The influence of air conduction value on the pumping of the fore-stage pump

Under the condition of shortening the process time, the vacuum pumping efficiency is very important. Usually the pumping efficiency accounts for more than half of the time in the entire process. Therefore, estimating the exhaust rate of the pipeline is necessary before designing the pipeline. Of course, the pumping volume also accounts for the majority of factor. Therefore, calculating the pump, pipe diameter and path to maximize the pumping efficiency is the primary goal of reducing the process time. Following is the calculation method:


     Consider the vacuum vessels with pressures P1 and P2, which are connected by pipes or simple holes, and the gas flow Q through here is:


      Among them, C is called Conductance, and its physical meaning is the ease of gas passing through pipes or apertures.

Compression Ratio

        In short, it is the pressure ratio between the inlet and the outlet. The ratio is calculated by the relationship of the Ideal-gas equation. However, the compression ratio has an absolute influence on the pumping efficiency of the pump.

Pumping time formula:

                              Seff=2.3 x V/t x logP1/P2

V is the volume of the cavity (m3), t is the rough pumping time (hr), and P1 and P2 are the pressure before and after pumping (Torr).

According to the state of viscous flow, the formula of pipeline air conduction can be known

Orifice air conduction: C0=20 x πD2/4

Round tube air conduction: Cp=180 x DP4/L

L is the tube length (cm), D is the tube diameter (cm), P is the average pressure (Torr)

Correct the pump exhaust rate formula by air conduction:


From this, the exhaust efficiency between the pipeline and the pump can be calculated, and the pipeline with the pump can be designed to achieve the highest exhaust efficiency.