Fluid Analysis Of Heat Transfer Fluids In Hot Oil Units

Fluid Analysis for Hot Oil Units

Organic Heat Transfer Fluids in hot oil units degrade over time due to thermal cracking and oxidation. The rate of degradation can be influenced by improper operating procedures, equipment layout and/or malfunctions or contamination from water or other material. A comprehensive Fluid Analysis Program consisting of:

  • Laboratory testing of the sample
  • Evaluation of system performance and laboratory data over time
  • Thorough discussion of results and recommendations to user can prolong the life of the system and the fluid

Thermal Cracking

Thermal cracking is the phenomena by which large oil molecules are decomposed into solid coke (90-95% 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.

The Distillation Range test establishes the relative amounts of large and small molecules in a sample by measuring the temperature at which certain volume fractions boil. An increase in the concentration of large and/or small molecules (which boil at higher and lower temperatures respectively) can be expected if thermal cracking has occurred. Corrective action or a fluid replacement should be considered when the change is greater than 10% as compared to new fluid.

Kinematic Viscosity of chemically similar fluids is proportional to the average molecular weight. Results may or may not deviate from new fluid due to the mix of large (high viscosity) and small (low viscosity) molecules and so this test alone is not a valid indicator of fluid condition. However, it does provide useful system information. Fluid replacement or other corrective action is recommended at a change of 50%.

Flash Point is the temperature at which the fluid vapor will flash when a small flame is passed a specified distance above the sample. The flash point may drop as smaller, more volatile molecules are formed. Corrective action such as a boilout or fluid replacement should be considered when the flash point decreases more than 100°F as compared to new fluid.

Pentane Insolubles measures the amount of coke and other particulate matter that is suspended in the fluid. The solids are removed by filtering, washed with pentane to remove fluid, dried and weighed. Thorough filtration of the fluid in the system should be considered at 0.4 % (wt.).


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.

The Total Acid Number (TAN) is a measure of the organic acid concentration in the fluid. New fluid has a TAN of 0.01. Complete fluid replacement is recommended at a TAN of 3.0.


Contaminants can catalyze fluid degradation and also result in severe operating and equipment problems.

The Karl Fischer Water test determines the amount of water present in the fluid. Generally, a level of 350 ppm will cause sufficient operating problems to require a system boilout.

A variety of analytical procedures are available for detecting and identifying non-aqueous contaminants such as leaked product. Contact MultiTherm® for more information.

Evaluation and Recommendations

The laboratory data provides only a snapshot of the fluid condition. The data must be put into a time perspective along with the operating history to obtain a complete system analysis. This allows corrective action to be implemented before the fluid life or equipment efficiency is compromised.


Fluid samples should be taken from a “live” part of the system, preferably from the heat user or near the suction side of the circulating pump. The fluid should be circulating at a temperature of 200°F (93°C). Flush the sample line thoroughly. Fill the bottle to within 2" of the top. Write the date line designation and purchase order number (if applicable) on the label. Include a Material Safety Data Sheet (MSDS) with the sample.

Do not take the sample from the expansion or drain tank.

It is important that the sample be put directly into the sample jar. Do not use another container to catch sample if fluid is too hot. Wait for system to cool before taking sample.

A one-quart sample bottle packed in a protective shipping container is available from MultiTherm®.