Fundamentals of Vacuum Technology (1248463), страница 10
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2.11).The gas ballast facility (see Fig. 2.13) prevents condensation of vapors inthe pump chamber of the pump. When pumping vapors these may only becompressed up to their saturation vapor pressure at the temperature of thepump. If pumping water vapor, for example, at a pump temperature of70 ¡C, the vapor may only be compressed to 312 mbar (saturation vaporpressure of water at 70 ¡C (see Table XIII in Section 9)). Whencompressing further, the water vapor condenses without increasing thePower of the drive motor [Watt]The gas ballast facility as used in the rotary vane, rotary plunger andtrochoid pumps permits not only pumping of permanent gases but alsoeven larger quantities of condensable gases.Pressure [mbar]1234Operating temp.
curve 1 32¡C,Operating temp. curve 2 40¡C,Operating temp. curve 3 60¡C,Operating temp. curve 4 90¡C,5 Theoretic curve for adiabaticcompression6 Theoretic curve for isothermcompressionFig. 2.11 Motor power of a rotary plunger pump (pumping speed 60 m3/h) as a function ofintake pressure and operating temperature.
The curves for gas ballast pumps of othersizes are similar.123456Toothed wheel fixed to the driving shaftToothed wheel fixed to the pistonElliptic pistonInner surface of the pumpDriving shaftEccentricFig. 2.12 Cross section of a trochoid pump24HomeinletDischargeGasballastCompressionVacuum generationGas ballastinletPositionof theleadingvaneonSucti1Ð2 Suction2Ð5 Compression3Ð4 Gas ballast inlet5Ð6 DischargeFig. 2.13 Working process within a rotary vane pump with gas ballastpressure.
No overpressure is created in the pump and the exhaust valve isnot opened. Instead the water vapor remains as water in the pump andemulsifies with the pumpÕs oil. This very rapidly impairs the lubricatingproperties of the oil and the pump may even seize when it has taken up toomuch water. The gas ballast facility developed in 1935 by Wolfgang Gaedeinhibits the occurrence of condensation of the vapor in the pump asfollows. Before the actual compression process begins (see Fig. 2.13), aprecisely defined quantity of air (Òthe gas ballastÓ) is admitted into thepumping chamber of the pump. The quantity is such that the compressionratio of the pump is reduced to 10:1 max.
Now vapors which have beentaken in by the pump may be compressed together with the gas ballast,before reaching their condensation point and ejected from the pump. Thepartial pressure of the vapors which are taken in may however not exceeda certain value. It must be so low that in the case of a compression by afactor of 10, the vapors can not condense at the operating temperature ofthe pump. When pumping water vapor this critical value is termed theÒwater vapor toleranceÓ.Shown schematically in Fig. 2.14 is the pumping process with and withoutgas ballast as it takes place in a rotary vane pump when pumpingcondensable vapors.a) Without gas ballast1) Pump is connected to the vessel, whichis already almost empty of air (70 mbar) Ð itmust thus transport mostly vapor particles2) Pump chamber is separated from thevessel Ð compression begins3) Content of pump chamber is already sofar compressed that the vapor condenses toform droplets Ð overpressure is not yetreached4) Residual air only now produces therequired overpressure and opens thedischarge valve, but the vapor has alreadycondensed and the droplets are precipitatedin the pump.b) With gas ballast1) Pump is connected to the vessel, whichis already almost empty of air (70 mbar) Ð itmust thus transport mostly vapor particles2) Pump chamber is separated from thevessel Ð now the gas ballast valve, throughwhich the pump chamber is filled withadditional air from outside, opens Ð thisadditional air is called gas ballast3) Discharge valve is pressed open, andparticles of vapor and gas are pushed out Ðthe overpressure required for this to occur isreached very early because of thesupplementary gas ballast air, as at thebeginning the entire pumping processcondensation cannot occur4) The pump discharges further air andvaporTwo requirements must be met when pumping vapors:1) the pump must be at operating temperature.Fig.
2.14 Diagram of pumping process in a rotary vane pump without (left) and with (right) gasballast device when pumping condensable substances.2) the gas ballast valve must be open.Where:(With the gas ballast valve open the temperature of the pump increases byabout 10 ¡C. Before pumping vapors the pump should be operated for halfan hour with the gas ballast valve open).pvapor= is the partial pressure of vapor at the intake ofthe pumppperm= is the total pressure of all pumped permanentgases at the intake of the pumppvapor,sat= is the saturation pressure of the pumpedvapor, depending on temperature (see Fig.
2.15)psum= pexhaust + Æpvalve + Æpexhaust filterÆpvalve= is the pressure difference across the exhaust valvewhich amounts depending on type of pump andoperating conditions to 0.2 ... 0.4 barSimultaneous pumping of gases and vaporsWhen simultaneously pumping permanent gases and condensable vaporsfrom a vacuum system, the quantity of permanent gas will often suffice toprevent any condensation of the vapors inside the pump. The quantity ofvapor which may be pumped without condensation in the pump can becalculated as follows:ppvapour< vapour , satpvapour + ppermp sum(2.1)Æpexhaust filter = is the pressure difference across the exhaust filteramounting to 0 ...
0.5 bar25HomeVacuum generationExample 1:With a rotary vane pump with an external oil mist filter in series, a mixtureof water vapor and air is being pumped. The following values are used forapplying eq. (2.1):ratio 3 parts of permanent gases to 1 part of water vapor, as indicated inexample 1, can be for guidance only. In actual practice it is recommendedto run up a rotary pump of the types described hitherto always with the gasballast valve open, because it takes some time until the pump has reachedits final working temperature.pexhaust = 1 barFrom eq. (2.1) follows for the permissible partial pressure pvapor of thepumped vapor the relationÆpvalve + Æpexhaust filter = 0.35 bar,temperature of the pump 70 ¡CHence:psum = 1.35 bar; pvapor sat (H2O)= 312 mbar(see Table XIII in chapter 9)pvapour ≤pvapour, sat·ppsum − pvapour, sat perm(2.2)This relation shows that with pperm = 0 no vapors can be pumped withoutcondensation in the pump, unless the gas-ballast concept is applied.
Thecorresponding formula is:Using eq. (2.1) follows:pvapour, H O312= 0. 23<pvapour, H O + pair 135022The pressure of the water vapor in the air/water vapor mixture must notexceed 23 % of the total pressure of the mixture.(p−p)pvapour ≤ B psum · Vapour, sat vapour, g.b. +p−pSsumvapour, satpvapour satpperm+psum − pvapour sat(2.3)Where:Example 2:Ethanoic acid is to be pumped with a rotary plunger pump.B= is the volume of air at 1013 mbarwhich is admitted to thepump chamber per unit time, called in brief the Ògas ballastÓpexhaustS= is the nominal speed of the pump (volume flow rate)psum= is the pressure at the discharge outlet of the pump, assumedto be a maximum at 1330 mbarpvapor, sat= Saturation vapor pressure of the vapor at the pumpÕs exhaust portÆpvalveÆpexhaust filterHence:psum= 1.1 bar (taking into consideration the flow resistanceof the pipes)= 0.25 bar= 0.15 bar(pressure loss in the oil mist trap)= 1.5 bar.By controlled cooling the pump and oil temperature is set at 100 ¡C.
Thesaturation pressure of the acid therefore is Ð seeFig. 2.15 Ð pvapor, sat = 500 mbar.From eq. (2.1) follows:pvapour,. acid0.5 1=<=pvapour,1.5 3. acid + pairSaturation vapor pressure [Torr]Saturation vapor pressure [mbar]Returning to the question of pumping water vapor in a mixture with air, theTemperature [¡ C]pvapor, g.b.
= is the partial pressure of any vapor that might be present inthe gas used as gas ballast (e.g. water vapor contained inthe atmospheric air when used as gas ballast)pperm= is the total pressure of all permanent gases at the inlet portof the pumpEq.
(2.3) shows that when using gas ballast (B - 0) vapors can also bepumped without condensation if no gas is present at the intake of thepump. The gas ballast may also be a mixture of non-condensable gas andcondensable vapor as long as the partial pressure of this vapor(pvapor, g.b.) is less than the saturation pressure pvapor, sat of the pumpedvapor at the temperature of the pump.Water vapor toleranceAn important special case in the general considerations made aboverelating to the topic of vapor tolerance is that of pumping water vapor.According to PNEUROP water vapor tolerance is defined as follows:ÒWater vapor tolerance is the highest pressure at which a vacuum pump,under normal ambient temperatures and pressure conditions (20 ¡C,1013 mbar), can continuously take in and transport pure water vapor.
It isquoted in mbarÓ. It is designated as pW,O.Applying eq. (2.3) to this special case means:pperm = 0 and pvapor, sat = ps (H2O), thus:Fig. 2.15 Saturation vapor pressures26HomeVacuum generationp W, O =p ( H O ) − pvapour, g.b.Bp sum s 2psum − ps ( H 2O )S(2.4)If for the gas ballast gas atmospheric air of 50 % humidity is used, thenpvapor, g.b. = 13 mbar; with B/S = 0.10 Ð a usual figure in practice Ð and psum(total exhaust pressure) = 1330 mbar, the water vapor tolerance pW,0 asfunction of the temperature of the pump is represented by the lowest curvein diagram Fig. 2.16. The other curves correspond to the pumping of watervapor-air mixtures, hence pperm = pair - O), indicated by the symbol pL inmillibar.
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