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2.8.1.3 Turbopump performance data

Gas loads

The gas loads (Formula 1-13), that can be displaced with a turbomolecular pump increase proportionally to pressure in the range of constant volume flow rate, and in the declining range reach a maximum that also is a function of the size of the backing pump. The maximum permissible gas loads depend upon the type of cooling and gas in question

Displacing heavy noble gases is problematic, because they generate a great deal of dissipated energy when they strike the rotor; and due to their low specific heat, only little of it can be dissipated to the housing. Measuring the rotor temperature and reducing the RPM enables the pump to be operated in the safe range. The technical data for the turbopumps specify the maximum permissible gas loads at nominal RPMs for hydrogen, helium, nitrogen and argon.

Critical backing pressure

Critical backing pressure is taken to mean the maximum pressure on the backing-vacuum side of the turbomolecular pump at which the pump's compression decreases. This value is determined as part of the measurements for determining the compression ratio in accordance with ISO 21360-1 by increasing the backing-vacuum pressure without gas inlet on the intake side. In the technical data for turbomolecular pumps, the maximum critical backing pressure is always specified for nitrogen.

Base pressure, ultimate pressure, residual gas

In the case of vacuum pumps, a distinction is made between ultimate pressure and base pressure (see also 2.1.3). While the pump must reach base pressure pb within the prescribed time under the conditions specified in the measurement guidelines, ultimate pressure pe can be significantly lower. In the HV range, base pressure is reached after 48 hours of bake-out under clean conditions and with a metallic seal. What is specified as the base pressure for pumps with aluminum housings is the pressure that is achieved without bake-out and with clean FPM seals. Corrosive gas-version pumps have a higher desorption rate, which can temporarily result in higher base pressures due to the coating on the rotor surface.

(Figure 2.23)
Figure 2.23: Typical residual gas spectrum of a turbomolecular pump

Dividing the backing pressure by the compression ratio yields the ultimate pressure.

Formula 2-15:

Ultimate pressure

Whether ultimate pressure will be achieved will hinge upon the size and cleanliness of both the equipment and the pump, as well as upon the bake-out conditions. After extreme bakeout (to 300 °C) only H2, CO and CO2 will be found in the residual gas. These are gases that are dissolved in the metal of the recipient and continuously escape.

The gas ballast in the backing pump that is being used should be activated occasionally to prevent hydrogen from accumulating in the backing-vacuum area. In many cases, the actual ultimate pressure will be a factor of the desorption conditions on the high vacuum side of the turbopump and its pumping speed, and not the compression ratios of the pumps.

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