Past and present: Milestones in development of Rotary Vane Pumps
Introduction
The functional principle of a rotary vane pump has remained virtually unchanged since its invention until today. For a long time RVPs were belt-driven at comparatively low rotational speeds. Starting from the 1960s direct drive was introduced starting with small pumps in the 10 m3/h class. This tripled the rotational speed to 1500 rpm and reduced the dimensions and weight of the pump, paving the way to many new applications.
The hammering sound of belt-driven rotary vane pumps was replaced by whisper quiet pumps. Compact size, high pumping speed and good base pressure were the success factors for the direct-driven models. The technical development was complemented by commercial success leading to step-by-step suppression of belt-driven models. After the transition to fast running pumps more than 40 years ago, further developments have been less drastic.
The growing importance of industrial design has changed the look of many pump models; most technical changes now take place under the skin of the pumps. However, these comparatively small changes can drastically improve pump operation in certain applications.
Figure 1: One of the first direct-driven RVPs from the 1960s in comparison to a current model of adixen’s Pascal series.
Motor
Previously a different motor had to be used for every voltage and frequency combination. As recently as the early 1990s some suppliers offered as many as five single phase and four three-phase motors to address all the existing voltage and frequency ranges. Since 1995 single phase and three phase universal motors are available that can be operated with any electrical supply world wide. This is an invaluable advantage for the export-oriented European industries regarding logistics and service.
Oil casing
At the other end of the pump body, there are large differences between old and new models. Pumps from the 1960s and 1970s had inlet and exhaust flanges – partly gas ballast also – on top of the oil casing. Current models group vacuum flanges on the pump body. Gas ballast, large reflecting oil sight glass and draining screw are all now located on the oil casing front end. This location allows for easy system integration.
Servicing older models still meant removal of the gas ballast, inlet and exhaust flanges prior to dismantling the oil casing for pump service. This is not required with current models.
Figure 2: A dismantled rotary vane pump from the 1960s. Servicing the pump is much more complicated …
Figure 3: ... compared to a current model.
After removal of the oil casing the next difference becomes visible. Previously pressed cork seals were used, these have been replaced by guided o-ring seals; this leads to improved tightness of the oil casing.
Oils and fluids
Why does a rotary vane pump need oil ?
Rotary vane pumps are mechanical pumps in which the oil performs several functions:
- Internal clearance reduction between moving parts for higher compression, this gives a lower base pressure
- Lubrication between moving parts
- Improved heat exchange between pumping module and oil casing cooling fins by transport of heat released by compression and friction
Vacuum beginners often compare rotary vane pumps on the basis of ultimate total pressure. However, the ultimate pressure that can be achieved with a rotary vane pump is independent of the pump quality but strongly depending on the vapour pressure of the oil used. In some applications special oils have to be used. This applied e. g. for pumping of pure oxygen or moisture.
Lubrication
In order to provide for sealing and lubrication during the pump compression phase the right amount of oil needs to be provided at all times. The precise amount of oil required is dependant on the pump model and application. Various lubrication mechanisms can be found on the marketplace. Forced lubrication is beneficial for pumping of condensable media whereas natural lubrication produces less oil mist at the exhaust of the pump.
The selection of the various pump models has a dramatic effect on pump temperature which is related to important parameters like water vapour capacity and oil back diffusion. Back diffusion is the phenomenon of oil migrating back through the vacuum line in opposite direction to the pumped flow. This is observed when the pump is operated close to ultimate pressure at elevated temperatures. Under these conditions a dynamic equilibrium is achieved where transported gas flow from the vacuum recipient and oil vapour pressure are equal.
Useful measures to avoid back diffusion are:
- Cooling traps
- Molecular sieves
- Carrier gas (leak valve in roughing line)
- Safety isolation valves
In oil-sensitive applications the most radical action is taken against oil back diffusion: pumping with a dry pump.
Vane material
The prohibition of vane materials containing asbestos has generated an extensive search for alternative materials by all major suppliers. Long-term qualifications have proven the suitability of modern asbestos-free materials. Figure 4 compares old and new vane materials.
Figure 4: Vane materials of old and new rotary vane pumps
Summary
Technical revolutions cannot be expected from a mature technical product like a rotary vane pump. Product development and model upgrading assesses in other areas like service-friendliness.
Despite all the prophecies of doom regarding replacement of rotary vane pumps by oil-free pumping solutions, the oil-sealed RVP still is the best solution in many vacuum applications and this will continue for years, indeed decades.
Last but not least the good old rotary vane pump still offers an important cost advantage compared to "modern" dry pumps. This does not also apply only to the purchase price but also its robustness and long-term stability, which is proven by the fact that many users are still operating 25 year-old rotary vane pumps.