If you’ve ever noticed that thick, dark buildup at the bottom of your reservoirs, tanks, or clinging to your piping, you’ve seen sludge. It’s one of the most preventable problems in heat transfer systems—and in this three-part series we’ll walk through its causes, how to avoid it, and how to clean it out if it appears.

Sludge is most often the result of oxidation, which also happens to be one of the leading drivers of fluid degradation in industries such as die casting, blow molding, plastics extrusion, and other operations where systems are exposed to oxygen in the atmosphere.

Put simply, oxidation occurs when hot fluid meets air. Most fluids begin oxidizing once they reach 200°F, and for every 15° above that, the rate of oxidation generally doubles.

So what impact does this have on your fluid?

As oxidation progresses, larger molecules form within the fluid. These eventually bond together, creating the thick deposits we call sludge. The fluid becomes heavier and more viscous, which makes it harder to circulate, reduces its heat transfer performance, and raises its acidity (TAN).

The end result is reduced system efficiency, lower productivity, and higher operating costs. The good news? Sludge can be avoided. We’ll explain how in Part II.