Introduction

There are many different types of oil mist eliminator technologies including wire mesh demisters, vane packs, centrifugal and squirrel-cage separators, electrostatic precipitators, and more. Each technology has different performance characteristics that impact overall effectiveness. Regardless of the technology, a primary goal is to eliminate visible oil mist emissions, often referred to “smoke".

Whether you're designing a vacuum system, building a gas compression skid, or maintaining other types of rotating equipment, eliminating oil mist emissions is critical to equipment longevity, safety, and environmental compliance. Coalescing filter elements play a critical role in separating fine oil aerosols from air and gas streams. This article is designed for engineers, maintenance personnel, and OEMs who need basic technical information of how coalescing elements work, the benefits they offer, and how to properly specify and maintain them. This article is broad and applicable across various industries.

What is the Efficiency Rating of an Oil Mist Eliminator?

Oil mist eliminator technologies have different efficiency ratings and performance characteristics making it critical to select the right type for the application. A one-size-fits-all approach will frequently cause problems in the field. Efficiency in oil mist eliminator systems is measured by comparing the upstream oil challenge or oil mist load entering the system to the downstream oil present after the filtration step. Efficiency is reported in the context of defined operating conditions. Depending on the technology used, manufactures of oil mist eliminators report efficiencies ranging from 50% to 99.99% and actual efficiencies can vary as operating conditions change. While efficiencies vary widely by technology, coalescing elements often achieve the highest performance.

What Are Coalescing Elements and How Do They Work?

Coalescing elements are high-efficiency filters engineered to remove fine liquid aerosols and submicron particles from air or gas. These filters work by intercepting fine oil mist particles as they pass through a densely packed fibrous media. Through mechanisms like direct interception and inertial impaction, the small droplets coalesce and merge into larger ones, which drain away by gravity.

This process is affected by several factors that influence coalescing effectiveness. The type of oil (mineral, semi, or full synthetic), its viscosity, operating pressure, and the operating temperature will significantly impact capture and coalescing effectiveness.

Why Are Coalescing Elements Used in Oil Mist Eliminators?

Systems that use coalescing filter element technology are known to offer some of the highest efficiencies, lowest carryover numbers, and best overall performance for all types of oil mist eliminators.

Typical oil mist droplets range between 0.1 to 1.0 microns. Factors like temperature, viscosity, and oil type affect droplet size and how easily they coalesce.

Oil mist elimination filter elements like the Solberg coalescing series achieve 99.97% or better efficiency at 0.3 microns. Any element-based oil mist eliminator system stating lower efficiency ratings most likely use a lower grade particulate filter marketed as a coalescer. Multiple test standards are used to determine carryover and efficiency ratings with ISO 12500-1 and ISO 8573-2 being the most common. It is important to verify performance data with the manufacturer.

When properly sized and optimized, high-efficiency coalescing elements will reduce fine mist to carryover levels to the 1-10 parts-per-million (PPM) range, or roughly 1- 10 mg/m3 oil mist concentration. However, an effective system must address the oil challenge in its entirety: pre-separation handles the heavier loads like slugs and large aerosol droplets, while the coalescing element is optimized for capturing the fine mist droplets that remain.

Manufacturers like Solberg provide multiple media pack configurations to meet specific efficiency targets and to match the operating environment, considering:

• Chemical compatibility
• Pressure drop limitations
• Installation requirements
• Regulatory compliance

How are Coalescing Filter Elements Different?

The media used in coalescing elements is different from the media in particulate or dust filters. Coalescing media contains more open fibers and uses depth to capture and combine fine droplets. Coalescing elements frequently incorporate specialized drain layers which have tighter fibers to capture the coalesced droplets and help with gravity draining. This differs from particulate filter media which focuses on blocking solid particles with limited capability to merge droplets and drain liquids. Traditional particulate filter media can quickly saturate and is much less effective at capturing and coalescing oil mist droplets. Additionally, coalescing media must be compatible with oils, chemicals, and temperature ranges specific to the application, often requiring specialized synthetic or fiberglass fibers with binders that resist chemical attack and thermal break down.

What Affects Performance of Coalescing Elements?

Typical coalescing applications operate between 20°C to 80°C (68°F to 176°F). This is the optimal temperature range for most standard coalescing filter media. Some specialized systems can handle up to 100°C (212°F) with custom-engineered materials and systems.

Performance factors:

• Oil Challenge and Flow Rate: Pre-separation for heavy aerosol loads and proper sizing for flow prevent early saturation and high back pressure. However, going bigger is not always better and all operating parameters must be considered to optimize performance.
• Oil Viscosity: Lower viscosity oils are harder to capture; specifying filters based on oil type is critical. (Typical Oil Grades used in vacuum pumps ranges from ISO 32 to ISO 100)
• Temperature: Higher temps reduce filter efficiency; oil droplets become smaller and less viscous, which can enable smaller droplets to pass through the media.
• Pressure: Most coalescing applications are on the discharge (positive pressure) side. Vacuum applications do exist and can be effective, and they involve more complex design considerations, especially as vacuum and temperature impact oil properties.
• High Particulate Contamination: Dust and solid particles will clog the coalescing media, especially when upstream filtration is inadequate. The right upstream solutions can significantly minimize or eliminate this issue.
• Moisture and Emulsions: Water and oil are immiscible. They do not mix and tend to form emulsions similar in consistency to mayonnaise that will cause draining issues. Emulsions will significantly degrade coalescing performance. The right upstream solutions can significantly minimize or eliminate this issue.
• Application-Specific Properties: Chemical compatibility, mounting location, external operating environment, and size constraints must all be factored in.

Is it Necessary to Drain Oil from an Oil Mist Eliminator?

Once a coalescing filter element captures oil droplets and drains into the housing, that oil must be removed continuously. Accumulated oil can increase back pressure, reduce overall system efficiency, and cause carryover. A properly designed coalescing system will include a drain connection to return oil to the pump, direct it into a sump, or send it to another designated collection point. Whether the recovered oil is reused or disposed of is up to the user, but the critical factor is ensuring the housing remains free of standing oil. Without proper drainage, even the most efficient coalescing element will not maintain optimal performance.

What is the Role of Pre-Separation for Oil Mist Eliminators?

Pre-separation is a crucial but often overlooked step. Removing slugs or larger droplets of oil before they hit the coalescing element will lower the oil challenge to the coalescing filter and lower the overall carryover numbers.

Techniques include:

• Baffles
• Demister Pads and Vane Packs
• Knock-out chambers
• Centrifugal separation
• Vertical Distance mounting

All these techniques can be effective at improving system efficiency and require special considerations for piping configurations and gravity. Skipping pre-separation can lead to oversaturation, where the coalescing media becomes so saturated that it can no longer perform efficiently, resulting in bypass and excessive carryover.

Can Heat Exchangers Remove Oil Mist?

Some high temp oil‑mist applications may require cooling of the air or gas stream to remove suspended oil mist and vapor. Cooling the gas before it hits the coalescing stage can significantly reduce oil carryover when paired with a high efficiency coalescing element. Heat exchangers, typically air‑cooled or shell‑and‑tube units lower the gas stream temperature, enabling oil droplets condense and coalesce more readily and settle out in bulk before reaching the final filter stage. Even with a properly sized heat exchanger, residual fine mist often remains, and an oil mist eliminator can serve a polishing step downstream.

Key Applications

Vacuum Pump Discharge

Oil-sealed rotary vane, screw, piston, and liquid ring pumps produce fine oil mist emissions. Coalescers are installed post-pump to capture exhaust emissions. Many OEM pump systems include an internal separator. Solberg often supplies a polishing coalescer downstream to handle residual mist not captured in the OEMs primary stage coalescing filter.

Crankcase Ventilation

Engine crankcases release oil-laden blowby gases. High efficiency filtration keeps oily mist out of the work environment and reduces emissions. In closed-loop systems, filtered gases may be routed back to the engine intake, so it’s critical to avoid contaminating turbochargers, intercoolers, or other engine components.

Gearbox and Turbine Lube Venting

Large rotating equipment often vents to atmosphere. Coalescers ensure these vents do not contribute to air contamination or safety issues.

Hydraulic Tank Breathers

Coalescers mounted as breathers can prevent atmospheric oil mist release during pressure equalization.

Compressor Scrubber and Separator Systems

In oil-lubricated compressor packages, coalescers are used in scrubbers to clean interstage or final discharge gas.

Gas Compression Packages (Natural and Renewable Gas)

These systems demand high-efficiency coalescing for safety, emissions, and process reliability.

Benefits of Proper Coalescing Element Use

• Environmental Safety: Prevent oil mist discharge to atmosphere, keeping operations in compliance with emission standards.
• Operational Savings: Capture and recycle expensive lubricants.
• Maintenance Reduction: Clean discharge protects downstream equipment and work areas.
• Workplace Safety: Keeps oil mist from settling on floors preventing slipping hazards.
• Equipment Longevity: Reduced fouling and corrosion from airborne oil.

FAQs: What Users Commonly Ask

Q: How often should I replace a coalescing element?
A: In a perfect world, with clean air/gas and no contaminants, it could last for many years. Most applications should see the coalescing element replaced at 1-2 times per year. If you notice visible emissions and “smoke”, increased pressure drop, or a decline in pump performance, it’s time for a change.

Q: Can I wash or reuse a coalescing element?
A:Replacing the element is always preferred to washing as a new element ensures maximum performance and reliability over time. Washing the used element can damage the media, reduce efficiency, contaminate oils, and cause emulsions, all of which are detrimental to equipment performance.

Q: What if my system operates above 80°C?
A: Solberg will work with you to develop a specialized coalescing system that will perform effectively in higher temperature conditions.

Q: Why do I still see smoke even with a coalescer installed?
A: Possible reasons include incorrect sizing, lack of pre-separation, high operating temperatures, or residual oil trapped in piping. Evaluating these factors is critical to solving performance issues.

Conclusion

Coalescing filter elements require thoughtful engineering consideration based on temperature, oil type, pressure, and expected mist load. With proper specification and maintenance, they protect your equipment, your people, and your bottom line.

If you have questions or need help selecting the right solution, contact Solberg’s filtration experts. We're here to help!