Exercises

Exercise 1: Investigate what the use of the turbo pump adds to the performance of the system. In the previous section (Pumpdown 2), you were able to pump down the reaction chamber  through the turbo pump with only the mech pump powered.  Now let's investigate what the use of the turbo adds to the performance of the system: Start/restart the simulator (stop if already running).  Turn on the mech pump.  Open V1, the cutoff valve separating mech pump from turbo pump.  Open V2, the gate valve separating turbo pump from chamber.  Turn on the turbo pump power and watch the various pressures change.    Observation: When V2 is opened, the reaction chamber, turbo pump chamber, and mech pump chamber pressures all decrease until they reach the base pressure of the mechanical pump (about 2x10-3 torr). Observation: When the turbo pump is turned on, the pressures in the reaction chamber and turbo pump decrease rapidly, in this case to the 10-9 torr range.  The mech pump pressure increases initially, then returns to its base pressure of about 2x10-3 torr.  The turbo clearly provides the extra pumping capability needed to reach very low pressures, as seen in both the reaction chamber and the inlet port of the turbo pump itself.  This enhanced pumping capability at low pressure arises from the multiblade design and its very high rotational speed.  The initial increase in mech pump pressure results from the burst of gas the turbo pumps immediately after being powered on.  NOTE: In the current simulation version, the turbo pump reaches full pumping speed immediately upon being turned on.  In reality, the high angular momentum of the turbo rotor takes a noticeable amount of time to achieve, so that the rotor speed and pumping capability increase smoothly over 1-2 minutes, even at sufficiently low pressures (<10 torr).  Correspondingly, if turbo power is turned off at low pressure, the turbo will continue to spin and thus pump for some comparable length of time.  Later versions of the simulator will incorporate these more complex dynamic effects.

Exercise 2: Pump down the reaction chamber using the full turbo pump system, i.e. using turbo and mech pumps together from the beginning. Start/restart the simulator (stop if already running).  Open V1, the cutoff valve separating mech pump from turbo pump.  Open V2, the gate valve separating turbo pump from chamber.  Then turn on both the mech pump and the turbo pump power, one immediately after the other, and watch the various pressures change.    Questions:  At what reaction chamber pressure will we observe the influence of the turbo pump?  Answers

 

Summary
The addition of the turbo pump to the mech pump  dramatically enhances the pumping capability of the system, substantially reducing the base pressures achievable from about the 10^-3 torr range to the 10^-9 torr range.

The influence of the turbo pump on system capability is seen below 50-100 torr.