Electrostatic Oil Cleaners and Hydraulic Oil Purification Systems

FRIESS electrostatic oil cleaners provide deep purification for non-conductive oils such as hydraulic and lubricating oil. They remove fine particles, sludge, resin and oxidation products and are suitable for injection-moulding machines, hydraulic power units, steelmaking equipment, hydropower stations and industrial hydraulic systems.

Key Advantages of Electrostatic Oil Cleaning for Hydraulic Oil

　Remove fine particles down to 0.01 μm

　Remove sludge, resin and oxidation products

　Extend oil service life and reduce hydraulic-system failures

Electrostatic deep-cleaning technology

How Does the FRIESS EFR Deep-Clean Hydraulic Oil?

Conventional filters primarily retain particles according to the pore size of the filter medium, while the FRIESS EFR electrostatic oil cleaner uses a high-voltage electrostatic field to capture contaminants in the oil. This purification method removes fine particles down to 0.01 μm and also captures soft contaminants such as sludge, resin and oxidation products, helping hydraulic oil remain clean over the long term. 0.01 μm fine particles　　10–20 years oil service life　　Up to about 70% fewer failures

DSD Electrostatic Oil Cleaner

Designed for industrial oils with water content below 500 ppm, the DSD uses electrostatic deep cleaning to remove fine particles, sludge and oxidation products, helping extend oil and equipment life.

DSD Electrostatic Oil Cleaner

Designed for industrial oils with water content below 500 ppm, the DSD uses electrostatic deep cleaning to remove fine particles, sludge and oxidation products, helping extend oil and equipment life.

Electrostatic Oil Cleaner Applications

FRIESS EFR D8 Reduces Hydraulic Oil Particles by About 90% in Gearbox Production

A European gearbox plant used a FRIESS EFR D8 electrostatic oil cleaner to reduce hydraulic-oil particles larger than 4 µm by about 90% in a 23-hour test.

FRIESS EFR D16 Electrostatic Oil Cleaning for an Open-Die Forging Press

Four FRIESS EFR D16 electrostatic oil cleaners treated 120,000 litres of HLP 46 hydraulic oil, removed oxidation products and reduced the MPC value from 60 to 8.

FRIESS EFR D8 Electrostatic Oil Cleaning for Injection Moulding Machines

A FRIESS EFR D8 electrostatic oil cleaner reduced hydraulic failures from two per year to zero, cut machine-filter consumption by 75% and eliminated oil changes.

FRIESS EFR D2 Oil Filtration for a Hydraulic Elevator

A FRIESS EFR D2 electrostatic oil cleaner restored 23-year-old hydraulic elevator oil, eliminating detectable oxidation products and reducing particle contamination.

Hydraulic Oil Purification Service: Reduce Failures, Oil Changes and Maintenance Costs

FRIESS hydraulic oil purification services combine oil analysis, EFR electrostatic cleaning and vacuum dehydration to reduce failures, extend oil life and lower operating costs.

FRIESS EFR D8 Reduces Hydraulic Oil Contamination by Over 90% in Wood Processing

FRIESS EFR D8 reduced wood-processing hydraulic oil contamination by over 90%, improving ISO 4406 cleanliness to 16/14/11.

Hydraulic Oil Particle Contamination: How It Causes Machine Failures

Particle contamination damages hydraulic components; FRIESS EFR electrostatic oil cleaning removes particles from 0.05 µm to over 100 µm and can reduce related failures by up to 90%.

FRIESS EFR D16 with Particle Monitoring for an Automotive Press Line

A FRIESS EFR D16 with automatic particle monitoring reduced hydraulic faults on four automotive presses by more than 80%, with payback in under six months.

Hydraulic Oil Purification for Hydropower Stations

Reliable Control and Actuation of Hydraulic Systems

Governors, gate actuators and auxiliary hydraulic systems in hydropower stations operate continuously for long periods. Fine particles, sludge and oxidation products in the oil impair valve response and accelerate wear on pumps and seals.

FRIESS electrostatic oil cleaners are available in different sizes to match tank volume and contamination load. Continuous offline purification does not interrupt normal equipment operation.

Typical applications: Governor systems, gate actuators, lubrication systems and large hydraulic power units

Hydraulic Oil Purification for Injection-Moulding Machines

Reduce Valve Sticking and Unplanned Hydraulic-System Downtime

Injection-moulding machines operate at high temperatures with continuous circulation and frequent directional changes. Hydraulic oil can develop oxidation products, sludge and extremely fine wear particles that impair the stability of proportional valves, servo valves and hydraulic pumps.

FRIESS electrostatic oil cleaners capture fine particles and soft contaminants that conventional filter elements often cannot retain, helping to extend the service life of both the oil and precision hydraulic components.

Typical applications: Injection-moulding machines, die-casting machines and other high-precision industrial hydraulic equipment

How FRIESS EFR Electrostatic Oil Cleaners Deep-Clean Hydraulic Oil

How Clean Hydraulic Oil Protects Reliable Equipment Operation

Technical Guide

01 · WEAR

Wear

02 · EROSION

Erosion

03 · OXIDATION

Oxidation

Contaminant particles catalyse oil oxidation and promote varnish, deposits and sludge, causing valves to stick and respond slowly.

High-speed oil flow carries particles into valve edges and nozzles, gradually cutting grooves that reduce control accuracy and increase internal leakage.

Particles entering lubrication clearances break down the oil film, damage pumps, valves and moving surfaces, and generate additional wear particles.

Fine particles in hydraulic systems cause wear and erosion and also accelerate oil oxidation. Continuously removing particles, sludge, varnish and deposits helps reduce failures and extend the service life of the oil and precision components.

1–4 µm

10–20 years

Approx. 70%

Typical lubricating-film thickness

Long-term oil-service example

Potential reduction in failures

Article Navigation

Jump directly to a section

01 How Contamination Damages Hydraulic Systems

02 Cleanliness and Oil Requirements

03 Conventional Filtration Methods

04 How EFR Electrostatic Purification Works

05 Maintenance, Sustainability and Economy

06 Selection Guide and FAQ

Why Filter and Purify Hydraulic Oil?

01 · RELIABILITY

Reduce Wear and Unplanned Downtime

Particles of every size, from coarse to extremely fine, accelerate wear on pumps, valves, pistons and seals, ultimately increasing maintenance and downtime costs.

02 · OIL SERVICE LIFE

Slow Oxidation and Varnish Formation

Continuously removing extremely fine particles, sludge and soft contaminants slows oil ageing and extends the life of both the oil and precision components.

03 · LIFE-CYCLE COST

Reduce Oil Purchasing, Replacement and Disposal

As long as the oil remains chemically within specification, maintaining cleanliness reduces complete oil changes, waste-oil disposal and the associated labour.

Key distinction: Built-in filters mainly control hard particles above a certain size; deep purification also removes ultrafine particles and soft deposits that accelerate oxidation.

Typical Consequences

Pump flow decreases
Internal valve leakage increases
Control accuracy gradually declines

A Wear: Breakdown of the Lubricating Film

The lubricating film between moving components can be only 1–4 µm thick. When contaminant particles enter the clearance, they disrupt the film, damage surfaces and create new wear particles, forming a cycle of contamination, wear and further contamination.

High-Risk Areas

Valve metering edges
Nozzles and throttle points
High-velocity flow-direction changes

B Erosion: Continuous Impact from High-Speed Particles

As hydraulic oil flows through the system at high velocity and pressure, particles strike valve edges, nozzles and other components with considerable energy, eroding surfaces and generating new contaminants. Continuous impact leaves grooves and wear marks.

Temperature Rule

Above 60 °C
For every approximately 10 °C increase
The oxidation rate roughly doubles

C Oxidation: Varnish and Deposits Form

Deposits and varnish are typical products of oil ageing. Particles and contaminants catalyse oxidation; the more contaminated the oil, the faster it normally ages.

Oxidation products reduce lubricating performance and thicken the oil, increasing flow resistance and energy consumption. Deposits can also form adhesive layers in clearances and on surfaces, causing valves to stick, respond slowly and lose control accuracy.

SECTION 01 | How Contamination Damages Hydraulic Systems

SECTION 02 | Cleanliness and Oil Requirements

Oil Cleanliness Must Meet System Requirements

Where Does the Cleanliness Target Come From?

Hydraulic-component manufacturers specify the oil-cleanliness level required for safe operation. Precision components such as proportional and servo valves are particularly sensitive to particle contamination.

Requirements vary by component, so the recommended cleanliness class is both a system-acceptance criterion and an important basis for selecting purification equipment.

Basic Selection Principle

Use the requirement of the
most contamination-sensitive component
as the purification target.

The more precise the component, the stricter its oil-cleanliness requirement usually is.

Oil Quality and Equipment-Warranty Requirements

Meeting the specified oil cleanliness is essential for technical reliability and may also affect the equipment warranty. Some manufacturers may reject warranty claims if the hydraulic oil does not meet their cleanliness requirements.

Select oil-purification equipment capable of continuously maintaining the target cleanliness class and verify performance through regular oil analysis.

More information: hydraulic-oil analysis and cleanliness-class assessment.

Hydraulic Oil Purification Basics

Maintenance Checklist

11 Ways to Reduce Failures and Operating Costs

For routine inspection and preventive maintenance

01

Follow the maintenance intervals and requirements specified by the manufacturer

02

Have trained personnel perform and document maintenance

03

Check oil condition regularly through oil analysis

04

Replace hydraulic oil only when its chemical condition is out of specification

05

Prevent water ingress and dewater the oil when necessary

06

Keep seals and hoses intact and replace damaged parts promptly

07

Keep the working environment clean to reduce contaminant ingress

08

Do not allow the oil level to fall below the specified minimum

09

Store hydraulic oil in clean, sealed and suitable containers

10

Use clean equipment and filter new oil during filling and top-up

11

Inspect the filtration system and oil cleanliness regularly

SECTION 03 | Conventional Hydraulic Oil Filtration Methods

Filtration methods can be classified by installation position or by filter medium and operating principle. Each performs a different task, and industrial hydraulic systems normally use a combination.

01 · PUMP INLET

Suction Filters

Prevents larger particles from entering the hydraulic pump; resistance must remain low to avoid insufficient suction or cavitation.

02 · PRESSURE LINE

Pressure Filters

Filters the oil before it reaches sensitive components; the housing and element must withstand system pressure.

03 · RETURN LINE

Return-Line Filters

Filters return oil before it enters the tank, preventing operating contaminants from being carried back into the reservoir.

04 · DEPTH MEDIA

Depth Filters

Uses a thick, porous medium to capture particles; soft contaminants can rapidly cause blockage.

05 · SURFACE SCREEN

Screen Filters

Retains solid contaminants with a fine screen; filtration accuracy is mainly determined by mesh size.

06 · OFFLINE CIRCUIT

Offline and Bypass Purification

Purifies continuously at a stable low flow, independent of main-system flow fluctuations, and can be serviced without stopping production.

SECTION 04 | EFR Electrostatic Purification Principle

Electrostatic Purification Principle

Capturing Contaminants with an Electrostatic Field Instead of Fine Pores

A high-voltage electrostatic field is created between two electrodes. It acts on charged contaminant particles in the oil, deflecting them toward the collector media. Because EFR uses a different operating principle, it can remove ultrafine particles that conventional filter media struggle to retain.

Generate the Electrostatic Field

High-voltage electrodes create a stable field inside the purification chamber.

Deflect the Particles

Charged particles of different sizes are acted upon by the field.

Capture in Collector Media

Particles and soft deposits are collected continuously.

Limitations of Conventional Filter Media

Pore Size Determines Retention Capability

Ultrafine particles can pass through conventional media, while soft contaminants such as sludge, resin and varnish can rapidly block the filter element.

How EFR Is Different

Regenerative Oil Purification

EFR continuously removes oxidation-promoting ultrafine particles and existing soft deposits, slowing oil ageing and helping restore oil performance.

Four Core Advantages of FRIESS EFR

Capture Ultrafine Particles

Removes particles finer than conventional depth filters can reliably retain.

Remove Soft Contaminants

Continuously removes sludge, resin, varnish and other oxidation deposits.

Continuous Offline Operation

Easy to connect, monitor and maintain without interrupting the main hydraulic system.

Low Consumable Use and Long Service Intervals

High-capacity collector media reduce consumable use and extend maintenance intervals.

SECTION 05 | Maintenance, Sustainability and Economy

The large electrostatic purification chamber provides high contaminant capacity and opens easily. Maintenance personnel can replace the collector media quickly and directly assess captured contamination by comparing the media before and after use.

Before Use

Clean, Large-Area Collector Media

Unused collector media provide a large effective collection area for continuously capturing fine particles and soft contaminants in the electrostatic field.

After Use

Contaminants Are Captured Visibly

Tar-like deposits, oxidation products and ultrafine particles accumulate on the originally white media, demonstrating EFR’s ability to capture resinous and fine contaminants that conventional filters cannot readily remove.

Maintenance Without Shutdown

The main system can continue operating while media are replaced in the offline circuit.

Approx. 2,000-Hour Inspection Interval

The actual inspection interval depends on the site contamination load.

Clear Maintenance Indication

The display shows operating time and maintenance reminders.

How Does Oil Purification Save Oil and Protect Resources?

↓

Extend Oil Life While Reducing Resource Consumption

Hydraulic oil consumes energy and generates carbon emissions throughout raw-material extraction, production, transport and waste-oil disposal. Reducing oil changes lowers purchasing, transport and disposal burdens at the same time.

SECTION 06 | Selection Guide and Frequently Asked Questions

More Than 50 Years of FRIESS Industrial Oil-Purification Experience

50+

Years of Oil-Purification Experience

FRIESS products are made in Germany and have operated reliably in numerous industrial applications for many years. We combine oil type, system volume and contamination load to provide durable, dependable and easy-to-maintain purification solutions.

When selecting hydraulic-oil purification equipment, assess purification performance, system compatibility and life-cycle cost—not only the initial purchase price.

Filtration Method

Review the existing suction, pressure and return-line filtration and determine whether offline deep purification is required.

Oil Volume and Treatment Capacity

Select the equipment size according to tank volume, system flow and contamination load.

Target Cleanliness

Use the cleanliness requirement of the most sensitive component as the purification target.

Contaminant Type

Consider both hard particles and soft contaminants such as sludge, resin and varnish.

Installation and Maintenance

Evaluate offline connections, installation space, consumables, maintenance intervals and whether shutdown is permitted.

Why Is Maintenance Simpler?

Clear, fast maintenance procedures help reduce labour and downtime costs. The FRIESS EFR display indicates operating time and maintenance intervals for timely contamination checks. Collector media and electrodes are readily accessible, and the hydraulic system can remain in operation during servicing. High contaminant capacity allows a typical inspection interval of about 2,000 operating hours; the actual interval depends on the contamination load.

How Do You Select the Right Filtration Combination?

The optimum filtration solution depends on the specific system. Most hydraulic systems combine several filtration methods to achieve the target cleanliness. Follow the oil and equipment manufacturers’ requirements when selecting equipment and setting maintenance intervals.

Conventional filters can control hard particles above a certain size but often cannot remove the ultrafine particles that accelerate oil ageing or soft contaminants such as varnish and resin. This is the principal application advantage of FRIESS EFR electrostatic oil purification.

Frequently Asked Questions: Selection, Operation and Cost

How Does Electrostatic Hydraulic-Oil Filtration Reduce Oil Consumption?

By slowing oil ageing and maintaining a consistently low contamination level, oil-change frequency can be reduced significantly. This lowers hydraulic-oil purchasing and waste-oil disposal volumes while extending the service life of the existing oil.

How Does Clean Oil Reduce Maintenance Costs?

Keeping the oil continuously clean reduces component wear and failures, lowering spare-parts replacement, top-up oil and maintenance labour costs. Continuous EFR purification combined with proper maintenance reduces total system operating cost.

What Is the Cost Impact of Continuous EFR Purification?

As long as the oil remains chemically within specification, continuous contaminant removal postpones complete oil changes and new-oil purchases while reducing damage to valves, pumps and other components and the risk of unplanned downtime.

What Is Varnish in Hydraulic Oil?

Varnish and resinous deposits are typical products of oil oxidation and ageing. Particles catalyse oxidation; above 60 °C, every increase of about 10 °C roughly doubles the oxidation rate. EFR continuously captures ultrafine particles and soft oxidation deposits.

How Is EFR Different from Conventional Filter Elements?

EFR is not directly limited by conventional filter-media pore size. It removes difficult-to-retain ultrafine particles and captures soft contaminants such as sludge, resin and varnish. Continuous offline operation gradually achieves a high level of system cleanliness.

What Is the Typical Payback Period for FRIESS EFR?

Payback mainly depends on system oil volume, the previous oil-change interval, component-failure frequency and the costs of oil purchasing, disposal and downtime. In large systems, avoiding one complete oil change can shorten payback significantly. Many projects achieve payback in about one year; the actual period should be evaluated using site data.

Replace the Oil or Continue Using It After Purification?

When viscosity, acid number, additive condition and other chemical parameters remain within specification, continued use after purification is usually more economical and reduces resource consumption and waste-oil disposal.

Replace the oil when: The oil has undergone irreversible chemical degradation or contamination is too severe for economical recovery.

How Long Does It Take to Reach the Target Cleanliness?

The time depends on system oil volume, initial contamination, contaminant type and existing filtration. Dirtier systems require a longer initial purification period; after the target is reached, the load required to maintain cleanliness drops substantially. A site-specific estimate can be based on oil analysis, tank volume and operating conditions.

How Does Clean Oil Help Reduce Maintenance Costs?

A substantial share of hydraulic-system damage is related to oil contamination. Keeping the oil continuously clean reduces component wear and failures, lowering spare-parts replacement, top-up oil and maintenance labour costs.

Continuous oil purification with FRIESS EFR, combined with proper maintenance, can significantly reduce the total operating cost of a hydraulic system.

What Is the Cost Impact of Continuous EFR Oil Purification?

As long as the hydraulic oil remains chemically within specification, continuous contaminant removal allows extended use with only leakage losses topped up. This postpones complete oil changes and new-oil purchases.

Clean oil also reduces damage to valves, pumps and other components and lowers the risk of unplanned downtime. Oil changes, repairs and downtime often represent a major share of hydraulic-system operating cost; reducing these expenses improves both equipment availability and overall economy.

What Is Varnish in Hydraulic Oil?

Varnish, resin and other oxidation products are typical signs of oil ageing caused by reactions between the oil and oxygen in the air. Particles and contaminants catalyse oxidation, so heavier contamination accelerates oil ageing.

Oil temperature also has a strong influence on oxidation. Below 60 °C the reaction is relatively slow; above 60 °C, every increase of about 10 °C roughly doubles the oxidation rate. Prolonged excessive oil temperature should therefore be avoided.

Conventional depth filters retain particles in the pores of the filter media. Ultrafine particles can pass through under pressure, while sticky varnish and resin deposits can rapidly block the element. EFR captures these contaminants with an electrostatic field rather than tiny pores, enabling continuous offline removal of ultrafine particles and soft oxidation deposits.

How Is EFR Different from Conventional Filter Elements?

FRIESS EFR is not directly limited by conventional filter-media pore size. The electrostatic field acts on charged particles of different sizes, removing ultrafine particles that conventional filters struggle to retain. It also captures soft oxidation deposits such as sludge, resin and varnish.

Although offline purification uses a relatively low flow rate, the unit can operate continuously and ultimately achieve a very high oil-cleanliness level—often cleaner than untreated new oil.

What Is the Typical Payback Period for FRIESS EFR?

The payback period of FRIESS EFR depends on:

System oil volume;
Previous oil-change interval;
Failure frequency of valves, pumps and other components;
Purchase and disposal cost per litre of hydraulic oil.

For systems with a large oil volume and high oil-change cost, avoiding one complete oil change may allow the equipment to pay for itself within two to three months. Many projects achieve payback in about one year, depending on savings in oil purchasing, waste-oil disposal, spare parts and downtime.

Replace the Oil or Continue Using It After Purification?

When contamination reaches a certain level, the oil must either be replaced or purified. Complete oil changes are expensive in large systems; as long as viscosity, acid number, additive condition and other chemical parameters remain within specification, continued use after purification is usually more economical and reduces resource consumption and waste-oil disposal.

If the oil has undergone irreversible chemical degradation or contamination is too severe for economical recovery, replace it in accordance with the oil and equipment manufacturers’ requirements.

How Long Does It Usually Take to Reach the Target Cleanliness?

The time required to reach the target cleanliness depends on system oil volume, initial contamination, contaminant type and existing filtration. Dirtier systems and oils require a longer initial purification period; after the target is reached, the treatment load and time needed to maintain cleanliness decrease substantially.

Because every system operates under different conditions, no universal time can be given without data. We can provide a more accurate estimate based on oil analysis, tank volume and operating conditions.

Beyond cost savings, using FRIESS oil skimmers, electrostatic oil cleaners, magnetic filters, coolant purification systems, oil-water separators, vacuum oil purifiers offers many other advantages.

We will be pleased to explain which solution best fits your application.