The Edwards “RV” (simply meaning Rotary Vane) laboratory sized oil sealed rotary vane vacuum pumps have been in the market for 25 years. They have a very unique design with no equal. This article will attempt to show the reasons for its design and introduction in 1993 and then explain the features of the vacuum pump that make it one of the best small vacuum pumps available today. This is not an official Edwards account, although the engineering related content is based on Edwards information, it contains my personal knowledge, experience and understanding from working with these pumps for many years.
In this article we will only discuss one and two-stage “medium vacuum” oil sealed rotary vane vacuum pumps that can produce a catalog ultimate vacuum of about 1 x 10-2 Torr (0.01 Torr or 10 microns) for a one stage model and about 1 x 10-3 Torr (0.001 Torr or 1 micron) for a two-stage model. We will review the Vee Belt Drive design and the Direct Drive design.
Smaller vacuum pumps such as those used in the heating, ventilating and air conditioning industry (HVAC) are not included as they are often only for intermittent use and do not have the design features built into the laboratory sized continuous running vacuum pumps used in industry and science. Larger rotary vane vacuum pumps, ones that require ball or roller bearings to support the weight of the rotor are not included either. Although they have many similar features to the laboratory sized vacuum pumps, they also have a variety of options to suit different applications.
In this article, we’re going to take a step away from vacuum pumps and systems and write about general applications that use vacuum in the process. There may be some applications you have heard about and some, hopefully, that may be new to you. Whenever a vacuum (a pressure lower than the surrounding atmospheric pressure) is used in a process it will generally fall into one of the Five Main Reasons for using Vacuum. In some cases, a process may use vacuum for two of the five reasons. This month I will discuss the first of these reasons, in no specific order.
First, a short explanation of the two vacuum measuring units used in this article. We know that standard atmospheric pressure is 14.7 lbs. in-2 and that your real life atmospheric pressure varies up and down a few percentage points from the standard depending on a) weather conditions in your area and, b) your altitude above sea level. Remember that “low” pressure is the same as “high” vacuum, and conversely “low” vacuum is the same as “high” pressure – but still below atmospheric pressure in the vacuum industry. I have written a small number and read many technical articles about vacuum and it is very difficult to ensure that these terms are consistent throughout. In the example below we also have to understand that the vacuum measuring units read in opposite directions and we have to change one of them. Does that sound confusing? Yes, it is.
By now, most maintenance departments should have a plan in place for schedules repairs and maintenance during the summer months, especially if the plant has a shutdown for vacation. The part of the vacuum furnace system that I know best is the vacuum system, so I will concentrate on this.
The summer maintenance schedule should cover all the vacuum pumps on a vacuum furnace. For some it will just be mechanical pumps, the rotary piston pumps and the Roots booster (or blower); for others it will include the oil diffusion pump or pumps and the small rotary vane holding pump, if used.
To maximize your investment and produce repeatable high-quality process results, it is mandatory to institute a thorough pump preventative maintenance program. In a pump’s normal operating life, nearly all unexpected vacuum pump failures can be prevented, and when carefully maintained, a vacuum pump will provide years of reliable service.
Be Organized and Document Everything. Start by generating a checklist of routine maintenance activities. The pump manufacturer typically provides this as part of the operating manual. Second, incorporate a maintenance log documenting all routine maintenance, repairs and component replacement. The log will play a critical role in diagnosing future problems, scheduling various maintenance activities, and stocking spare parts. Install a running hour meter to document the number of operating hours on the pump between service activities and enter this into the log. If possible, measure and electronically record the vacuum before and after the pump during every operating cycle, as well as the pumpdown times and ultimate vacuum level achieved in the furnace. This information is often collected and stored as data points in the furnace’s data acquisition system for process reasons. It can also be useful for planning and scheduling maintenance. Changes in these variables can be programmed into the furnace control system to notify the operator when a maintenance inspection or a specific maintenance activity is necessary.
Booster pumps (aka Roots blowers or intermediate stage vacuum pumps) fall into the class of dry, gas transfer pumps. As a dry pump, they do not introduce oil or water into the pumped gas stream. Gas transfer is accomplished by mechanically means, that is the transfer of pumped gas molecules, rather than their collection, as with, foe example, cryogenic or turbomolecular pumps.
The words “Roots blower” are synonymous with these pumps and the reason they are also commonly referred to as booster pumps is that they are mounted at the inlet of a primary/backing pump (such as a rotary vane, claw pump, or screw pump). They “boost” the performance of the primary pump improving pumpdown speed (much as a relay race in which the baton is passed from one runner to the next). The combination of Root blower and primary pump provides roughly a seven-fold increase in pumping speed and a ten-fold increase in pressure, in comparison to a primary pump alone.
Just as vacuum pumps can be considered the heart of the vacuum furnace, so too can the oil be thought of as 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.
Oil Formulations – 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, 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, as 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.