Calculating Temperature at which 100ppm HF Gas Condenses

Ok, lets work this through from first principles.

The dew point is where the vapour pressure of the liquid HF equals the partial pressure in the gas mixture.

Say we have 100 ppm by mass of HF in dry N2. That gives us the mass fraction HF as .0001. By adjusting for the molecular weights of each gas, 20 and 28 respectively, we have a mole fraction HF of .00014. So the partial pressure is .00014 atmospheres, providing your stream is at atmospheric pressure.

Now we need to know the vapour pressure of HF. This can be found on the following graph:

http://encyclopedia.airliquide.com/images_encyclopedie/VaporPressureGraph/Hydrogen_Fluoride_Vapor_Pressure.GIF

Looking at that graph, our value of .00014 is off scale. We could extend the graph by various means, but we can take a quick estimate and say that the temperature will be around -100 C. I suspect that will be sufficiently accurate for your purposes?

In the case of SF6, the mole fraction will be higher, but the SF6 would begin to condense before the HF did, I think. I'll leave that one to you.