THERMAL STABILITY OF ORGANIC HEAT TRANSFER FLUIDS: ASTM D6743
EXPLANATION
Heat transfer fluids degrade when exposed to sufficient high temperatures. The amount of degradation increases as the temperature increases or the length of exposure increases, or both. Due to reaction and rearrangement, degradation products can be formed. Degradation products include high and low boiling components, gaseous decomposition products, and products that cannot be evaporated. The type and content of degradation products produced will change the performance characteristics of a heat transfer fluid. In order to evaluate thermal stability, it is necessary to quantitatively determine the mass percentages of high and low boiling components, as well as gaseous decomposition products and those that cannot be vaporized, in the thermally stressed heat transfer fluid. This test method differentiates the relative stability of organic heat transfer fluids at elevated temperatures in the absence of oxygen and water under the conditions of the test.
The users should determine to their own satisfaction whether the results of this test method correlate to field performance. Heat transfer fluids in industrial plants are exposed to a variety of additional influencing variables. Interaction with the plant's materials, impurities, heat build-up during impaired flow conditions, the temperature distribution in the heat transfer fluid circuit, and other factors can also lead to changes in the heat transfer fluid. This test method provides an indication of the relative thermal stability of a heat transfer fluid, and can be considered as one factor in the decision making process for fluid selection. The accuracy of the results by this test method depend very strongly on how closely the test conditions are followed.
The procedure is applicable to heat transfer fluids at temperatures both above and below their boiling points. It is applicable to fluids with maximum bulk operating temperature between 500° F and 850° F. The procedure shall not be used to test a fluid above its critical temperature. In this test method the volatile decomposition products are in continuous contact with the fluid during the test. This test will not measure the thermal stability threshold (the temperature at which volatile oil fragments begin to form), but instead will indicate bulk fragmentation occurring for a specified temperature and testing period. Because potential decomposition and generation of high pressure gas may occur at temperatures above 500° F, this test method should not be used for aqueous fluids or other fluids which generate high pressure gas at these temperatures. The applicability of this test method to siloxane based heat transfer fluids has not been determined. DIN Method 51528 covers a test method that is similar to this test method.
TEST SUMMARY
The test fluid is charged in a thermal stability test cell purged with nitrogen. The cell is tightly sealed to remove and preclude introduction of oxygen and water from the atmosphere. The fluid is heated in an oven at a given temperature and for a given period of time. The boiling range of the heated fluid is determined by gas chromatography, and is compared to the boiling range of pure unused fluid.
TEST PRECISION
No data are available at present regarding the repeatability or reproducibility of this test method. This procedure has no bias because the value of thermal stability is defined only in terms of this test method.
