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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Diffusion pump</span></span>

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<p><b>Diffusion pumps</b> use a high speed jet of vapor to direct gas <a href="Molecule" title="Molecule">molecules</a> in the pump throat down into the bottom of the pump and out the exhaust. They were the first type of high <a href="Vacuum_pump" title="Vacuum pump">vacuum pumps</a> operating in the regime of <a href="Free_molecular_flow" title="Free molecular flow">free molecular flow</a>, where the movement of the gas molecules can be better understood as <a href="Diffusion" title="Diffusion">diffusion</a> than by conventional <a href="Fluid_dynamics" title="Fluid dynamics">fluid dynamics</a>. Invented in 1915 by <a href="Wolfgang_Gaede" title="Wolfgang Gaede">Wolfgang Gaede</a>, he named it a <i>diffusion pump</i> since his design was based on the finding that gas cannot diffuse against the vapor stream, but will be carried with it to the exhaust.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> However, the principle of operation might be more precisely described as <b>gas-jet pump</b>, since diffusion also plays a role in other types of high vacuum pumps. In modern textbooks, the diffusion pump is categorized as a <a href="Vacuum_pump#Momentum_transfer" title="Vacuum pump">momentum transfer</a> pump.

</p><p>The diffusion pump is widely used in both industrial and research applications. Most modern diffusion pumps use <a href="Silicone_oil" title="Silicone oil">silicone oil</a> or <a href="Polyphenyl_ether" title="Polyphenyl ether">polyphenyl ethers</a> as the working fluid.

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<div class="mw-heading mw-heading2"><h2 id="History">History</h2></div>

<p>In the late 19th century, most vacuums were created using a <a href="Sprengel_pump" title="Sprengel pump">Sprengel pump</a>, which had the advantage of being very simple to operate, and capable of achieving quite good vacuum given enough time. Compared to later pumps, however, the pumping speed was very slow and the <a href="Vapor_pressure" title="Vapor pressure">vapor pressure</a> of the liquid mercury limited the ultimate vacuum.

</p><p>Following his invention of the <a href="Holweck_pump" class="mw-redirect" title="Holweck pump">molecular pump</a>, <a href="Wolfgang_Gaede" title="Wolfgang Gaede">Wolfgang Gaede</a> invented the diffusion pump in 1915,<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> and originally used <a href="Mercury_(element)" title="Mercury (element)">elemental mercury</a> as the working fluid. After its invention, the design was quickly commercialized by <a href="Leybold_GmbH" title="Leybold GmbH">Leybold</a>.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> It was then improved by <a href="Irving_Langmuir" title="Irving Langmuir">Irving Langmuir</a><sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> and W. Crawford. <a href="Cecil_Reginald_Burch" title="Cecil Reginald Burch">Cecil Reginald Burch</a> discovered the possibility of using <a href="Silicone_oil" title="Silicone oil">silicone oil</a> in 1928.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Oil_diffusion_pumps">Oil diffusion pumps</h2></div>

<p>An oil diffusion pump is used to achieve higher vacuum (lower pressure) than is possible by use of <a href="Positive_displacement_vacuum_pump" class="mw-redirect" title="Positive displacement vacuum pump">positive displacement</a> pumps alone. Although its use has been mainly associated within the high-vacuum range, down to

1<span style="margin:0 .15em 0 .25em">×</span>10<sup>−9</sup> <a href="Bar_(unit)" title="Bar (unit)">mbar</a> (1<span style="margin:0 .15em 0 .25em">×</span>10<sup>−7</sup> <a href="Pascal_(unit)" title="Pascal (unit)">Pa</a>), diffusion pumps today can produce pressures approaching

1<span style="margin:0 .15em 0 .25em">×</span>10<sup>−10</sup> mbar (1<span style="margin:0 .15em 0 .25em">×</span>10<sup>−8</sup> Pa) when properly used with modern fluids and accessories. The features that make the diffusion pump attractive for high and ultra-high vacuum use are its high pumping speed for all gases and low cost per unit pumping speed when compared with other types of pump used in the same vacuum range. Diffusion pumps cannot discharge directly into the atmosphere, so a mechanical forepump is typically used to maintain an outlet pressure around

0.1 mbar (10 Pa).

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<p>The oil diffusion pump is operated with an oil of low <a href="Vapor_pressure" title="Vapor pressure">vapor pressure</a>. The high speed jet is generated by boiling the fluid and directing the vapor through a jet assembly. Note that the oil is gaseous when entering the nozzles. Within the nozzles, the flow changes from <a href="Laminar_flow" title="Laminar flow">laminar</a> to <a href="Supersonic" class="mw-redirect" title="Supersonic">supersonic</a> and <a href="Free_molecular_flow" title="Free molecular flow">molecular</a>. Often, several jets are used in series to enhance the pumping action. The outside of the diffusion pump is cooled using either air flow, water lines or a water-filled jacket. As the vapor jet hits the outer cooled shell of the diffusion pump, the working fluid condenses and is recovered and directed back to the boiler. The pumped gases continue flowing to the base of the pump at increased pressure, flowing out through the diffusion pump outlet, where they are compressed to ambient pressure by the secondary mechanical forepump and exhausted.

</p><p>Unlike <a href="Turbomolecular_pump" title="Turbomolecular pump">turbomolecular pumps</a> and <a href="Cryopump" title="Cryopump">cryopumps</a>, diffusion pumps have no moving parts and as a result are quite durable and reliable. They can function over pressure ranges of

1<span style="margin:0 .15em 0 .25em">×</span>10<sup>−10</sup> to 1<span style="margin:0 .15em 0 .25em">×</span>10<sup>−2</sup> mbar (1<span style="margin:0 .15em 0 .25em">×</span>10<sup>−8</sup> to 1 Pa). They are driven only by <a href="Convection" title="Convection">convection</a> and thus have a very low energy efficiency.

</p><p>One major disadvantage of diffusion pumps is the tendency to backstream oil into the vacuum chamber. This oil can contaminate surfaces inside the chamber or upon contact with hot filaments or electrical discharges may result in carbonaceous or siliceous deposits. Due to backstreaming, oil diffusion pumps are not suitable for use with highly sensitive analytical equipment or other applications which require an extremely clean vacuum environment, but mercury diffusion pumps may be in the case of ultra high vacuum chambers used for metal deposition. Often <a href="Cold_trap" title="Cold trap">cold traps</a> and <a href="Baffle_(heat_transfer)" title="Baffle (heat transfer)">baffles</a> are used to minimize backstreaming, although this results in some loss of pumping speed.

</p><p>The oil of a diffusion pump cannot be exposed to the atmosphere when hot. If this occurs, the oil will oxidise and has to be replaced. If a fire occurs, the smoke and residue may contaminate other parts of the system.

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<div class="mw-heading mw-heading3"><h3 id="Oil_types">Oil types</h3></div>

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</style><div role="note" class="hatnote navigation-not-searchable">See also: <a href="Polyphenyl_ether#Ultra-high-vacuum_fluids" title="Polyphenyl ether">Polyphenyl ether § Ultra-high-vacuum fluids</a></div>

<p>The least expensive diffusion pump oils are based on <a href="Hydrocarbon" title="Hydrocarbon">hydrocarbons</a> which have been purified by double-distillation. Compared with the other fluids, they have higher vapor pressure, so are usually limited to a pressure of

1<span style="margin:0 .15em 0 .25em">×</span>10<sup>−6</sup> <a href="Torr" title="Torr">Torr</a> (1.3<span style="margin:0 .15em 0 .25em">×</span>10<sup>−4</sup> Pa). They are also the most likely to burn or explode if exposed to oxidizers.

</p><p>The most common <a href="Silicone_oil" title="Silicone oil">silicone oils</a> used in diffusion pumps are <a href="Trisiloxane" class="mw-redirect" title="Trisiloxane">trisiloxanes</a>, which contain the chemical group Si-O-Si-O-Si, to which various <a href="Phenyl_group" title="Phenyl group">phenyl groups</a> or <a href="Methyl_group" title="Methyl group">methyl groups</a> are attached. These are available as the so-called 702 and 703 blends, which were formerly manufactured by <a href="Dow_Corning" title="Dow Corning">Dow Corning</a>. These can be further separated into 704 and 705 oils, which are made up of the isomers of tetraphenyl tetramethyl trisiloxane and pentaphenyl trimethyl trisiloxane respectively.<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup>

</p><p>For pumping reactive species, usually a <a href="Polyphenyl_ether" title="Polyphenyl ether">polyphenyl ether</a> based oil is used. These oils are the most chemical and heat resistant type of diffusion pump oil.

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<div class="mw-heading mw-heading2"><h2 id="Steam_ejectors">Steam ejectors</h2></div>

<div role="note" class="hatnote navigation-not-searchable">Main article: <a href="Vacuum_ejector" title="Vacuum ejector">Vacuum ejector</a></div>

<p>The steam ejector is a popular form of pump for vacuum <a href="Distillation" title="Distillation">distillation</a> and <a href="Freeze-drying" class="mw-redirect" title="Freeze-drying">freeze-drying</a>. A jet of steam entrains the vapour that must be removed from the vacuum chamber. Steam ejectors can have single or multiple stages, with and without <a href="Condenser_(steam_turbine)" class="mw-redirect" title="Condenser (steam turbine)">condensers</a> in between the stages. While both steam ejectors and diffusion pumps use jets of vapor to entrain gas, they work on fundamentally different principles - steam ejectors rely on viscous flow and mixing to pump gas, whereas diffusion pumps use molecular diffusion. This has several consequences. In diffusion pumps, the inlet pressure can be much lower than the static pressure of jet, whereas in steam ejectors the two pressures are about the same. Also, diffusion pumps are capable of much higher compression ratios, and cannot discharge directly to atmosphere.

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<ul><li><a href="Turbomolecular_pump" title="Turbomolecular pump">Turbomolecular pump</a></li>

<li><a href="Vacuum_pump" title="Vacuum pump">Vacuum pump</a></li>

<li><a href="Aspirator_(pump)" class="mw-redirect" title="Aspirator (pump)">Aspirator (pump)</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFD._G._Avery_and_R._Witty1947" class="citation journal cs1">D. G. Avery and R. Witty (1947). "Diffusion pumps: a critical discussion of existing theories". <i><a href="Proc._Phys._Soc." class="mw-redirect" title="Proc. Phys. Soc.">Proc. Phys. Soc.</a></i> <b>59</b> (6): <span class="nowrap">1016–</span>1030. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1947PPS....59.1016A">1947PPS....59.1016A</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1088%2F0959-5309%2F59%2F6%2F313">10.1088/0959-5309/59/6/313</a>.</cite></span>

</li>

<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite id="CITEREFGaede,_W.1915" class="citation journal cs1">Gaede, W. (1915). <a rel="nofollow" class="external text" href="https://zenodo.org/record/1447291">"Die Diffusion der Gase durch Quecksilberdampf bei niederen Drucken und die Diffusionsluftpumpe"</a>. <i><a href="Annalen_der_Physik" title="Annalen der Physik">Annalen der Physik</a></i>. <b>46</b> (3): 357. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1915AnP...351..357G">1915AnP...351..357G</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1002%2Fandp.19153510304">10.1002/andp.19153510304</a>.</cite></span>

</li>

<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><cite id="CITEREFSella2009" class="citation web cs1">Sella, Andrea (2009-04-28). <a rel="nofollow" class="external text" href="https://www.chemistryworld.com/opinion/classic-kit-gaedes-diffusion-pump/3004912.article">"Classic Kit: Gaede's diffusion pump"</a>. <i>Chemistry World</i><span class="reference-accessdate">. Retrieved <span class="nowrap">2019-08-03</span></span>.</cite></span>

</li>

<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><cite id="CITEREFLangmuir1916" class="citation journal cs1">Langmuir, Irving (1916). <a rel="nofollow" class="external text" href="https://babel.hathitrust.org/cgi/pt?id=mdp.39015075043383&view=1up&seq=1104">"The Condensation Pump: An Improved Form of High Vacuum Pump"</a>. <i>General Electric Review</i>. <b>19</b>: <span class="nowrap">1060–</span>1071.</cite></span>

</li>

<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><cite id="CITEREFC._R._Burch1928" class="citation journal cs1">C. R. Burch (1928). <a rel="nofollow" class="external text" href="https://doi.org/10.1038%2F122729c0">"Oils, greases and high vacua"</a>. <i><a href="Nature_(journal)" title="Nature (journal)">Nature</a></i>. <b>122</b> (3080): 729. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1928Natur.122..729B">1928Natur.122..729B</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.1038%2F122729c0">10.1038/122729c0</a></span>. <a href="S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:4126707">4126707</a>.</cite></span>

</li>

<li id="cite_note-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-6">^</a></b></span> <span class="reference-text"><cite class="citation book cs1">"Pump Fluids". <i>A User's Guide to Vacuum Technology</i>. Hoboken, NJ, USA: John Wiley & Sons, Inc. 2004-12-07. pp. <span class="nowrap">229–</span>246. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1002%2F0471467162.ch13">10.1002/0471467162.ch13</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0-471-46716-8</bdi>.</cite></span>

</li>

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<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="http://www.public.asu.edu/~aomdw/GLASS/DIFFUSION_PUMP.html">An oil diffusion pump built from glass by the Arizona State University Main</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="Further_reading">Further reading</h2></div>

<ul><li><cite id="CITEREFHablanian1994" class="citation book cs1">Hablanian, M. H. (1994) [1983]. <i>Diffusion Pumps : Performance and Operation</i>. AVS Monograph Series (2nd ed.). New York, NY: American Vacuum Society. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>1-56396-384-1</bdi>.</cite></li></ul></div><div><div class="zim-footer">

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