The two fluids I considered are methyl alcohol or methanol, and acetone. Both are common, relatively low toxicity, but flammable fluids that might be useful in organic Rankine cycle engines. Acetone boils at 56.5 Celsius, and produces a pressure of 10 atmospheres at 144.5 Celsius. It has a critical temperature of 235 C and a critical pressure of 47 atmospheres. Both materials show a relatively high autoignition temperature, with acetone showing an autoignition temperature over 800 K, indicating it is the more stable of the two compounds.
Explosive mixtures are the solar engineers worst nightmare.
Because air cannot generally be completely excluded from field operating equipment, I was interested in determining which of the two chemicals might be the safer material to use if the criterion is to minimize explosion hazard. On the other hand, for methanol, the vapor pressure at 21.2 C is about 100 Torr. These numbers were derived from Perry's Chemical Engineering Handbook. One had acetone. One had methanol.