Organic fluids are used in rotary vane, piston, screw, scroll, lobe, turbomolecular, and diffusion pumps. In a rotary vane or piston pump fluids provide a vacuum seal between the moving surfaces and lubricate and cool the low-speed bearings and sliding surfaces. Screw, scroll, lobe, and turbomolecular pumps use oil or a synthetic fluid to lubricate and cool low-, medium-, or high-speed bearings; however, none of these pumps uses fluid to make the vacuum seal. Diffusion pumps vaporize liquids to form supersonic jets that transfer momentum to gas molecules. Ideally each fluid should be thermally stable, and chemically inert, have a low vapor pressure, and, when necessary, be a good lubricant. Unfortunately every desirable attribute cannot be realized simultaneously so that some compromise is required in formulating or choosing a fluid for any application.
The fluids used in vacuum pumps are highly refined mineral oils and synthetic esters, silicones, ethers, and fluorocarbons. In this chapter we review fluid properties, pump fluid types, selection, and reclamation.
13.1 FLUID PROPERTIES
Vapor pressure and lubricating ability are the two most important properties of a vacuum pump fluid. Low vapor pressure is necessary to avoid oil vapor transport to the vacuum chamber, and mechanical pump fluids need to be good lubricants. In Chapter 18 we discuss rheological properties-absolute and kinematic viscosity and viscosity index. Here we review vapor pressure measurements and other physical and chemical properties of vacuum pump fluids.
13.1.1 Vapor Pressure
Regardless of its other qualities, a pump fluid, lubricant, or additive is of no use if its vapor pressure is so high that it contaminates the working region of the vacuum chamber. A minimum vapor pressure is necessary 229