Different vacuum pump oil formulations are used in different pumps. The chemistry and properties of the oil must match the style of pump and its operating conditions.
Diffusion Pump Oil
Diffusion pumps operate by boiling the pump oil into a dense vapor and forcing it through multiple-angled jet nozzles that capture the pumped gas and carry it to the pump outlet through kinetic action (Fig. 1). To serve this purpose, the oil must be designed with specific properties.
Diffusion-pump oils are silicones, hydrocarbons, esters, perfluorals or polyphenyl ethers (Table 1). The primary properties of these oils are high molecular weight, low vapor pressure and low chemical reactivity.
Since the temperature of the boiler that generates the oil vapor ranges from approximately 190-280°C (374-536°F), the oil must have a boiling temperature lower than this. Oils having lower molecular weights tend to boil at the lower end of this range. The oil must be changed at proper intervals, otherwise it can break down over time and contaminate the pump (Fig. 2).
Contamination can also occur if the oil is not rated for the operating temperature, in which case oxidation or thermal breakdown can occur. Other selection criteria for the pump fluid include low vapor pressure at room temperature, low toxicity and reasonable cost. To prevent overheating and thermal breakdown of the oil and resulting pump contamination, an interlock may be incorporated into the vacuum chamber to automatically shut the pump down if the temperature exceeds a certain value. Oils with low boiling points also tend to have lower thermal breakdown temperatures. It is important to properly select the oil considering expected temperature and other operating parameters.
Traps to Reduce Backstreaming
In order to reduce backstreaming, one or more oil traps, baffles or cold traps are typically located between the pump and the furnace. A baffle is made of high-conductivity metal and is in tight contact with the pump walls to promote good heat transfer away from the pumped gas. In higher-capacity pumps, the baffle is water cooled. Baffles reduce the pumping speed somewhat but can reduce backstreaming by about 90-95%. In diffusion-pump applications where oil contamination of the chamber must be minimized, liquid-nitrogen-cooled cold traps are used.
1. Herring, Daniel H., Vacuum Heat Treatment, Volume I, BNP Media, 2012
2. Herring, Daniel H., Vacuum Heat Treatment, Volume II, BNP Media, 2016
3. Joaquim, Manuel E. and Bill Foley, “Inside a Vacuum Diffusion Pump,” white paper, Santovac Fluids Inc. and Variain Inc., Vacuum Technologies