4.9.1.3 Turbopump performance data

Gas loads

The gas loads, $q_{pV}=S \cdot p=\frac{dV}{dt} \cdot p$ (Formula 1-16),

that can be displaced with a turbomolecular pump increase proportionally to pressure in the range of constant volume flow rate. In the declining branch of the pumping speed curve the maximum displaced gas loads can continue to rise but they will reach thermal limits that depend on the size of the backing pump. The maximum permissible gas loads also depend on the pump temperature (cooling and/or heated pump) and the type of 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.

Measurement of the rotor temperature by the manufacturer enables gas type-dependent process windows to be recommended for safely operating the turbopumps. The technical data for the turbopumps specify the maximum permissible gas loads at nominal rpms for hydrogen, helium, nitrogen, argon and CF₄. A reduction in the rotation speed allows higher gas throughputs.

Pumps in the HiPace series with pumping speeds > 1,000 l · s^-1 are equipped with rotor temperature monitoring and protect themselves from overheating.

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 21360-1:2012 by increasing the backing 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 Section 4.1.3). While the pump must reach base pressure $p_b$ within the prescribed time under the conditions specified in the measurement guidelines, the ultimate pressure $p_e$ 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 FKM 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.

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

Whether ultimate pressure will be achieved will depend upon the size and cleanliness of both the equipment and the pump, as well as upon the bake-out conditions. After extreme bakeout (to over 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. A typical residual gas spectrum of clean, baked out equipment is shown in Figure 4.28.

Figure 4.28: Typical UHV residual gas spectrum (turbopump)

In the backing pump used, the gas ballast should be switched on at regular intervals to prevent the accumulation of hydrogen in the fore-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.