There Is No Perfect Refrigerant!
Posted on Wed, Oct 19, 2011
Part 3 of the Series: Keys to a Successful Refrigerant Transition
By Ed Kueper, GEA Sr. Associate
At first glance it might seem that water would be the perfect refrigerant for vapor compression refrigeration systems. It is non-toxic, non-flammable, chemically stable, inexpensive, widely available, large latent heat of vaporization, and environmentally friendly. But look a little closer and it doesn’t look so good. Its applicability range is unacceptably limited. Because of its freezing point, water cannot continuously produce refrigeration below 32F (0C). Further, for use in the “air conditioning” range, the entire cycle would operate below atmospheric pressure, the evaporator in particular in a deep vacuum. Also, due to its vapor’s large specific volume, the compressor and heat exchangers would have to be large and costly.
The list of desirable attributes noted above, however, is not complete. To be added to the list would be suitable lubricant for the compressor, compatibility with materials of construction and lubricants, cost of manufacture, possible hazardous products of refrigerant breakdown, reasonable pressures, energy efficiency of the refrigerant in producing cold, and others. These filters for suitability in combination trap and exclude most refrigerant candidate compounds.
There are many chemical compounds that can be used in vapor compression refrigeration systems, in the sense that they are capable of producing cooling. All have some negative characteristic for some or all applications. Therefore some compromises must be made in selecting and applying each refrigerant candidate. And while the many applications permit several refrigerant candidates, as an industry it is desirable to narrow the list of available refrigerants to a small number. For example, R134 and R134a are very similar but not identical. So we choose to standardize on R134a and ignore R134. We don’t want service vehicles to have to inventory a dozen different refrigerants.
Examples of some current rejection reasons include:
Toxicity - Sulfur dioxide, ammonia
Flammability – Propane, ammonia, R152a
Governmental/ environmental regulation – R11, R12
Toxicity of Breakdown Products – R22
Incompatibility with Materials of systems – ammonia
Inefficiency – water, CO2
Too High or Low Boiling Points – R113, R13, water
Too Small Market for Refrigerant producers – 245ca
Question:
Why do we blend refrigerants? Because some aspect of one refrigerant is desirable while another aspect of that same refrigerant is undesirable or intolerable. Example: R32 is comparatively energy efficient as a refrigerant but it is flammable. So we blend R32 in equal parts with R125, which is non-flammable but less efficient than R32, to produce R410A, a non-flammable, reasonably efficient refrigerant. Conclusion: All refrigerant blends are compromises and therefore not “perfect”.
(To see part 1 of the rerigerant transtion series click here)