Forevacuum pump

In this chapter, those pumps that are frequently encountered throughout the range of vacuum pressures are dealt with (see Table 3.1). Where necessary, to support the calculations, the operating principles and pump characteristics are reviewed. With gas-transfer pumps operating in the HV/UHV range (typically diffusion or turbomolecular pumps), continuous operation of backing (forevacuum) pumps is required for efficient performance. In such cases, the combination is considered. 

Gas-transfer pumps such as turbomolecular and diffusion pumps are extensively employed in the HV/UHY range. Both types require backing (forevacuum) pumps with the appropriate characteristics to enable their efficient performance. Aspects of the operation of diffusion pumps (Examples 3.10-3.15) and turbomolecular pumps (Examples 3.16-3.19) were considered. 

Fig. 41. Stock s Universal System, a) forevacuum pump t) diffusion pump o) fore vacuum reservoir d) drying agent.

The sample temperature was set using a special thermostat that could control the temperature in the interval 0-40 °C. In order to prevent the formation of condensate on the sample and electrodes, the entire test fixture (Agilent Model 1645IB) was arranged in a vacuum chamber (Figure 7.4) evacuated by forevacuum pump. The sample temperature was measured by a chromel-alumel thermocouple, one junction of which was fixed with an epoxy glue on the lower electrode, in the immediate vicinity of the sample. The second junction was placed into a water-ice mixture at 0 ° C. The thermo emf in the thermocouple was measured by a digital millivoltmeter. 

High vacuum flange Pump casing Forevacuum flange Safety valve for gas dixharge Thermal radiation shield Baffle... 

In addition to the vacuum valves, which perform solely an isolation function (fully open - fully closed position), special valves are needed for special functions. Typical are variable leak valves, which cover the leakage range from 10" ° cm /s (NTP) up to 1.6 10 cm /s (NTP). These valves are usually motor driven and suitable for remote control and when they are connected to a pressure gauge, the process pressures can be set and maintained. Other special valves fulfill safety functions, such as rapid, automatic cut-off of diffusion pumps or vacuum systems in the event of a power failure. For example, SECUVAC valves belong to this group. In the event of a power failure, they cut off the vacuum system from the pumping system and vent the forevacuum system. The vacuum system is enabled only after a certain minimum pressure (about 200 mbar) has been attained once the power has been restored. 

Changing the pump fluid The interior section will be extracted from the pump and the contaminated pump fluid poured out. The interior section and the pump body are then cleaned with pure petroleum ether (naphtha). The interior section and pump body of mercury pumps should have been cleaned beforehand with a clean brush use a bottle brush for the nozzle bores. Ensure that all the nozzle orifices are properly cleaned. It is advantageous to evaporate all solvent residues in a drying kiln. Then the inside section is inserted once again and the fresh pump fluid is installed through the forevacuum port. It is necessary to ensure that the upper nozzle cover is not moistened with pump fluid Do not install too much pump fluid ... 

Forevacuum pressure is too high Check the forevacuum allow the gas ballast pump to run for a longer period of time w/ith the gas ballast valve open. It may be necessary to change the oil in the forepump. 

At the high pressure end of the S vs pMet curve, (here labelled, overload region also known as the forepressure break-down region), S declines from iSmax until a critical pressure is reached in the backing line (critical backing pressure or forevacuum tolerance) which, if exceeded, causes the pumping action of the diffusion pump to cease. 

For classic turbomolecular pumps, Km.dx is also dependent on the forevacuum pressure, decreasing at values of p orevSLC 10-3 mbar (the pressure at which the mean free path of the gas becomes less than the blade distance (see Figure 3.11)). 

Using the same experimental set-up as described in Example 3.16 but with a different type of turbomolecular pump, k 2 = 8 x 108 was obtained with pUi forevac = 5 x 10 2mbar. If H2 had been admitted instead of N2 to the same forevacuum pressure, estimate pHaiV. 

Pumping capacity, the required forevacuum and the final vacuum have to be taken into consideration for the correct selection of a vacuum pump [ 12]. Pumping speed is defined as the gas volume L or m fed per time unit. It also depends on the pressure at the pump s intake port the more the pressure is reduced, the smaller the pumping speed. Vapour jet pumps, such as steam ejectors or oil ejectors, can compress the sucked-off air only up to approximately 1 mbar. This pressure limit is also called stability of forevacuum . The final vacuum is defined by the vapour pressure of the propellant. 

Forevacuum 2) = Pump stages 3) = Ion source 4) = GC column 5) Heating for oil diffusion pump 6) = Pump oil 7) = Ion path 8) = Quadrupole rods (separation of ions in the electrical field 9) Secondary emission, electron multiplier (based on Hewlett Packard)... 

---