Overview
ISO 6521-3:2019 specifies minimum requirements for lubricants for refrigerating compressors (Family D, Class L). It defines categories DRA, DRB, DRC, DRD, DRE, DRF and DRG and covers mineral- and synthetic-based oils intended for all types of refrigerating compressors. The standard focuses on oil/refrigerant interaction - including miscibility, solubility, thermal and chemical compatibility, and practical handling such as sampling and water content control. ISO 6521-3 is intended to be read alongside ISO 6743-3 (classification of compressor lubricants).
Key topics and technical requirements
- Categories and fluid types: Defines seven refrigerating compressor oil categories (DRA–DRG) by miscibility with specific refrigerant families (e.g., ammonia R717, HFCs, CFC/HCFC, CO2, hydrocarbons). Typical base fluids listed include highly refined mineral oils, polyalphaolefins (PAO), polyol esters, polyalkylene glycols (PAGs) and polyvinyl ethers.
- Compatibility with refrigerant: Oils must not chemically react under compressor temperatures and must meet compatibility requirements for the specific refrigerant.
- Miscibility and solubility: Standards require that oils exhibit appropriate miscibility at evaporator temperatures (to avoid phase separation and trapped oil) and maintain adequate viscosity under compressor pressure/temperature (VPT or Daniel plots).
- Water content and handling: Maximum water limits for fresh oils are specified by oil chemistry (PAGs are more hygroscopic than esters or hydrocarbons). Gas-tight packaging and vacuum drying/degassing before compressor fill are emphasized.
- Sampling and testing: Sampling per ISO 3170 and a suite of test methods are referenced (e.g., ISO 3104 for kinematic viscosity, ISO 2592/2719 for flash points, ISO 6296/Karl Fischer methods for water, ISO 6245 ash, DIN 51538 ammonia resistance).
- Specification tables: Detailed property tables (Annex A) give viscosity grades (ISO 3448) and other required parameters for each ISO-L-DR category.
Applications and who uses this standard
ISO 6521-3 is used by:
- Refrigeration and HVAC-R engineers specifying compressor lubricants.
- Compressor manufacturers to set lubricant compatibility and performance requirements.
- Lubricant manufacturers and suppliers for product classification and formulation guidance.
- Procurement/specification writers and maintenance teams ensuring correct oil selection for systems using ammonia, HFCs, CO2, hydrocarbons or legacy CFC/HCFC refrigerants.
- Testing labs performing VPT, miscibility, water content and compatibility tests.
Keywords: ISO 6521-3:2019, refrigerating compressor oils, compressor lubricant specifications, oil-refrigerant miscibility, VPT diagram, polyol esters, polyalkylene glycols, ISO 6743-3.
Related standards
Open preview (PDF)Lubricants, industrial oils and related products (Class L) — Family D (compressors) — Part 3: Specifications of categories DRA, DRB, DRC, DRD, DRE, DRF and DRG (lubricants for refrigerating compressors)