Forevacuum pressure

The forevacuum pressure also influences the vapor jet and becomes detrimental if its value exceeds a certain critical limit. This limit is called maximum backing pressure or critical forepressure. The capacity of the... 

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. 

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. 

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. 

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. 

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. 

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. 

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