Just as vacuum pumps can be considered the heart of the vacuum furnace, the oil is its circulatory system. The selection and properties of the oil are critical to proper furnace operation. Pump oil serves different purposes in different types of pumps and even has different functions within the same pump. In addition to lubrication, it helps provide the seal on rotary-vane and other wet pumps and serves as the media to propel the pumped gas via kinetic action in diffusion pumps.
Different pump-oil formulations are specifically designed for different pump applications, and careful consideration must be given to the oil selection. Typical motor oil, for example, is not sufficiently refined for use in a vacuum pump. It has insufficient resistance to chemical attack and contains additives that may be detrimental to the process being performed in the vacuum furnace.
In addition, the viscosity must be considered. Lower-viscosity oils are used for lower operating temperatures and for smaller pumps, and medium-viscosity oils are used for medium to large pumps. Temperature resistance is also critical since many pumps operate at high temperatures, and the oil must be rated for these temperatures. Many of the oils used in vacuum pumps are not traditional oils at all but made of silicone or other non-hydrocarbon fluids.
The oil vapor pressure is important in all vacuum pumps because the oil is exposed to the gas being pumped from the chamber. If the oil vapor pressure is too high, it will vaporize when exposed to vacuum, and the vapor can contaminate the vacuum chamber (referred to as backstreaming). For this reason, the vapor pressure is one of the factors dictating the ultimate vacuum the pump can achieve.
Rotary Pump Oil
The rotary-vane pump utilizes an eccentrically mounted rotor that contains two or more vanes that propel the pumped gas through the pump as the rotor turns (Fig. 1). The vanes seal against the pump housing (stator) while rotating, relying on a thin film of pump oil to help provide the seal.
The oil used in a rotary-vane pump does not only provide lubrication of the pump rotor bearings. It must also: provide a seal between the vanes and the rotor (the "Duo seal"); generate the seal between the tips of the vanes and the stator (Fig. 2); provide cooling of the stator by transferring heat to the outer casing; and offer corrosion protection of the metal parts from the gas being pumped.
Oils designed specifically for rotary pumps are distilled mineral oils to which hydrogen atoms have been attached to any loose molecules in the chain. This process, referred to as hydro-treating, provides a strong, stable formulation with a low vapor pressure. For applications where the vacuum pump may be exposed to reactive or corrosive gases carried in the pumped gas, specially engineered oil is used, which has been further processed to remove impurities.
Where a high concentration of oxygen or other chemically reactive gases are present, highly inert, man-made lubricants are recommended. These perfluoropolyether (PFPE) fluids have good temperature resistance but must not be exposed to temperatures above 280°C, at which point they release toxic vapors. PFPE fluids are available under the trade names Fomblin (Solvay Solexis) and Krytox (Dupont). If the incorrect oil is used in a chemically aggressive environment, it will break down and leave a tar-like residue that will block the internal passageways and cause pump overheating and failure resulting from insufficient lubrication.