ISO 6743-12:1989

Overview

ISO 6743-12:1989 specifies the classification of heat transfer fluids within the broader ISO 6743 family for lubricants, industrial oils and related products (class L). It defines product categories (Family Q) and a uniform designation system for fluids used in heat transfer applications. The standard is intended to be read together with ISO 6743-0 (general rules) and uses viscosity grades defined in ISO 3448.

Key topics and requirements

• Category coding: Each product is given a two‑letter code starting with “Q” (e.g., QA, QB). The second letter identifies the specific category; the pair forms a standard code for the fluid family.
• Temperature- and system-based categories: Categories in the detailed classification address typical operating conditions and system types:
  • Q A - Open-circuit heat transfer fluids; bulk temperature up to <250 °C; fire risk and open container considerations.
  • Q B - Closed-circuit fluids for general heat transfer; bulk temperature up to <300 °C.
  • Q C - Closed-circuit fluids with forced circulation for higher temperatures >300 °C and <320 °C; systems require filtration and circulation.
  • Q D - Synthetic fluids for particularly high temperatures (bulk measured with forced circulation >320 °C).
  • Q E - Cooling circuits where fluid must perform at low temperatures > −30 °C and <200 °C with low viscosity at low temperature and adequate thermal stability.
• Designation format: Products can be designated in full (ISO‑L‑QA 220) or abbreviated (L‑QA 220); the numeric suffix indicates viscosity grade per ISO 3448.
• Practical system requirements: Notes on safety and system design - fire risk assessment, expansion tanks and venting for closed systems, filtration and circulation for high‑temperature units, and hygiene compliance for heat transfer in food applications.
• Measurement note: Temperatures are bulk oil temperatures measured in the heater discharge line; film temperatures on heater surfaces can be higher.

Applications and users

• Who uses it: lubricant and heat‑transfer fluid manufacturers, OEMs of heaters and heat‑transfer equipment, maintenance engineers, plant safety and procurement teams, and regulatory/specification writers.
• Where applied: industrial heating and cooling systems, hot oil loops, thermal oil heaters, closed‑circuit process heat systems, and applications requiring classification for specification, procurement, testing, or compliance (including food‑service heat exchangers).

Related standards

• ISO 6743-0 - General rules for classification of lubricants and related products (required companion document).
• ISO 3448 - Viscosity classification used to indicate fluid viscosity grades in designations.

Keywords: ISO 6743-12, heat transfer fluids, lubricant classification, industrial oils, ISO 3448, thermal stability, closed-circuit, open-circuit, high-temperature fluids.