Vacuum diffusion pump, commonly known as oil diffusion vacuum pump or oil diffusion pump, is a vacuum pump that uses low-pressure, high-speed and directional oil vapor jet to evacuate air. Its working principle is as follows:
1. Working principle
Heating and evaporation:
The electric heater (electric furnace) heats the liquid working medium (pump oil) in the pump chamber (oil pan) to turn it into steam.
The pump oil is heated to boiling temperature (about 200℃) in a vacuum, generating a large amount of oil vapor.
Steam jet:
Oil vapor is ejected at high speed through the pump core nozzle installed in the pump chamber to form a low-density, high-speed steam jet.
The steam jet is ejected from the umbrella nozzle (such as the I-level nozzle) at supersonic speed, and its speed gradually increases, and the pressure and density gradually decrease.
Gas diffusion and carrying:
The pumped gas enters the steam jet through diffusion.
Since the pumped gas above the steam jet needs to diffuse into the steam jet due to the density difference, it is carried by the jet to the water-cooled pump wall.
Condensation and reflux:
After the working steam reaches the water-cooled pump wall, it condenses into liquid and flows back to the oil pot along the pump wall for recycling.
After the oil vapor is condensed on the cooled pump wall, it returns to the oil pot and is reheated, and the cycle works in this way.
Gas compression and discharge:
The pumped gas accumulates and compresses at the pump wall, and is finally carried away by the lower jet to achieve step-by-step compression.
Finally, the pumped gas is discharged from the vacuum system by the front pump.
II. Influencing factors
The main factors affecting the performance of the diffusion pump include:
Oil vapor reflux: It can be effectively prevented by adding a cold trap to the vacuum system.
Back diffusion of gas molecules: By adding a Roots pump or a dual mechanical pump as a front pump, increasing the pumping speed, and reducing the pressure at the outlet of the diffusion pump, the back diffusion of gas molecules and the reflux of oil vapor can be reduced.
Pump oil cracking and decomposition: Using qualified diffusion pump oil can prevent cracking and decomposition.
Pump dirtiness: The repeated circulation of pollutants left in the pump will affect the performance of the pump. The diffusion pump should be cleaned and the pump oil should be replaced.
3. Structural composition
The oil diffusion pump is mainly composed of a pump body, a diffusion nozzle, a steam duct, an oil pan, a heater, a diffuser, a cooling system and an injection nozzle.
4. Application scenarios
The oil diffusion pump is the main equipment for obtaining high vacuum. It is widely used in vacuum smelting, vacuum coating, space simulation experiments and some vacuum systems that are not sensitive to oil pollution.
In summary, the working principle of the vacuum diffusion pump oil is to generate a steam jet by heating the pump oil, use the steam jet to carry and compress the pumped gas, and finally discharge the vacuum system by the front stage pump. This process involves multiple links such as heating, evaporation, diffusion, carrying, condensation and reflux.