6
Application Engineering
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AE4-1383 R5
© 2012 Emerson Climate Technologies, Inc.
Printed in the U.S.A.
Note: For performance of ZF*K5E models with other
refrigerants, refer to the Online Product Information at
www.emersonclimate.com
Discharge Temperature Control with Vapor Injection
Although using vapor injection offers some inherent
compressor cooling, when using the ZF*K5E scrolls
with R-407A/C/F and vapor injection additional cooling
is required to operate across the whole operating
map of the compressor. To provide this extra cooling
a t-fitting and DTC valve should be installed onto the
compressor's injection port. The t-fitting will meter
liquid from the DTC valve into one side of the fitting,
while vapor flows in through the otherside. See
Figure
4C
at the end of this bulletin for a example schematic.
This is different than the current method used on other
Copeland vapor injected scrolls (ZF*KVE models) which
use the Copeland Demand Cooling to inject liquid in the
vapor line of the compressor based on a discharge line
temperature reading.
Note! Just as with liquid injection operation, when using
the DTC valve with vapor injection ensure that the
thermal bulb is well insulated.
When using vapor injection with R-404A/R-507, the
DTC valve and t-fitting are not required. A discharge line
thermistor is supplied with the CoreSense diagnostics
assembly (more information on CoreSense diagnostics
is found later in this bulletin). The thermistor should
be placed no more than 6 inches (15.2 cm) from the
discharge of the compressor. The thermistor should be
well insulated to ensure accurate temperature sensing
on the discharge line.
System Configuration
There are two methods of controlling refrigerant
fl
ow at
the EVI heat exchanger - downstream and upstream
extraction.
Downstream Extraction
The downstream extraction is the preferred method
employed in the United States. In downstream
extraction the TXV is placed between the liquid outlet
and vapor inlet of the heat exchanger. The advantage
of downstream extraction is that subcooling is ensured
because the liquid is further subcooled as it
fl
ows
through the heat exchanger. Therefore, more subcooled
liquid enters the TXV which increases the probability
that the valve will not hunt. The disadvantage with this
method is that it is not as ef
fi
cient as the upstream
method; however, the difference is too small for
practical purposes. See
Figure 4.
Upstream Extraction
In upstream extraction the TXV is placed between
the condenser and the heat exchanger. The TXV
regulates the
fl
ow of subcooled refrigerant out of the
condenser and into the heat exchanger. With this type
of con
fi
guration there is a potential for
fl
ash gas which
would cause the valve to hunt. See
Figure 5.
Heat Exchanger Piping Arrangements
Best subcooling effect is assured if counter
fl
ow of
gas and liquid is provided as shown (see
Figure 6
).
In order to guarantee optimum heat transfer, the plate
heat exchanger should be mounted vertically and
vapor should exit it at the top.
For more information on applying ZF*K5E scrolls with
an economized vapor injection (EVI) circuit refer to
AE4-
1327,
Economized Vapor Injection (EVI) Compressors
.
Accumulator Requirements
Due to the Copeland Scroll compressor's inherent
ability to handle liquid refrigerant in
fl
ooded start and
defrost operation conditions, accumulators may not
be required. An accumulator is required on single
compressor systems with refrigerant charges over
17 lbs. On systems with defrost schemes or transient
operations that allow prolonged, uncontrolled liquid
return to the compressor, an accumulator is required
unless a suction header of suf
fi
cient volume is used to
prevent liquid migration to the compressor.
Superheat Requirements
In order to assure that liquid refrigerant does not return
to the compressor during the running cycle, attention
must be given to maintaining proper superheat at the
compressor suction inlet. Emerson recommends a
minimum of 20°F (11°C) superheat, measured on the
suction line 6 inches (152mm) from the suction valve,
to prevent liquid refrigerant floodback.
Another method to determine if liquid refrigerant is
returning to the compressor is to accurately measure
the temperature difference between the compressor
oil crankcase and the suction line. During continuous
operation we recommend that this difference be a
minimum of 50°F (27°C). This “crankcase differential
temperature” requirement supersedes the minimum
suction superheat requirement in the last paragraph. To
measure oil temperature through the compressor shell,
place a thermocouple on the bottom center (not the side)
of the compressor shell and insulate from the ambient.
During rapid system changes, such as defrost or ice
harvest cycles, this temperature difference may drop
