Technical
BITZER REFRIGERANT REPORT:
PART 2
Republished with permission of Bitzer
Stratospheric ozone depletion as well as atmospheric greenhouse effect due to refrigerant
emissions have led to drastic changes in the refrigeration and air conditioning technology
since the beginning of the 1990s.
This is especially true for the area of commercial refrigeration and
air conditioning systems with their wide range of applications.
Please see part 1 in the August 2020 issues of RACA Journal for other
references.
ASPECTS ON THE DEVELOPMENT OF HFO AND HFO/
HFC REFRIGERANTS
The decision to use the ‘low global warming potential (GWP)’
refrigerant R1234yf in mobile air conditioning systems for passenger
cars also led to the development of alternatives for further mobile
applications as well as stationary refrigeration, air conditioning and
heat pump systems.
Primary objectives are the use of single-component refrigerants and
of mixtures with significantly reduced GWP and similar thermodynamic
properties as the HFCs currently used predominantly. An essential basic
component for this is R1234yf (CF3CF=CH2). This refrigerant belongs
to the group of hydro-fluoro-olefins (HFO), which is unsaturated HFCs
with molecular double bonds.
This group of HFOs also includes another substance called
R1234ze(E), which has been mainly used as a propellant for PU foam
and aerosol. R1234ze(E) differs from R1234yf in its molecular structure.
Both substances are the preferred choice in terms of their properties
and are also used as basic components in HFO/HFC blends.
The Global Warming Potential is very low − R1234yf with GWP 4 and
R1234ze(E) with GWP 7. However, these refrigerants are flammable
(safety class A2L), meaning the refrigerant quantity in the system must
be considered in light of the installation location. In addition, there
remain open questions concerning the long-term stability in stationary
systems where long life cycles are common.
Furthermore the volumetric refrigerating capacity is relatively low;
for R1234yf it is close to the level of R134a, and more than 20% lower
for R1234ze(E). There is also some uncertainty concerning flammability.
In safety data sheets, R1234ze(E) is declared as non-flammable.
However, this only applies to its transport and storage. When used
as a refrigerant, a higher reference temperature for flammability
tests of 60°C applies. At this temperature, R1234ze(E) is flammable
and therefore classified in safety class A2L, like R1234yf. R1234ze(E)
is sometimes referred to as a R134a substitute, but its volumetric
refrigerating capacity is more than 20% lower than that of R134a or
R1234yf. The boiling point (-19°C) also greatly restricts the application
at lower evaporation temperatures. Its preferred use is therefore in
liquid chillers and high temperature applications. The list of further
potential HFO refrigerants is relatively long. However, there are only
few substances that meet the requirements in terms of thermodynamic
properties, flammability, toxicity, chemical stability, compatibility with
materials and lubricants.
These include, for example, the non-flammable (safety group A1)
low-pressure refrigerants R1336mzz(Z), R1233zd(E) and R1224yd(Z).
These are primarily an option for liquid chillers with large turbocompressors,
and they can be used with positive displacement
compressors in high-temperature applications. R1233zd(E) and
R1224yd(Z) belong to the group of HCFO (hydrochloro-fluoro-olefins);
they have a (very) low ozone depletion potential (ODP).
Upon release into the atmosphere, however, the molecule
rapidly disintegrates. On the other hand, there are currently
no candidates from the HFO family with similar volumetric
refrigerating capacity such as R22/R407C, R404A/R507A and
R410A available for commercial use. Direct alternatives for these
refrigerants with significantly lower GWPs must therefore be
‘constructed’ as a mixture of R1234yf and/or R1234ze(E) with HFC
refrigerants, possibly also small proportions of hydrocarbons, CO₂ or
other suitable molecules.
Though, due to the properties of the HFC refrigerants suitable as
blend components, flammability and GWP are related diametrically
to one another. In other words: Blends as alternatives to R22/R407C of
GWP < approx. 900 are flammable. This is also true with alternatives
for R404A/R507A in blends of GWP < approx. 1300 and for R410A in
blends of GWP < approx. 2000. The reason for this is the high GWP of
each of the required non-flammable components.
For R134a alternatives, the situation is more favourable. Due to
the already quite low GWP of R134a, a blend with R1234yf and/or
R1234ze(E) allows a formulation of non-flammable refrigerants with a
GWP of approx. 600. Thus, primarily two directions for development
are pursued: o Non-flammable HFC alternatives (blends) with GWP
values according to the above mentioned limits – safety group A1.
Regarding safety requirements, these refrigerants can then be
utilised similar to currently used HFCs. Flammable HFC alternatives
(blends) with GWP values below the above mentioned possible limits –
according to safety group A2L (for refrigerants of lower flammability).
This group of refrigerants is then subject to charge limitations
according to future requirements for A2L refrigerants.
Meanwhile, there are development projects using refrigerant
components with a much higher volumetric refrigerating capacity
and pressure than R1234yf and R1234ze(E). These can then be used to
‘formulate’ mixtures with R32 as an alternative to R410A, which are
optimised for certain properties.
Continued in the next issue of RACA Journal. RACA
www.hvacronline.co.za RACA Journal I September 2020 49