 21  °C to 343  °C prior to entering the distillation column. A series of shell and tube heat exchangers typically exchange heat between crude oil and other oil streams to heat the crude to 260  °C prior to heating in a furnace. Fouling occurs on the crude side of these exchangers due to asphaltene insolubility. The nature of asphaltene solubility in crude oil was successfully modeled by Wiehe and Kennedy. 18] The precipitation of insoluble asphaltenes in crude preheat trains has been successfully modeled as a first order reaction by Ebert and Panchal[19] who expanded on the work of Ke rn and Seaton. Cooling Water Fouling. Cooling water systems are susceptible to fouling. Cooling water typically has a high total dissolved solids content and suspended colloidal solids. Localized precipitation of dissolved solids occurs at the heat exchange surface due to wall temperatures higher than bulk fluid temperature. Low fluid velocities (less than 3  ft/s) allow suspended solids to settle on the heat exchange surface.Cooling water is typically on the tube side of a shell and tube exchanger because it's easy to clean. To prevent fouling, designers typically ensure that cooling water velocity is greater than 0. 9 m/s and bulk fluid temperature is maintained less than 60  °C. Other approaches to control fouling control combine the â€Å"blind† application of biocides and anti-scale chemicals with periodic lab testing. Maintenance Plate heat exchangers must be disassembled and cleaned periodically. Tubular heat exchangers can be cleaned by such methods as acid cleani ng, sandblasting, high-pressure water jet, bullet cleaning, or drill rods.In large-scale cooling water systems for heat exchangers, water treatment such as purification, addition of chemicals, and testing, is used to minimize fouling of the heat exchange equipment. Other water treatment is also used in steam systems for power plants, etc. to minimize fouling and corrosion of the heat exchange and other equipment. A variety of companies have started using water borne oscillations technology to prevent biofouling. Without the use of chemicals, this type of technology has helped in providing a low-pressure drop in heat exchangers. In nature HumansThe human nasal passages serve as a heat exchanger, which warms air being inhaled and cools air being exhaled. You can demonstrate its effectiveness by putting your hand in front of your face and exhaling, first through your nose and then through your mouth. Air exhaled through your nose is substantially cooler. [20][21] In species that have e xternal testes (such as humans), the artery to the testis is surrounded by a mesh of veins called the pampiniform plexus. This cools the blood heading to the testis, while reheating the returning blood. Birds, fish, marine mammalsCountercurrent exchange conservation circuit Further information: Countercurrent exchange in biological systems â€Å"Countercurrent† heat exchangers occur naturally in the circulation system of fish, whales and other marine mammals. Arteries to the skin carrying warm blood are intertwined with veins from the skin carrying cold blood, causing the warm arterial blood to exchange heat with the cold venous blood. This reduces the overall heat loss in cold waters. Heat exchangers are also present in the tongue of baleen whales as large volumes of water flow through their mouths. 22][23] Wading birds use a similar system to limit heat losses from their body through their legs into the water. Carotid rete The carotid rete is a counter-current heat exchangi ng organ in some ungulates. The blood ascending the carotid arteries on its way to the brain, flows via a network of vessels where heat is discharged to the veins of cooler blood descending from the nasal passages. The carotid rete allows Thomson's gazelle to maintain its brain almost 3 °C cooler than the rest of the body, and therefore aids in tolerating bursts in metabolic heat roduction such as associated with outrunning cheetahs (during which the body temperature exceeds the maximum temperature at which the brain could function). [24] In industry Heat exchangers are widely used in industry both for cooling and heating large scale industrial processes. The type and size of heat exchanger used can be tailored to suit a process depending on the type of fluid, its phase, temperature, density, viscosity, pressures, chemical composition and various other thermodynamic properties.In many industrial processes there is waste of energy or a heat stream that is being exhausted, heat exch angers can be used to recover this heat and put it to use by heating a different stream in the process. This practice saves a lot of money in industry, as the heat supplied to other streams from the heat exchangers would otherwise come from an external source that is more expensive and more harmful to the environment. Heat exchangers are used in many industries, including: * Waste water treatment Refrigeration * Wine and beer making * Petroleum refining In waste water treatment, heat exchangers play a vital role in maintaining optimal temperatures within anaerobic digesters to promote the growth of microbes that remove pollutants. Common types of heat exchangers used in this application are the double pipe heat exchanger as well as the plate and frame heat exchanger. In aircraft In commercial aircraft heat exchangers are used to take heat from the engine's oil system to heat cold fuel. 25] This improves fuel efficiency, as well as reduces the possibility of water entrapped in the fu el freezing in components. [26] Early 2008, a Boeing 777 flying as British Airways Flight 38 crashed just short of the runway. In an early-2009 Boeing-update sent to aircraft operators, the problem was identified as specific to the Rolls-Royce engine oil-fuel flow heat exchangers. [26] Other heat exchangers, or Boeing 777 aircraft powered by GE or Pratt and Whitney engines, were not affected by the problem. [26]

American History In Literature Annotated Bibliography

American History In Literature - Annotated Bibliography Example Berkin, Carol. Making America: A History of the United States. Brief Sixth ed. Boston, MA: WADSWORTH CENGAGE Learning, 2014 This bibliography demonstrates history as a vibrant process twisted by human expectations, tricky choices, and frequently the startling consequences. The book encourages readers to reason historically and to grow into citizens who value their past. The straightforward narrative and clear chronology emphasize communication over intimidation making if very interesting to read. Berkin, Carol. Making America: A History of the United States. Brief Sixth ed. Boston, MA: WADSWORTH CENGAGE Learning, 2014 This book acknowledges President Abraham Lincoln consideration of other approaches to remedying the menace of slavery during his reign. The book uses various artistic devices to present a bigoted and flawed notion of colonization as a temporary fixture of U.S. As such, the historical book remembers Lincoln as having discarded his backup of colonization when he signed the declaration.

Murder of JonBenet Ramsey Case Study Example | Topics and Well Written Essays - 750 words

Murder of JonBenet Ramsey - Case Study Example Prior to the discovery of the body, the Ramsey's found a ransom note in the basement from â€Å"a foreign faction† demanding $118,000.00 for the safe return of Jon Benet (â€Å"JonBenet Ramsey Would Have Turned 22 Years Old This Week; Murder Case Remains Unsolved†, 2012). This particular piece of information pertaining to the ransom amount was a red flag for the police at the time. The amount was the exact amount that John had just received as a bonus earlier in the year, this coincidence led to the speculation that somehow, the person involved in the crime knew the Ramsey's on a personal level (Stuart, 2012). But without a follow up call on the ransom note, the Ramsey family decided that it would be in the best interest of all concerned to inform their family and friends of the situation while also preparing to follow the instructions for the ransom demand. The police began an intensive search for the child within hours only to have her father inform them that he had f ound the body of the child in the basement. The official cause of death is listed as asphyxiation due to craniocerebral trauma. But a more in-depth look at the autopsy records reveals that the child also suffered from a skull fracture and severe blunt trauma. Evidence gathered at the crime scene included a garrote made from tweed and the broken handle of a paintbrush that is believed to have been used to penetrate the child's vagina. Although there was penetration, the police could not accurately declare if sexual assault was one of the horrors that the child suffered in the hands of her abductors (Montaldo, 2013). Although a majority of the evidence collected by the police indicated that a stranger had caused the death of JonBenet, there were still certain things that led the district attorney to question the abduction theory because of the way the child's body was recovered at home. Somehow, the evidence that the DA was seeing did not add up to an abduction scenario. Rather, the s ights of the police investigating the case, and the media for that matter, then centered on the possibility that one or both parents of JonBenet had caused her death. Although the authorities and media fed the public lines that led those outside of the case to believe that Patsy and John had possibly murdered their daughter in cold blood, the courts saw otherwise. Patsy, who died of cancer in 2006, and John were exonerated by the federal courts in May 2003 due to lack of strong evidence to support the parent murderer theory (Montaldo, 2013). Their total exoneration came as new DNA evidence in the case was presented, proving that traces of DNA not belonging to family members was found on the preserved clothing of JonBenet (Gardner, 2010). With the discovery of new DNA evidence in the case, one can clearly see that, had the police not set upon a trial by publicity on the Ramsey's and concentrated instead on actual police investigation, they would have caught the actual perpetrator(s) in the case. During the course of the first investigation of the case, the police had conducted more than a few blunders that influenced the case and set upon creating a publicity machine using false leaked information to crucify the suffering parents instead. First of all, because Boulder, Colorado was known back then as a relatively peaceful and safe place for its

Motivation and Pay Case Study Example | Topics and Well Written Essays - 3000 words

Motivation and Pay - Case Study Example Many variables are found to be considerably related to indices of absence, the results appear to be unstable across situations and time. Every incentive program is based on a formula for enhancing motivation that engages four fundamental variables: effort, performance, outcomes, and satisfaction. The logic behind these programs goes something like this: employees at Gap Inc. will put in the accurate quantity of effort to meet performance hopes if these part time employees at Gap Inc. obtain the types of outcomes that include pay raises and promotions which will provide part time employees satisfaction. In simpler words, Gap Inc. should provide its employees what they want, and employees will work hard to get it. Conversely, the problem with most incentive programs like of Gap Inc. is that they center exclusively on the submission of outcomes and overlook the three beliefs that are the key to making the motivation solution work: The first conviction compacts with the relationship between employee effort and performance. The second compacts with the relationship between performance and outcomes. And the third compacts with the relationship between outcomes and satisfaction. These three beliefs form the basis of the belief system of motivation and performance. Accepting that these beliefs are decisive preconditions for motivatio... Accepting that these beliefs are decisive preconditions for motivation helps to explain why incentive programs generally yield such lackluster results like in case of Gap Inc. Since employees do not always hold these beliefs to be true, attempts to improve motivation by using incentives cannot make the grade, even when the incentives are highly desirable ones. Types of Motivation Problems One cannot do it Motivation problem: Lack of confidence Associated feelings: Self-doubt Anxiety Frustration Outcomes are not tied to one's performance Motivation problem: Lack of trust Associated feelings: Skepticism Disbelief Mistrust Outcomes will not be satisfying to one Motivation problem: Lack of satisfaction Associated feelings: Anger Rebelliousness Low Morale and Absenteeism At Gap Inc. a major transformation attempt only makes difficult the situation. If any of three beliefs are shaky to begin with, organisational change at Gap Inc. can weaken them even further. The result is often serious motivation and performance problems, at a time when organisations can least afford them, and a resultant surge in the negative emotions associated with change. When an employee believes 'one cannot do it' for example, one may develop a lack of self-confidence and begin to experience many of the unpleasant feelings that go along with it: self-doubt, anxiety, and frustration. About a year into the change effort, one manager portrayed the inner turmoil one went through by comparing the restructuring to building a ship at the same time one is trying to sail it. (Mele, 2003) Worker beliefs that 'outcomes are not tied to one's performance' can also escort to noteworthy motivation

Reflection Essay Example | Topics and Well Written Essays - 750 words - 23

Reflection - Essay Example The development of the sporting activity was promoted by the technological advancement after the world war two, it was further supported by the car industry involvement in the sporting activity like Ford. From the video, it is relatively clear that the sporting activity was promoted by the increased interest of the teenagers in Southern California from the year 1937. These were further promoted by the development of the southern California timing association that was involved in organizing events. Young people in schools were crazy with the sporting activities with each student wanting to own a car. Those who had cars spent most of their time in the garage to ensure that they had working cars that can be used for car racing. The parents and the authorities could not be able to stop the behaviour that was getting rampant among youths in South California. It can be said, with relative conviction that the current view of the sporting activity is different from what people thought of it in the past. The drag racing had gained a bad reputation among parents, police and media. This was due to the fact there were a number of accidents that were involved at that time. The fatal one was when Bruce, Nick and Dennis lost their lives while racing in Los Angeles. Further the activity was not a sport, hence did not get that attention of a sporting activity. The racing activity attracted people who were there just there for fun. For instance, at this time, there was no timing of the events, there were no prices given to the winners and entrance fee. People participated just to win and have fun. Thus, I think that that was the reason as to why there was a poor reputation of the sporting adding to the fact that there were a number of road accidents associated with it. The period post-World War II, gave rise to more sophisticated cars that were developed from the then technological advancement. Most vehicle producing companies had invested in

Legislative Analysis Essay Example | Topics and Well Written Essays - 1000 words

Legislative Analysis - Essay Example 2454, The American Clean Energy and Security Act of 2009, which establishes a federal cap-and-trade program to reduce GHG emissions (Key Provisions for Climate Legislation). GHG Emission the United States of America produces and emits a significant portion of GHG and helps developing Global warming throughout the world. It is estimated that of the total energy use Homes account for about 35% of the potential efficiency gains while the industrial sector accounts for 40% and the commercial sector 25%. Of the total emission of GHG in USA, California emits a robust figure of GHG. California produces roughly 1.4 percent of the world's and 6.2 percent of the total U.S., greenhouse gases (California Climate Change Portal). The state has been working on and finding solutions of impacts towards people due to climate since 1988. The executive order of 2005 issued by Arnold Schwarzenegger on climate change kicked into high gear to further advance clean renewable energy and other solutions to lower our state's greenhouse gas (GHG) emissions. Again, most importantly, the California Global Warming Solutions Act of 2006 established the first-in-the-world comprehensive program of regulatory and market mechanisms to achieve real, quantifiable, cost-effective reductions of GHG. Provision has also been framed to prevent the unscrupulous trend of the polluters. To this end, the Act reveals that Polluters would balance out some of their emissions by purchasing carbon "offsets," which are official certificates that greenhouse gas emissions have been avoided or taken out of the air The provision of setting up a New Clean Energy Deployment Administration is certainly a wise decision to monitor and improve the overall situation. Recommendations and Concluding Remarks The biggest opportunity to improve US energy situation is a major investment program to make homes and businesses more efficient. An investment of $520 billion in improvements like sealing products and replacing inefficient appliances could produce $1.2 trillion in savings on energy bills by 2020(Galbraith, 2009). If such a program can be carried out over the next decade, the country could save around 23 % of total energy use. However, to improve the existing situation further notice on the Act and undertaking following actions are necessary. The Senator/ Government should rightly consider about the Act so that U.S. climate policy become successful in both reducing GHG emissions and addressing the cost concerns of consumers. Efforts should also be made so that framing necessary legislation is possible that reduces GHG emissions 80 percent below current emissions levels by 2050, while providing strong, effective consumer-protection measures to help reduce electricity price increases as we transition to a low-carbon future. Provisions should be made to allocate emissions allowancesto the electric power sector that will help toreduce price increases to all electricity customers-large and small, rural, urban, and suburban-without compromising environmental gains. Again, necessary steps should be undertaken to provide some allowances to that Merchant coal generators to help mitigate price increases in wholesale electricity markets. There is nothing denying that setting reasonable and

UK employee relations have seen significant changes over the last 30 Essay

UK employee relations have seen significant changes over the last 30 years. Evaluate the importance of these changes and how the - Essay Example One of the other changes that have occurred since 1980 is the growth of non-standard employment, which includes short fixed-term contract, temporary employment, freelance work, outwork, homework and part-time work (Millward et al., 2000, p. 44). As far as part-time employment is concerned, 25% of firms had part-time workers in 1980, while 44% of the firms had part-time workers in 1998 (Millward et al., 2000, p. 44). The ever-increasing use of part-time workers has implications for unions, as part-timers are much less likely to be a part of a union, and workplaces that employ a substantial number of part-time workers are less likely to recognize trade unions (Millward et al., 2000, p. 45). With regards to workers with short fixed-term contracts, this number rose from 19% of all firms answering they had at least one employee with a short fixed-term contract in 1980 to 35% answering this question in 1998 (Millward et al., 2000, p. 46). With regards to temporary employment, the percentag e increased from 20% of all firms answering in the affirmative whether they had at least one temporary worker in 1980 to 28% answering this same question in 1998 (Millward et al., 2000, p. 47). With regards to free-lance workers, the data is unclear as to whether this number has risen or fallen between the years 1980 and 1998 (Millward et al., 2000, p. 48). Employee relations have also undergone a big shift since 1980. One of the shifts is in regards to whom manages the employees – in 1980, employees were managed by a generalist; in 1998, the shift was towards management by a specialist in employee relations and line managers. In 1980, generalists accounted for 75% of all managers; in 1998, they accounted for only 46% (Millward et al., 2000, p. 52). One of the reasons for this shift is the increased prevalence of foreign firms, as they rely more upon employee relations specialists, such as human resource managers than domestic workplaces (Millward et al., 2000, p. 54). What t he shift towards employing ER specialists for employee relations shows is that firms increasingly see employee relations as a position that requires a professional with formal credentials (Millward et al., 2000, p. 81). Another major change is that more women are managing employee relations – just 12% of employee relations positions were held by women in 1980, while 39% of employee relations positions were held by women in 1998 (Millward et al., 2000, p. 59). Another of the broad changes has concerned unionism. In particular, unionism has fallen considerably since 1980. In 1980, 73% of workplaces had one or more union members; that number fell to 54% in 1998 (Millward et al., 2000, p. 84). Moreover, the density of union members fell from 47% in 1980 to 36% in 1998, with the fall most steep in the private sector, with the public sector showing much less of a decline (Millward et al.., 2000, p. 103). With the decline in numbers came an attendant decline in clout, with collectiv e bargaining coverage declining during the period between 1984-1998 (Millward et al., 2000, pp. 180-181). In 2004, around 34% of all employees were union members, and there were still roughly 36% of workplaces with a union, mainly in the public sector (Kersey et al., 2004). However, the decline of unions did not mean that employees lost their voice – there simply was a shift from union representatives being the voice of employees to the implementation of channels where employees can communicate directly with