The purpose of use of the vacuum system determines the required vacuum degree and pumping time, and then the appropriate vacuum pump is selected accordingly. This paper describes the calculation of the pumping time in different vacuum ranges.
1. Calculation of Pumping Time in Atmospheric Pressure-Low Vacuum Range
The low-vacuum area refers to a vacuum degree in the range of 100 KPa to 0.2 KPa. In the connection zone between the vacuum chamber and the pump in the low-vacuum area, when the gas molecules are viscous, the pumping time can be calculated by the initial pressure p1, reached pressure P2, pumping speed S and volume V (including piping).
p1—initial pressure (atmospheric pressure) [Pa]
P2 — reached pressure [Pa]
t — pumping time [min]
Se—actual pumping speed [L/min]
Taking into account the bottleneck effect of the conduit and valve, the actual pumping speed can be roughly estimated to be 80% of the theoretical pumping speed.
2. Calculation of Pumping Time in the Mid-Vacuum Range
The high-vacuum and ultra-high vacuum area refers to a vacuum of 200 Pa to 0.2 Pa. The gas molecules in the ducts in the middle-vacuum range are in an intermediate state of viscous flow and molecular flow, the pumping speed cannot be calculates simply as that of low vacuum range or that of high vacuum explained in the third section below. Under normal circumstances, the pumping time is calculated separately in two ways, and then the result with the larger calculated value is taken.
Vacuum pumping factors to consider:
(1) Reached vacuum
(2) Pumping speed
(4) Actual pumping speed
(5) Gas emission rate
(6) Leakage rate
When the vacuum chamber is evacuated with a vacuum pump, the pressure in the chamber decreases rapidly, but after a certain period of time, the pressure decreases gradually and tends to a constant value. The main reason for this phenomenon is the surface deflation of the material. As shown in Fig.1, different areas of pressure change are respectively called space pumping and surface pumping. In order to further increase the vacuum degree, the commonly used countermeasures are as follows:
Fig.1 Relationship between pressure and pumping time
(1) Select materials that emit less gas on the surface.
(2) Reducing the surface area of the material by means of electropolishing, etc., and subsequently reducing the adsorption of gas molecules.
(3) The cavity is baked to promote the release of adsorbed gas on the surface.
3. Calculation of Pumping Time in the High Vacuum and Ultra-High Vacuum Range
The high-vacuum and ultra-high vacuum field here refers to a vacuum degree below 0.2 Pa. For the high vacuum field, the gas discharge from the vessel wall and the objects in the vessel must be fully considered. Therefore, the calculation of the pumping time and the pumping speed is different from that of low vacuum.
p(t) —reached pressure
Se—actual pumping speed
Qg(t)—the amount of gas released inside the chamber
The amount of gas released Qg(t) decreases with time t. At the beginning of the calculation, assuming a pumping time, the degree of vacuum achieved is obtained based on the amount of bleed air at the time. If the calculation result p(t) is not consistent with the desired degree of vacuum, the time is re-assumed and the amount of gas released according to the new hypothetical time is calculated again. Repeatedly, eventually let p(t) be within the required vacuum range.
The calculation of pumping time in the high vacuum area is much more complicated than that in the low vacuum area. When the inner surface of the vacuum chamber is subjected to alcohol cleaning and baking at 150-200°C, the gas emission of the latter will be reduced by about 10%. Therefore, the vacuum degree that reached by same pump is also higher.
The shape and material of the components in the vacuum chamber also greatly affect the degree of reached vacuum and the pumping time. If a resin-based material is used, the degree of reached vacuum will be two to three orders of magnitude worse than the mere discharge of the gas from the metal surface. When internal screws are used, the residual gas in the thread portion is released slowly with the pumping time. In order to accelerate the gas discharge of the threaded part, punch a hole in the center of the screw, or open a vent on the side of the thread (Fig.2). Therefore, the more complex the internal structure, the more factors affect the vacuum.
Fig.2 Schematic drawing of the thread pumping.