Single compressors
System design recommendations
Crankcase heater / sump heater:
When the
compressor is idle, the oil temperature in the
sump of the compressor must be maintained
at no lower than 6 K above the saturation
temperature of the refrigerant on the low-
pressure side. This requirement ensures that
the liquid refrigerant is not accumulating in the
sump. Refer to the flood back test criteria.
Liquid line solenoid valve
(LLSV): An LLSV
may be used to isolate the liquid charge on the
condenser side, thereby preventing against
charge transfer or excessive migration to the
compressor during off-cycles. When installed,
EXV also ensures this function. The quantity of
refrigerant on the low-pressure side of the system
can be further reduced by using a pump-down
cycle in association with the LLSV.
Pump-down cycle
: A pump-down cycle
represents one of the most effective ways to
protect against the off-cycle migration of liquid
refrigerant. Once the system has reached its set
point and is about to shut off, the LLSV on the
condenser outlet closes. The compressor then
pumps the majority of the refrigerant charge into
the condenser and receiver before the system
stops on the low pressure pump-down switch.
This step reduces the amount of charge on the
low side in order to prevent off-cycle migration.
The recommended low-pressure pump-down
switch setting is 1.5 bar below the nominal
evaporating pressure. It shall not be set lower
than 2.3 bar.
An external non return valve is recommended to
avoid liquid flood back from the high-pressure
side.
Liquid receiver
: Refrigerant charge optimization
varies with compressor speed. To avoid flash
gas at low speed, a receiver may be necessary.
Receiver dimensioning requires special attention.
The receiver shall be large enough to contain part
of the system refrigerant charge, but shall not
be too large, to avoid refrigerant overcharging
during maintenance operations.
Liquid floodback during
operation
Danfoss recommends the use of an electronic
expansion valve for all air conditioning
applications with VZH028-044 compressors. An
EXV has two key benefits: it provides modulating
control of the system under varying load
conditions, and it protects the compressors from
flood back during adverse running conditions.
Exceptions to the use of EXV’s with Danfoss
variable speed compressors must be approved by
Danfoss application engineering.
Su
m
p
Su
pe
rh
ea
t
Saturated Suction
Floodback Requirement
°
C
-30
-25
-20
-15
-10
-5
0
5
10
15
20
0
2
4
6
8
10
12
14
16
18
20
22
ACCEPTABLE
UNACCEPTABLE
A crankcase heater is only effective if capable of
sustaining this level of temperature difference.
Tests must be conducted to ensure that the
appropriate oil temperature is maintained under
all ambient conditions (temperature and wind).
Provide separate electrical supply for the
heaters so that they remain energized even
when the machine is out of service (eg. seasonal
shut-down).
Refer to section “Crankcase heating function” for
details and settings of crankcase heating function
integrated in the drive.
31
FRCC.PC.034.A2.02
Application Guidelines
