Q: What physical properties make for a good refrigerant for compression cycle operation (e.g. Freon)?
A: Early refrigerants like sulfur dioxide and ammonia became notorious. Some folks kept the refrigerator in the back yard or on the porch because of the leaks of such noxious gases that frequently occurred. When returning home from a long absence, it was customary to air out the house if you kept your refrigerator indoors.
Unfortunately, dichlorodifluoromethane (R-12) was not an entirely suitable replacement for refrigerant, since leaks of it deplete the earth’s ozone layer.
Some refrigerants in limited use today that may see more use in the future are carbon dioxide and nonhalogenated hydrocarbons (although the latter are potentially explosive).
Ideally, we would like the refrigerant to undergo phase change at convenient temperatures and pressures, to be nontoxic, to be environmentally safe, and to be noncorrosive.
Q: Why does the air conditioner in my car and in my house produce liquid water as a byproduct?
A: Psychrometry (do not confuse with psychometry) is the study and measurement of gas-vapor-liquid systems. People that study the air/water vapor/liquidwater system have tabulated extensive psychrometric charts telling us something that you might have an intuitive feel for; hot air can hold more water vapor that cold air. You see this condensation on the inside surface of a cold window on a winter day or on the outside of an iced drink glass on a hot day.
Typically, air conditioners subcool the air to a temperature below room temperature. During this process, liquid water is produced and may be caught in a drain pan and piped to a harmless location (clogged drain-pan drains are a common source of attic flooding). The air is then modestly warmed before being released to the spaces to be cooled.