There are three main reasons why any hydrocarbon heat transfer fluid will break down or degrade.

Thermal Cracking. Thermal cracking is the phenomena by which large oil molecules are decomposed into solid coke - 90 to 95 percent carbon - and small, lower boiling molecules. Certain of these molecules, which are reactive, combine to produce even larger molecules than those in the original fluid and increase the viscosity of the fluid.

All fired and electric immersion-type heaters are capable of exceeding the maximum recommended film temperature of a heat transfer fluid under certain conditions. Exceeding the maximum film temperature for a period of time can cause excessive cracking and premature fluid failure.

Oxidation.All organic heat transfer fluids react with air to form organic acids. This oxidation rate is low at ambient conditions but increases rapidly with temperature. These acids can undergo free radical polymerization, which will increase the fluid viscosity and ultimately can result in deposits. 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.

Contamination.Contaminants can catalyze fluid degradation and also result in severe operating and equipment problems. 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 (mill scale, weld spotter,slag, etc.) and protective coatings are removed before assembly. Pressure test system with either heat transfer fluid or inert gas. Never pressure test with water.

  • Daily Operation: Always use fresh fluid to top off system. Fluid “burped” out the vent or collected in drip pans should be discarded. Do not mix fluids. Make sure spare drums are kept in a covered shelter, and do not allow water to gather on top of drums during storage.