Evaporator Control With Electronic Expansion Valve - Emerson E2 User Manual

ESR is in this state and the suction pressure of compressor(s) serving it can be floated
further upward.
In summary, E2 floating suction pressure control coupled with ESR fixture
temperature control provides the following benefits over conventional EPR control:
•
Energy savings of up to 8.0% due to higher compressor suction pressures
possible.
•
Automatic temperature adjustment to accommodate all seasonal and
other system and load effects.
•
Remote temperature set point changes.
•
Tight fixture temperature control and reduced product shrinkage,
increased shelf life and shorter required defrost duration.
•
Knowledge of valve position in addition to case temperature reading
provides another useful source of data for diagnosing problems on site or
remotely.

EVAPORATOR CONTROL WITH ELECTRONIC EXPANSION VALVE

As mentioned near the start of this article, higher compressor suction pressures
and lower suction return gas temperatures lower compressor power and refrigeration
system energy usage. Therefore, it is important that the expansion valves that control
the flow of refrigerant into fixture evaporator coils do so in a manner that results in the
lowest gas superheat at the exit of evaporator coil possible, without allowing liquid
refrigerant to pass from the coil in the suction line and possibly flood and damage the
compressors. Inadequate refrigerant flow into an evaporator coil, as evidenced by high
coil exit suction superheat temperatures, reduces evaporator coil heat transfer
effectiveness and heat transfer rates, requiring as a result a larger temperature
difference between the evaporator coil and the fixture air flow to remove an equal
amount of heat. This means higher evaporator superheat conditions require lower EPR
or ESR and compressor suction pressure settings to achieve the same case air.
Mechanical thermal expansion valves (TXVs) are typically used to meter
refrigerant into evaporator coils and managed outlet superheat. These valves have
numerous limitations. First of all, they must be set manually using a difficult process not
well understood and executed by many service technicians. When setting these valves,
the service technician is forced to open up the refrigerated fixture, disturbing its normal
air flow and heat transfer and making proper valve setup almost impossible. Once the
TXV is initially adjusted, any change in refrigeration system conditions including liquid
pressure or sub-cooling, evaporator suction pressure, case load and case airflow will
result in the valve no longer being properly set and superheats that are either too high or
too low. During cold weather, lower liquid refrigerant temperatures at TXVs cause liquid
refrigerant to flood through evaporator coils. An experimental study has shown that the
flooding can be as high as 5 lbs/hr per display case. With 100 cases in a typical
supermarket, the total extra load on the refrigeration compressors can be as high as 500
lb/hr, which results in approximately 5 kW of additional compressor power. This results
in a 10-15% parasitic energy loss. During warmer periods when condensing and liquid
temperatures rise, conventional TXVs can starve refrigerated case evaporators, which
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