How Do We Know Our Heat Transfer Fluids
are Long Lasting?

Introduction

Heat transfer fluid temperature ratings are traditionally considered to be the maximum use temperature that the fluid can withstand with an acceptable rate of thermal degradation.  Radco has done thermal stability comparison studies between many heat transfer fluids, including between synthetic fluids and “hot oils.”  If you would like to see a comparison between Radco’s XCELTHERM® brand of heat transfer fluids and what you are using now, just click here and e-mail Pete, our product manager, with the brand name of the current fluid and your bulk operating temperature.

When we formulate a fluid, we start by identifying those which have been in use for many years and are accepted as having a long life.  Using laboratory testing, we benchmark degradation rates of these fluids and they become our minimum standard.  Fluid ratings can be very subjective.  As you can see in the example below, the “Typical Hot Oil” in this test was rated at 600ºF, even though it severely degrades at that temperature. Radco always rates fluids conservatively to take the risk out of choosing a fluid.  Typical degradation of all fluids we tested was about 4% at the fluid’s highest recommended operating temperature.  Xceltherm® fluids from Radco typically have a degradation rate of 2% in a side by side comparison. 

What is the Ampule Test?

The Ampule Test is generally accepted as an industry standard to test relative thermal stability between heat transfer fluids. A standard ASTM-type Ampule Test is in development and the data generated by such testing has proven reliable over the course of many years.

Variations between methods exist as to the composition and diameter of the ampules, the method by which the oxygen is removed from the ampule, the sample size, and the temperature run. However, with the exception of those methods which do not properly provide for oxygen removal, the relative results are reliable as long as uniform temperatures are maintained across the oven.

Radco Methodology for Comparing Thermal Stability of Heat Transfer Fluid

Thermal stability can be defined as the time rate of change of the chemical composition of a fluid from its initial components to its degradation products at a given temperature. To compile this information over a range of temperatures, fluid samples are capped with nitrogen to remove oxygen and sealed. They are heated to specific temperatures over a specific period of time (two weeks in our current data) and compared. The amount of degradation is then plotted on a graph of degradation by percent of weight vs. temperature to indicate the relative ability of each fluid to withstand use at specific temperatures.

Conclusion

Some suppliers feel that they can predict actual system performance from Ampule Test data. Radco feels that due to the tremendous differences in system design and operating parameters in the marketplace, the direct prediction of thermal fluid life from ampule data in any given system is difficult. However, Radco strongly feels that the data can serve to define the thermal stability performance differences between fluids in any given system.