Fundamentals of Vacuum Technology (1248463), страница 15
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At partial pressures of air that aremore than about 5 % of the total pressure at the exit of the condenser, amarked accumulation of air is produced in front of the condensersurfaces. The condenser then cannot reach its full capacity (See also theaccount in Section 2.2.3 on the simultaneous pumping of gases andvapors).b) the water vapor pressure at the condenser exit Ð that is, at the inlet side ofthe gas ballast pump Ð should not (when the quantity of permanent gasdescribed in more detail in Section 2.2.3 is not pumped simultaneously)be greater than the water vapor tolerance for the gas ballast pumpinvolved. If Ð as cannot always be avoided in practice Ð a higher watervapor partial pressure is to be expected at the condenser exit, it isconvenient to insert a throttle between the condenser exit and the inletport of the gas ballast pump.
The conductance of this throttle should bevariable and regulated (see Section 1.5.2) so that, with full throttling, thepressure at the inlet port of the gas ballast pump cannot become higherthan the water vapor tolerance. Also, the use of other refrigerants or adecrease of the cooling water temperature may often permit the watervapor pressure to fall below the required value.pp1 + pv1 = pp2 + pv2(2.23a)As a consequence of the action of the condenser, the water vapor pressurepD2 at the exit of the condenser is always lower than that at the entrance;for (2.23) to be fulfilled, the partial pressure of air pp2 at the exit must behigher than at the entrance pp1, (see Fig.
2.43), even when no throttle ispresent.The higher air partial pressure pp2 at the condenser exit is produced by anaccumulation of air, which, as long as it is present at the exit, results in astationary flow equilibrium. From this accumulation of air, the (eventuallythrottled) gas ballast pump in equilibrium removes just so much as streamsfrom the entrance (1) through the condenser.All calculations are based on (2.23a) for which, however, information on thequantity of pumped vapors and permanent gases, the composition, and thepressure should be available. The size of the condenser and gas ballastpump can be calculated, where these two quantities are, indeed, notmutually independent.
Fig. 2.42 represents the result of such a calculationas an example of a condenser having a condensation surface of 1 m2, andat an inlet pressure pv1, of 40 mbar, a condensation capacity that amountsto 15 kg / h of pure water vapor if the fraction of the permanent gases isvery small. 1 m3 of cooling water is used per hour, at a line overpressure of3 bar and a temperature of 12 ¡C.
The necessary pumping speed of thegas ballast pump depends on the existing operating conditions, particularlythe size of the condenser. Depending on the efficiency of the condenser,the water vapor partial pressure pv2 lies more or less above the saturationpressure pS which corresponds to the temperature of the refrigerant. (Bycooling with water at 12 ¡C, pS, would be 15 mbar (see Table XIII in Section9)). Correspondingly, the partial air pressure pp2 that prevails at thecondenser exit also varies. With a large condenser, pv2 Å pS, the air partialpressure pp,2 is thus large, and because pp · V = const, the volume of airinvolved is small.
Therefore, only a relatively small gas ballast pump isCondensation capacity [kg á h-1]1 Inlet of the condenser2 Discharge of the condenser3 See textIntake pressure pD1Fig. 2.42 Condensation capacity of the condenser (surface area available to condensation 1m2) as a function of intake pressure pD1 of the water vapor. Curve a: Cooling watertemperature 12¡C.
Curve b: Temperature 25 ¡C. Consumption in both cases 1 m3/h at3 bar overpressure.39HomeVacuum generationtwo following cases:PÕv2Pv1Pv2Pp2PÕp2Pp1Fig. 2.43 Schematic representation of the pressure distribution in the condenser. The full linescorrespond to the conditions in a condenser in which a small pressure drop takesplace (ptot 2 < ptot 1). The dashed lines are those for an ideal condenser (ptot 2 ≈ ptot 1).pD: Partial pressure of the water vapor, pL: Partial pressure of the airnecessary.
However, if the condenser is small, the opposite case arises:pv2 > pS · pp2, is small. Here a relatively large gas ballast pump is required.Since the quantity of air involved during a pumping process that usescondensers is not necessarily constant but alternates within more or lesswide limits, the considerations to be made are more difficult. Therefore, it isnecessary that the pumping speed of the gas ballast pump effective at thecondenser can be regulated within certain limits.1. Pumping of permanent gases with small amounts of water vapor. Herethe size of the condenser Ð gas ballast pump combination is decided on thebasis of the pumped-off permanent gas quantity. The condenser function ismerely to reduce the water vapor pressure at the inlet port of the gasballast pump to a value below the water vapor tolerance.2.
Pumping of water vapor with small amounts of permanent gases. Here,to make the condenser highly effective, as small as possible a partialpressure of the permanent gases in the condenser is sought. Even if thewater vapor partial pressure in the condenser should be greater than thewater vapor tolerance of the gas ballast pump, a relatively small gas ballastpump is, in general, sufficient with the then required throttling to pump awaythe prevailing permanent gases.Important note: During the process, if the pressure in the condenser dropsbelow the saturation vapor pressure of the condensate (dependent on thecooling water temperature), the condenser must be blocked out or at leastthe collected condensate isolated.
If this is not done, the gas ballast pumpagain will pump out the vapor previously condensed in the condenser2.1.6 Fluid-entrainment pumpsb) Next to the large pump for rough pumping a holding pump with lowspeed is installed, which is of a size corresponding to the minimumprevailing gas quantity. The objective of this holding pump is merely tomaintain optimum operating pressure during the process.Basically, a distinction is made between ejector pumps such as water jetpumps (17 mbar < p < 1013 mbar), vapor ejector vacuum pumps(10-3 mbar < p < 10-1 mbar) and diffusion pumps (p < 10-3 mbar). Ejectorvacuum pumps are used mainly for the production of medium vacuum.Diffusion pumps produce high and ultrahigh vacuum. Both types operatewith a fast-moving stream of pump fluid in vapor or liquid form (water jet aswell as water vapor, oil or mercury vapor).
The pumping mechanism of allfluid-entrainment pumps is basically the same. The pumped gas moleculesare removed from the vessel and enter into the pump fluid stream whichexpands after passing through a nozzle. The molecules of the pump fluidstream transfer by way of impact impulses to the gas molecules in thedirection of the flow. Thus the gas which is to be pumped is moved to aspace having a higher pressure.c) The necessary quantity of air is admitted into the inlet line of the pumpthrough a variable-leak valve. This additional quantity of air acts like anenlarged gas ballast, increasing the water vapor tolerance of the pump.However, this measure usually results in reduced condenser capacity.Moreover, the additional admitted quantity of air means additional powerconsumption and (see Section 8.3.1.1) increased oil consumption.
Asthe efficiency of the condenser deteriorates with too great a partialpressure of air in the condenser, the admission of air should not be infront, but generally only behind the condenser.In fluid-entrainment pumps corresponding vapor pressures arise duringoperation depending on the type of pump fluid and the temperature as wellas the design of the nozzle. In the case of oil diffusion pumps this mayamount to 1 mbar in the boiling chamber. The backing pressure in thepump must be low enough to allow the vapor to flow out. To ensure this,such pumps require corresponding backing pumps, mostly of themechanical type.
The vapor jet cannot enter the vessel since it condensesat the cooled outer walls of the pump after having been ejected through thenozzle.If the starting time of a process is shorter than the total running time,technically the simplest method Ð the roughing and the holding pump Ð isused. Processes with strongly varying conditions require an adjustablethrottle section and, if needed, an adjustable air admittance.Wolfgang Gaede was the first to realize that gases at comparatively lowpressure can be pumped with the aid of a pump fluid stream of essentiallyhigher pressure and that, therefore, the gas molecules from a region of lowtotal pressure move into a region of high total pressure.
This apparentlyparadoxical state of affairs develops as the vapor stream is initially entirelyfree of gas, so that the gases from a region of higher partial gas pressure(the vessel) can diffuse into a region of lower partial gas pressure (thevapor stream). This basic Gaede concept was used by Langmuir (1915) inthe construction of the first modern diffusion pump. The first diffusionpumps were mercury diffusion pumps made of glass, later of metal.
In theIn practice, the following measures are usual:a) A throttle section is placed between the gas ballast pump and thecondenser, which can be short-circuited during rough pumping. The flowresistance of the throttle section must be adjustable so that the effectivespeed of the pump can be reduced to the required value. This value canbe calculated using the equations given in Section 2.2.3.On the inlet side of the gas ballast pump a water vapor partial pressure pv2is always present, which is at least as large as the saturation vaporpressure of water at the coolant temperature.
This ideal case is realizable inpractice only with a very large condenser (see above).With a view to practice and from the stated fundamental rules, consider the40HomeVacuum generationSixties, mercury as the medium was almost completely replaced by oil. Toobtain as high a vapor stream velocity as possible, he allowed the vaporstream to emanate from a nozzle with supersonic speed.
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