So, you make or fix refrigeration units? Or, maybe you have a different application that requires very quick cooling. Some of the gasses used in the past include sulfur dioxide and ammonia gas. They proved to be too dangerous because back in those times, the manufacturers hadn't come up with self defrost models. People would jab at the thick frost with picks, trying to defrost faster, but bust open the coil. That let the toxic gas out. Of course, I'm guessing that you'd rather not see a combustible gas leak emitting from your fridge or air conditioner.