ChemCases: Refrigerants - 3. Early Refrigerants

With the engineering of a mechanical refrigerator in hand, various refrigerants were tried. Of primary importance was their relationship of vapor pressure to boiling point. In order to obtain an internal temperature in the cold chamber of the refrigerator low enough to preserve, yet not freeze food, refrigerants boiling in the range of -10 to -40^oC./760 torr were needed. This temperature can be obtained with a wide variety of liquified gases whose boiling points are in that range.

But the number of candidates decreases considerably when their vapor pressures, toxicity and flammability are taken into consideration.

Ammonia (NH3) and sulfur dioxide (SO2) were initially chosen as refrigerants for commercial use. Both have desirably high latent heats of vaporization. However, they have the obvious drawbacks (to the chemist, at least) of being toxic and corrosive, and with NH3, flammable. The Frigidaire Division, now a member of the Electrolux Group, but then a division of General Motors, realized that their use would considerably limit markets for mechanical refrigerants. And serious industrial accidents are documented when they were used.

The EPA requires that Material Safety Data Sheets (MSDS) be prepared for chemicals in commercial use. The DuPont Company, whose emphasis on safety dates back to their origins as an explosives manufacturer, maintains an MSDS Search Engine that you can use to obtain MSDS information

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Compressor Diagram

Compounds with high vapor pressures would require heavy-duty equipment to contain them. To illustrate, carbon dioxide (CO2) would seem to be ideal for many reasons. Carbon dioxide is low in cost, non-toxic, non-flammable and relatively non-reactive.

You can read a Material Safety Data Sheet on carbon dioxide.

But carbon dioxide's vapor pressure at -25^oC, the temperature at which we want to expand the gas and remove the heat from the refrigerated substances, is about 15 atm. At the higher temperatures at which we want to lose heat to the surroundings, above room temperature, the vapor pressure is above 60 atm. However, early on CO2 was used on naval vessels where safety considerations are of utmost importance.

Therefore, Frigidaire recruited three scientists, Robert McNary, Thomas Midgley and Albert Henne, to try to identify safe, effective refrigerants. Henne had been studying the synthesis of fluorocarbons since the 1920's. They quickly concluded that small molecules having C-F bonds (fluorocarbons) were the best bets. And CF2Cl2, dichlorodifluoromethane had the best properties.

The very strong C-F bond imparts inertness, low toxicity and non-flammability to the fluorocarbons. The low cohesive forces that fluorocarbons possess lead to low boiling points, viscosities and surface tensions that are desirable in refrigerants. However, this causes them to have latent heats of vaporization which are lower than most other potential refrigerants. In addition to the desirable properties already mentioned, fluorocarbons have been found to have good heat capacities and thermal conductivities.

Intermolecular Forces:

Let's look at some of the physical properties of carbon dioxide, ammonia, sulfur dioxide and CF2Cl2. We can learn much about the relationship between intermolecular forces (cohesive forces) and physical properties.

	MW	bp^oC/ 760torr	Latent Heat	Flammable?	Pressure at 0^oC	Pressure at 50^oC
NH3	17	-34	24kJ/mole	Yes	4atm	20atm
CO2	44	-78	25kJ/mole	No	35atm	>>60atm
SO2	64	-10	25kJ/mole	No	2atm	9atm
CF2Cl2	121	-30	22kJ/mole	No	3atm	12atm

We order the gases by molecular weight.

Compare the boiling points of ammonia and CF2Cl2. These two compounds have about the same boiling point, yet one has MW 17, the other 121. Ammonia molecules are held together by hydrogen bonds. Ammonia behaves as if it had a higher molecular weight. Only the weaker London dispersion forces hold the CF2Cl2 molecules together.
Consider the boiling points of carbon dioxide and sulfur dioxide. What accounts for the huge difference in boiling point. Molecular weight accounts for part of the difference. But carbon dioxide is linear, sulfur dioxide is bent. The bent SO2 has a dipole and the dipole-dipole interaction increases the intermolecular forces and raises its boiling point.

Initially, there was concern about the toxicity of fluorocarbons based on earlier experiences of other researchers. This proved to be a false concern related to toxic impurities present in early samples. Midgley dramatically demonstrated the low toxicity and non-flammability of purified CF2Cl2 (CFC*-12) at a 1930 American Chemical Society meeting by inhaling the gas and then using the inhaled vapors to extinguish a lighted candle!

For these many reasons, CFC-12 quickly became the refrigerant of choice for most applications.

* - Chlorofluorocarbon
Latent Heat of Vaporization: The higher the latent heat of vaporization, the more heat can be removed in refrigeration per mole of refrigerant gas.