Another class of low temperature heat transfer fluids is based on naturally derived terpenes such as d-limonene. D-limonene is the major component in citrus fruit oil and is present in trace quantities in orange juice. It is recovered in commercial quantities by distilling orange oil, which is obtained from citrus peels. Being derived from the citrus industry, d-limonene is considered a safe and environmentally friendly heat transfer fluid and is preferred in many food and pharmaceutical processes. The published melting point of d-limonene is approximately -140°F (-96°C). But, testing has shown that below -108°F (-78°C), it becomes a thick white, gel-like substance that is impossible to pump. In addition, at elevated temperatures, d-limonene oxidizes rapidly in the presence of air. This oxidation triggers the acidification and polymerization of the molecules.
So, hypothetically, it could be used in an all-stainless setup without those concerns. There are stainless centrifugal and air diaphragm pumps that could be used to pump it through stainless lines to and from stainless jackets. I’ve never setup a system of this scale with these specific components, but in an effort to attain max efficiency, if the heat transfer properties, viscocity, and degradation factors exceed that of glycol and ethanol, it may be worth trying, in my case.
d-limonene can be expensive, but I’m not concerned about the quality necessarily as a heat transfer fluid, and so an industrial grade purchased in bulk should be reasonably priced. If I don’t have to replace it as frequently, it might just pay for itself.