Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. The best-known brand name of PTFE-based formulas is Teflon by Chemours. Chemours is a spin-off of DuPont, which discovered the compound in 1938. Another popular brand name of PTFE is Syncolon® by Synco Chemical Corporation. PTFE is a fluorocarbon solid, as it is a high molecular weight compound consisting wholly of carbon and fluorine. PTFE is hydrophobic: neither water nor water-containing substances wet PTFE, as fluorocarbons demonstrate mitigated London dispersion forces due to the high electronegativity of fluorine. PTFE has one of the lowest coefficients of friction of any solid. PTFE is used as a non-stick coating for other cookware, it is nonreactive because of the strength of carbon–fluorine bonds, so it is used in containers and pipework for reactive and corrosive chemicals. Where used as a lubricant, PTFE reduces friction and energy consumption of machinery, it is used as a graft material in surgical interventions.
It is frequently employed as coating on catheters. PTFE was accidentally discovered in 1938 by Roy J. Plunkett while he was working in New Jersey for DuPont; as Plunkett attempted to make a new chlorofluorocarbon refrigerant, the tetrafluoroethylene gas in its pressure bottle stopped flowing before the bottle's weight had dropped to the point signaling "empty." Since Plunkett was measuring the amount of gas used by weighing the bottle, he became curious as to the source of the weight, resorted to sawing the bottle apart. He found the bottle's interior coated with a waxy white material, oddly slippery. Analysis showed that it was polymerized perfluoroethylene, with the iron from the inside of the container having acted as a catalyst at high pressure. Kinetic Chemicals patented the new fluorinated plastic in 1941, registered the Teflon trademark in 1945. By 1948, DuPont, which founded Kinetic Chemicals in partnership with General Motors, was producing over two million pounds of Teflon brand PTFE per year in Parkersburg, West Virginia.
An early use was in the Manhattan Project as a material to coat valves and seals in the pipes holding reactive uranium hexafluoride at the vast K-25 uranium enrichment plant in Oak Ridge, Tennessee. In 1954, Collette Grégoire, the wife of French engineer Marc Grégoire urged him to try the material he had been using on fishing tackle on her cooking pans, he subsequently created the first non-stick pans under the brandname Tefal. In the United States, Marion A. Trozzolo, using the substance on scientific utensils, marketed the first US-made PTFE-coated pan, "The Happy Pan", in 1961. However, Tefal was not the only company to utilize PTFE in nonstick cookware coatings. In subsequent years, many cookware manufacturers developed proprietary PTFE-based formulas, including Swiss Diamond International, which uses a diamond-reinforced PTFE formula. Other cookware companies, such as Meyer Corporation's Anolon, use Teflon nonstick coatings purchased from Chemours. Chemours is a 2015 corporate spin-off of DuPont.
In the 1990s, it was found that PTFE could be radiation cross-linked above its melting point in an oxygen-free environment. Electron beam processing is one example of radiation processing. Cross-linked PTFE has improved radiation stability; this was significant because, for many years, irradiation at ambient conditions has been used to break down PTFE for recycling. This radiation-induced chain scission allows it to be more reground and reused. PTFE is produced by free-radical polymerization of tetrafluoroethylene; the net equation is n F2C=CF2 → −n−Because tetrafluoroethylene can explosively decompose to tetrafluoromethane and carbon, special apparatus is required for the polymerization to prevent hot spots that might initiate this dangerous side reaction. The process is initiated with persulfate, which homolyzes to generate sulfate radicals: 2− ⇌ 2 SO4•−The resulting polymer is terminated with sulfate ester groups, which can be hydrolyzed to give OH end-groups; because PTFE is poorly soluble in all solvents, the polymerization is conducted as an emulsion in water.
This process gives a suspension of polymer particles. Alternatively, the polymerization is conducted using a surfactant such as PFOS. PTFE is a thermoplastic polymer, a white solid at room temperature, with a density of about 2200 kg/m3. According to Chemours, its melting point is 600 K, it maintains high strength and self-lubrication at low temperatures down to 5 K, good flexibility at temperatures above 194 K. PTFE gains its properties from the aggregate effect of carbon-fluorine bonds, as do all fluorocarbons; the only chemicals known to affect these carbon-fluorine bonds are reactive metals like the alkali metals, at higher temperatures such metals as aluminium and magnesium, fluorinating agents such as xenon difluoride and cobalt fluoride. The coefficient of friction of plastics is measured against polished steel. PTFE's coefficient of friction is 0.05 to 0.10, the third-lowest of any known solid material. PTFE's resistance to van de
An expansion tank or expansion vessel is a small tank used to protect closed water heating systems and domestic hot water systems from excessive pressure. The tank is filled with air, whose compressibility cushions shock caused by water hammer and absorbs excess water pressure caused by thermal expansion; the modern vessel is a small tank divided in two by a rubber diaphragm. One side therefore contains water; the other, the dry side, contains air under pressure, a Schrader valve for checking pressures and adding air. When the heating system is empty or at the low end of the normal range of working pressure, the diaphragm is pushed against the water inlet. An older style of expansion tank was larger, oriented horizontally, had no rubber diaphragm separating the water from the air pocket; this now obsolete style would transfer air from the tank to the highest point in the system, due to air dissolving in the water, coming out of solution elsewhere in the system. This in turn required periodic draining of the expansion tank, as well as periodic bleeding of the system, to maintain its effectiveness.
The rubber diaphragm in modern expansion tanks prevents this undesired transfer of air, helps maintain low levels of oxygen within the pipes, reducing corrosion in the system. When expansion tanks are used in domestic hot water systems, the tank and the diaphragm must conform to drinking water regulations and must be capable of accommodating the required volume of water. In the past, domestic plumbing systems contained more air than they do and the trapped air acted as a crude expansion tank. In new and upgraded systems, expansion tanks are used more than in the past. In the UK, prior to the use of sealed expansion tanks, "open" tanks were installed in the roof space to accommodate the water's expansion. This, without effective loft insulation, could fall below freezing, could cause the pipework supplying the tank to freeze. However, with good pipe and tank insulation, this was in practice quite rare. Although such systems were remarkably trouble free, there are concerns about the potability of water from roof tanks due to the possibility of contamination.
The other major disadvantage is that the water pressure from a roof tank is lower than mains water pressure, making the use of mixer taps sometimes unpredictable. Domestic hydronic heating and cooling systems include an expansion tank to buffer pressure changes due to expansion and contraction of the water they use for heat transfer. A minimum pressure of 4-5 psig at the top of a closed hydronic system is suggested. In Europe the design and the construction of expansion tanks are ruled by EN 13831 according to Pressure Equipment Directive 97/23/EC. An expansion tank known as "overflow bottle", is used in the cooling system of most internal combustion engines, to allow the coolant, the antifreeze, the air in the system to expand with rising temperature and pressure; the tank is called a "coolant recovery tank", since it prevents venting and permanent loss of coolant, by allowing it to be sucked back into the cooling system as the engine cools. Similar devices are used in large-scale pumping stations, where they may be called expansion chambers or hydrophores, to maintain an pressure and to reduce the effects of water hammer.
The moving parts of a machine are those parts of it that move. Machines include fixed parts; the moving parts have constrained motions. Moving parts do not include any moving fluids, such as coolant or hydraulic fluid. Moving parts do not include any mechanical locks, switches and bolts, screw caps for bottles etc. A system with no moving parts is described as "solid state"; the amount of moving parts in a machine is a factor in its mechanical efficiency. The greater the number of moving parts, the greater the amount of energy lost to heat by friction between those parts. For example, in a modern automobile engine 7% of the total power obtained from burning the engine's fuel is lost to friction between the engine's moving parts. Conversely, the fewer the number of moving parts, the greater the efficiency. Machines with no moving parts at all can be efficient. An electrical transformer, for example, has no moving parts, its mechanical efficiency is above the 90% mark. Two means are used for overcoming the efficiency losses caused by friction between moving parts.
First, moving parts are lubricated. Second, the moving parts of a machine are designed so that they have a small amount of contact with one another; the latter, in its turn, comprises two approaches. A machine can be reduced in size, thereby quite reducing the areas of the moving parts that rub against one another. Lubrication reduces wear, as does the use of suitable materials; as moving parts wear out, this can affect the precision of the machine. Designers thus have to design moving parts with this factor in mind, ensuring that if precision over the lifetime of the machine is paramount, that wear is accounted for and, if possible, minimized; the scientific and engineering discipline that deals with the lubrication and wear of moving parts is tribology, an interdisciplinary field that encompasses materials science, mechanical engineering and mechanics. As mentioned, wear is a concern for moving parts in a machine. Other concerns that lead to failure include corrosion, thermal stress and heat generation, fatigue loading, cavitation.
Fatigue is related to large inertial forces, is affected by the type of motion that a moving part has. A moving part that has a uniform rotation motion is subject to less fatigue than a moving part that oscillates back and forth. Vibration leads to failure when the forcing frequency of the machine's operation hits a resonant frequency of one or more moving parts, such as rotating shafts. Designers avoid these problems by calculating the natural frequencies of the parts at design time, altering the parts to limit or eliminate such resonance, yet further factors that can lead to failure of moving parts include failures in the cooling and lubrication systems of a machine. One final, factor related to failure of moving parts is kinetic energy; the sudden release of the kinetic energy of the moving parts of a machine causes overstress failures if a moving part is impeded in its motion by a foreign object. For example, consider a stone caught on the blades of a fan or propellor, or the proverbial "spanner/monkey wrench in the works".
The kinetic energy of a machine is the sum of the kinetic energies of its individual moving parts. A machine with moving parts can, mathematically, be treated as a connected system of bodies, whose kinetic energies are summed; the individual kinetic energies are determined from the kinetic energies of the moving parts' translations and rotations about their axes. The kinetic energy of rotation of the moving parts can be determined by noting that every such system of moving parts can be reduced to a collection of connected bodies rotating about an instantaneous axis, which form either a ring or a portion of an ideal ring, of radius a rotating at n revolutions per second; this ideal ring is known as the equivalent flywheel. The integral of the squares of the radii all the portions of the ring with respect to their mass ∫ a 2 d m expressible if the ring is modelled as a collection of discrete particles as the sum of the products of those mass and the squares of their radii ∑ k = 0 n m k × a k 2 is the ring's moment of inertia, denoted I.
The rotational kinetic energy of the whole system of moving parts is 1 2 I ω 2, where ω is the angular velocity of the moving parts about the same axis as the moment of inertia. The kinetic energy of translation of the moving parts is 1 2
A mechanical or physical shock is a sudden acceleration caused, for example, by impact, drop kick, earthquake, or explosion. Shock is a transient physical excitation. Shock describes matter subject to extreme rates of force with respect to time. Shock is a vector; the unit g is conventionally used. A shock pulse can be characterised by its peak acceleration, the duration, the shape of the shock pulse; the Shock response spectrum is a method for further evaluating a mechanical shock. Shock measurement is of interest in several fields such as Propagation of heel shock through a runner’s body Measure the magnitude of a shock need to cause damage to an item: ‘’’fragility’’’. Measure shock attenuation through athletic flooring Measuring the effectiveness of a shock absorber Measuring the shock absorbing ability of package cushioning Measure the ability of an athletic helmet to protect people Measure the effectiveness of shock mounts Determining the ability of structures to resist seismic shock: earthquakes, etc.
Determining whether personal protective fabric attenuates or amplifies shocks Verifying that a Naval ship and its equipment can survive explosive shocks Shocks are measured by accelerometers but other transducers and high speed imaging are used. A wide variety of laboratory instrumentation is available. Field shocks are variable and have uneven shapes. Laboratory controlled shocks have uneven shapes and include short duration spikes. Governing test methods and specifications provide detail about the conduct of shock tests. Proper placement of measuring instruments is critical. Fragile items and packaged goods respond with variation to uniform laboratory shocks. For example MIL-STD-810G Method 516.6 indicates: ‘’at least three times in both directions along each of three orthogonal axes”. Shock testing falls into two categories, classical shock testing and pyroshock or ballistic shock testing. Classical shock testing consists of the following shock impulses: half sine, sawtooth wave, trapezoid. Pyroshock and ballistic shock tests are not considered classical shocks.
Classical shocks can be performed on Electro Dynamic Shakers, Free Fall Drop Tower or Pneumatic Shock Machines. A classical shock impulse is created; this abrupt change in direction causes a rapid velocity change. Use of proper test methods and Verification and validation protocols are important for all phases of testing and evaluation. Mechanical shock has the potential for damaging an item or an element of the item: A brittle or fragile item can fracture. For example, two crystal wine glasses may shatter. A shear pin in an engine is designed to fracture with a specific magnitude of shock. Note that a soft ductile material may sometimes exhibit brittle failure during shock due to time-temperature superposition. A ductile item can be bent by a shock. For example, a copper pitcher may bend; some items may appear to be not damaged by a single shock but will experience fatigue failure with numerous repeated low-level shocks. A shock may result in only minor damage. However, cumulative minor damage from several shocks will result in the item being unusable.
A shock may not produce immediate apparent damage but might cause the service life of the product to be shortened: the reliability is reduced. A shock may cause an item to become out of adjustment. For example, when a precision scientific instrument is subjected to a moderate shock, good metrology practice may be to have it recalibrated before further use; some materials such as primary high explosives may detonate with mechanical impact. When glass bottles of liquid are dropped or subjected to shock, the water hammer effect may cause hydrodynamic glass breakage; when laboratory testing, field experience, or engineering judgement indicates that an item could be damaged by mechanical shock, several courses of action might be considered: Reduce and control the input shock at the source. Modify the item to improve its toughness or support it to better handle shocks. Use shock absorbers, shock cushions to control the shock transmitted to the item. Cushioning reduces the peak acceleration by extending the duration of the shock.
Plan for failures: accept certain losses. Have redundant systems available, etc. DeSilva, C. W. "Vibration and Shock Handbook", CRC, 2005, ISBN 0-8493-1580-8 Harris, C. M. and Peirsol, A. G. "Shock and Vibration Handbook", 2001, McGraw Hill, ISBN 0-07-137081-1 ISO 18431:2007 - Mechanical vibration and shock ASTM D6537, Standard Practice for Instrumented Package Shock Testing for Determination of Package Performance. MIL-STD-810G, Environmental Test Methods and Engineering Guidelines, 2000, sect 516.6 Brogliato, B. "Nonsmooth Mechanics. Models and Control", Springer London, 2nd Edition, 1999. Response to mechanical shock, Department of Energy, Shock Response Spectrum, a primer, A Study in the Application of SRS
Hydraulic shock is a pressure surge or wave caused when a fluid a liquid but sometimes a gas, in motion is forced to stop or change direction suddenly. This phenomenon occurs when a valve closes at an end of a pipeline system, a pressure wave propagates in the pipe; this pressure wave can cause major problems, from vibration to pipe collapse. It is possible to reduce the effects of the water hammer pulses with accumulators, expansion tanks, surge tanks, blowoff valves, other features. Rough calculations can be made either using the Zhukovsky equation, or more accurate ones using the method of characteristics. In the 1st century B. C. Marcus Vitruvius Pollio described the effect of water hammer in lead pipes and stone tubes of the Roman public water supply. Water hammer was exploited before there was a word for it. In 1796, French inventor Joseph Michel Montgolfier built a hydraulic ram for his paper mill in Voiron. In French and Italian, the terms for "water hammer" come from the hydraulic ram: coup de bélier and colpo d'ariete both mean "blow of the ram".
As the 19th century witnessed the installation of municipal water supplies, water hammer became a concern to civil engineers. Water hammer interested physiologists who were studying the circulatory system. Although it was prefigured in work by Thomas Young, the theory of water hammer is considered to have begun in 1883 with the work of German physiologist Johannes von Kries, investigating the pulse in blood vessels. However, his findings went unnoticed by civil engineers. Kries's findings were subsequently derived independently in 1898 by the Russian fluid dynamicist Nikolay Yegorovich Zhukovsky, in 1898 by the American civil engineer Joseph Palmer Frizell, in 1902 by the Italian engineer Lorenzo Allievi; when a pipe is closed at the outlet, the mass of water before the closure is still moving, thereby building up high pressure and a resulting shock wave. In domestic plumbing this is experienced as a loud banging resembling a hammering noise. Water hammer can cause pipelines to break. Air traps or stand pipes are sometimes added as dampers to water systems to absorb the damaging forces caused by the moving water.
In hydroelectric generating stations, the water traveling along the tunnel or pipeline may be prevented from entering a turbine by closing a valve. For example, if there is 14 km of tunnel of 7.7 m diameter full of water travelling at 3.75 m/s, that represents 8,000 megajoules of kinetic energy that must be arrested. This arresting is achieved by a surge shaft open at the top, into which the water flows; as the water rises up the shaft its kinetic energy is converted into potential energy, which causes the water in the tunnel to decelerate. At some hydroelectric power stations, such as the Saxon Falls Hydro Power Plant In Michigan, what looks like a water tower is one of these devices, known in these cases as a surge drum. In the home, a water hammer may occur when a dishwasher, washing machine or toilet shuts off water flow; the result may be heard as some shuddering. On the other hand, when an upstream valve in a pipe closes, water downstream of the valve attempts to continue flowing creating a vacuum that may cause the pipe to collapse or implode.
This problem can be acute if the pipe is on a downhill slope. To prevent this and vacuum relief valves or air vents are installed just downstream of the valve to allow air to enter the line to prevent this vacuum from occurring. Other causes of water hammer are pump check valve slam. To alleviate this situation, it is recommended to install non-slam check valves as they do not rely on gravity or fluid flow for their closure. For vertical pipes, other suggestions include installing new piping that can be designed to include air chambers to alleviate the possible shockwave of water due to excess water flow. Steam distribution systems may be vulnerable to a situation similar to water hammer, known as steam hammer. In a steam system, a water hammer most occurs when some of the steam condenses into water in a horizontal section of the piping. Steam picks up the water, forming a "slug", hurls this at high velocity into a pipe fitting, creating a loud hammering noise and stressing the pipe; this condition is caused by a poor condensate drainage strategy.
Where air filled traps are used, these become depleted of their trapped air over a long period through absorption into the water. This can be cured by shutting off the supply, opening taps at the highest and lowest locations to drain the system, closing the taps and re-opening the supply. On turbocharged internal combustion engines, a fluid hammer can take place when the throttle is closed while the turbocharger is forcing air into the engine. A pressure relief valve placed before the throttle prevents the air from surging against the throttle body by diverting it elsewhere, thus protecting the turbocharger from pressure damage; this valve can either recirculate the air into the turbocharger's intake, or it can blow the air into th
Infrared radiation, sometimes called infrared light, is electromagnetic radiation with longer wavelengths than those of visible light, is therefore invisible to the human eye, although IR at wavelengths up to 1050 nanometers s from specially pulsed lasers can be seen by humans under certain conditions. IR wavelengths extend from the nominal red edge of the visible spectrum at 700 nanometers, to 1 millimeter. Most of the thermal radiation emitted by objects near room temperature is infrared; as with all EMR, IR carries radiant energy and behaves both like a wave and like its quantum particle, the photon. Infrared radiation was discovered in 1800 by astronomer Sir William Herschel, who discovered a type of invisible radiation in the spectrum lower in energy than red light, by means of its effect on a thermometer. More than half of the total energy from the Sun was found to arrive on Earth in the form of infrared; the balance between absorbed and emitted infrared radiation has a critical effect on Earth's climate.
Infrared radiation is emitted or absorbed by molecules when they change their rotational-vibrational movements. It excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines transmission of photons in the infrared range. Infrared radiation is used in industrial, military, law enforcement, medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space such as molecular clouds, detect objects such as planets, to view red-shifted objects from the early days of the universe. Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, to detect overheating of electrical apparatus. Extensive uses for military and civilian applications include target acquisition, night vision and tracking.
Humans at normal body temperature radiate chiefly at wavelengths around 10 μm. Non-military uses include thermal efficiency analysis, environmental monitoring, industrial facility inspections, detection of grow-ops, remote temperature sensing, short-range wireless communication and weather forecasting. Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers to 1 millimeter; this range of wavelengths corresponds to a frequency range of 430 THz down to 300 GHz. Below infrared is the microwave portion of the electromagnetic spectrum. Sunlight, at an effective temperature of 5,780 kelvins, is composed of near-thermal-spectrum radiation, more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, 32 watts is ultraviolet radiation. Nearly all the infrared radiation in sunlight is shorter than 4 micrometers. On the surface of Earth, at far lower temperatures than the surface of the Sun, some thermal radiation consists of infrared in the mid-infrared region, much longer than in sunlight.
However, black body or thermal radiation is continuous: it gives off radiation at all wavelengths. Of these natural thermal radiation processes, only lightning and natural fires are hot enough to produce much visible energy, fires produce far more infrared than visible-light energy. In general, objects emit infrared radiation across a spectrum of wavelengths, but sometimes only a limited region of the spectrum is of interest because sensors collect radiation only within a specific bandwidth. Thermal infrared radiation has a maximum emission wavelength, inversely proportional to the absolute temperature of object, in accordance with Wien's displacement law. Therefore, the infrared band is subdivided into smaller sections. A used sub-division scheme is: NIR and SWIR is sometimes called "reflected infrared", whereas MWIR and LWIR is sometimes referred to as "thermal infrared". Due to the nature of the blackbody radiation curves, typical "hot" objects, such as exhaust pipes appear brighter in the MW compared to the same object viewed in the LW.
The International Commission on Illumination recommended the division of infrared radiation into the following three bands: ISO 20473 specifies the following scheme: Astronomers divide the infrared spectrum as follows: These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges, hence different environments in space; the most common photometric system used in astronomy allocates capital letters to different spectral regions according to filters used. These letters are understood in reference to atmospheric windows and appear, for instance, in the titles of many papers. A third scheme divides up the band based on the response of various detectors: Near-infrared: from 0.7 to 1.0 µm. Short-wave infrared: 1.0 to 3 µm. InGaAs covers to about 1.8 µm. Mid-wave infrared: 3 to 5 µm (defined by the atmospheric window and covered by indium antimonide and mercury cadmium telluride and by lead