Fundamentals of Vacuum Technology (1248463), страница 38
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The vessel (11) can be evacuatedby means of a Roots pump (14) or a diffusion pump (15), both of whichoperate in conjunction with the backing pump (1). The Roots pump is usedin the medium vacuum range and the diffusion pump in the high vacuumrange. The valves (3), (8) and (16) are operated electropneumatically. Theindividual components are actuated from a control panel with pushbuttons.The pump system is to be protected against the following malfunctions:The measures to be taken in order to forestall such malfunctions will bediscussed in the same order:a) power failurea) Measures in the event of power failure: All valves are closed so as toprevent admission of air to the vacuum vessel and protect the diffusionpump against damage.b) Protection in the event of a drop in pressure in the compressed airnetwork: The compressed air is monitored by a pressure monitoringdevice (5).
If the pressure falls under a specified value, a signal caninitially be emitted or the valves can be automatically closed. In thiscase, a sufficient reserve supply of compressed air is necessary (notshown in Fig. 3.20), which allows all valves to be actuated at least once.c) Measures in the event of failure of cooling water to the diffusion pump:The cooling water is monitored by a flow or temperature monitoringdevice (6) and (7). If the flow of cooling water is inadequate, the heaterof the diffusion pump is switched off and a signal is given; the valve (8)closes.b) drop in pressure in the compressed air networkc) failure of cooling water to the diffusion pumpd) fault in diffusion pump heating systeme) failure of backing pumpf) pressure rise in the vessel above a maximum permissible valued) Protection against failure of the diffusion pump heater: Interruption of thediffusion pump heating system can be monitored by a relay.
If thetemperature rises above a maximum permissible value, a temperaturemonitoring device (6) responds. In both cases the valve (8) closes and asignal is given.e) Protection in the event of backing pump failure: Belt-driven backingpumps must have a centrifugal switch which shuts down the entiresystem in the event of belt breakage or another malfunction. Monoblockpumps for which the drive is mounted directly on the shaft can bemonitored by current relays and the like.f) Protection against a pressure rise in the vessel above a certain limitvalue: The high vacuum monitoring device (10) emits a signal when aspecified pressure is exceeded.1 Backing pump2 Backing pressure monitoringdevice3 Electropneumatic valve4 Compressed air connection5 Pressure monitoring device6 Temperature monitoringdevice7 Cooling water monitoringdevice891011121314151617Electropneumatic valveRecorderHigh-vacuum monitoring deviceVesselHigh-vacuum gaugeLimit switchesRoots pumpDiffusion pumpElectropneumatic valveVenting valveg) Ensuring the critical forepressure of the diffusion pump: When a certainbacking pressure is exceeded, all valves are closed by the backingpressure monitoring device (2), the pumps are switched off and again asignal is given.
The position of the valves (3), (8) and (16) is indicatedon the control panel by means of limit switches (13). The pressure in thevessel is measured with a high vacuum gauge (12) and recorded with arecorder (9). Protection against operating errors can be provided byinterlocking the individual switches so that they can only be actuated ina predetermined sequence. The diffusion pump, for example, may notbe switched on when the backing pump is not running or the requiredbacking pressure is not maintained or the cooling water circulation is notfunctioning.In principle, it is not a big step from a system protected against allmalfunctions to a fully automatic, program-controlled plant, though thecomplexity of the electrical circuits, of course, increases significantly.Fig. 3.20 Schematic diagram of a high vacuum pump system with optional operation of a Rootspump or a diffusion pump89HomeVacuum measurement3.5.3 Pressure regulation and control inrough and medium vacuum systemsControl and regulation have the function of giving a physical variable Ð inthis case the pressure in the vacuum system Ð a certain value.
Thecommon feature is the actuator which changes the energy supply to thephysical variable and thus the variable itself. Control refers to influencing asystem or unit through commands. In this case the actuator and hence theactual value of the physical variable is changed directly with a manipulatedvariable.
Example: Actuation of a valve by means of a pressure-dependentswitch. The actual value may change in an undesirable way due toadditional external influences. The controlled unit cannot react to thecontrol unit. For this reason control systems are said to have an openoperating sequence. In the case of regulation, the actual value of thephysical variable is constantly compared to the specified setpoint andregulated if there is any deviation so that it completely approximates thesetpoint as far as possible.
For all practical purposes regulation alwaysrequires control. The main difference is the controller in which the setpointand the actual value are compared. The totality of all elements involved inthe control process forms the control circuit. The terms and characteristicvariables for describing control processes are stipulated in DIN 19226.Generally a distinction is made between discontinuous control (e.g. twostep or three-step control) with specification of a pressure window, withinwhich the pressure may vary, and continuous control (e.g.
PID control) witha specified pressure setpoint, which should be maintained as precisely aspossible. We have two possible ways of adjusting the pressure in a vacuumsystem: first, by changing the pumping speed (altering the speed of thepump or throttling by closing a valve); second, through admission of gas(opening a valve). This results in a total of 4 procedures.Discontinuous pressure regulationAlthough continuous regulation undoubtedly represents the more elegantprocedure, in many cases two-step or three-step regulation is fullyadequate in all vacuum ranges.
To specify the pressure window, two orthree variable, pressure-dependent switch contacts are necessary. It doesnot matter here whether the switch contacts are installed in a gauge withdisplay or in a downstream unit or whether it is a pressure switch withoutdisplay. Fig. 3.21 illustrates the difference between two-step regulationthrough pumping speed throttling, two-point regulation through gasadmission and three-point regulation through a combination of pumpingspeed throttling and gas admission. Figures 3.22 and 3.23 show the circuitTwo-step regulation throughpumping speed throttling➀➁➂➃➄Gauge with two switching pointsThrottle valveVacuum pumpPump valveVacuum vesselFuR, MpSmaxSminPVR1K1MFuseMains connection 220 V/50 HzSwitching point for maximum valueSwitching point for minimum valuePump valveAuxiliary relay for pump valveRelay contact of R1Measuring and switching deviceFig. 3.22 Two-step regulation through pumping speed throttlingand structure of the two two-step regulation systems.
In the case of twostep regulation through pumping speed throttling (Fig. 3.22), voltage issupplied to pump valve 4, i.e. it is open when the relay contacts are in therelease condition. At a level below the upper switching point the valveremains open because of the self-holding function of the auxiliary relay.Only at a level below the lower switching point is the relay latchingreleased. If the pressure subsequently rises, the valve is opened again atthe upper switching point.In the case of two-step regulation through gas admission, the inlet valve isinitially closed.
If the upper pressure switching point is not reached, nothingchanges; only when the pressure falls below the lower switching point, dothe Òmake contactsÓ open the gas inlet valve and actuate the auxiliary relaywith self-holding function simultaneously. Return to the idle state withclosing of the gas inlet valve is not effected until after the upper switchingpoint is exceeded due to the release of the relay self-holding function.Three-step regulation throughgas admissionpumping speed throttlingand gas admissionPressurePressurePressurepAtmpAtmpAtmpmaxpmittepmaxpminpminTimepminTimeFig.
3.21 Schematic diagram of two-step and three-step regulationTime➀ Gauge with two switchingpoints➁ Variable-leak valve➂ Inlet valve➃ Gas supply➄ Throttle valve➅ Vacuum pumpFuR, MpSmaxSminEVR2K2MFuseMains connection 220 V/50 HzSwitching point for maximum valueSwitching point for minimum valueInlet valveAuxiliary relay for inlet valveRelay contact of R2Measuring and switching deviceFig. 3.23 Two-step regulation through gas admission90HomeVacuum measurement➀ Gauge with three switchingpoints➁ Variable-leak valve➂ Variable-leak valve➃ Inlet valve➄ Gas supply➅ Throttle valve➆ Vacuum pump➇ Pump valve➈ Vacuum vesselFuR, MpSmaxSmitteSminTPVEVR1R2K1K2MFuseMains connection 220 V/50 HzSwitching point for maximum valueSwitching point for mean valueSwitching point for minimum valueTORROSTAT¨ S 020Pump valveInlet valveAuxiliary relay for pump intervalAuxiliary relay for inlet intervalRelay contact of R1Relay contact of R2Measuring and switching deviceFig.
3.24 Three-step regulation systemFig. 3.26 LEYBOLD-A series, equipment with level and interval triggersFig. 3.24 shows the corresponding three-step regulation system which wascreated with the two components just described. As the name indicates, twoswitching points, the lower switching point of the regulation system throughpumping speed throttling and the upper switching point of the gas inletregulation system, were combined.Continuous pressure regulationWe have to make a distinction here between electric controllers (e.g.
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