Technical
R134a concentration in the blend in evaporators, with
reduced performance and malfunctioning of the expansion
valve (example, insufficient suction gas superheat).
Resulting design criteria
With regard to system technology, previous experience with
R22 can only be utilised to a limited extent.
The distinctive temperature glide requires a particular
design of the main system components, for example
evaporator, condenser, expansion valve. In this context it must
be considered that heat exchangers should preferably be laid
out for counterflow operation and with optimised refrigerant
distribution. There are also special requirements with regard
to the adjustment of control devices and service handling.
Furthermore, the use in systems with flooded evaporators
is not recommended as this would result in a severe
concentration shift and layer formation in the evaporator.
R410A AS SUBSTITUTE FOR R22
In addition to R407C, the near-azeotropic mixture listed
by ASHRAE as R410A is available and widely used for
medium-sized capacities in air conditioning and heat
pump applications. An essential feature indicates nearly
50% higher volumetric refrigerating capacity (Figure
21) compared to R22, but with the consequence of a
proportional rise in system pressures (Figure 22).
At high condensing temperatures, energy consumption/
COP initially seems to be less favourable than with R22.
This is mainly due to the thermodynamic properties. On the
other hand, very high isentropic efficiencies are achievable
(with reciprocating and scroll compressors), so that the real
differences are lower.
Another aspect are the high heat transfer coefficients in
evaporators and condensers determined in numerous test
series, resulting in especially favourable operating conditions.
With an optimised design, it is quite possible for the system to
achieve a better overall efficiency than with other refrigerants.
Because of the negligible temperature glide (< 0.2K), the
general usability is similar to that of a pure refrigerant.
Resulting design criteria
The fundamental criteria for HFC blends also apply to the
system technology with R410A. However, the high-pressure
levels have to be considered (43°C condensing temperature
already corresponds to 26 bar abs.).
Compressors and other system components designed
for R22 are not suitable for this refrigerant (or only to a
limited extent). Though, suitable compressors and system
components are available.
When considering covering usual R22 application ranges,
the significant differences in the thermodynamic properties
(for example pressure levels, mass and volume flow, vapour
density) must be taken into account. This also requires
considerable constructional changes to compressors, heat
exchangers, and controls, as well as measures of tuning
vibrations. There are stricter safety requirements, for
example affecting the quality and dimensions of piping
and flexible tube elements (for condensing temperatures
of approximately 60°C/40 bar). Another criterion is the
relatively low critical temperature of 73°C. Irrespective of
the design of components on the high-pressure side, the
condensing temperature is thus limited.
R410A complies with the requirements of the EU F-Gas
Regulation, which will only allow refrigerants with
www.hvacronline.co.za RACA Journal I August 2020 49