Heat transfer fluid oxidation is one of the degradation pathways that maintenance personnel and facility operators often worry about. It is very important to understand this phenomenon and the different metrics of measurement. Relatherm Heat Transfer Fluids provides one of the most comprehensive resources on fluid oxidation. In this article, we will be reviewing measurement metrics for thermal fluid oxidation. These include Total Acid Number (TAN) and Infrared (IR) Absorbance.
Total Acid Number
Total Acid Number (TAN) is one of the most common metrics used by laboratory analysts, engineers and maintenance personnel to characterize the amount of degradation that a heat transfer oil sample has seen. TAN is a measurement of acidity in a heat transfer fluid sample. TAN tests capture the weak organic acids and strong inorganic acids present within oil. The higher the acidity, the higher the fluid degradation and additive depletion. TAN is determined by the amount of Potassium Hydroxide (KOH) (in milligrams) required to neutralize the acid in 1 gram of heat transfer fluid.
The standard method of determining the TAN value of a sample in the laboratory is by titration analysis. The endpoint is determined by potentiometric or photometric titration. TAN values can range from 0.01 to higher than 1.0, which is the recommended condemning limit set by Relatherm. A high TAN value corresponds to the formation of sludge that impact heat transfer negatively. This is a messy mix of ketones, esters, aldehydes, carbonates and carboxylic acids, Sludge is a maintenance nightmare of course. In some other advanced cases, the corrosive acids formed can impact the integrity of the piping and process equipment.
Infrared (IR) Absorbance
Fourier Transform Infrared (FTIR) Spectroscopy is used to determine heat transfer oil oxidation. In basic terms, an electrically heated glow bar is used as an Infrared (IR) radiation source, which is passed through a heat transfer fluid sample. The measurement approach relies on Beer Lambert’s Law which states the amount of IR absorbed is proportional to the concentration of the absorbing species and the distance the IR light has to travel through it.
The chemical constituents in the sample absorb some of the IR light at reproducible and specific wavenumbers. The amount of energy in the IR beam is related to its wavelength; the smaller the wavelength the more energy. Different components exhibit varying infrared absorbances that are evident on the FTIR spectra.
Essentially, the spectra obtained for a virgin heat transfer fluid sample will be significantly different from that obtained for an oxidized sample. A comparison of the resultant spectra with a baseline spectra already obtained for a virgin sample reveals the presence of oxidation byproducts. By-products present in a used heat transfer fluid sample can include ketones, esters, aldehydes, carbonates and carboxylic acids. Chemical bonds within each molecule exhibit characteristic IR absorptions. This is a more direct test for oxidation byproducts because these components feature distinctly at known wavelengths.