Fundamentals of Vacuum Technology (1248463), страница 39
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PIDcontrollers) with a proportional valve as actuator and mechanical diaphragmcontrollers. In a regulation system with electric controllers the coordinationbetween controller and actuator (piezoelectric gas inlet valve, inlet valvewith motor drive, butterfly control valve, throttle valve) is difficult because ofthe very different boundary conditions (volume of the vessel, effectivepumping speed at the vessel, pressure control range).
Such control circuitstend to vibrate easily when process malfunctions occur. It is virtually impossible to specify generally valid standard values.To avoid the complicated installation with auxiliary relays, many units offer afacility for changing the type of function of the built-in trigger values viasoftware. Initially one can choose between individual switching points (socalled Òlevel triggersÓ) and interlinked switching points (Òinterval triggersÓ).These functions are explained in Fig. 3.25.
With interval triggers one canalso select the size of the hysteresis and the type of setpoint specification,i.e. either fixed setting in the unit or specification through an externalvoltage, e.g. from 0 Ð 10 volts. A three-step regulation system (withoutauxiliary relay), for example, can be set up with the LEYBOLDMEMBRANOVAC of the A series. Fig. 3.26 shows different units of the newLEYBOLD A series, which, although they function according to differentmeasuring methods, all display a uniform appearance.Many control problems can be better solved with a diaphragm controller.The function of the diaphragm controller (see Fig. 3.27) can be easilyderived from that of a diaphragm vacuum gauge: the blunt end of a tube orpipe is either closed off by means of an elastic rubber diaphragm (forreference pressure > process pressure) or released (for reference pressure< process pressure) so that in the latter case, a connection is establishedbetween the process side and the vacuum pump.
This elegant and more orless ÒautomaticÓ regulation system has excellent control characteristics (seeFig. 3.28).process chamberconnectionmeasuringconnectionreferencecontrol chamberdiaphragmcontroller seatmeasuring connectionfor reference chamberreference pressureadjustment valveFig. 3.25 Diagram of level triggers and interval triggerspump connectionFig. 3.27 Principle of a diaphragm controller91HomeVacuum measurementP1 = process pressure, P2 = pressure in pump, Pref = reference pressureFig. 3.28 Control characteristics of a diaphragm controllerTo achieve higher flow rates, several diaphragm controllers can beconnected in parallel.
This means that the process chambers and thereference chambers are also connected in parallel. Fig. 3.29 shows such aconnection of 3 MR 50 diaphragm controllers.To control a vacuum process, it is frequently necessary to modify thepressure in individual process steps. With a diaphragm controller this canbe done either manually or via electric control of the reference pressure.Electric control of the reference pressure of a diaphragm controller isrelatively easy because of the small reference volume that always remainsconstant.
Fig. 3.31 shows such an arrangement on the left as a picture andon the right schematically, see 3.5.5 for application examples withdiaphragm controllers.To be able to change the reference pressure and thus the process pressuretowards higher pressures, a gas inlet valve must additionally be installed atthe process chamber. This valve is opened by means of a differentialpressure switch (not shown in Fig. 3.31) when the desired higher processpressure exceeds the current process pressure by more than the pressuredifference set on the differential pressure switch.DC Diaphragm controllerP Vacuum pumpM Measuring and switching devicePS Pressure sensorV1 Pump valveV2 Gas inlet valveTH ThrottleRC Reference chamberPC Process chamberCV Internal reference pressure control valveFig.
3.30 Control of vacuum drying processes by regulation of the intake pressure of thevacuum pump according to the water vapor tolerance3.5.4 Pressure regulation in high andultrahigh vacuum systemsIf the pressure is to be kept constant within certain limits, an equilibriummust be established between the gas admitted to the vacuum vessel andthe gas simultaneously removed by the pump with the aid of valves orthrottling devices. This is not very difficult in rough and medium vacuumsystems because desorption of adsorbed gases from the walls is generallynegligible in comparison to the quantity of gas flowing through the system.Pressure regulation can be carried out through gas inlet or pumping speedregulation.
However, the use of diaphragm controllers is only possiblebetween atmospheric pressure and about 10 mbar.In the high and ultrahigh vacuum range, on the other hand, the gasevolution from the vessel walls has a decisive influence on the pressure.Setting of specific pressure values in the high and ultrahigh vacuum range,therefore, is only possible if the gas evolution from the walls is negligible inrelation to the controlled admission of gas by means of the pressureregulating unit. For this reason, pressure regulation in this range is usuallyeffected as gas admission regulation with an electric PID controller.Piezoelectric or servomotor-controlled variable-leak valves are used asactuators. Only bakeable all-metal gas inlet valves should be used forpressure regulation below 10-6 mbar.Fig.
3.29 Triple connection of diaphragm controllers92HomeVacuum measurement3.5.5 Examples of applications withdiaphragm controllers1) Regulation of a drying/distillation process, taking into account themaximum water vapor tolerance of a vane type rotary pumpIn a drying process it is frequently desirable to carry out drying solely bymeans of vacuum pumps without inserting condensers. In view of thelimited water vapor tolerance of vacuum pumps Ð approx. 30 mbar as a ruleÐ this would result in condensation of the vapors produced within thevacuum pump, given non-throttled or non-regulated pumping speed. Onecan avoid this through process-dependent remote control of a diaphragmcontroller with auxiliary control valves and a measuring and switchingdevice with a pressure sensor at the inlet connection of the vacuum pump ifthe intake pressure is adapted to the pumps water vapor tolerance throughautomatic monitoring of the intake pressure of the vacuum pump and bythrottling the pumping speed.
Fig. 3.30 shows the principle of thisarrangement.Mode of operation: Starting from atmospheric pressure with the processheating switched off, valve V1 is initially open (maximum switching pointexceeded) so that atmospheric pressure also prevails in the referencechamber.The diaphragm controller is therefore closed. When the system is startedup, the connecting line between the vacuum pump and pump valve V2 isfirst evacuated.
As soon as the pressure drops below the maximumswitching point, valve V1 closes. When the pressure falls below theminimum switching point, valve V2 opens.In this manner the pressure in the reference chamber is slowly lowered, thethrottling of the diaphragm controller is reduced accordingly and thus theprocess pressure is lowered until the quantity of process gas is greater thanthe quantity conveyed by the pump so that the minimum switching point isagain exceeded. Valve V2 closes again.
This interaction repeats itself untilthe pressure in the process chamber has dropped below the minimumswitching point. After that, valve V2 remains open so that the process canbe brought down to the required final pressure with a completely opendiaphragm controller.The material to be dried is usually heated to intensify and speed up thedrying process.
If a certain amount of water vapor is produced, the intakepressure rises above the two switching points. As a result, valve V2 firstcloses and V1 opens. Through incoming air or protective gas the pressurein the reference chamber is raised and the throughput at the diaphragmcontroller thus throttled until the intake pressure of the vacuum pump hasdropped below the set maximum switching point again. Then valve V1closes.Depending on the quantity of vapor that accumulates, the throughput of thediaphragm controller is set by increasing or decreasing the referencepressure in each case so that the maximum permissible partial water vaporpressure at the inlet connection of the vacuum pump is never exceeded.As soon as the pressure in the process chamber drops below the setminimum switching point towards the end of the drying process, valve V2opens and remains open.
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