Fundamentals of Vacuum Technology (1248463), страница 29
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2.76 Diagram for graphically determining a suitable backing pumpof 4 · 10-1 mbar is made with throughput characteristic 2. This correspondsto the two-stage rotary plunger pump with a nominal pumping speed of100 m3/h. Therefore, this pump is the correct backing pump for the 6000 l/sdiffusion pump under the preceding assumption.However, if the pumping process is such that the maximum throughput of9.5 mbar á l/s is unlikely, a smaller backing pump can, of course, be used.This is self-explanatory, for example, from line b in Fig. 2.76 b, whichcorresponds to a maximum throughput of only 2 mbar l/s.
In this case a25 m3/h two-stage rotary-plunger pump would be sufficient.2.3.3 Determination of pump-down timefrom nomogramsIn practice, for instance, when estimating the cost of a planned vacuumplant, calculation of the pump-down time from the effective pumping speedSeff, the required pressure p, and the chamber volume V by formulaspresented would be too troublesome and time-consuming. Nomograms arevery helpful here. By using the nomogram in Fig. 9.7 in Section 9, one canquickly estimate the pump-down time for vacuum plants evacuated withrotary pumps, if the pumping speed of the pump concerned is fairlyconstant through the pressure region involved.
By studying the examplespresented, one can easily understand the application of the nomogram.The pump-down times of rotary vane and rotary piston pumps, insofar asthe pumping speed of the pump concerned is constant down to therequired pressure, can be determined by reference to example 1.In general, Roots pumps do not have constant pumping speeds in theworking region involved. For the evaluation of the pump-down time, itusually suffices to assume the mean pumping speed. Examples 2 and 3 ofthe nomogram show, in this context, that for Roots pumps, the compressionratio K refers not to the atmospheric pressure (1013 mbar), but to thepressure at which the Roots pump is switched on.In the medium vacuum region, the gas evolution or the leak rate becomessignificantly evident.
From the nomogram 9.10 in Section 9, thecorresponding calculations of the pump-down time in this vacuum regioncan be approximated.In many applications it is expedient to relate the attainable pressures at anygiven time to the pump-down time. This is easily possible with reference tothe nomogram 9.7 in Section 9.As a first example the pump-down characteristic Ð that is, the relationshippressure p (denoted as desired pressure pend) versus pumping time tp Ð isderived from the nomogram for evacuating a vessel of 5 m3 volume by thesingle-stage rotary plunger pump E 250 with an effective pumping speed ofSeff = 250 m3/h and an ultimate pressure pend,p = 3 · 10-1 mbar whenoperated with a gas ballast and at pend,p = 3 · 10-2 mbar without a gasballast. The time constant τ = V / Seff (see equation 2.36) is the same inboth cases and amounts as per nomogram 9.7 to about 70 s (column 3).For any given value of pend > pend,p the straight line connecting the Ò70 spointÓ on column 3 with the (pend Ð pend,p) value on the right-hand scale ofcolumn 5 gives the corresponding tp value.
The results of this procedureare shown as curves a and b in Fig. 2.77.It is somewhat more tedious to determine the (pend,tp) relationship for acombination of pumps. The second example discussed in the followingdeals with evacuating a vessel of 5 m3 volume by the pump combinationRoots pump WA 1001 and the backing pump E 250 (as in the precedingexample). Pumping starts with the E 250 pump operated without gas ballastalone, until the Roots pump is switched on at the pressure of 10 mbar. Asthe pumping speed characteristic of the combination WA 1001/ E 250 Ð incontrast to the characteristic of the E 250 Ð is no longer a horizontal70HomeVacuum generationstraight line over the best part of the pressure range (compare this to thecorresponding course of the characteristic for the combination WA 2001 / E250 in Fig. 2.19), one introduces, as an approximation, average values ofSeff, related to defined pressure ranges.
In the case of the WA 1001/ E 250combination the following average figures apply:Seff = 800 m3/h in the range 10 Ð 1 mbar,Seff = 900 m3/h in the range 1 mbar to5 · 10-2 mbar,Seff = 500 m3/h in the range 5 · 10-2 to5 · 10-3 mbarThe ultimate pressure of the combination WA 1001 / E 250 is:Pend,p = 3 · 10-3 mbar. From these figures the corresponding time constantsin the nomogram can be determined; from there, the pump-down time tpcan be found by calculating the pressure reduction R on the left side ofcolumn 5.
The result is curve c in Fig. 2.77.Computer aided calculations at LEYBOLDOf course calculations for our industrial systems are performed by computerprograms. These require high performance computers and are thus usuallynot available for simple initial calculations.2.3.4 Evacuation of a chamber wheregases and vapors are evolvedThe preceding observations about the pump-down time are significantlyaltered if vapors and gases arise during the evacuation process. With bakeout processes particularly, large quantities of vapor can arise when thesurfaces of the chamber are cleared of contamination.
The resultingnecessary pump-down time depends on very different parameters.Increased heating of the chamber walls is accompanied by increaseddesorption of gases and vapors from the walls. However, because thehigher temperatures result in an accelerated escape of gases and vaporsfrom the walls, the rate at which they can be removed from the chamber isalso increased.The magnitude of the allowable temperature for the bake-out process inquestion will, indeed, be determined essentially by the material in thechamber.
Precise pump-down times can then be estimated by calculationonly if the quantity of the evolving and pumped vapors is known. However,since this is seldom the case except with drying processes, a quantitativeconsideration of this question is abandoned within the scope of thispublication.2.3.5 Selection of pumps for dryingprocessesFundamentally, we must distinguish between short-term drying and dryingprocesses that can require several hours or even days. Independently ofthe duration of drying, all drying processes proceed approximately as inSection 2.24As an example of an application, the drying of salt (short-term drying) isdescribed, this being an already well-proven drying process.Drying of saltFirst, 400 kg of finely divided salt with a water content of about 8 % bymass is to be dried in the shortest possible time (about 1 h) until the watercontent is less than 1 % by mass.
The expected water evolution amounts toabout 28 kg. The salt in the chamber is continuously agitated during thedrying process and heated to about 80 ¡C. The vacuum system isschematically drawn in Fig. 2.78.Fig. 2.77 Pumpdown time, tp, of a 5 m3 vessel using a rotary plunger pump E 250 having anominal pumping speed of 250 m3/h with (a) and without (b) gas ballast, as well asRoots/rotary plunger pump combination WA 1001 / E250 for a cut-in pressure of 10mbar for the WA 1001 (e).During the first quarter of drying time far more than half the quantity ofwater vapor is evolved.
Then the condenser is the actual main pump.Because of the high water vapor temperature and, at the beginning of thedrying, the very high water vapor pressure, the condensation efficiency ofthe condenser is significantly increased. In Fig. 2.78 it is understood thattwo parallel condensers each of 1 m2 condensation surface can togethercondense about 15 l of water at an inlet pressure of 100 mbar in 15 min.However, during this initial process, it must be ensured that the water vaporpressure at the inlet port of the rotary piston pump does not exceed 6071HomeVacuum generation2. PredryingDuring predrying Ð depending on the pressure region in which the work iscarried out Ð about 75 % of the moisture is drawn off.
This predrying shouldoccupy the first third of the drying time. The rate at which predryingproceeds depends almost exclusively on the sufficiency of the heat supply.For predrying 1 ton of paper in 5 h, 60 kg of water must be evaporated; thatis, an energy expenditure of about 40 kWh is needed to evaporate water.Since the paper must be heated to a temperature of about 120 ¡C at thesame time, an average of about 20 kW must be provided.
The mean vaporevolution per hour amounts to 12 kg. Therefore, a condenser with acapacity of 15 kg/h should be sufficient. If the paper is sufficiently preheated(perhaps by air-circulation drying) before evacuation, in the first hour ofdrying, double vapor evolution must be anticipated.1 Vacuum chamber with saltfilling2 RUVAC WA 501,3 Condensers4 Throttle valve5 Rotary plunger pumpFig. 2.78 Vacuum diagram for drying of salt.
Pump combination consisting of Roots pump,condenser and rotary plunger pump for stepwise switching of the pumping process(see text)mbar (see Section 2.15 for further details). Since the backing pump hasonly to pump away the small part of the noncondensable gases at thisstage, a single-stage rotary piston pump TRIVAC S 65 B will suffice. Withincreasing process time, the water vapor evolution decreases, as does thewater vapor pressure in the condenser. After the water pressure in thechamber falls below 27 mbar, the Roots pump (say, a Roots pump RUVACWA 501) is switched in. Thereby the water vapor is pumped more rapidlyout of the chamber, the pressure increases in the condensers, and theircondensation efficiency again increases. The condensers are isolated by avalve when their water vapor reaches its saturation vapor pressure.
At thispoint, there is a water vapor pressure in the chamber of only about 4 mbar,and pumping is accomplished by the Roots pump with a gas ballastbacking pump until the water vapor pressure reaches about 0.65 mbar.From experience it can be assumed that the salt has now reached thedesired degree of dryness.Drying of paperIf the pumps are to be of the correct size for a longer process run, it isexpedient to break down the process run into characteristic sections.
As anexample, paper drying is explained in the following where the paper hasan initial moisture content of 8 %, and the vessel has the volume V.1. EvacuationThe backing pump must be suitably rated with regard to the volume of thevessel and the desired pump-down time. This pump-down time is arrangedaccording to the desired process duration: if the process is to be finishedafter 12 Ð 15 h, the pump-down time should not last longer than 1 h. Thesize of the backing pump may be easily calculated according to Section2.3.1.3. Main dryingIf, in the second stage, the pressure in a further 5 h is to fall from 20 toabout 5.3 mbar and 75 % of the total moisture (i.e., 19 % of the totalmoisture of 15 kg) is to be drawn off, the pump must, according toequations (2.37) and (2.38), have a pumping speed ofSeff =V · ∆pt·pAccording to equation 1.7, 15 kg of water vapor corresponds at 15 ¡C to aquantity of water vapor ofV · ∆p =m · R · T 15 ·83.14 · 288 ≈=M18≈ 20000 mbar · m3Seff =subsequently20000= 750 m 3/h5 · 5.
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