Refrigerants for the 21st Century
15. Refrigerator Redesign

In this short section, we limit discussion to HFC-134a since its successful adaptation to home refrigerators and auto air conditioners was required to replace the stratospheric O3 - damaging CFC-12. However, all other potential CFC's replacements either have or are being carefully characterized, and system changes made to make them operative.

At first glance, HFC-134a would appear to be a very good match for CFC-12. However, it has other properties that lead to refrigerators and air conditioners which would be less energy efficient than the old CFC-12 units. Among these are the thermochemical properties of specific heat, thermal conductivity and latent heat of vaporization. Also important are viscosity and lower compressor efficiency.

Fortunately, the refrigeration engineer is not limited by these inherent molecular deficiencies of HFC-134a. By redesigning the condenser, evaporator and compressor , the HFC-134a unit is capable of operating as efficiently, if not more so, than the old CFC-12 unit. This is so important to our environment if we hope to minimize emissions of carbon dioxide (CO2) from energy generation. But these equipment modifications add to the overall cost of purchasing HFC-134a refrigerators and retrofitting auto air conditioners.

Returning to the vapor pressure - boiling point relationship between HFC-134a and CFC-12, the subtle difference between the two has necessitated:

• Evaporators having greater surface areas for refrigeration (lower vapor pressure region).

• Sturdier construction for the condensers in auto air conditioners (higher vapor pressure region).
  

In addition to the physico-chemical differences between HFC-134a and CFC-12, another property difference - solvency - leads to further modifications for the HFC-134a unit. Intermolecular forces such as London Dispersion Forces play a major role.   CFC-12, because of the C-Cl bond, has different solvent properties than HFC-134a.  This became apparent when compressor lubricants were tested for the HFC-134a refrigerant systems.  Hydrocarbon and aromatic hydrocarbon oils had been used successfully with CFC-12.  However with the hydrocarbon oils, refrigerant systems based on HFC-134a failed to function with the efficiency and long life required.

Lubricant solubility in the refrigerant is important to ensure that its circulation through the system does not deplete the lubricant in the compressor.  Lack of solubility tends to deposit the lubricant permanently in other locations in the system, especially in the evaporator.  HFC-134a is a very poor solvent for the hydrocarbon oils.  Lubricants with oxygen in their structures such as a family of oils called polyol esters are preferred for HFC-134a. 

These slight differences in solubility characteristics between CFC-12 and HFC-134a demonstrate the broadly stated principle that "like dissolves like".  HFC-134a has a higher dipole moment than CFC-12 because of the electron withdrawal of the three fluorine atoms on the carbon adjacent to the carbon bonded to the single fluorine.  HFC 134-a dissolves well only in oxygen containing lubricants that themselves have a higher dipole moment than the hydrocarbon lubricants.

Plastics and rubbery materials called elastomers are used in the refrigerant systems to make gaskets, seals and, especially in auto air conditioners, for hose connections.  A number of the plastics and elastomers have low dipole moments like CFC-12 refrigerant.  We might expect that CFC-12 would dissolve in such materials causing them to swell in service and resulting in penetration of CFC-12 through the elastomer or plastic.  Elastomers such as butyl rubber, silicone rubber and natural rubber swell unacceptably and cannot be used in CFC-12 refrigeration systems.

However these three elastomers show much lower swelling when exposed to HFC-134a.  This molecule is more "unlike" the elastomers and does not dissolve in or penetrate them to any great extent.  Therefore, these elastomers can be considered for use in HFC-134a refrigeration systems.

CFC-12 and HFC-134a are, however, mutually soluble in each other.  Further they form a complex known as an azeotrope when mixed.   Azeotropes behave as if they are single entities, having a constant boiling point, different from that of each of the two components.  They cannot be separated into the components by distillation.  With the CFC-12/HFC-134a azeotrope, the boiling point of the azeotrope is lower than either of the two components; the azeotrope exerts a higher refrigeration system pressure as a result.

This phenomenon can play havoc with any system designed for the pure refrigerant if one refrigerant is used to make up for losses of the other ("topping off").  Often when these higher pressures are noted, the exasperated refrigeration mechanic merely vents the troublesome azeotrope to the atmosphere, adding yet more CFC-12 to the environment, eventually to damage the O3 in the stratosphere.  Correction of the azeotrope "mistake" by reclaimers of CFC-12 is usually not possible.  They must incinerate the unfortunate mixture.

One final concern for refrigerant systems using HFC-134a is the choice of dessicant to remove traces of water that may be present in the refrigeration equipment.  Water can lead to corrosion and improper compressor performance.  Dessicants that had been selected for CFC-12 are unsuitable for HFC-134a.  The original dessicants chemically degrade the new refrigerants.  The refrigeration engineers adapting systems for the use of HFC-134a have developed alternate dessicants that function to remove water without chemical degradation.

By careful engineering design, it is possible to construct an HFC-134a refrigeration system for the 21st century. But the cost penalties - from a more complex manufacturing process for HFC-134a, more F per molecule and engineering redesign - are real. It's a bargain for ensuring our future well-being - and that of generations to come.