Sirella or Evisa? How to Choose Between an Oil-Free and a Lubricated Vacuum Pump

A practical selection guide for industrial and process engineers

Choosing between a Sirella dry claw pump (oil-free) and an Evisa rotary vane pump (oil-lubricated) is one of the most consequential decisions you will make when specifying a vacuum system. It is rarely a question of which pump is “better”, both are proven MIL’S vacuum pumps available through our MIL’S page – where we have been UK distributors for years –  but rather which is right for your specific process pressure, cleanliness requirements, energy budget, and maintenance regime.

At Girovac, we have been advising on industrial vacuum solutions since 1983. In our 40-year history, we have seen that getting this choice right leads to years of reliable, efficient service. Conversely, selecting the wrong technology often results in persistent contamination issues, failure to reach target vacuum levels, or unnecessary costs associated with managing waste oil.

This guide walks you through the decision the way we would during a technical consultation: technology first, followed by the eight factors that decide the outcome, and finally a clear “choose this if…” verdict by application.

The short answer

Choose an oil-free Sirella if:  You require maximum process cleanliness, seek lower long-term running costs, and your process operates within the rough-vacuum range (typically from atmospheric pressure down to approximately 50 mbar).

Choose a lubricated Evisa if:  You need to achieve a deeper vacuum (reaching into the 10⁻² mbar region), require high water-vapour tolerance, or are using the unit as a backing pump for a Roots or turbomolecular stage.

Everything below explains the “why” behind these choices and covers the technical nuances where the answer may not be so straightforward.

First, understand the fundamental technology

The two ranges pump air using fundamentally different mechanisms, and almost every performance trade-off flows from this primary distinction.

The Evisa: An oil-sealed rotary vane pump

The Evisa is a single-stage, oil-lubricated rotary vane pump. Inside the pumping chamber, an eccentrically mounted rotor carries sliding vanes. As the rotor spins, these vanes sweep gas toward the exhaust, compressing it until the discharge valve opens.

In this system, oil performs three critical roles: it lubricates the moving vanes, it dissipates heat, and , most importantly , it acts as a fluid seal between the vanes and the pump housing. This oil seal is the reason a rotary vane pump can reach a genuinely deep vacuum level. However, it is also the reason the pump requires regular oil changes and generates a waste stream.

The Sirella: A dry claw pump

The Sirella utilises two claw-shaped rotors that counter-rotate inside a housing. These rotors mesh precisely without ever making contact with each other or the housing walls. Gas is trapped, compressed, and expelled purely by the geometry of the claws.

Because there is no rubbing contact in the pumping chamber, there is no need for internal lubrication or sealing fluid. This “dry” operation ensures that no oil can contaminate the gas stream. The trade-off is that without a liquid seal, the pump cannot close its internal clearances as tightly, which limits its ultimate vacuum depth compared to the Evisa.

Head-to-head at a glance

Note: Figures are indicative and model-dependent. MIL’S reserves the right to change specifications without notice. Always consult our technical team for exact figures against your specific duty point.

The eight factors that decide it

1. Target vacuum level

This is often the deciding factor. If your process requires a pressure below ~50 mbar, an oil-free claw pump will likely struggle to maintain that level on its own. The lubricated Evisa reaches into the 10⁻² mbar region, roughly a thousand times deeper than the Sirella. We always recommend mapping your actual process pressure first. Many industrial duties, such as vacuum handling or packaging, live comfortably in the rough-vacuum band where the Sirella excels.

2. Cleanliness and contamination risk

In pharmaceutical, food, medical, and analytical applications, even trace hydrocarbon contamination can be catastrophic. While the Evisa features integrated oil mist filters, there remains a small risk of oil backstreaming (oil vapour migrating into the vacuum chamber), particularly during idling. Because the Sirella has no liquid in the gas path, the air remains pristine.

3. Flow rate and throughput

If your bottleneck is the speed at which you can evacuate a large volume, rather than the depth of the vacuum, the Evisa range offers higher nominal flow rates (up to 600 m³/h) compared to the Sirella (typically up to 260 m³/h).

4. Energy and running costs

With no sealing fluid to create drag, dry claw pumps are inherently more energy-efficient in continuous operation. Furthermore, many Sirella models (such as the Sirella.RS) feature integrated variable-frequency drives (VSD), allowing the pump to match its speed to demand. This can lead to significant reductions in your annual electricity spend.

5. Maintenance and Total Cost of Ownership (TCO)

You must look past the initial purchase price to the five-year cost. An Evisa requires scheduled oil changes and filter replacements. A Sirella has no oil to change and no vanes to replace. Because the rotors never touch, wear is effectively eliminated, leading to higher uptime and lower servicing costs over time.

6. Environmental impact

For facilities working toward ISO 14001 or other environmental targets, the Sirella removes an entire waste stream. No oil means no hazardous waste disposal paperwork and no risk of site contamination from leaks or spills.

7. Explosive atmospheres (ATEX)

If your application involves handling potentially explosive gases, MIL’S offers Evisa ATEX models certified under Directive 2014/34/EU. While dry claw pumps can be adapted, the Evisa range is the established standard for Zone 1 and Zone 21 industrial duties.

8. Water-vapour handling

The Evisa is explicitly designed with a high water-vapour pumping capacity and an adjustable gas ballast. This makes it the superior choice for processes involving moisture, such as drying or impregnation. The Sirella is better suited for dry gas service.

Application Guide: Which should you choose?

Choose the oil-free Sirella if you are in:

• Pharmaceutical & Life Sciences: Where hydrocarbon contamination is strictly unacceptable.
• Food & Beverage: For clean, oil-free packaging and handling.
• Electronics & Optics: Sensitive environments where any residue would spoil the product.
• Medical Vacuum: Centralised systems requiring clean, low-maintenance, continuous duty.

Choose the lubricated Evisa if you are in:

• Metallurgy & Heat Treatment: Robust, deep-vacuum duties that require high reliability.
• Vacuum Furnaces & Degassing: Applications needing pressures well below 50 mbar.
• High-Vacuum Backing: When serving as a primary pump for a Roots blower or turbomolecular stage.
• Heavy Industrial Environments: Where high throughput and water-vapour tolerance are essential.

When the answer is “both”

In many complex systems, the answer is a combination. For example, an Evisa might act as the backing pump for a high-vacuum stage, while a Sirella manages the primary rough-vacuum handling.

We also frequently package the oil-free Sirella into Indusvac centralised vacuum plants. These systems use multiple pumps controlled by a central Millenium unit, providing a whole facility with clean vacuum while only running the capacity required at any given moment.

Your five-point selection checklist

Before contacting us for a quote, please have the following information to hand:

1. Target process pressure: The exact level in mbar you need to maintain.
2. Required flow rate: The volume of air you need to move per hour.
3. Cleanliness requirement: Is any amount of oil contact acceptable?
4. Duty cycle: Will the pump run intermittently, single-shift, or 24/7?
5. Environment: Are there any ATEX zoning requirements or space constraints?

Talk to a specialist before you specify

Girovac is the authorised UK distributor for MIL’S / Frenes vacuum equipment. We don’t just sell pumps; we design systems and provide full repair and servicing support from our North Walsham workshop via our Repairs & Servicing page. Whether you need a brand-new Sirella, a reconditioned Evisa, or a bespoke system design, our team has the expertise to ensure you get the right tool for the job.

If you are unsure which technology fits your process, we invite you to contact our engineering team through our Contact page for a no-obligation consultation.

Send your duty point details to [email protected] and we will provide a technical recommendation and quote.

Frequently Asked Questions