Emerson E2 User Manual page 9

Figure 2: Conventional mechanical suction pressure regulator-based evaporator control.
Fixed suction pressure control has several inherent problems. Control settings
such as those in the above example are generally based on and therefore only correct
for the highest fixture load condition anticipated, that is during warm weather conditions
and occupied store hours, when warmer store temperatures, higher humidities, and
shopping activity all contribute to maximum fixture refrigeration load conditions. The
same settings are not appropriate for other load conditions, the best example of which
often occur during winter months. During the winter, colder liquid refrigerant leaving the
condenser and reduced fixture evaporator loads due to cooler and dryer store conditions
dramatically change the required evaporator suction temperature and pressure
requirement. Typically, evaporator pressures can be raised at least 2 psig above those
required in the summer.
maintain the same fixture and product temperatures might be as high as 47 psig, with a
required compressor operating suction pressure of 45 psig. Unfortunately, because both
the compressor and EPR settings are fixed, the standard refrigeration control system
over-cools product in winter, often even freezing thinner cuts of meat, and wastes
substantial energy.
CPC's latest generation of E2 refrigeration controllers provide a solution to this
problem by supporting the use of a stepper-motor driven, electronic EPR (the ESR) that
uses a digital reading of fixture temperature as the basis on which to automatically
readjust and control the ESR valve and resulting evaporator pressure and temperature.
As a result, the fixture is held at the exact temperature required irrespective of system
operation and load conditions. At the same time, the E2 controller, when used to manage
compressor on/off cycling, continually readjusts the compressor suction pressure
setpoint so that it just satisfied the pressure requirement at the ESR valve. The E2
control with the ESR valve is equivalent to having a service technician standing by at all
times at the refrigeration unit, adjusting the EPR to control fixture temperature at the
constant setpoint desired and adjusting the compressor control to just satisfy the suction
pressure requirement at the EPR. In practice, usually there are multiple fixture groups
(circuits) connected to each compressor "rack" system, which normally include multiple
compressors. Each fixture group may require slightly different suction pressures to
maintain the exact temperature required by each fixture group and type. Again the E2
with ESRs allows the refrigeration system to deliver the exact temperature required by
every fixture group under all load and system conditions, at the same time the E2 directly
controls compressor to maintain suction pressures at the highest level possible while
satisfying the pressure requirements of all the ESRs. Recalling our rule of thumb for
effect of changes in suction pressure on compressor energy use, floating suction
In above example, the EPR setting required in winter to
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