MACS Service Reports - 2016 - JUN5

Another Factor

(IHX) spliced between the high-pressure liquid and
low-pressure (suction) vapor lines.
So in normal operation with the IHX, heat is transferred from the hot liquid to the cold vapor, which
further sub-cools the liquid refrigerant, improving
its efficiency in absorbing heat when it vaporizes
in the evaporator. Of course it also adds superheat
to the vapor. Superheat is the heat added, causing an increase in temperature, as the refrigerant
vaporizes, and is the basis for expansion valve operating calibration. Inasmuch as the increased superheat occurs downstream from where it's sensed
by the expansion valve, it would seem that either
the system needs to sense the superheat close to
the compressor, or some engineering adjustment to
compensate must be made. Well until we have elec-

Also entering the picture is the variable displacement compressor. As should be obvious, the temperature drop across the condenser also will be
affected by ambient temperature and the percentage of compressor displacement. Today's variable
compressors, typically regulated by an external
solenoid-type control valve (others by an internal
pressure-modulated valve) will show a performance difference based on displacement.
The important thing to understand is that at full
displacement, depending on all the other factors
(ambient, airflow, etc.) the temperature drop not
only will be greater than at a lower level of displacement, but that drop (temperature differential)
could be well outside the usual 20-50° F. rule of
thumb. That's a function of the peak performance
of the compressor at full displacement of course.
And in fact, the temperature differential across the
temperature differential across the condenser assembly might be outside our 20-50° F. rule of thumb
even at somewhat less than full displacement.

The Condenser and the Sub-cool Condenser

Tim Iezzi

Now let's look closer at the sub-cool factor in a
specific case. MACS trainer Tim Iezzi, reporting at
the recent MACS training event, presented his data
on that 2013 Kia Optima with the Halla electronic
variable compressor (Halla, a Korean HVAC supplier, has been merged with Visteon and the product line is now branded "Hanon"). The system is,
by today's standards, reasonably conventional:
electronically-controlled (solenoid valve) variable
displacement compressor, with a liquid line pressure transducer and an Internal Heat Exchanger

Figure 10: Chart shows readings on all the temperature sensors at 100%
compressor displacement, as commanded by the duty cycle valve.

MACS

tronic expansion valves with remote temperature
sensors to do that (they're coming), we have some
imperfection in the calibration of course.
But let's look at what happened to the condenser
temperature drop - including the sub-cool condenser section - at full displacement, just after
starting the A/C. Iezzi had connected six testers:
four temperature sensors, a meter to measure solenoid voltage and a tester to display (and command)
the solenoid duty cycle to change compressor displacement (FIG. 9). He recorded 163° F. at the condenser inlet, 103° F. at the condenser outlet, a drop
of 60° F. See FIG. 10.
Iezzi's testing for the MACS presentation, done
on a hot day (85° F. ambient), showed that when
the system had been running a while and the duty
cycle of the compressor solenoid valve dropped to
about half after initial cooldown, the temperature
drop across the condenser did fall within the 20-50°
F. rule-of-thumb range: it was 125° F. at the condenser inlet, 90° F. at the condenser outlet, a drop
of 35°. FIG. 11.

Figure 9: BMACS member Tim Iezzi's instrumentation to check temperature drops across both the condenser and the Internal Heat Exchanger
(IHX) is shown, left to right. In addition to the condenser "in and out"
and liquid line side "in and out" through the IHX, there are two testers
checking the compressor solenoid valve, one for voltage, the next for duty
cycle. Continuing to right is the suction line "in and out" through the
IHX and finally the ambient and center vent air temperature readings.

June 2016

MACS Service Reports

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