[Mobile Master Tech]

I also found some interesting references to the human safety and maximum Coefficient of Performances (COP) of some refrigerants, apparently the Carnot efficiency limits for a single stage:

CO2-COP 2.96 (safe to 30,000ppm)
R410a-COP 4.41 (no safety concerns)
R134a COP 4.60 (no safety concerns)
R290/propane COP 4.74 (flammable between 20,000 and 100,000ppm)
R717/Ammonia COP 4.84 (safe to 300ppm, which is beyond what is endurable for even conditioned persons. Doesn't play nice with copper)

CO2 seems to be the laggard and has extra cost involved with engineering for the the high pressure, but it apparently works extremely well in multistage cooling systems cooling to very low temps and is much more efficient than others there. Not much use for us, though.

R410a requires very little flow for a given amount of heat moved, but its supercritical temp (temperature above which it won't condense to a liquid) is very low, so it is nearly useless at condensing temps above 125F-not good for heat pumps providing hot water.

Ammonia is a completely different ball of wax and there isn't much equipment made that is useful for us to hack.

R134a and the flammables have good COP (especially at higher condensing temps) and many other technical good habits. They are cheap, easy to use and easy to find. On the bad side, R134a has Global Warming Potential, requires more flow to move a given amount of heat and the flammables are, well, flammable.

A blend between R134a and one or more of the flammables minimizes the GWP by using less R134a, minimizes flammability risk by using less flammables, and has technical attributes that are greater than any component alone. They are near-azeotropic, so you don't end up with much temperature glide or problems due to changing the fraction of each component during charging.

Perdue University published some good papers on blends. 45% R290/55% R134a had greater heat moving capacity than R290 alone and looked very promising. Anyone tried any blends?

Craig
The MMT