System information

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© 2015 Emerson Climate Technologies, Inc.
AE4-1287 R8
Performance data for Demand Cooling compressors
includes the effects of injection when it is
required. The approximate conditions where injection
occurs are shown in Figures 2, 3, and 6. At the conditions
where Demand Cooling is operating, the performance
values are time averages of the instantaneous values,
since small fl uctuations in suction and discharge
conditions occur as the Demand Cooling injection
valve cycles.
Demand Cooling System Design
When Demand Cooling operates, it 'diverts'
refrigeration capacity in the form of injected saturated
refrigerant from the evaporator to the compressor
(See Figure 7 for a typical single system schematic).
The effect of this diversion on evaporator capacity
is minimal because the diverted capacity is used to
cool the gas entering the compressor. As the gas is
cooled, it naturally becomes more dense, increasing
the mass fl ow through the compressor, which partly
compensates for the capacity diverted from the
evaporator.
If there is substantial heat gain along the suction
line, injection may result in a substantial loss in
evaporator capacity during Demand Cooling operation.
In order to minimize this loss, good practice indicates
Demand Cooling operation be kept to a minimum
through proper system design and installation practices.
There are three areas which can be addressed to
minimize the impact of Demand Cooling operation on
performance.
1. Compressor Return Gas Temperature: Suction lines
should be well insulated to reduce suction line heat
gain. Return gas superheat should be as low as
possible consistent with safe compressor operation.
2. Condensing Temperatures: It is important when
using R22, R-407 A/C/F, or R448A/449A as a
low temperature refrigerant that condensing
temperatures be minimized to reduce compression
ratios and compressor discharge temperature.
3. Suction pressure: Evaporator design and system
control settings should provide the maximum
suction pressure consistent with the application
in order to have as low a compression ratio as
possible.
Demand Cooling Compressors
No new compressor models have been introduced
for Demand Cooling. Instead, existing low
temperature Discus CFC-502 compressors have been
modifi ed for use with R-22, R-407 A/C/F, or R-448A/449A
and Demand Cooling. The modifi cations are the addition
of an injection port on the compressor body and a
temperature sensor port in the head of the compressor.
The locations of these ports are critical and were
determined through an extensive development program.
The R-22, R-407 A/C/F, or R-448A/449A rating data
includes the effects of Demand Cooling injection when
operating conditions require it based on 65 °F return
gas.
Condenser Sizing
Condensers should be sized using conventional
methods. Demand Cooling has virtually no effect on
system heat of rejection.
Demand Cooling System Components
The Demand Cooling System (see Figure 1) consists
of: The Demand Cooling Temperature Sensor (TS),
The Demand Cooling Module (CM), and the Injection
Valve (lV).
The TS uses a precision Negative Temperature
Coeffi cient (NTC) Thermistor (thermistor resistance
drops on temperature rise) to provide temperature
signals to the CM.
The IV meters refrigerant fl ow from the liquid line
to the compressor. The IV solenoid receives on-off
signals from the CM. When compressor cooling is
required the solenoid is energized and opens the IV
orifi ce to deliver saturated refrigerant to the compressor
for cooling. The valve orifi ce is carefully sized to
meet the requirements of each body style of Discus
compressors.
The CM has three functional groups:
A. The Input signal and calculator circuits
compare the temperature sensor input signal to an
internal set-point and decide whether to energize
the IV solenoid or, in the case of a problem, the
CM alarm relay.
B. The output signal to the IV is controlled by
an electronic switch connected to the IV
solenoid so that, when required, refrigerant
vapor can be metered to the compressor to
prevent compressor overheating. One side of
the electronic switch is connected internally to
'L1' and the other side to output terminal 'S' (see
Figure 6).