Editor's Note: CR4 would like to thank Ed Keuper for contributing
this third installment in GEA Consulting's Refrigerant series. Click here for Part 2.
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 32ºF
(0ºC). 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".
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