In-Service Heat Transfer Fluids
by David Doyle, CLS, OMA I, OMA II
General Manager, Tribology

A fluid that can perform realibly is required for heat transfer systems or heat exchangers using hot-oil systems. Heat transfer fluids can be formulated with highly refined mineral oils or silicon based fluids, this short subject will mainly address mineral oil based fluids. Heat transfer systems that use a mineral oil medium are used in a variety of industries:

              • Industrial manufacturing
              • Asphalt processing
              • Food and beverage
              • Chemical processing
              • Plastics and polymers

These fluids are required to operate within a wide temperature range for cold startups and heat transfer applications. The correct fluid should have a low vapor pressure to resist thermal cracking and be able to reach high temperatures without boiling or reaching the vapor phase.

Proper system performance and maintenance is also necessary for fluid life integrity and required fluid heat transfer properties. Water contamination can occur when air is allowed to enter and moisture condenses during cooling. When water heats up in the system a popping or snapping noise can sometime be heard in the piping. Low points in the system can collect water. If it is possible to drain a sample from one of these low points the system can be inspected for a buildup of water contamination. Leaks in the piping that allows the fluid to absorb into porous insulation can raise the internal temperature of the insulation as the fluid heats and becomes oxidized. This can also lead to a fire danger. It is important the reservoir design allows fluid expansion between cooling when shutdown and during heating when starting up. The reservoir should prevent exposure to air and not allow the fluid to get too hot during residence time. Caution should be used when mixing different heat transfer fluid products. Heat transfer properties can be adversely affected and additive compounding chemistry can become compromised. The safest approach when changing to another heat transfer fluid product is to ensure the system is clean and dry before changing products.

Thermal cracking and oxidation is one of the primary concerns of heat transfer fluids in service. Thermal cracking occurs when larger oil molecules decompose into coke particles, which is mostly carbon, and smaller oil molecules with a lower boiling point. These smaller ‘cracked ‘molecules can react together and combine to produce even larger molecules than those in the original fluid. This increases the viscosity of the fluid and hinders heat transfer properties. The larger oxidative molecules can also form polymerized molecules which develops sludge, varnish and insolubles that also reduces heat transfer performance of the fluid.

Thermal cracking can be minimized by:

  • Maintaining circulating fluid flow and consistent fluid pressure through the heater
  • Bring cold fluid up to temperature slowly after circulation begins
  • Avoid sudden shutdowns without allowing the fluid to cool down first.
  • Monitor the combustion chamber for proper flame propagation and burner alignment
  • Properly maintain system instrumentation
  • Daily top-up with fresh fluid and not reuse old fluid
  • Perform regular system cleaning

As well as routinely inspecting the heat transfer system, periodic fluid samples should be taken from the system while the fluid is circulating. In-line sampling from the suction side of the circulating pump, return line, or near the heater are suitable locations. The fluid should be circulating at a temperature of 200°F (93°C) when sampling occurs.

ASTM Standard D5372 ‘Standard Guide for Evaluation of Hydrocarbon Heat Transfer Fluids’ provides the following list for testing in-service mineral oil heat transfer fluids.

  • Viscosity
  • Carbon Residue
  • Copper Strip Corrosion
  • Pentane Insolubles
  • Flash Point
  • Total Acid Number
  • Color
  • Water Content
  • Pour Point
  • Spectrochemical Analysis
  • Oxidation Number

ALS Tribology can provide a testing routine on a monthly, quarterly, or annual basis that is cost effective and meets system reliability requirements. Please feel free to contact one of our laboratories or customer service representatives for further information.