Current industry activity is strong in production of enclosed cleaning machines which allow cleaning solvents to be used under vacuum conditions. This year and next, I’ll devote several columns to that technology. I’ll try to focus on what’s not commonly known; what’s unusual; and what users should know. This month and next, I’ll cover the engine powering the vacuum vapor degreaser—a vacuum pump.
A VACUUM IS…
…a volume of space which is, to a degree, void of matter. It is technically impossible to produce a vacuum by suction, because fluids1 can’t be pulled. Rather, the vacuum- producing device expands the volume of space to be evacuated into some larger volume, thereby reducing the pressure in the volume being evacuated. So, a vacuum pump doesn’t pump a vacuum.
TYPES OF VACUUM PUMPS
There are many types of vacuum pumps designed to produce various levels of vacuum for specific types of research, development, and production work, and for use in enclosed cleaning machines. The four types most commonly used with enclosed cleaning machines are described below.
Rotary Vane Vacuum Pumps (also called fixed displacement pumps) are positive displacement devices. This means that the volume of movement within the pump is the same volume of fluid which will be removed from the work chamber during every pump revolution. Of course, the mass of fluid being removed within the work chamber during every revolution depends chiefly upon the pressure and temperature within the work chamber, and so varies within the cycle time of evacuation.
The most common type is a single vane rotating inside of a circular cavity. The motor and the cavity are not concentric—their centers are offset (eccentric). There is a movable vane attached to the rotor, which can slide on springs toward and away from the rotor— always contacting the cavity wall. An illustration is shown in Figure 1.2
JET EDUCTOR VACUUM PUMPS
These have no parts which rotate, or move. An eductor effect of a converging-diverging nozzle is to convert the pressure energy of a motive fluid to kinetic energy.
That creates a low pressure zone that draws in and entrains a suction fluid (some of the contents of the work chamber). This is the mechanism by which a vacuum is created in the work chamber.
After the combined fluid (suction and motive fluids) passes through the throat (diffuser) of the jet eductor, it expands through the diverging (or exit) nozzle. In that nozzle, kinetic energy (basically velocity) is converted back to pressure energy, so the pressure is increased at the exit as velocity is reduced. A commercial unit is shown as Figure 2.
Liquid Ring Vacuum Pumps, a popular type of rotating vane positive displacement vacuum pump, is often used in vacuum enclosed cleaning machines.
When this pump is not operating, it is partially full of a liquid—usually water. When the eccentricallymounted rotor is activated, the vanes lift water from the bottom of the casing and fling it to the circumference of the casing. This is centrifugal force in action—forcing the liquid to form a stable ring. While the rotor is not mounted coaxially with the casing, the axis of the ring of liquid is coaxial with the casing.
The volume of liquid within this pump is just sufficient for all vanes to be wetted with liquid at all times. This is assured by the continuous flow of liquid to the pump. Usually liquid is removed from the pump, externally cooled, and returned to the pump.
The point of entry of vapor from the work chamber into the pump is within the rotor—not the casing—the light gray zone shown in Figure 3.
The liquid ring pump like other vacuum pumps creates an increased volume into which the contents of the work chamber expands. That lowers the pressure. Removal (and pressurization) of those contents complete the general functions of a vacuum pump.
I’ll cover another type of pump commonly used in vapor degreasers, the rotary piston vacuum pump, and compare these units for service in enclosed vacuum vapor degreasers.
- This is a somewhat arbitrary definition of a “fluid.” Gases or liquids can’t be pulled. But solid metals can be melted and drawn into tubing and solid polymers can be melted and pulled into fibers.
- The image of Figure 2 is courtesy of Gardner Denver Nash, http://www.gdnash.com/.
John Durkee is the author of the book Management of Industrial Cleaning Technology and Processes, published by Elsevier (ISBN 0- 0804-48887). He is the author of the forthcoming book Solvent Cleaning for the 21st Century, also to be published by Elsevier, and is an independent consultant specializing in critical cleaning. You can contact him at PO Box 847, Hunt, TX 78024 or 122 Ridge Road West, Hunt, TX 78024; 830-238-7610; Fax 612-677-3170; or firstname.lastname@example.org.