Determination of Treatment Air Volume for Oil Mist Purifiers: Standards, Methods and Applications

The treatment air volume of an oil mist purifier is the key performance parameter that determines capture efficiency, purification capacity and fan energy consumption in metalworking and machining facilities. This article explains how oil mist purifier treatment air volume determination is performed, covering applicable standards such as ISO 5801, ISO 3966, GB/T 1236 and GB/T 16157, instrumentation requirements, calculation methods and acceptance criteria as carried out by accredited third-party laboratories.

Technical Overview

Oil mist purifiers, also referred to as oil mist collectors or mist eliminators, are air pollution control devices engineered to remove airborne oil mist, coolant aerosols, smoke and fine solid particulates generated by CNC machine tools, grinding machines, gear cutting equipment, heat treatment furnaces and die casting operations. The treatment air volume, typically expressed in cubic meters per hour (m³/h) or cubic feet per minute (CFM), quantifies the volume of contaminated air that the purifier can process per unit of time under rated operating conditions, and it directly defines the capture velocity at the pollution source, the residence time inside the purification stages and the pressure load on the fan.

Accurate determination of the treatment air volume is critical for the following reasons:

• Capture performance: the air volume extracted from a machine enclosure must exceed the internal mist generation rate in order to maintain slight negative pressure and prevent oil mist from escaping into the workshop.
• Purification efficiency: both electrostatic collection cells and mechanical filter media are velocity sensitive, and operation far above or below the design face velocity measurably reduces collection efficiency.
• Energy and acoustics: fan input power rises steeply with flow rate, so verified air volume data allows the system resistance, fan selection and noise level to be optimized.
• Compliance: verified air volume is a declared parameter in product type tests, acceptance tests and occupational hygiene assessments conducted by accredited third-party laboratories.

Depending on the purpose of the measurement, the determination is performed either as a laboratory type test using a standardized airway conforming to ISO 5801 or GB/T 1236, or as an in-situ field measurement in the connected ducting following the pitot static tube velocity area method of ISO 3966 and GB/T 16157. Both approaches rely on calibrated, traceable instrumentation and documented density correction, and results are reported with the measurement uncertainty, commonly within ±3% to ±5% at a 95% confidence level.

Testing Samples

Electrostatic Oil Mist Purifiers

Units that ionize the oil aerosol with high-voltage electrodes and collect the charged droplets on parallel plate cells. The air volume is verified at rated discharge voltage and rated fan speed, with pre-filters, metal mesh demisters and after-filters installed in their operating positions, because their initial resistance directly influences the delivered flow.

Mechanical and Filter Media Oil Mist Purifiers

Units based on inertial impaction, centrifugal separation, coalescing media, cartridge filtration or HEPA final stages. Because the pressure drop of these units rises as they load with oil, the treatment air volume is normally determined in the clean, unloaded state, and optionally repeated at the recommended replacement pressure drop to document the minimum guaranteed flow.

Centrifugal Oil Mist Collectors

Machines using a high-speed rotating drum or impeller that flings oil droplets against the casing wall, commonly mounted directly on machining centers. The air volume of these units is strongly coupled to rotor speed and internal flow path resistance, so rotational speed is recorded together with the measured flow.

Complete Installed Systems and Duct Networks

Single-machine ducted installations and centralized multi-machine purification systems, in recirculation or exhaust configuration. Field samples require an accessible straight duct section of sufficient length for the measuring plane, and the operating status of dampers, machine doors and interconnected machines is documented during the test.

Testing Items

The primary deliverable of the determination is the rated treatment air volume; the supporting measured quantities required to make the result valid and traceable include:

• Rated treatment air volume under specified operating conditions (m³/h or CFM)
• Mean air velocity at the measuring cross section (m/s)
• Velocity distribution and uniformity across the measuring plane
• Duct dimensions and internal cross-sectional area (m²)
• Static pressure at the purifier inlet and outlet, and total device pressure drop (Pa)
• Dynamic or velocity pressure readings at each traverse point (Pa)
• Ambient temperature, relative humidity and barometric pressure for air density correction
• Fan rotational speed (r/min) and motor operating parameters such as voltage, current and frequency
• Derived air volume versus static pressure resistance curve across the operating range
• Measurement uncertainty statement and instrument calibration traceability

Testing Methods

Pitot Static Tube Velocity Area Method

This is the reference method for in-duct field measurement, specified in ISO 3966 and GB/T 16157. A calibrated S-type or L-type pitot static tube connected to a calibrated digital micromanometer is inserted into a straight duct section located preferably at least six duct diameters downstream and three diameters upstream of elbows, dampers or other flow disturbances. Velocity pressure is measured sequentially at a grid of traverse points arranged on perpendicular diameters of circular ducts using the log-linear or equal-area annular layout, typically comprising at least twenty points. Each point velocity is calculated from the pitot equation v = √(2 × Δp / ρ), where Δp is the velocity pressure and ρ is the air density. The mean of all point velocities multiplied by the internal duct cross-sectional area yields the volume flow, summarized as Q = 3600 × v × A, where Q is the treatment air volume in m³/h, v is the mean velocity in m/s and A is the duct cross section in m².

Anemometer Traverse Method

For low-velocity ducts, short straight sections or direct face measurement at the purifier inlet and outlet, calibrated vane anemometers or hot-wire thermal anemometers are traversed over an equal-area grid of the measurement plane. The arithmetic mean of the point velocities multiplied by the effective flow area gives the air volume. This method is convenient for on-site verification but requires careful sealing of probe access openings, regular calibration and zero verification, and its measurement uncertainty is generally slightly higher than that of the pitot tube method.

Standardized Airway and Nozzle Chamber Method

For laboratory type testing, the purifier is installed on a standardized test airway conforming to ISO 5801 or GB/T 1236, and the air volume is derived from the differential pressure across one or more calibrated flow nozzles in a chamber, together with the air density upstream of the nozzles. By varying the auxiliary fan and throttling device, the complete characteristic curve of the purifier from free delivery to shut-off can be mapped, providing both the rated air volume at the design resistance point and the air volume versus static pressure relationship needed for system matching.

Density Correction, Calculation and Reporting

Air density at the measuring plane is computed from the measured dry-bulb temperature, duct static pressure, water vapor pressure and barometric pressure following the procedures in ISO 3966. Where required by the specification, the measured air volume is converted to standard atmospheric conditions, for example 20°C and 101.325 kPa, and clearly labeled as a standard volume flow. The test report states the instrument list with calibration certificates, measuring plane location, traverse point layout, individual pressure readings, the density correction calculation, the final air volume with its expanded uncertainty, and the operating conditions of the purifier including fan speed and filter loading status.

Applications

Determination results for treatment air volume are used throughout the lifecycle of oil mist purification equipment:

• Factory and site acceptance testing of newly purchased oil mist purifiers against contractual rated values
• Type testing and product certification programs conducted by accredited third-party laboratories, reported together with purification efficiency
• Research and development of fan and filter matching, casing aerodynamics and air volume versus resistance optimization
• Occupational hygiene assessments of workshop oil mist exposure and indoor air quality improvement projects
• Emission compliance verification for purified exhaust discharged to the atmosphere through stacks
• Predictive maintenance, in which a measured decline of air volume below the rated band indicates filter clogging, electrostatic cell fouling, duct leakage or fan belt slip

Typical industrial scenarios include CNC turning and milling centers, machining centers, gear hobbing and broaching machines, surface and internal grinding, induction hardening and quenching tanks, aluminum die casting, cold heading, and lubrication oil separation at screw compressor stations.

FAQ

What is the treatment air volume of an oil mist purifier?

It is the volume of oil-mist-laden air, expressed in m³/h or CFM, that the purifier can process per unit of time under rated voltage, rated fan speed and specified resistance conditions. It is the fundamental sizing parameter used to match the purifier to the machine enclosure volume, the mist generation rate and the required capture velocity.

How is the air volume calculated from measured velocity?

The volume flow is the product of the mean air velocity and the flow cross-sectional area: Q = 3600 × v × A, where Q is the air volume in m³/h, v is the area-weighted mean velocity in m/s obtained from traverse measurements, and A is the internal duct or face area in m². The factor 3600 converts seconds to hours.

What deviation from the rated air volume is acceptable?

Laboratory type tests usually require the measured air volume to agree with the declared rating within ±5% under the specified resistance. For field measurements on installed systems, an acceptance band of approximately ±10% is commonly applied, because duct losses, damper settings and machine door positions introduce additional variability.

Which standards govern the determination?

Laboratory fan performance testing is performed per ISO 5801 and GB/T 1236, in-duct velocity area measurement per ISO 3966 and GB/T 16157, with Japanese equipment often referenced to the corresponding JIS fan testing standard. The choice depends on whether the purpose is product type testing or on-site verification.

How often should the air volume be measured?

At commissioning and acceptance of the installation, after every filter replacement or electrostatic cell cleaning, and at least annually as part of a preventive maintenance program. Significant or recurring shortfalls compared with previous readings indicate progressive fouling or mechanical degradation that should be corrected.

Can the air volume be measured while the machine is producing oil mist?

Yes, pitot static tube and anemometer measurements are largely unaffected by oil aerosol at typical workshop concentrations, although heavy wetting of the probe should be avoided by using S-type pitot tubes and periodic purge checks. For heavily loaded airstreams, laboratory determination on a standardized airway under clean air conditions is preferred for rating purposes.