1 June, 2014

Vacuum Pumps

This is my project so plz elaborate it help me. Do you know what I found? Vacuum pumps are combined with chambers and operational procedures into a wide variety of vacuum systems. Sometimes more than one pump will be used (in series or in parallel) in a single application. A partial vacuum, or rough vacuum, can be created using a positive displacement pump that transports a gas load from an inlet port to an outlet (exhaust) port. Because of their mechanical limitations, such pumps can only achieve a fairly crude partial vacuum. To achieve a more-perfect vacuum, other techniques must then be used, typically in series (usually following an initial fast “pump down” using a positive displacement pump). Some examples might be use of an oil sealed rotary vane pump backing a diffusion pump, or a dry scroll pump backing a turbo molecular pump. There are other combinations depending on the vacuum quality desired. Achieving truly high vacuum is difficult because all of the materials exposed to the vacuum must be carefully evaluated for their out gassing and vapor pressure properties. For example, oils, greases, rubber, or plastic used to form gaskets and seals must not boil off when exposed to the vacuum, or the gases they produce would prevent the creation of the desired degree of vacuum. Often, all of the surfaces exposed to the vacuum must be baked at high temperature to drive off adsorbed gases. Out gassing can also be reduced simply by desiccation prior to vacuum pumping. High vacuum systems generally require metal chambers with metal O-ring seals such as Klein flanges or ISO flanges. The system must be clean and free of organic matter to minimize out gassing. All materials, solid or liquid, have a small vapor pressure, and their out gassing becomes important when the vacuum pressure falls below this vapor pressure. As a result, many materials that work well in low vacuums, such as epoxy, will become a problematic source of out gassing when attempting to achieve high vacuums. With these standard precautions, vacuums of 1 mPa are easily achieved with off-the-shelf molecular pumps. With careful design and operation, 1 µPa is possible. Several types of pumps may be used in sequence or in parallel. In a typical pump down sequence, a positive displacement pump would be used to remove most of the gas from a chamber, starting from atmosphere (760 Torr, 101 kPa) to 25 Torr (3 kPa). Then a Absorption pump would be used to bring the pressure down to 10-4 Torr (10 mPa). A cryopump or turbo molecular pump would be used to bring the pressure further down to 10-8 Torr (1 µPa). An additional ion pump can be started below 10-6 Torr to remove gases which are not adequately handled by a cryopump or turbo pump, such as helium or hydrogen. Ultra high vacuum generally requires custom-built equipment, strict operational procedures, and a fair amount of trial-and-error. Ultra-high vacuum systems are mostly made of stainless steel with metal-gasketed con flat flanges. The system is mostly baked, preferably under vacuum, to temporarily raise the vapor pressure of all out gassing materials in the system and boil them off. If necessary, this out gassing of the system can also be performed at room temperature, but this takes much more time. Once the bulk of the out gassing materials are boiled off and evacuated, the system may be cooled to lower vapor pressures and minimize residual out gassing during actual operation. Some systems are cooled well below room temperature by liquid nitrogen to shut down residual out gassing and simultaneously cryopump the system. In ultra-high vacuum systems, some very odd leakage paths and out gassing sources must be considered. The water absorption of aluminum and palladium becomes an unacceptable source of out gassing, and even the absorptivity of hard metals such as stainless steel or titanium must be considered. Some oils and greases will boil off in extreme vacuums. The porosity of the metallic chamber walls may have to be considered, and the grain direction of the metallic flanges should be parallel to the flange face. The impact of molecular size must be considered. Smaller molecules can leak in more easily and are more easily absorbed by certain materials, and molecular pumps are less effective at pumping gases with lower molecular weights. A system may be able to evacuate nitrogen, (the main component of air,) to the desired vacuum, but the chamber could still be full of residual atmospheric hydrogen and helium. Vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special out gassing problems. Vacuum pumps are used in many industrial and scientific processes including: * The production of most types of electric lamps, vacuum tubes, and CRTs where the device is either left evacuated or re-filled with a specific gas or gas mixture * Semiconductor processing, notably ion implantation and sputter deposition * Electron microscopy * Medical processes that require suction * Mass spectrometers to create an ultra high vacuum between the ion source and the detector * Vacuum engineering Vacuum may be used to power mechanical devices. In diesel-engined automobiles, a pump fitted on the engine (usually on the camshaft) is used to produce vacuum. In gasoline-powered automobiles, instead, vacuum is obtained as a side-effect of the operation of the engine and the flow restriction created by the throttle plate. This vacuum may then be used to power: * The booster for the power brakes * Motors that move dampers in the ventilation system * The throttle driver in the cruise control servomechanism * Controlling other devices (such as turbo geometry, EGR, etc. ) In an aircraft, the vacuum source is often used to power gyroscopes in the various flight instruments. To prevent the complete loss of instrumentation in the event of an electrical failure, the instrument panel is deliberately designed with certain instruments powered by electricity and other instruments powered by the vacuum source.

Some of you may have wondered what the difference is between a 1 stage and 2 stage pump. In this video I go over the difference between the two, hopefully he. . .


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1 Comment

  1. Mandy Nguyen May 29, 2014 at 1:25 am #
    A vacuum pump takes the air out of a container (evacuation). When the air volume is removed, there is less pressure on the inside. creating the lower pressure.