Service Life Of A Heat Transfer Fluid

The service life of a heat transfer fluid and the operating efficiency of the process can be increased by minimizing thermal cracking, oxidation and contamination*.

Thermal Cracking

All fired and electric immersion type heaters are capable of exceeding the maximum recommended film temperature of the heat transfer fluid under certain condition. Exceeding the maximum film temperature for a period of time can cause excessive cracking and premature fluid failure. Excessive cracking can be minimized as follows:

Maintain Design Fluid Velocity at all Times through the Heater - Pressure drop across the entire system should be calculated when sizing pumps. System by-pass valve response should be tuned to maintain design fluid flow rate under all process conditions. Filters and strainers should be properly located and monitored to prevent blinding.

Bring Cold Systems Up to Temperature Slowly - Cold fluid can overheat if heater operates at full power from start-up. Fluid temperature should be increased in 25°F (15°C) increments until fluid viscosity is less than 10 CPS (check fluid property tables). Make sure this procedure does not heat up the system more rapidly than manufactures recommendations.

Avoid Sudden Shutdowns - Allow fluid to circulate until the heater outlet temperature is a maximum of 250°F. If repeated power failures occur, consider connecting pump to auxiliary power source.

Maintain System Instrumentation - Failure of high temperature and/or low flow alarms not only can cause overheating but can create potential for equipment fires.

Check the Combustion Chamber - Improper flame propagation or burner alignment can cause overheating but can create potential for equipment fires.


Severe fluid oxidation can create significant equipment problems. In many cases, fouling or corrosion of the expansion tank are the first signs that a problem exists if routine fluid analyses have not been performed. Minimizing oxidation is relatively simple:

Keep the Expansion Tank Below 140°F - Smoke emitting from the expansion tank vent line is usually a sign that the expansion tank is too hot. Install either a cold seal pot on the expansion tank vent or blanket the tank with a low pressure inert gas. Never continuously operate a system with the warm up valve open. The tank itself and lines leading to it should be left uninsulated. A thermal loop seal, reduced diameter piping (1/3 main pipe diameter) between the expansion tank and main loop or thermal buffer tank will reduce thermal currents.

Minimize Exposure of Hot Fluid to Air - install either a cold seal pot on expansion tank vent or blanket the tank itself with low pressure inert gas.

Maintain Positive Net Pump Suction Head (NPSH) at All Times - High vacuum due to flow restrictions (such as plugged strainers) can cause seal wear and allow air to be sucked into fluid.


Contaminants can promote fluid degradation as well as cause operational problems. Contaminants can enter the system in several ways:

New Systems - Make sure that all fabrication debris (such as mill scale, weld spotter and slag) or protective coatings are removed before assembly. Pressure test system with either heat transfer fluid or inert gas - Never pressure test with water.

System Cleaning** - Complete draining from all system low points is required when using organic based solvents or flushing fluids. At least one full charge of new heat transfer fluid should be used to flush the system if complete draining is not possible. Water based cleaners must be completely rinsed from the system with fresh water. Residual water should be removed by draining and then purging with hot, dry nitrogen down to a -20°F dew point or vacuum. Boiling off residual water through the expansion tank is not recommended since it will cause fluid degradation.

Daily Operation - Always use fresh fluid to top off system. Fluid “burped” out the vent or collected in drip pans should be discarded. Don’t mix fluids - if you have a problem, call MultiTherm® at 800-225-7440.

*See Fluid Analysis for more detail on mechanism and detection of fluid degradation.
** See System Cleaning Information for a review of cleaning procedures.