Oil-sealed or dry vacuum pumps: what should you consider?

Choosing between an oil-sealed and a dry vacuum pump starts with the gases being handled and the conditions the process needs. Cleanliness matters, but so do working pressure, vapour load, maintenance and how the equipment is used.

The descriptions cover a wide range of pumps. A useful comparison moves from broad categories to technologies and models suited to the application.

What the descriptions mean

In an oil-sealed rotary vane pump, oil contributes to sealing and lubrication inside the pumping mechanism. Its condition is part of the pump’s performance.

A dry pump operates without an oil seal in its pumping chamber. Depending on the design, lubricants may still be used in bearings or gears separated from the process gas. “Dry” does not necessarily mean the entire machine contains no lubricant or needs no maintenance.

Diaphragm, scroll, screw and multistage Roots designs also differ substantially. They should not be treated as interchangeable.

How sensitive is the process to contamination?

If oil vapour could affect a sample, coating or measurement, state that requirement before selecting equipment. A suitable dry pump can remove a source of oil contamination from the pumping arrangement. Chamber materials, seals, previous use and cleaning still influence cleanliness.

An oil-sealed pump may be suitable where the process permits it and any necessary controls are in place. Assess normal operation, shutdown and restart. Describe the contamination limit or the effect you need to avoid; “must be clean” is difficult to translate into a specification.

What will pass through the pump?

Clean air presents a different duty from water vapour, solvent vapour or gas carrying particles. With an oil-sealed pump, substances entering the oil may affect lubrication and achievable pressure. Gas ballast can help with suitable condensable vapours within the stated limits, but oil still needs monitoring.

A dry pump avoids contamination of a sealing oil charge in the pumping chamber. It can still be affected by condensation, deposits, corrosion or particles. Suitability depends on design, materials and operating conditions. Either type may need upstream separation, condensation or filtration.

Compare performance at the working pressure

Check pumping-speed curves, permitted continuous conditions and gas or vapour limits. Ultimate pressure alone is not enough. For a cycling system, evacuation time may be decisive. For a continuous process, maintaining pressure while gas enters may matter more.

For turbomolecular backing duty, assess the complete combination. The backing pump must maintain an acceptable forevacuum pressure at the required throughput.

What will maintenance involve?

Oil-sealed pumps require attention to oil level and condition, with changes and servicing according to the manufacturer’s instructions and duty. Dry pumps have their own requirements, which may include diaphragms, seals, bearings or other wearing components.

Ask what routine work is required, whether the installed pump will be accessible and how servicing affects the working schedule.

Look beyond the purchase price

Compare complete installations, including traps, filters, exhaust treatment, controls and connections. Energy use, consumables, servicing and downtime vary by model and application. Neither category is automatically cheaper to own, quieter or lower in vibration.

For an existing installation, explain what the replacement should improve. Send Girovac the process conditions, existing model, operating pressure and maintenance history. For a comparison within the MIL’S range, see the SIRELLA and EVISA guide.